# pressurised systems and non pressurised systems, whats the difference?



## ihookem (Feb 19, 2009)

I was looking into an OWB but an leaning towards a woodgun because it's a gassifier and is ok to put outside and has a big firebox with a 26" log capacity. I was told it has to be pressurised but I don't know much at all between the two. Can anyone give me advise on the two systems?


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## Nofossil (Feb 19, 2009)

Pressurized means that you don't need a heat exchanger between the boiler and the rest of your system. Since there's no ongoing supply of dissolved oxygen in the water, components (including the boiler) will last much longer, and water treatment is minimal to nonexistent. As far as I know, all indoor systems are pressurized. Any indoor boiler can be installed in a suitable outbuilding - many have done it. I assume that you've read up on the advantages of gasification. If you aren't planning on heat storage, you might want to at least design the system with the option of adding it later.


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## Northwoodsman (Feb 19, 2009)

A pressurized/closed boiler system operates at approx. 15 psi vs. a non-pressurized/open system which operates without pressure. Because the water in a pressurized system is not exposed to the atmosphere, it does not require the water treatment that a typical non-pressurized, open system does. Also, a pressurized boiler, because it is not exposed to polutents and oxygen, will typically last much longer than an open system.

The next question you have to ask yourself is do you want to go with a stainless steel boiler construction or a regular steel boiler construction.

There are several threds on the forum that discuss the pros and cons of each but I think you'll find the majority would choose regular steel over stainless. 

Stainless steel has a very bad habbit of cracking at the welds due to its extremely high expansion rate at elevated temperatures (thus building up internal stresses in the metal itself) and thus many people have experienced cracking at the welds of stainless boilers and thus subsiquent leaking.

Finally, when going with a gasifiaction boiler it is extremely critical to have adequate thermal storage. This storage allows the boiler to operate at its peak (90%+) efficiency and also reduces the amount of times a wood boiler must be filled with wood. I have an EKO40 gasification system with 1,000 gallons of storage and have been extremely impressed with the systems' operation and minimal wood consumption. Even on cold days (0-10 F) I am able to go approx. 18-20 hours before having to start a new fire and charge the thermal storage tanks back up. Two weekends ago when our temps in northern Michigan were up to 55-60F I was able to go 30-35 hours before making a fire. Thus, the thermal storage is extremely important in the spring and fall and allows the boiler to run at optimum efficiency where as a boiler without the storage will cycle rapidly and thus loose a large portion of its efficiency. 

I am heating a 2,500 sq. ft home at 73 F, 76 gallons of hot water for showers and a 350 gallon hot tub.

(BLATANT ADVERTISING PLUG REMOVED - MODERATOR)

Thanks,

NWM


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## Piker (Feb 19, 2009)

Dittos to what has been previously posted about closed systems.

There are some boilers on the market that do not require thermal storage... although it is very desireable for the shoulder seasons as stated above.  The flexibilty of not absolutely having to have storage allows you to install systems in stages.  Plenty of people put their boiler in and run it for a year or two without storage, and then add it later.  I believe Nofossil has done this, as am I this season.  If you plumb the provision for storage in when you first install the boiler, it's pretty simple to just hook it up and open a valve when you get around to getting the tanks in.

cheers


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## ihookem (Feb 19, 2009)

So  a closed system is better but must be more expensive right? I got a quote for a Woodgun 100 w/ stainless and it's 10k and the dealer said it would be another 8k for the install so i'm looking at 18k if he does everything. I can make the shed,dig the trench, pull the pex between the joists but it will still might be at  14k. At this point I won't get my money back for 15 years. I can even take the 14k and get 350 dollars in interst and that pays Januarys heat bill to boot. There has to be a cheaper way and still get hydronic heat. A forced air is cheaper and works but how much cheaper? The only thing I know about hydronic heat is that you use pipes  to carry btu's and it is a very warm 72 degrees and forced air natural gas is a very cold 72 degrees. Does this quote seem high? This guy knows everything about every furnace I asked about (he said he will install any furnace or OWB. don't matter to him but he likes the Woodgun the best but likes EKO and Orlan Ekos almost as much.


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## Piker (Feb 19, 2009)

Installation prices between closed and open systems aren't really that different.  If you already have a boiler system installed in the house, the closed loop ad-ons are usually cheaper because no heat exchange is necessary, and the rest of the system is already there.

You could probably save some money if you bought and installed an indoor gasifier, providing you have space and a chimney for it...  No ditch, no underground pex, no extra building.  That having been said, you will likely still not get a turnkey gasser system installed for much less than 10k no matter what.  The cost of the equipment to install the boilers is expensive.  

If you are only using $1000 per year worth of fuel for heating right now, your payback will definitely be long term.  Don't forget to factor in time and/or money spent acquiring firewood.  I believe it would still be a practical investment for you over the long haul.

Don't forget that the added comfort of a radiant system is worth 'something.'  On top of that, people who buy gassers don't usually tend to keep their homes cool when they can keep them 75 for minimal cost.  Our home is heated with both forced air and radiant floor heat.  If we put the temp above 72 in the radiant zones, it's almost uncomfortably warm... while the forced air portion of the home is always set at 74 when we are home.  It's cozy all the time... and if you're in to cozy, then cozy has value.

Again, take your time and don't get frustrated at preliminary numbers.  Do the research and find an installation that gives a payback that you are comfortable with.  There really is something out there for everyone.

cheers


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## fabguy01 (Feb 20, 2009)

????? everyone seems to think that a pressurized system is not open to oxogen getting into the water. How can that be? H2O is water part oxogen part hydrogen if there were no oxogen in your system you would be heating with hydrogen :bug:


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## Piker (Feb 20, 2009)

fabguy01 said:
			
		

> ????? everyone seems to think that a pressurized system is not open to oxogen getting into the water. How can that be? H2O is water part oxogen part hydrogen if there were no oxogen in your system you would be heating with hydrogen :bug:



Closed systems will still corrode, but at a much slower rate than open systems.  I think the difference is the amount of O2 that is suspended in H20.  I think... not a chemist here.

I have been considering the idea of putting some anti corrosive agent in all the systems I install just as a precaution...  most of the HVAC guys around here don't do that.  Not sure if the precaution is warranted or not.  

cheers


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## stee6043 (Feb 20, 2009)

fabguy01 said:
			
		

> ????? everyone seems to think that a pressurized system is not open to oxogen getting into the water. How can that be? H2O is water part oxogen part hydrogen if there were no oxogen in your system you would be heating with hydrogen :bug:



Ummmm....there is no "loose" oxygen in a water molecule.  It's bonded with hydrogen to form water.  It's not just sitting there chillin' with hydrogen waiting to split.

Your theory above would be similar to assuming table salt is likely "HIGHLY FLAMMABLE" because it has sodium in it.  Sodium alone is a highly reactive metal when exposed to water.  But I think you'll be hard pressed to find someone who has experienced spontaneoulsy combusting table salt when they threw a dash in a pot of water....


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## Der Fuirmeister (Feb 20, 2009)

ihookem said:
			
		

> So  a closed system is better but must be more expensive right? I got a quote for a Woodgun 100 w/ stainless and it's 10k and the dealer said it would be another 8k for the install so i'm looking at 18k if he does everything. I can make the shed,dig the trench, pull the pex between the joists but it will still might be at  14k. At this point I won't get my money back for 15 years. I can even take the 14k and get 350 dollars in interst and that pays Januarys heat bill to boot. There has to be a cheaper way and still get hydronic heat. A forced air is cheaper and works but how much cheaper? The only thing I know about hydronic heat is that you use pipes  to carry btu's and it is a very warm 72 degrees and forced air natural gas is a very cold 72 degrees. Does this quote seem high? This guy knows everything about every furnace I asked about (he said he will install any furnace orOWB. don't matter to him but he likes the Woodgun the best but likes EKO and Orlan Ekos almost as much.




Have you found one near by to look at?  I asked Pat at the AHS factory where I could go look at one in WI and got no response.  Same question for Jim at Garn.....no response.  Both received my inquiry ten days ago (confirmed email reply), but must be too busy to respond to a potential customer.


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## nic89 (Feb 20, 2009)

I thought you would still have heat exchangers in your pressurized tank that go to the house? Keeping the boiler water separated from house water.
That way your oxygen in your water in the boiler would be driven off in time due to the heat..


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## Nofossil (Feb 20, 2009)

nic89 said:
			
		

> I thought you would still have heat exchangers in your pressurized tank that go to the house? Keeping the boiler water separated from house water.
> That way your oxygen in your water in the boiler would be driven off in time due to the heat..



Not typically. In a pressurized system, the water in storage is the same water that's in the boiler. Domestic hot water is in a smaller and separate tank.


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## racinrick (May 13, 2009)

what type of boiler life difference are you speaking of between pressurized systems and non-pressurized?  I've got two rough numbers from two different contractors for a system for my house, one with pressurized tanks and one for non-pressurized and the price difference is substantial (6-7k).


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## tom in maine (May 13, 2009)

Boilers are not going to see the difference, since they are both pressurized and de-aerated. Both storage systems will not affect the life of the boiler.
An unpressurized tank offers you the convenience of getting the storage into a basement that is already built that does not have any other easy access for getting a propane tank or other large vessel into that location. Of course, a site built tank can be welded up in a basement, but if it is a pressure vessel, it's life expectancy might not be the same as a professionally built pressure tank.
I will leave it to others to comment on the longevity of used propane tanks. I would assume they should last.

Our (unpressurized) tanks have been around in one form or another for about 25 years. They are rebuildable, if that was necessary and are easily moveable--once emptied.


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## Singed Eyebrows (May 14, 2009)

Hi Ihookem; I wouldn't worry about the stainless steel welds cracking in a Woodgun as you have been warned about. They have been welding 304 for about 25 years.  Yes others may have troubles with this. Have you seen the number of stays in a SS Woodgun, it's four times as many as my old Energymate & this was ASME. I do not hear of welds cracking in the SS Woodguns, at least not recently.You are in Wis. as I, IPS tanks will weld up a pressure tank very reasonably if you go this route. My friend in Milwaukee just bought a SS E140 Woodgun with 4 of the better options & although it isn't hooked up I'm sure you could look at it. I also know of a E140 SS Woodgun that has been running in Wisconsin for about 3 years & the owner is very pleased with it. I saw him load green wood in it & all there was just  some steam out the chimney on startup. I am quite sure you could look at this. / Also, Der Fiur Meister, if you PM me I can give you the phone numbers of the 2 Woodgun owners after ok'ing it with them. I guess I got to sell Patricks boilers for him, Randy


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## Duetech (May 14, 2009)

stee6043 said:
			
		

> fabguy01 said:
> 
> 
> 
> ...



There is no loose oxygen in the molecule H2O but in the water mass extra oxygen molecules can be present just like dissolved minerals and various other chemicals like pollutants. Fish and aquatic creatures that use oxygen would otherwise render H2O into<maybe, either CHO or just CH and lakes would get explosive. Many ground water sources readily have extra oxygen (sub surface drainage) and rivers and streams, via turbulence, mix extra air into the water stream. Heat will often cause that extra air to gather to the side of a pan before the water boils. Oxygen robbing water treatment chemicals have a very usable place in a boiler system.


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## Gooserider (May 14, 2009)

Let me attempt to clarify this a oxygen question a little bit...

The way we usually encounter water, it is exposed to air, and has a certain amount of air dissolved into it, including the approximately 21% O2 that is part of normal air.  Tap water, well water, and pretty much any other water you would use to fill a boiler will have this dissolved air in it to a greater or lesser extent.

In a pressurised system, the water in it is sealed away from contact with the air, and two things happen - the first is that the combination of heating and cooling cycles, plus various mechanical devices in the boiler circuit remove the dissolved air from the water.  The second is that any remaining free oxygen will start to attack the iron in the boiler and plumbing and start to corrode it - however since there is very little oxygen, the amount of corrosion will be minimal, just a thin film.  That corrosion (aka as "rust") also chemically removes the free oxygen from the boiler water.  What remains is all bound in the water molecules as H2O, and so long as the Ph of the boiler water is in the right range, it's pretty much chemically inert, and won't cause any further problems.  Adding an oxygen scavenger to the boiler water is helpful in this process, but not absolutely essential - the more important function of a boiler additive would be to adjust the Ph of the water in order to keep it in the right range.

In an open system, the water is exposed to the air, and as such will continue to absorb air into solution, thus constantly replenishing the supply of free oxygen.  This free oxygen can and will attack any iron or steel it finds in the system as it circulates, causing additional rust - this rust can lead to either blockages in passages, or eventual failure of system components - Therefore in a non-pressurised system should be made with as few ferrous metal components as possible, and the use of an oxygen scavenging water treatment is much more important.

The same problem with oxygen happens with any top-up water added to a pressurized system, which is why the standard advice is to avoid draining and refilling a system as much as you can, and also to minimize the water additions...  Keeping free O2 out of the boiler water is also why one should only use O2-barrier type PEX in a boiler system.

Hope this helps understanding...

Gooserider


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## racinrick (May 14, 2009)

VERY informative.  For once, something actually made sense to me.  What is the typical life expectancy of a boiler with each system?


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## Gooserider (May 14, 2009)

racinrick said:
			
		

> VERY informative.  For once, something actually made sense to me.  What is the typical life expectancy of a boiler with each system?



Varies a great deal - depends on the boiler, details of the system, the care given, and so on, however I've seen numbers thrown around that suggest on the order of 20-30 years for a pressurised system, considerably less for a non-pressurised.  This can be somewhat compensated for by design if a unit was intended to be run non-pressurised - for instance Garns are claimed to have very long lives if properly installed and maintained.  OTOH, some fossil fuel units can supposedly be destroyed by excess oxygen in a matter of months...

I would tend to say that the basic guidelines should be never to run a system other than the way it was designed to be run - pressurising a non-pressure designed boiler would be dangerous at best, running a pressure designed boiler non-pressurised is likely to greatly shorten its life.  Secondly, if running non-pressured, you need to be fanatical about maintaining water quality.  Last, whatever sort of system it is, take all possible steps to limit O2 infiltration - use barrier PEX on all systems, don't tolerate seepage that leads to adding more makeup water (you should very seldom, if ever, need to add make-up water to a pressurised system once it is fully up and running, if you do, find out why and fix it)  In a non-pressurised system minimize the area of any vents while still allowing the unit to breathe, consider adding some type of 'sealant' to the exposed water surface, such as parafin or non-flammable oil of some sort (if safe to do so)

If I was designing a system, I would try to go pressurised as much as possible, though I would consider strongly using non-pressurised storage with either a heat exchanger or pressurised coils in the tank - somewhat easier to build, and getting the storage out of the pressure loop will greatly reduce the size / cost of the required expansion tank...

Gooserider


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## btwncentres (May 15, 2009)

Hi trying to find time to go through past forums...any one and only book out there that would be helpful?....also it seems that with open storage HX's must be used...is it totally wrong to have say a 2000 gallon open  storage and pull water directly from it to your building/bulidings ? reasons why? think I can think of some but there certainly seems to be some tricky stuff involved....
thanks......


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## Gooserider (May 15, 2009)

Btwncentres said:
			
		

> Hi trying to find time to go through past forums...any one and only book out there that would be helpful?....also it seems that with open storage HX's must be used...is it totally wrong to have say a 2000 gallon open  storage and pull water directly from it to your building/bulidings ? reasons why? think I can think of some but there certainly seems to be some tricky stuff involved....
> thanks......



You are correct - any time you go from pressure to non-pressure there must be some sort of heat exchange interface - this can be a flat plate exchanger, a "sidearm" unit, or a set of coils in the storage tank, and possibly other options...

Books - generally the one book that gets pointed at is Modern Hydronic Heating, 2nd Edition by John Siegenthaler, P.E. - It is regarded as the "Bible" on the subject of hydronics, and while probably less ambiguous than the Bible, it is similar in size, complexity, and being a tough read...  Also it's a relatively expensive book.  There are other books that also get pointed at, but with far less concensus, most noticably the titles by Dan Hollohan of HeatingHelp.com fame...

As to your question about pulling water directly from a storage tank for heating, in theory it would work, but it's probably a less than optimal design....

1. You will be using oxygenated water - this means that you need to either avoid ferrous components like cast iron circs and other fittings in that circuit (and the non-cast iron versions of these tend to be MUCH more expensive) or plan to spend more on chemistry to keep the corrosion under control.

2. It means you would need to heat up all the storage before getting useful heat to the load - can be a problem if things have cooled down for any reason.  Putting the storage as just one of the loops in a pressured system can let you give priority to the house first, then worry about heating the storage after the other loads are satisfied.

3. Because of the lack of oxygen, pressured systems are very sterile from a biological standpoint - little or nothing will grow in the system water.  In a non-pressured system, even with the high temperatures, you can sometimes get some really strange "wildlife" growing in your system, with the potential for getting thick enough to clog up the plumbing = expensive and difficult repairs...  Because of the many metals in the system and other issues, it can be a challenge to stop such growth by use of chemistry....  If you just have a tank w/ coils in it, any growth is mostly a non-problem as long as it doesn't stink...  If you have a plate exchanger based setup you would have more of a concern, but still it's a case of the less plumbing exposed, the fewer potential problems...

4. The more you circulate the water, the greater challenge it would be to maintain good stratification - it can be done, it just increases the complication of the tank plumbing design...

Bottom line - If I had to do a "blank sheet" setup where I needed to do an open storage tank, I would choose a setup that put coils in a tank of non-circulated water, but otherwise kept the number of distinct heating loops to a minimum*.  (Others might choose other options) I feel that this is something that gives maximum flexibility in storing and extracting heat, maximizes stratification, and is the least likely to cause problems.

*meaning that I would keep all the plumbing I could in one pressurized zone - I would probably end up with three independent sets of fluid - 
1. The pressurized water circuit (boiler, house heat loads, DHW heating, storage heat exchange, etc. which could be configured in multiple zones and other interconnected loops), 
2. The glycol circuit (Solar heating panels, garage heating, and any other freeze prone items that would be better with the use of a glycol antifreeze solution instead of plain water)
3. The storage tank - non pressured, non circulating

Gooserider


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## 91220da (May 15, 2009)

Hi Guys,
Any reccomendations on where, or what type, of oxygen scrubbing additive to buy.  I am running a non pressurized open system (loop) to my pressurized oil furnace in the basement through a flat plate hx.  I went with the open system because my wood boiler is in the garage with no make up or recovery water availiable.  I have a 55 gallon expansion/holding tank as a resevoiur and the loop is filled with about 30 gallons of water glycol mix.   The glycol supposedly contains a corrosion inhibitor but your discussion got me interested in the oxygen scrubbing addative ?????


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## Gooserider (May 16, 2009)

Nothing specific, might be worth talking to some of the Garn guys or the OWB folks since they run open by design...  Some stuff I've seen suggested that it may depend on your individual water chemistry - there are supposedly labs out there that will (for a price) analyze a sample of your boiler water and recommend an optimal chemical mix, but I don't have any details on who / where they are...

Gooserider


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## fabguy01 (May 17, 2009)

91220da said:
			
		

> Hi Guys,
> Any reccomendations on where, or what type, of oxygen scrubbing additive to buy.  I am running a non pressurized open system (loop) to my pressurized oil furnace in the basement through a flat plate hx.  I went with the open system because my wood boiler is in the garage with no make up or recovery water availiable.  I have a 55 gallon expansion/holding tank as a resevoiur and the loop is filled with about 30 gallons of water glycol mix.   The glycol supposedly contains a corrosion inhibitor but your discussion got me interested in the oxygen scrubbing addative ?????


http://www.woodboilersolutions.com/


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## pybyr (May 17, 2009)

91220da said:
			
		

> Hi Guys,
> Any reccomendations on where, or what type, of oxygen scrubbing additive to buy.  I am running a non pressurized open system (loop) to my pressurized oil furnace in the basement through a flat plate hx.  I went with the open system because my wood boiler is in the garage with no make up or recovery water availiable.  I have a 55 gallon expansion/holding tank as a resevoiur and the loop is filled with about 30 gallons of water glycol mix.   The glycol supposedly contains a corrosion inhibitor but your discussion got me interested in the oxygen scrubbing addative ?????



My boiler is not open (though my storage will be), and, on the advice of a number of pros, I went with "8 way boiler chemical" in my system

www.rectorseal.com/files/172/ds8way.pdf

I got it at the local F.W. Webb, and it was very modestly priced.  A couple of quarts treated my boiler and all associated plumbing.

I would tend to think it would work OK in your application, too, as long as you occasionally draw samples or look at it through a sight tube to see that the indicator color still looks good.  It is recommended for steam as well as hydronic systems, and steam systems are, for all practical purposes, "open."

I found in talking to some local installers (regular heating guys, not wood-boiler-specific) that it seems common that they just use regular domestic water, with no treatment.  For my money, after a lot of $$$ and sweat putting in my Econoburn, I didn't want to skimp on $13 +/- of chemicals to extend the useful life of my investment.


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## pybyr (May 17, 2009)

Gooserider said:
			
		

> Books - generally the one book that gets pointed at is Modern Hydronic Heating, 2nd Edition by John Siegenthaler, P.E. - It is regarded as the "Bible" on the subject of hydronics, and while probably less ambiguous than the Bible, it is similar in size, complexity, and being a tough read...  Also it's a relatively expensive book.  There are other books that also get pointed at, but with far less concensus, most noticably the titles by Dan Hollohan of HeatingHelp.com fame...
> 
> Gooserider



Goose- I think you raise good points all around.  Only thing I'll note is that I managed to get a copy of Siegenthaler's masterwork by way of interlibrary loan, as I was not ready or able to throw well over $100 to purchase a book sight unseen.  I found it to be a GREAT book, and spent several weeks devouring it.  Athough I did not need to buy it, on account of the fact that I gleaned what I needed for my system, I was really impressed with it and would say that it took my understanding of both the nitty gritty and big picture of hydronics from clueless newbie to bold and empowered designer and construct-er of my own system.


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## Gooserider (May 17, 2009)

Glad to hear the positive review - Have to say I haven't read it myself, but was going by the comments of other folks here, and on some of the sites that sell it...

Ironically, there was a recent post pointing to a Canadian industry magazine that had a couple of Sigenthaler articles in it, and some ways I was less impressed by the layouts in those articles than I've been by some of the ones I've seen here...  Hopefully the book would be better, or maybe there were subtleties I wasn't getting...

Gooserider


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## Nofossil (May 17, 2009)

> *meaning that I would keep all the plumbing I could in one pressurized zone - I would probably end up with three independent sets of fluid -
> 1. The pressurized water circuit (boiler, house heat loads, DHW heating, storage heat exchange, etc. which could be configured in multiple zones and other interconnected loops),
> 2. The glycol circuit (Solar heating panels, garage heating, and any other freeze prone items that would be better with the use of a glycol antifreeze solution instead of plain water)
> 3. The storage tank - non pressured, non circulating



You have described my system EXACTLY. Having run it in this configuration for three years, I'm contemplating a switch to pressurized storage. In my case, the issue is that any heat exchanger loses transfer capacity as the temperature differential decreases. That means that with boiler outlet water at 180, it's really easy to get storage from 120 to 130, really hard to get it from 160 to 170, and impossible to get it from 170 to 180.

The ability to have additional in-tank heat exchangers is the reason I went with open storage, and it's an important advantage. In addition to the charge/discharge coil, I have a DHW preheat coil and a coil for extracting heat from the solar panels. If I go pressurized, I'll probably do some large surface area sidearm variant for both of these functions.

I have the Siegenthaler book. It's invaluable, and I'd suggest that if you can't read and understand what he's saying you should spend some extra time getting system design help from someone who has that level of knowledge. It may not be your local plumber, though....

I don't disagree with Siegenthaler on any major points, but I find it interesting that in one section he goes to great lengths to show the total cost of a circulator including electrical power consumption over the system lifetime, making the point that the kWh add up over the years. Then, he completely fails to mention that primary / secondary systems consume two to five times the electricity of other designs, or that zone valves consume a tiny fraction of the power used by circulators.


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## SolarAndWood (May 17, 2009)

nofossil said:
			
		

> You have described my system EXACTLY. Having run it in this configuration for three years, I'm contemplating a switch to pressurized storage. In my case, the issue is that any heat exchanger loses transfer capacity as the temperature differential decreases. That means that with boiler outlet water at 180, it's really easy to get storage from 120 to 130, really hard to get it from 160 to 170, and impossible to get it from 170 to 180.



Is your primary motivation to increase the effective capacity of your storage?


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## Gooserider (May 17, 2009)

nofossil said:
			
		

> > *meaning that I would keep all the plumbing I could in one pressurized zone - I would probably end up with three independent sets of fluid -
> > 1. The pressurized water circuit (boiler, house heat loads, DHW heating, storage heat exchange, etc. which could be configured in multiple zones and other interconnected loops),
> > 2. The glycol circuit (Solar heating panels, garage heating, and any other freeze prone items that would be better with the use of a glycol antifreeze solution instead of plain water)
> > 3. The storage tank - non pressured, non circulating
> ...



Sounds reasonable, though I assume that one can slightly improve on the in-tank heat exchanger performance by using more tubing and / or better performing exchange tubes...  I was thinking of using some stuff like what these guys make - would that do better than plain copper pipe?  In actual practice, I don't know that I have a lot of choice, as MA currently requires ASME stamps on pressure vessels, and I would have a great deal of difficulty in getting any kind of sizeable / affordable pressure tanks into our basement in any case...  



> I have the Siegenthaler book. It's invaluable, and I'd suggest that if you can't read and understand what he's saying you should spend some extra time getting system design help from someone who has that level of knowledge. It may not be your local plumber, though....



No problem with either idea, and didn't mean in my comments that his book was incomprehensible, just that many of the comments suggested that it was a technically oriented book written for those with some level of knowledge on the subject, and as such was not an "easy reader" book...



> I don't disagree with Siegenthaler on any major points, but I find it interesting that in one section he goes to great lengths to show the total cost of a circulator including electrical power consumption over the system lifetime, making the point that the kWh add up over the years. Then, he completely fails to mention that primary / secondary systems consume two to five times the electricity of other designs, or that zone valves consume a tiny fraction of the power used by circulators.



Exactly!  You put your finger on the biggest thing that bothered me about his suggestions in the articles I mentioned - he seems to be very fond of throwing lots of circs into a system, which IMHO defeats the purpose - why take money away from the oil / gas companies in order to give it to the electric companies?  IMHO one of the desired design standards should be to minimize the amount of electricity that is needed to distribute the heat.  

This is also a potential issue I have with some of the designs I see that seem to want to have circs running 24/7 - I suppose if one is talking a really tiny circ it might be possible for it to consume less power running all the time than a larger circ running intermittently, but I would really want to see some hard numbers on that, as from what I've seen even the larger Taco's don't seem to be all that high power draw...

Gooserider


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## DaveBP (May 17, 2009)

> I don’t disagree with Siegenthaler on any major points, but I find it interesting that in one section he goes to great lengths to show the total cost of a circulator including electrical power consumption over the system lifetime, making the point that the kWh add up over the years. Then, he completely fails to mention that primary / secondary systems consume two to five times the electricity of other designs, or that zone valves consume a tiny fraction of the power used by circulators.



Yeah. Ironic, isn't it. 

I hope in future editions of the book he will give more attention to comparing the different overall strategies in terms like electric power usage and maintenance and repair costs. 

(And I also hope that someone I know buys it so I can borrow it and not buy it).


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## pybyr (May 17, 2009)

Gooserider said:
			
		

> I was thinking of using some stuff like what these guys make - would that do better than plain copper pipe?
> Gooserider



I'd looked into that product as it had been highly recommended by several knowledgeable people on here (Joe Brown and/ or Hot Rod Bob Rohr).  The cost was pretty staggering.  For my planned DHW loop within my unpressurized storage, I ended up getting about 70 ft of corrugated 3/4 stainless tubing through East Coast Metal Hose-

http://www.eastcoastmetalhose.com/home.htm

The 70 FT will run in loops around the upper perimeter of my 6x6 by 5 ft high tank

My aim/ expectation is that the corrugation of the stainless should increase the surface area, and also increase the turbulence (and thus surface contact of the fluid) within the tubing.

The HX between the boiler system and the tank will be a 5x12x70 plate FlatPlate.

I've got most of this stuff on hand and the tank already partially assembled- just need to re-ignite my motivation to finish the storage and connect it together.  My Econoburn did a pretty nice job this past winter, once I put it on line on 1-15-09 even without storage, but it all ought to get even better with storage.


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## Gooserider (May 20, 2009)

Interesting looking stuff, but the only stuff I saw looked like the braided stainless (or bronze) tubing I use for brake lines on my bike, wonderful stuff but it has a lining inside it, with the metal braid around the outside for strength, and I would expect it to have fairly poor heat transfer characteristics...  If it is more of a corugated pipe like a longer version of the expansion joints they were picturing, it might work very well...

Gooserider


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## pybyr (May 20, 2009)

I've already got it in my cellar-- it is corrugated 300-series stainless, with no braid, nor any non-metallic lining- and the vendor that I mentioned brazed NPT pipe thread fittings on the ends of the 70 FT of corrugated for me.


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## tom in maine (May 21, 2009)

I am not quite getting the rationale of using corrugated pipe or other materials instead of smooth copper for heat exchangers.
Maybe it is because I have spent a lot of time and energy over the past 30 years checking out these alternatives.
There are a lot of research papers done on this whole issue and every time (I would say except where someone is trying to sell their product)
the cost/benefit of smooth copper is much better than any extended surface tubings. 
Since we manufacture smooth copper hx, I guess you can say the same about us, but NREL and a bunch of engineering schools have done the testing.

Now, if you own the stuff, an extended surface material might make sense, but most folks are buying this stuff--for a lot of money. I think when you factor in the ease of fabrication, smooth copper is a great immersed hx. Smooth PEX can work also at 3-6x the lineal footage.
Extended surface materials are usually using refrigerants or are other applications other than water/water. 

The configuration of the heat exchanger is significant, but when properly designed in a regular tank, performance can be very good.
Corrguations do not do as much as we would like to believe in this application.

Give it a try, but I would suggest trying to research it more, if you are planning on spending a bunch of money in high hopes of gaining some great performance gain.


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## Gooserider (May 21, 2009)

Tom in Maine said:
			
		

> I am not quite getting the rationale of using corrugated pipe or other materials instead of smooth copper for heat exchangers.
> Maybe it is because I have spent a lot of time and energy over the past 30 years checking out these alternatives.
> There are a lot of research papers done on this whole issue and every time (I would say except where someone is trying to sell their product)
> the cost/benefit of smooth copper is much better than any extended surface tubings. <snip>



Interesting - I will admit I haven't done any testing, but I would have thought there would be benefits to using the increased surface materials - ideally with surface extensions on both sides...  On the outside I would have expected the benefit to come from increased surface area for heat exchange, and on the inside, from the surface increasing the turbulence in the flow, bringing hotter water into contact with the tube walls, and breaking up any laminar flow patterns, again increasing the heat transfer rate - presumably reducing the amount of tubing length needed in the exchanger.

Of course, I guess that if you put enough feet of smooth pipe in the tank, the above advantages would cancel out... Depending on the relative costs the smooth pipe might be cheaper.  

Which case is it, that the extended surface stuff doesn't give as much increase in heat transfer as one would expect, or that the cost differential isn't good enough to justify it?

(Would there be advantages in hitting the salvage yards to see if one can find used stuff, or is it flat not worth the effort?)

Gooserider


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## pybyr (May 21, 2009)

Tom- not to disagree with you- you obviously have a depth in this subject that exceeds mine- I'd be interested in links to the NREL and academic studies you mention, and I expect that others would also.

I sourced the stainless corrugated tubing for the DHW loop partly because my tank is also stainless and I didn't want to get into issues about dissimilar metals setting up long-term funky dielectric/ corrosion factors over the long haul.

One commercial application I can think of that does use corrugated stainless is the Haase heat tank.  But oh my goodness, is that expensive


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## tom in maine (May 21, 2009)

The one seminal paper is one that I have in my office. I will get it scanned in and email it to you.
Davidson, at the University of Minnesota has done a lot of research for NREL on immersed heat exchangers.
You might Google some of them. Their focus has been on inexpensive smooth polymer hx, but after all the work done, NREL has collaborated on a warm climate solar collector with integral storage that uses a smooth copper hx. And for good reason, as you said, corrugated exchangers are expensive.

As I recall, there is a barrier effect in corrugated tubing that negates the extended surface benefits in immersed water/water applications and also induces more flow restriction than one would want to see. My engineer friends get into Reynolds numbers, etc. and I start to glaze over. Bottom line is with Dick Hill, my retired ME friend, who developed the first gasifiers, who says that corrugated tubing is not worth looking at in our applications.

Once I found some papers corroborating what he told me, I have focused on using smooth copper. 
Is it perfect? Absolutely not. But nothing in this world is.


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## Gooserider (May 21, 2009)

Don't know just why but I have less trouble believing what you say about the outside of the exchanger than I do about the inside...  Just out of curiosity do you know if anyone has tried doing anything with "turbulator" type inserts in the exchanger pipes?

Gooserider


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## tom in maine (May 21, 2009)

Turbulators on the inside would work well, except how do you put them in a normal hx?
You need to get turbulent flow in the coils since that is part of the equation. 
I suspect extended surface on the outside is worthwhile, it is inside where the problem arises.

I think much of this issue is plain old dollars and cents. Extended surfaces are too expensive and do not give the advantage
that everyone thinks they do in a water immersed water to water heat exchanger.

There is a lot of subtle physics taking place in tanks with coils, between volume of the coils, temps, stratification, flow, and 
coil thermal performance. The only way to test this is to cut and try. 
The engineers I dealt with had weird ideas about heat exchangers. We had many coils that did not work and sorted it out initially by trial
and error in solar drainback tanks. 

There is no greater motivator than a pissed off customer who cannot get enough hot water in January in Maine!

You don't have to believe me, but if you are looking at a big bill to buy extended surface heat exchangers, you might want to compare it to
smooth copper. There are enough people here who are using them that you can easily calculate the costs.

Trendsetter and Haase use extended surface heat exchangers and they cost a lot of money. I will bet they use them to look impressive
and imply some sort of high tech performance. Or they never did any homework and met a smooth corrugated sales person.


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## DaveBP (May 21, 2009)

When I was scheming on how to make my copper tube heat exchangers with all the tube benders available at work I had the notion to mount some 1/4" dowel pins at opposing 45 degree angles on the jaws of a vise and indent the tubing 1/8" or so every few inches to create a helical pattern along the length of the tubing. The idea was to make a spiral turbulator effect along the water path that would produce a spiral water flow inside the tubing without restricting the flow too much ( through 3/4" tubing). Going to revolutionize the industry, I was. 

Checked into the price of corrugated tube and fin tube and figured you could buy a whole lot of plain old copper tube for the same money. That's when I thought of "enhancing" the interior turbulence of the copper tubing. 

Of course, like a number of other times I was going to revolutionize the industry with a bright idea of my own, I got distracted (in this case by a pair of 500 gallon propane tanks) and never did manage to change the world. Alas, next time....


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## in hot water (May 21, 2009)

A lot of changes in heat transfer design these days.  I noticed several of the European tank manufacturers switching some, if not all of the coils to corrugated.  It's a surface area game, no question.  Same with the turblators inside the tube.  Check out copper tube boiler tubes and you will see the devices they insert into the tube to "stir the water.

I read an article on Delphi recently.  They build radiatiors, oil coolers and condensors for the automotive industry.  They have been adding turbulators to their products to increase transfer, just as we do for the EKO tubes.  Same with the HVAC industry.  Trying to squeeze any performance increase is what heat transfer engineers do, the quest goes on.

The Triangle Tube tank in tank indirect and solar tanks use a corrugated inner tank to get more surface area.

To compare fairly you would need coils built from the same materials.  I've not seen corrugated copper coils used in tanks.  Finned copper, yes, but smooth coils have been shown to out-perform fined if the surface area is exactly the same. The German Solar Energy Society has some good data on smooth vs finned copper heat exchanger tubes. that subject.  Copper is a better conductor, than stainless so comparing copper to stainless would not be fair.

I've been working with a company in Ohio that builds HX tube and turboltors as a way to increase performance of solar coil tanks.  See the picture of one of the sample tubes they sent us for testing.

Here is another company that deals with turbulators www.alcotwin.com/turbulators.html

Yes, it comes down to what is not only do-able, but at what cost, for what performance increase.  And certainly the pressure drop and increased pumping power needs to be considered.

I suspect high copper costs drive manufacturers to look at other materials, like stainless.

Yet another example of surface are can be seen in flat plate exchanger.  Now if you really want performance, skip the tubes in a tank and consider a small plate stye.  Lots of performance from a small block.  Surface area, turbulance, and two pump flows are the key.

 hr


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## Dune (May 25, 2009)

Sorry to jump into this so late, but I think the primary consideration would be the heat conduction rate of the metals involved. Since copper's rate of heat conduction is aproximately 6(six) times that of stainless steel, there is not much of a question. As far as finned conductors, these would make more sense in a water to air system, since most of the conduction is from the edge of the fins, effectively adding very little surface area in a water to water system. I have to agree with Tom, the most economical and practical system is smooth copper tube.


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## DaveBP (May 25, 2009)

I want to see the machine that makes the stuff in Hot Rod's photo.


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## Nofossil (May 25, 2009)

Dunebilly said:
			
		

> Sorry to jump into this so late, but I think the primary consideration would be the heat conduction rate of the metals involved. Since copper's rate of heat conduction is aproximately 6(six) times that of stainless steel, there is not much of a question. As far as finned conductors, these would make more sense in a water to air system, since most of the conduction is from the edge of the fins, effectively adding very little surface area in a water to water system. I have to agree with Tom, the most economical and practical system is smooth copper tube.



Heat transfer is an astonishingly complex process, and almost any first-approximation mathematical model is wrong - sometimes shockingly wrong. In a fluid-to-fluid heat exchanger, turbulence and convection are both important. Here's a way to look at it. First, a static model of a coil-in-tank system:

Imagine a hot water molecule on the inside of the pipe, at an average distance from the pipe surface - say 1/4". We need to transfer the heat from that water molecule to a cold water molecule that's an average distance from the outside ot the pipe - say 6". There are 5 thermal barriers to deal with:

1) The 1/4" of water between the hot molecule and the pipe wall.
2) The boundary layer between the water and copper (or stainless)
3) The metal of the pipe itself
4) The boundary layer on the outside of the pipe
5) the 6" of water between the outside of the pipe and the average 'cold' water molecule.

Of these, the water itself is by FAR the biggest insulator. My guess is that the boundary layers are next, and the metal pipe is last, whether it's copper or stainless.

There is another factor, of course - how much time you have before the hot molecule is circulated away.

On the outside of the pipe, this model is too simple. Still fluids naturally establish convection currents as heated fluid rises and cool fluid falls. This serves to bring fresh cooler fluid into contact with the heated surface and reduce the average distance between the hot surface and the average cool water molecule. However, there is a maximum convection velocity which is much too slow to  accomplish optimum heat transfer. For that reason, anything that increases the amount of external surface area will help dramatically by bringing more cool water in contact with the heat source.

On the inside of the pipe, the static model is also too simple. For the diameters and flows that we're going to have, flow is very turbulent and water molecules are continuously bouncing around from the center to the outside. The only thing that can help here is more surface are and more time. A larger diameter pipe helps in both cases. Because the flow is already turbulent, additional turbulence enhancers probably don't make much difference.

If you wanted to get hard data, what you'd want to do is measure temps at multiple spots and look at the temperature differences:

1) fluid, center of pipe
2) fluid, at pipe wall
3) pipe wall, inside surface
4) pipe wall, outside surface
5) fluid, at outside of pipe wall
6) fluid, short distance above pipe

I suspect that you'd find virtually no difference in the first two, showing that the fluid is turbulent and well mixed. There would be a larger drop between the fluid at the pipe wall and the pipe wall itself (both inside and outside), but virtually none between the inner and outer surfaces of the pipe. The biggest drop would be the fluid at the outside pipe wall vs. fluid an inch or two away.

If I'm right, that argues for the largest diameter pipe that you can do, with fins on the outside if possible.


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## tom in maine (May 25, 2009)

Nofossil, you have hit the nail on the head, mostly.
There are a lot of variables that are constantly changing in a seemingly static tank environment.

Many folks just throw a lot of copper into the tank until it works. That is pretty expensive in the best of times and suggests lack of engineering by the manufacturer.
I believe many of the extended surface heat exchangers suffer from boundary layer effects and fluted and corrugated heat exchangers can affect flow
rates. I think a lot of these extended tubes were originally designed for condensing or evaporating refrigerants--a whole different animal than that water to water heat exchange environment inside a tank.

Fins might help, but when one factors in the cost and the possibility of improper orientation of the fins or more importantly, the damage that fins can reek on a liner, I suspect barrier effects and cost cannot overcome the cost effectiveness of smooth copper.
As one who has analyzed this for over 25 years, I keep coming back to smooth copper. Would like to think there are other options, but other than plate hx with extra pumps, there are no real good choices other than a well designed smooth coil in an immersed environment.

The one time I think larger tubing diameter might be more helpful is with PEX, but that might only afford you some preheated volume before the rules of thermodynamics come and bite you and force you to use 3-6x the surface area of copper.

Have a Safe and Happy Memorial Day.
Let's Remember all our Veterans and  the 9/11 people, too.


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