# pressurized vs. non-pressurized storage



## chuck172 (Mar 2, 2011)

I'm in my third year of heating with my Tarm and I never suspected that my Domestic hot water system is lacking. According to Chris @Tarm:
"Pressurized storage tanks do not normally have domestic hot water coils in them.  Most options being used in North American are simple steel tanks, either factory fabricated for the purpose or modified LP tanks, etc.  In most cases these tanks do not have a heat exchange coil in them and it is difficult to add one.  This means that the heat exchange for domestic hot water production has to take place outside of the tank, typically this is done with an indirect water heater.  The heat exchange coil that is built into the indirect is sized assuming 180 degree supply water temperature.  This is not a problem when you are heating the indirect with the wood boiler or fossil fuel boiler, but when you are heating the indirect from storage, you will be using lower temperatures than that and the performance of the indirect drops off quickly.  Even if the tank starts out at 180 degrees (or more), once you start to get below, say 160 degrees (150?), you canâ€™t expect much hot water production from the indirect.  This is all perfectly practical, but it does mean that you need to fire the boiler frequently enough to maintain high enough tank temperatures to meet your domestic hot water demand when you are running off of storage for a significant portion of the time, like in Summer.

An unpressurized tank system normally incorporates a heat exchange coil for domestic hot water production.  It is easy to â€˜oversizeâ€™ this coil so that it has enough surface area to produce acceptable amounts of domestic hot water at lower thermal storage tank temperatures, say down to 120 degrees tank temp.  This coil is normally supplied directly from the well or city cold water supply and feeds into a more traditional water heater.  The cold incoming water is heated, or pre-heated, before it gets to the water heater and the water heater only fires when the pre-heat from the thermal storage tank is insufficient.  In an ideal, but expensive, scenario, the in-tank domestic hot water coil feeds a normal indirect water heater.  This allows you to pre-heat the incoming cold water as described and simultaneously heat it via the coil built into the indirect.  This gives even more heat exchange surface area and so allows for even lower tank temps.  One of the things that people are looking for when they are thinking about heating domestic hot water with the wood boiler in the Summer, in my experience, is long coast times between wood boiler firings.  I would suggest that the unpressurized tank system I described means the difference between firing once or twice per week and firing two or three times a week with the pressurized system I described.

There are of course many variables including storage volume and how much hot water is needed (retired couple or four kids and a ton of laundry?).  Also, there are clearly other options like shell-in-tube or flat plate heat exchangers that can be used with pressurized storage to take advantage of lower tank temperatures.  And, of course, if you are living in Europe , you can just buy a pressurized storage tank(s) that have over sized coils built in!

As I say, none of this is a hard and fast rule about pressurized vs unpressurized storage, but given the market as we see it and the technology available here, I think they are good rules of thumbs.  I will be very interested to hear othersâ€™ thoughts on this, it is something that is very important to the industry and consumer and something that we continue to struggle with".  Chris.

I have a 45 gallon superstore that seemed to work well with my pressurized storage. Now I'm wondering if I can upgrade my DHW system. I plan on burning all summer.


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## Nofossil (Mar 2, 2011)

It's a good discussion. My current candidate for best of all worlds:

1) Pressurized storage, to start. Much better performance, more usable heat from same size tank.

2) Indirect DHW tank, plumbed as a zone in the heating system so that it can be heated with wood or backup heat source.

3) DHW tank mounted near but above pressurized storage with sidearm heat exchanger plumbed to thermosiphon from storage. That SHOULD result in raising the temp of the DHW tank to the temp of top-of-storage automagically, but allowing DHW to remain hotter than storage if storage heat is withdrawn for space heating purposes.


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## chuck172 (Mar 2, 2011)

Chris (from Tarm)  claims that the coils in the indirect hot water tanks are designed for 180* boiler water temp. and are somewhat lacking when used with lower temperatures as seen with our wood-boiler pressurized storage systems.


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## Singed Eyebrows (Mar 2, 2011)

When IPS built my tank I had a 3' diameter 2 turn stainless domestic coil put in. Even with this easy install it was about $300.00 for the loop. I know my tank is not typical, it is an option though, Randy


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## ewdudley (Mar 2, 2011)

nofossil said:
			
		

> It's a good discussion. My current candidate for best of all worlds:
> 
> 1) Pressurized storage, to start. Much better performance, more usable heat from same size tank.
> 
> ...



+1

And I would note that the power of the sidearm will increase with elevation, so if the sidearm was above the storage tank and the DHW tank was above that in a first-floor closet, then so much the better.


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## dzook (Mar 2, 2011)

We just finished a project with a Vedolux 37 where the owner used a 1000 gallon propane tank and had it modified with two domestic coils installed into the top to the tank. the boot/flange that held the coil was mounted almost horizontally into the tank but right at the top. With two domestic coils, doubling the surface area, the tank seems to give good domestic water with the top of tank down to 140deg.
 BTW the lines going into the domestic coils were redone neater after this photo was taken.


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## Jeff S (Mar 2, 2011)

I wish I thought of it before I insulated my tanks but I think if you wrapped your propane storage tanks with several loops of copper or maybe even pex you would have a effective way to preheat your domestic water.I believe the circumference of a 500 gal. tank is close to 10 ft.You could easily wrap several hundred ft.around one of these things for a large reserve and since all would be insulated ,the water in the tube should be the same temp as the tank.All that would be left would be to temper it.


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## tom in maine (Mar 2, 2011)

If you have a SuperStor in your system, this can draw off your pressure tank and it would allow you to heat the Superstor with relatively cool or warm water.
As an example, if your pressure tank is only 118F you can pump that tank water into the SuperStor and heat it to about 118F. It might take a while, but it is do-able and you do not need to modify anything. An external plate hx will do the same thing with a regular water heater tank. This scenario gets around the limitation of using an in tank coil in a pressure tank. Most pressure tank coils are not large enough to perform very well, at say 2 gpm, when the tank is cooler than 130F.

All this being said, an unpressurized tank does not suffer this same limitation since we can install whatever size heat exchanger we want.
Our standard DHW hx will deliver usable hot water at 3 gpm with relatively cool tanks, being below 120F. 
We wind up dealing with a lot of apartment building and motel systems that require very large gpm ratings for solar DHW systems. It is a simple matter to install a very large hx in an unpressurized tank to fit any flow rate. They can be simply retrofitted also. Just pop the cover and install whatever you need.

I know that a lot of you like the perceived economy of old LP tanks, but for convenience and service and simplicity (read that very small expansion tank), an unpressurized system can offer a lot.

Maybe it is not your cup of tea, but it does work very well if properly designed.


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## jimdeq (Mar 3, 2011)

Chuck172,  I have a sidearm heat exchanger off of 1200 gallons pressurized storage.  I dont think I have it plumbed correctly because it sure seems like it takes a long time to recharge.  It runs off a Grundfos Alpha.  The sidearm is 48" of 1.5" inch copper with a 3/4" inside.  It runs off a zone valve.  I have a stainless tee threaded into the bottom of my propane fired hot water heater.  Sidearm goes into the side off the tee and A419 aquastat set at 130 is threaded into the end of the tee.  The problem is that it seems like Alpha is constantly pumping to the sidearm.  I know it is working because water heater rarely fires,but it seems like I could do something more efficient.  Any thoughts anyone?


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## in hot water (Mar 3, 2011)

chuck172 said:
			
		

> Chris (from Tarm)  claims that the coils in the indirect hot water tanks are designed for 180* boiler water temp. and are somewhat lacking when used with lower temperatures as seen with our wood-boiler pressurized storage systems.



There are numerous tanks that are approved for higher than 180F in the coil.  The tank we sell is approved for up to 200F in the coil as well as the tank temperature. 

 But you really should not have to drive them that hot to get acceptable performance.  Most indirects, regardless of the brand will have 15- 18 square feet of HX coil surface.  Some use finned coils, some are smooth, some use corrugated stainless.

If you really need more performance you can buy dual coil indirects and series the two coils.  We offer a tank with 36 feet if 1-1/2" diameter lower coil, with an upper coil option of another 30 feet.  With a large diameter coil you can move a lot of flow thru the coil.  We have tested ours with a 14 gpm flow in the coil.  14 gpm of 180- 200F boiler input should cover just about any residential DHW application you can throw at it.

Some of the reverse indirects used to be rated with 200F boiler supply to get the high output numbers.  Make certain to have a good listed thermostatic mix valve if you plan on running those high temperatures or with those side arm, long tube type HX that run wild, with no temperature limit.  it's possible to get your DHW up close to boiler temperature, that can be extremely dangerous!

hr


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## chuck172 (Mar 3, 2011)

But how do the indirects (superstor) do when the pressurized storage temps drop to say 140*, and I'm looking to heat dhw to 120*?
Is there generally enough square feet of coil in the water heater tank to efficiently  do the job?


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## Rick Stanley (Mar 3, 2011)

chuck172 said:
			
		

> But how do the indirects (superstor) do when the pressurized storage temps drop to say 140*, and I'm looking to heat dhw to 120*?
> Is there generally enough square feet of coil in the water heater tank to efficiently  do the job?



I'm gonna guess, no. But we'll see what the heating guys say. I have an oil boiler with a dhw coil in it. I've been keeping that boiler at 140 all winter with my Garn. Dhw production SEEMED ok and heating SEEMED ok. I ran out of wood a few days ago and am now using oil and keeping the boiler at 180. There's no comparison. Waay better showers (2 family house) and the house is warmer.
   Anyways, I don't know why an indirect would work any better............?? bU t wHa t dew eyE nO


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## tom in maine (Mar 3, 2011)

chuck172 said:
			
		

> But how do the indirects (superstor) do when the pressurized storage temps drop to say 140*, and I'm looking to heat dhw to 120*?
> Is there generally enough square feet of coil in the water heater tank to efficiently  do the job?



Yes, the circulator might run longer to heat up the tank, but as long as there is a reasonable temperature differential,
it will do the job.


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## Nofossil (Mar 3, 2011)

Indirect DHW heaters usually have a coil sized to provide a heat load of around 100K btu/hr at 180 degrees to match the output of a typical fossil boiler. That also provides rapid recovery after your teenager has taken a 45 minute shower.

You can run cooler water through and still transfer heat, but it will take exponentially longer as the supply temp approaches the DHW temp. It helps to run the circ at really low speed to make the most of whatever stratification you have in storage.

In reference to other post on long time to heat DHW with sidearm - that's certainly true. Sidearm HX has VERY little surface area and relies on thermosiphoning for the DHW <-> sidearm transfer. Works well when you have hot water flowing through the sidearm anyway as you would when heating other zones. Pretty slow if that's the only reason you're running a circulator. That's the reason form my thermosiphon approach above - slow heat transfer, but operates 24/7 as needed with no circulator. I don't have that setup, but that's what I'd do if I were starting over.


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## tom in maine (Mar 3, 2011)

"You can run cooler water through and still transfer heat, but it will take exponentially longer as the supply temp approaches the DHW temp. It helps to run the circ at really low speed to make the most of whatever stratification you have in storage."

Since this hardware is already in place, it should not cost much to try it with the temps discussed.

A thermosyphon is great if you can locate all the hardware properly.

OR one could consider a proper unpressurized tank with a DHW coil. ;-)


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## leaddog (Mar 3, 2011)

Jeff S said:
			
		

> I wish I thought of it before I insulated my tanks but I think if you wrapped your propane storage tanks with several loops of copper or maybe even pex you would have a effective way to preheat your domestic water.I believe the circumference of a 500 gal. tank is close to 10 ft.You could easily wrap several hundred ft.around one of these things for a large reserve and since all would be insulated ,the water in the tube should be the same temp as the tank.All that would be left would be to temper it.



This would work IF you had the tanks vertical and you wraped the coils at the top where you had the high temp. BUT if the tank is horizonel you would tend to try and make the tank all the same temp and loose stratification. Not a good thing. A horizonal tank could have 100* water at the bottem and 180* at the top.
leaddog


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## Grover59 (Mar 3, 2011)

I just use a flat plate hx, I have an electric hot water tank with the electric coil disabled, and I use the thermostat on the tank to start the pumps to the hx when the dmh tank temp gets low. I have set the thermostat on the dmh tank at 120 this gives me about 140 at the faucet. My 750 gal non pressurized wooden freak show tank may get down to 120 and I still have plenty of heat for dmh. 
      I love flat plate heat exchangers they seem to give you a lot of flexibility may not be as efficient, but they really seem to work well. I don't have any coils in my big tank, I just use dip tubes to move the water, I don't seem to see many others that do it this way. 

Steve


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## bupalos (Mar 3, 2011)

Regarding the external heat exchanger, that's a very interesting idea. Someone needs to try it. I definitely think pex would be better bang for the buck there.


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## Chris Hoskin (Mar 3, 2011)

anyone ever have any problems with a flat plate clogging because of sediment or mineral build up when being used to heat domestic?


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## Hydronics (Mar 3, 2011)

nofossil said:
			
		

> Indirect DHW heaters usually have a coil sized to provide a heat load of around 100K btu/hr at 180 degrees to match the output of a typical fossil boiler. That also provides rapid recovery after your teenager has taken a 45 minute shower.
> 
> You can run cooler water through and still transfer heat, but it will take exponentially longer as the supply temp approaches the DHW temp. It helps to run the circ at really low speed to make the most of whatever stratification you have in storage.
> 
> In reference to other post on long time to heat DHW with sidearm - that's certainly true. Sidearm HX has VERY little surface area and relies on thermosiphoning for the DHW <-> sidearm transfer. Works well when you have hot water flowing through the sidearm anyway as you would when heating other zones. Pretty slow if that's the only reason you're running a circulator. That's the reason form my thermosiphon approach above - slow heat transfer, but operates 24/7 as needed with no circulator. I don't have that setup, but that's what I'd do if I were starting over.



I have often wondered how well a flat plate would work as a side arm -basically replace the sidearm with a flat plate placed at the bottom of the DHW tank and a copper tubing riser tied into the top of the tank. By placing it low on the DHW tank you'd get the greatest delta T and therefore the greatest change in temp / density which would maximize flow through the domestic side of the hx. Even the smallest flat plate has more hx area than most sidearms. Thoughts?


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## Grover59 (Mar 3, 2011)

I can say this thermosiphoning happens quite easily my loops to and from the tank are long and up and down and I have to put check valves in to stop the flow. Most all of the loops have zone valves now so it is not a problem. Even my floor heat would circulate through the 300 feet of pipe quite easily. Knock on wood but at this time I have not had any problems with crud in my hx, I have heard some people have, I am sure a strainer would be a good idea particularly in the dmw loop where you could get rust and I do get rust. 

Steve


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## dpsfireman (Mar 4, 2011)

There is another way to stop unwanted convection and that is by using heat traps. The beauty of this setup is that when the zone valves are closed they stop convection. However, when the power goes off and my valves open, it takes a while but it will still convect supplying some heat when the power is off and I don't have the generator running. They work great and there are no moving parts!


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## Bad Wolf (Mar 7, 2011)

Has anyone tried a hybrid system? If you took a propane tank for a presurized system and installed a access something like a small "man-way". That is cut, say a 10" hole in the tank, take a piece of 10" pipe and contour one end to match the curvature of the tank, weld a flange with holes into to the other end, match the curved end to the tank and weld. You could then install a prefabricated DHW coil into the tank. The system only runs at 14 PSI so I would think a good welder could ensure it would be solid.


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## ewdudley (Mar 7, 2011)

Greg H said:
			
		

> Has anyone tried a hybrid system? If you took a propane tank for a presurized system and installed a access something like a small "man-way". That is cut, say a 10" hole in the tank, take a piece of 10" pipe and contour one end to match the curvature of the tank, weld a flange with holes into to the other end, match the curved end to the tank and weld. You could then install a prefabricated DHW coil into the tank. The system only runs at 14 PSI so I would think a good welder could ensure it would be solid.



Sure!  There've been a few hearth guys who have reported success with this idea.

Many if not most of the European-style purpose-built pressurized storage tanks incorporate a DHW coil in the top (and optionally a solar coil in the bottom), typically with internal duct/baffle systems to let the cooled water fall away to the correct stratification level.

Here's a nice field expedient version at post #5 of this thread:

https://www.hearth.com/econtent/index.php/forums/viewthread/72019/#835793

And post #3 of this thread, where the water is pumped from storage, or from a fossil fuel boiler, to small buffer with a DHW coil:

https://www.hearth.com/econtent/index.php/forums/viewthread/66601/#760078

--ewd


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## chuck172 (Mar 7, 2011)

How about stacking a small tank with a coil in it  on top of the large storage tank. I'm using a 500 gallon propane tank for storage now.  Going with the "hybrid idea" as Greg proposed, how about setting a 50 gallon tank with a dhw coil,  ontop of the 500 gallon. Stratification would guarantee the 50 gal. tank the hottest water. Size the coil to provide all the dhw needed.


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## chesterewers (May 17, 2011)

lots of ideas here on this site, I am planning to add heat storage to tarm 402, I currently just run from the tarm to a forced air heat exchanger and back. Where would I go with the plumbing from that to hot water storage? My idea for hot water storage was a pump sucking from a pex or copper tube that goes to the bottom of the non presurized heat storage tank, the return line would than be also near the bottom to make sure no air is gained, so there would be no need for any flat plate heat exchanger or copper coil in the hot water storage tank. Just doesn't make sense to use it if the water is already circulating in the tank like I described. Anyone else do that? Thanks


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## Burning Alaska (Mar 29, 2013)

How is this thread two years dead? It seems like a pretty important topic for wood boiler design. Although there are some good bits of info on this thread, for the most part it seems to have derailed into a discussion about indirect DHW heaters and heat exchangers. I’d really like to hear some good arguments on pressurized vs. non-pressurized heat storage systems. One particular aspect that I have not seen addressed anywhere is whether or not non-pressurized heat storage tanks are prone to cause or contribute to moisture problems in the building in which they are located. This is just one example of what I’m sure are many as yet unaddressed topics related to pressurized vs. non-pressurized heat storage.


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## Burning Alaska (Mar 29, 2013)

For anyone else in my boat, there’s a few posts here on the topic.


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## Chris Hoskin (Mar 29, 2013)

Burning Alaska, I would not say that unpressurized systems are PRONE to add moisture to the room, but it is a possibility where it is not with a pressurized system. 

Incidentally, in the two years since this thread was started, we have begun carrying reverse indirect water heaters that have about three times the heat exchange surface area of a traditional indirect and so work well at low supply temps (as from storage).  We have found these to be an excellent match with pressurized thermal storage and especially in a low temp heating system where you would like to be able to draw tank temps down to 110.  In the past, with a traditional indirect, you would have to maintain higher tank temps in order to make decent volumes of hot water - in other words, domestic hot water production was the limiting factor, not space heating.  Chris


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## maple1 (Mar 29, 2013)

What would the brand/model of that heater be?

(For those in other market areas...).

That is one feature that made me decide against an indirect here - the bigger one was I couldn't find one with electric elements for backup DHW heating. I'm always eyeing the future - these electric tanks only last so long....


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## Burning Alaska (Mar 29, 2013)

Chris Hoskin said:


> Burning Alaska, I would not say that unpressurized systems are PRONE to add moisture to the room, but it is a possibility where it is not with a pressurized system.
> 
> Incidentally, in the two years since this thread was started, we have begun carrying reverse indirect water heaters that have about three times the heat exchange surface area of a traditional indirect and so work well at low supply temps (as from storage). We have found these to be an excellent match with pressurized thermal storage and especially in a low temp heating system where you would like to be able to draw tank temps down to 110. In the past, with a traditional indirect, you would have to maintain higher tank temps in order to make decent volumes of hot water - in other words, domestic hot water production was the limiting factor, not space heating. Chris


 
What’s “reverse” about a reverse indirect water heater? Is it a separate heat exchanger unit for the storage tank, or storage tank and hx in one (in which case, what’s the difference from a “regular” indirect?)? Can it only be used with pressurized storage, or can it also be used with non-pressurized?

Thanks


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## Chris Hoskin (Apr 1, 2013)

reverse indirect:  instead of running hot boiler water through a heat exchanger inside a 40 gallon (typical) tank filled with domestic water, a reverse indirect runs boiler water through the 40 gallon vessel and the domestic hot water runs through the coil.  In other words, the boiler water and domestic water are on opposite sides of the heat exchanger - or reversed - compared to traditional indirect.  In addition to larger heat exchange surface area, these tanks typically have larger supply and return ports allowing for higher btu input.  Typical indirect would have 3/4" ports whereas the reverse indirect might have 1 1/4".


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## Nofossil (Apr 1, 2013)

Wow - the IS an old thread! It's great to see new and innovative products and ideas. More for us crazy folk to tinker with.

I'm an increasing fan of s-l-o-w heat transfer for DHW. It doesn't matter if it takes a long time to transfer heat into DHW as long as you can keep ahead of demand. Traditional 'bang-bang' systems let DHW run down almost to cold, then have to heat it back up REALLY fast so that you don't get a cold shower. If you can instead just thermosiphon a couple thousand BTU per hour as needed and keep it 'topped off', you'll be fine.


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## maple1 (Apr 1, 2013)

That's what I'm doing with my sidearm and 80 gallon electric tank.

It's almost time though to get back at that project & finish it right - I need to do some more insulating, and put in a thermal switch and maybe a check valve to stop reverse flow when my storage gets too low.


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## rkusek (Apr 2, 2013)

Here is my plan. Build a sidearm similar to this one.
http://www.ebay.com/itm/HIGH-OUTPUT...ltDomain_0&hash=item27c94b4cdb#ht_1420wt_1093
It's only 24" long but has 7 smaller copper tubes inside of the 1.5" shell. I would try this one for $85 but I don't like how close the drinking water gets to the boiler water and the chance of contamination if a solder joint fails. Picture a 1.5" shell with two 1.5"x1"x1.5" tees on the ends, then end caps on both ends with 7 holes drilled through the caps. The 3/8" tubes would extend through the caps on both ends and connect to a 3/4" manifold with seven 3/8" ports for the DHW connections to my 80 gal electric water heater. Any kind of leak whether it be boiler or DHW would easily be visible and would have almost no chance of drinking water contamination. The heat transfer should be about 1.75 times that of the typical old style 48" sidearm by my calculations. Also, placing it at the bottom 2 foot of the water heater should greatly improve the thermosiphon action when compared to the old style because of the elevation difference. Right?

Instead of a 7 port manifold on each end (which I don't believe already exists) it might be cheaper to add 2 more endcaps with 7 holes turned the other way an inch or so away from the other caps and then reduce this down from 1.5" to 3/4" to connect the DHW side. Sketch on this to come later.


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## maple1 (Apr 2, 2013)

That's the sidearm I'm using. And I have it situated just above the bottom inlet of my electric tank. The second time, that is - I first had it at the top & it didn't work so good. Mine works very good - but as I noted above I think it'll transfer heat the other way just as good if your storage gets low on temps. Which mine is doing now once in while now that we're creeping into shoulder season.


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## tom in maine (Apr 2, 2013)

Although I make my living manufacturing  coiled DHW hx's, I would like to add that if you want to produce a sidearm hx, a single pass one, without any joints in the boiler stream would work reasonably well.
It would take longer to heat the tank, but a little more time for a simpler, safer hx is no big deal.
BTW, if the eBay hx did leak, it would over-pressurize the boiler side and the potential of cross contamination is low, because your boiler PR valve would lift.


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## rkusek (Apr 2, 2013)

You're right.  The water pressure would be way more than the pr blowoff.  I may still look at making my own.  Going to get a mixing valve too.  Someone posted a link awhile back for cheap simple differential controller board that I might add as well.


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## chewy (Apr 4, 2013)

Does anyone have any pics or links of a complete unpressurized  system install?  I don't understand why you can't use a tank with an open expansion tank above it and not have to use heat exchangers in it.  

Erin


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## Burning Alaska (Apr 4, 2013)

chewy said:


> Does anyone have any pics or links of a complete unpressurized system install?


 
+1


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## Nofossil (Apr 4, 2013)

chewy said:


> Does anyone have any pics or links of a complete unpressurized system install? I don't understand why you can't use a tank with an open expansion tank above it and not have to use heat exchangers in it.
> 
> Erin


 
http://www.nofossil.org - click 'system design' and 'heat storage' on the left side.


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## chewy (Apr 4, 2013)

Nofossil,  first off that is an awesome website  I read through the heat storage part and it said you don't pull water from the tank correct?  What do you do if your zones are calling for heat and also the tank needs heat?  Can you do both at once or does the boiler heat the tank only?

Erin


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## Nofossil (Apr 5, 2013)

The water in storage stays there and never moves. I built a simple little control system that prioritizes heat transfer ;-)

If the boiler has heat available and DHW is cold, it heats up DHW to a usable level first.
If any of the space heating zones need heat, they're satisfied next.
The hot tub gets attention once the space heating zones are satisfied.
Next, the DHW tank is heated up to 160.
Finally, heat is dumped to storage.
If at any point in the sequence the boiler output starts to get too high, the next load in sequence is added. Storage never 'needs' heat - it's just there to absorb heat if the other zones don't need any.


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## bmblank (Apr 5, 2013)

In my case my dhw is just a coil running through my storage. Furnace just automatically heats storage and radiant pump draws the water from the hot side, so it'll get heat before the storage. Essentially, storage furnace and radiant are all in parallel with each other.


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## Hansson (Apr 6, 2013)

Found this drawing in a Swedish forum
http://www.byggahus.se/forum/attachment.php?attachmentid=6452&d=1171210146
It non presurized.


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## dswitham (Apr 6, 2013)

tom in maine said:


> BTW, if the eBay hx did leak, it would over-pressurize the boiler side and the potential of cross contamination is low, because your boiler PR valve would lift.


 
Not in everyone's house, although mine is probably a rare case. My domestic water pressure is only 20 psi, not enough to set off my boiler PR valve.
We can get it higher if we want to turn on the pump but I hate the wasted electricity just to get higher pressure.  Our water is gravity fed  from a spring way up on the hill behind our house.


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## maple1 (Apr 6, 2013)

Even 20psi could be more than boiler system pressure.

My boiler system maxes out at 15psi when full hot. 8-10 when cold. Domestic goes from 25-35, or so.


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## bmblank (Apr 6, 2013)

Yeah, but the blow off valve I'd probably at 30psi.
I've got a 40-60 pressure switch on my well. That would do it for sure.


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