# Initial Experiences with Plenum Heat Exchangers



## rickh1001 (Nov 17, 2008)

I just completed installing the last of 3 plenum heat exchangers, into oil hot air furnaces that serviced our converted barn/home (two furnaces - one at each end), and a smaller farmhouse 50 ft away.  Although the weather has just started turning colder (it is about 30F today), it just seems to me they are not putting out the heat equivalent to the oil burners they are replacing.  I am using 16X18" units for the two furnaces in the barn.  These are "rated" at about 129,000 btu/h.  I measured the air temp coming up at the register at about 140F, when the boiler system was heated to about 175F.  As oil burners, I used to put in a 0.7 gal/min nozzle, or about 97,000 btu/h, and they would run for fairly short periods then shut off.  However, the heat exchangers seem to run forever, even with only a 30F outdoor temp, as though they are not really putting out that much heat.  I think they won't be able to handle it when it hit 0F or below.  I certainly don't believe they are putting out more heat than the oil burners did.  I haven't measured the exact supply/return temps on each of the zones yet - but both lines are very hot.  I am running 1" copper lines, and have should at least 8 gal/min as calculated by several approaches.  The supply and return line temps don't feel that different (this is very unscientific at this point, and I don't expect my hands to accurately detect 20F differentials), but it seems the exchangers are not extracting as much heat as they should, rather than it being a supply problem.  I feel I am not getting anywhere near the same heat out of the heat exchangers that I did with the oil burners, that theoretically should be putting out less heat.

I know this is a complex question, but in general, what experiences have others had on using these plenum heat exchangers of this general size?  

There is a downside to using plenum heat exchangers that I am starting to see.  Because they seem to run much longer, the fan electricity is going to add up over the winter.  Eventually, I want to replace them with baseboard and floor radiant heat, which will be much more efficient electricity-wise.  These units allowed me to get the system going quickly, and retain a backup heat source.  However, as I am watching their performance, I am not impressed so far.  

Does this match up with others, or did I do something wrong on the install?


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## Tony H (Nov 17, 2008)

I noticed the same with my system running one heat ex 20x19 . The output air is not as warm as with the gas and it runs longer. I turned the speed on the pump to high and this seemed to help some , the temp out the outlet water is much cooler than the input on mine. Since this is my first year I am also wondering how it will work when the temps get  way colder


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## pybyr (Nov 17, 2008)

I haven't yet got my water-air HX running, but before I bought it, I looked into them a lot, and found that a lot of the specs are dependent not only on water temp and flow but also air flow, and it seemed to me that some of the specifications used to market them (at least the ones marketed on ebay and/ or outdoor boiler supply sites) tend to assume parameters that seemed over-optimistic to me.  

That's why I ended up going with a big 4-row coil that I ordered from Nationwide Coils after Ross there helped me by modelling it under a variety of temps and flows (lots of the ones generally marketed seem to be 2-row coils)


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## pybyr (Nov 17, 2008)

one other question/ suggestion to check- are you guys certain that you have these hooked up to run in counterflow- that is, the hot water goes in the part of the coil nearest where the air exits, and cool water comes out nearest where cold air enters?  

that'll keep max temp differential between the two media (air and water) at all points, which is a much more efficient way to transfer maximum heat than the other way 'round


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## Tony H (Nov 17, 2008)

pybyr - Let me get this straight it makes a difference to the heat exchange temp gain based on the direction of flow thru the exchanger ? Mine is installed with the hot entering the lower side closest to the air input and cooler water leaves at the top where the heated air exits. This appears to be backwards from your example.
The water is much cooler leaving the exchanger so I know it's transfering alot of its heat but how much am I losing because it's flowing the wrong direction ?
It can be changed but it won't be fun to take apart all that stuff again but is it worth it in your opinion ? 
Another fix I have considered would be to add another coil located downstream to tramsfer more heat .
Thanks
Tony


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## rickh1001 (Nov 17, 2008)

pybyr, 

Great idea on bringing the supply in at the top and the return out the bottom, to maximize heat differentials.  I am kicking myself for not thinking of it that way.  Instead, I brought the supply into the bottom, and out the top, thinking only of forcing any air out more easily.  However, what you say makes perfect sense.  Looks like I have some re-soldering ahead of me!  

Overall though, I am still getting the sense that the heat exchangers I have won't be enough for -20 deg temps.  I am starting to think about tapping off the supply lines to some baseboard units, while keeping the plenum exchangers, to get more of the heat out of the various zones.


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## applewood (Nov 17, 2008)

I'm planning to go coil in plenum also so I'm wondering which 4 coil unit you ordered and how expensive they are? May decide to go directly to under floor pex.


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## Medman (Nov 17, 2008)

I also am only going to be using a coil in the plenum of my furnace.  It is my only option because I have a slab-on-grade house with exterior walls of concrete block. (Original structure is from 1960 and was built as a cottage.)  All exterior walls have 2x6 insulated walls sistered outside the structure, but inside access for piping, etc. is limited.  It is a downflow configuration with all supply ducts run in the slab with all returns in the attic space (all new and insulated as of last week).
I purchased a new air handler with a variable speed DC fan to replace the old furnace, partly because I was concerned about the cost of running the fan all the time.  I plan on running the fan at low speed all the time, with the speed increasing when the thermostat calls for heat.  Hot water will only flow through the coil when the t-stat calls for heat as well.  This hopefully will keep the ducts in the slab warm and keep the returns in the attic warm.
The slab is over two feet thick and under the furnace room it is over four feet thick - I had to jackhammer a new hole for the supply plenum to locate it in a more convenient spot.  This mass will absorb a lot of heat.  I hope it holds the heat as well.

Firing should happen this week, but no later that Dec. 1. (fingers crossed!)


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## jgrimm (Nov 17, 2008)

The charts I have seen for water to air heat exchangers show the "rated" outputs at the highest airflow, gpm and water temp along with the lowest entering air temp.  Yours is probably rated the same.  For example, 1400 CFM, 12 gpm, 205 deg water, and 55 deg air.  None of these numbers are realistic, your HX dealer should know this.  My real life experience with a 140btu rated HX is closer to 71k calculated from 7 GPM and ~20deg water temperature drop.

Do you know what your GPM is?

Changing to counter flow will help.  When you do that add a thermometer on the inlet and outlet.


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## pybyr (Nov 17, 2008)

Tony H said:
			
		

> pybyr - Let me get this straight it makes a difference to the heat exchange temp gain based on the direction of flow thru the exchanger ? Mine is installed with the hot entering the lower side closest to the air input and cooler water leaves at the top where the heated air exits. This appears to be backwards from your example.
> The water is much cooler leaving the exchanger so I know it's transfering alot of its heat but how much am I losing because it's flowing the wrong direction ?
> It can be changed but it won't be fun to take apart all that stuff again but is it worth it in your opinion ?
> Another fix I have considered would be to add another coil located downstream to tramsfer more heat .
> ...



any exchange process (heat, chemical, etc.) is rendered a lot more efficient if you can maintain counterflow (your kidneys work on this principle)--

here's an explanation

http://en.wikipedia.org/wiki/Countercurrent_exchange

in the liquid to air heat exchanger setting, counterflow means that the coldest air hits the coldest portion of the coil, but, based on that difference, still picks up some heat.  

as that partly-warmed air moves through the fins, it heads to warmer fins & tubes, where it picks up still more heat along the way.

without counterflow, your cold air picks up whatever heat it can pick up in a single pass by the hottest coil, and then it's literally downhill all the rest of the way through the exchanger


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## pybyr (Nov 17, 2008)

applewood said:
			
		

> I'm planning to go coil in plenum also so I'm wondering which 4 coil unit you ordered and how expensive they are? May decide to go directly to under floor pex.



if you go with the coil, call Ross at "Nationwide Coils," and have your data on needed BTU outputs, water temperatures and air CFMS handy.  he can then work with you to figure out what most closely meets your needs and at what cost, and then he'll work directly with the manufacturer to have a coil made, fast, to not only fit your thermal parameters, but also your duct sizes and available space


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## mtfallsmikey (Nov 17, 2008)

Depending on entering hot water temp, etc. 130-140 deg. outlet air temp. is about right. An oil/gas furnace (especially older ones) has leaving air temps close to 190 deg.


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## rickh1001 (Nov 17, 2008)

pybyr

I have the same question as TonyH, and I wondering if during your research, you came up with any numbers regarding the relative efficiencies of the overall heat transfer, if you plumb it counterflow or "normal"?  Do you think it will it be a 5% type of improvement, or 50%?  

It won't be all that hard to re-do the piping near each HX, but I just convinced myself a few days ago that I was all done, the air is out of the system and everything is starting to purr.  Hate to shut everything down and redo the plumbing unless it is worth it. 

Also, I have no idea of the cfm for the air handler on the furnace.  All three furnaces I am using are similar systems, which are normal sized household furnaces rated at (I think) about 110,000 btu.  I can try to track down the manuals or go online, but does anyone know the ballpark cfm figures for such household furnaces?  All three zones are running 8-10 gal/min, and the storage tank temp runs anywhere from 160 - 180F.  

I can say that a seat of the pants estimate of the btu's I am getting out of the HX is that they are only putting out 50-60% of what the oil burners did - even though I was using only a 0.7 gal/min nozzle in each one.  So I would guess I am seeing 40-50,000 btu max from each of the HX's, just guessing from the way the house warms up as compared to the oil burners.  I am going to need to get more heat than that out of them, for the really cold weather.


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## pybyr (Nov 17, 2008)

boilerman said:
			
		

> pybyr
> 
> I have the same question as TonyH, and I wondering if during your research, you came up with any numbers regarding the relative efficiencies of the overall heat transfer, if you plumb it counterflow or "normal"?  Do you think it will it be a 5% type of improvement, or 50%?
> 
> ...



others around here have the actual formal technical horsepower (whether by education, experience, or both)- that I don't- to tell you _how_ big a difference counterflow makes.  I do know that it is always used in formal design of exchange circuits

and think of it this way- I don't know if you have a 2 row coil or a 4 row coil, but if you have a 2-row coil in non-counterflow, then once the air has passed the first row of tubes, the second can't add any serious heat, because the fluid there is cooler than the coil that the air has already been through- so you're losing a substantial chunk of the coil's effective area.  the same effect would be even more extreme if you have a 4 row coil, because only one out of 4 rows of coils is really able to do its best to heat the air passing through.

if you get the make and model of the furnace and then call the furnace manufacturer, it should not be a big deal for them to look up and tell you the CFMs; that's what I did.  

While you are at it, also find out from the furnace manufacturer how much back pressure the blower can take- because, especially if you need to back up and order a new coil or coils, you'll want to be able to take that into consideration, too, so that your coil is not choking off the blower's ability to move air.  Or, you may want to even try to measure what sort of air pressure you have on either side of your existing coil, because if it is somehow restricting the air flow your blower is able to achieve, then that'll also be crippling your net heat output.

Most of the coil vendors gave me "huh" responses to efforts to identify or account for such variables; 

Ross at Nationwide was able to work with me to come up with a coil (24x24x 4 rows of 1/2 tube) that should move a heckuva lot of BTUs even when my water from storage starts getting on the cooler end of its range, and while introducing very little back pressure for my relatively slow moving 1600cfm blower in my ThermoPride.  And it was delivered quickly - sooner than they even promised.


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## Donl (Nov 17, 2008)

Damn!  Just when I thought I had everything running really well I find out that I need to reverse the feeds to my water to air HX. Well, that shouldn't be too difficult. Thanks for the info.  I have been getting 140-150 degree air temps with concurrent flow. I'll post what temps I get after I switch to counter flow. 

Don


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## pybyr (Nov 18, 2008)

Don L said:
			
		

> Damn!  Just when I thought I had everything running really well I find out that I need to reverse the feeds to my water to air HX. Well, that shouldn't be too difficult. Thanks for the info.  I have been getting 140-150 degree air temps with concurrent flow. I'll post what temps I get after I switch to counter flow.
> 
> Don



my first reaction after reading your post was that if you have everything running really well, then why mess with success, regardless of what's optimum in theory- but then again, if your system is new, and hasn't been through the coldest times yet, you may want to switch your coilto counterflow before then


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## pybyr (Nov 18, 2008)

Abolish the Federal Reserve said:
			
		

> The charts I have seen for water to air heat exchangers show the "rated" outputs at the highest airflow, gpm and water temp along with the lowest entering air temp.  Yours is probably rated the same.  For example, 1400 CFM, 12 gpm, 205 deg water, and 55 deg air.  None of these numbers are realistic, your HX dealer should know this.  My real life experience with a 140btu rated HX is closer to 71k calculated from 7 GPM and ~20deg water temperature drop.
> 
> Do you know what your GPM is?
> 
> Changing to counter flow will help.  When you do that add a thermometer on the inlet and outlet.



that's one of the beauties of dealing with some folks who are really pros at this- not just people making a sales pitch based on a best-case-scenario-that-never-really-happens set of assumptions- 

and Ross at Nationwide Coils  http://www.nationwidecoils.com/ (who is basically a distributor for Precision Coils  http://www.precision-coils.com/  , who also took some time on the phone with me, but do not sell direct to end users) is someone who can and will will help you model the real variables that are likely to actually represent your particular system.  

In the part of the trade that Nationwide/ Precision serve, "sort of works" means out of business, so they need to know how to get it right.  

My only surprise, pleasantly, was that Precision was glad to speak with me on the phone, and after I found that they did not sell to end users, and asked who was a "rep" who'd be good at dealing with a DIY-er like me, pointed me to Ross, and he was all of the above


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## Donl (Nov 18, 2008)

pybyr said:
			
		

> Don L said:
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I could leave it as is, but that's not me.  My curiosity alone  needs to know how much a difference it will make. Even though everything is running ok, as I learn, I will make changes. That's the fun of it!    Reversing the flow in the HX may result in faster house warm ups and less run time of the blower motor. That's a good thing.

Don


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## pybyr (Nov 18, 2008)

Don L said:
			
		

> pybyr said:
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glad to know that I'm not the only one that ticks that way


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## Tony H (Nov 18, 2008)

pybyr said:
			
		

> Don L said:
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I for one am glad both you guys think that way, I am in a if it's working fine don't F it up mode. I am very interested in what happens on your system Don L . Are you able to measure input and output water temps before and after ? That would give a rough idea of the increase in heat transfered  by changing the flow direction.  I know there are alot of new systems coming online including mine and this seems like a good thing to know.


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## Donl (Nov 18, 2008)

Tony H said:
			
		

> pybyr said:
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I'll try to take a few measurements before and after to see if we can see any meaningful change. My worry is that there are so many variables involved that it will be difficult to get the before and after parameters just right. I think the best indication of improvement will show up in higher plenum air temperatures.    I'll let you know what happens. I hope to do this tomorrow sometime.

Don


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## rickh1001 (Nov 18, 2008)

You know, I have been thinking a bit about this whole plenum HX approach.  It was the quickest and easiest way to get the system going.  Now I am starting to see some of the shortcomings of this approach.  First, the good side.  My wife has not hassled me for a week or more, since we actually have sensible heat, and I temporarily no longer have to endure the terrible fate that having taken out a home equity loan, that in fact the new system will not work.  It works, and we have burned no oil for many weeks now.  The flip side is, that come mid-January, when it is -20F and the winds are blowing, that those meager little HX's are going to have us all wrapped in blankets.  As they (used to) say, no one gets fired for buying IBM.  At the end of the day, the new system has to work.  I think the plenum HX approach gets one started, but by far isn't appearing to be the best long term solution.  I am hoping the dog eats the electric bill for running the blower motors so long, before we get it.  Plus, it is almost impossible to think about running the system in a prolonged power outage, without a generator, and the air handler has to be hooked up to the backup, rather than just a circulator. 

So I am thinking about alternatives, and Dan Holohan's love of radiant heating comes to mind.   For our converted barn, we have major volume, with 13 ft ceilings in the 35 ft half of it, and conventional 8 ft ceilings in the other half/bedroom area.  Everything I have read says that convective heat is worst in tall ceilings.  Radiative heat is the key here.  Ideally, I would put in a second floor on top of the existing one with radiant heat underneath - but things have been a little busy, and I am still waiting for the numerous blisters to clear up after soldering so many joints on the main heating system. So forget a new floor for a few years.  But Holohan got me thinking, that I have plenty of wall space and ceiling space available.  Instead of this somewhat ridiculous little HX stuffed into the plenum of the oil burner, why not put up expansive panels of radiant heat surfaces?  It makes no difference if they are on the floor, the walls or the ceilings.

As I have been thinking the problem over, radiant heating is inherently much more suitable for our wood heat systems.  We all seem to struggle to maintain even 170F, yet radiant systems work with temps of <100F.  So why struggle with an inefficient approach, of trying to heat to 180F or so, then extract the heat out with a water/air HX?  As I look around the rooms, forgetting the floors, there is all kinds of space on the walls and ceilings to put in attractive panels, that could radiate more than enough heat for each room.  For example, right now looking to my right, we have a map/bulletin board roughly 4X6 feet.  I could make that instead a 4X6 ft radiative panel, and still put the corkboard and map over it.  As Holahan noted, it doesn't matter if the heat source is on the floor, the walls or the ceilings - it is the same heat.  Heat doesn't rise - it transmits via radiation, convention and conduction.  Hot air rises (both politically and thermodynamically unfortunately).  So I am thinking rather than put up with the problems of the HX, I will start to figure how to put radiant heat surfaces into the room.  Eventually, I will use the same zone plumbing to feed these, and eliminate the plenum HX and its power-hungry fan, and be able to heat the whole house with only a UPS to feed the EKO and the zone circs.  

The Hx's were an OK first step to get the system up and running - but I don't see them as a viable long term heat solution.


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## pybyr (Nov 18, 2008)

boilerman said:
			
		

> You know, I have been thinking a bit about this whole plenum HX approach.  It was the quickest and easiest way to get the system going.  Now I am starting to see some of the shortcomings of this approach.  First, the good side.  My wife has not hassled me for a week or more, since we actually have sensible heat, and I temporarily no longer have to endure the terrible fate that having taken out a home equity loan, that in fact the new system will not work.  It works, and we have burned no oil for many weeks now.  The flip side is, that come mid-January, when it is -20F and the winds are blowing, that those meager little HX's are going to have us all wrapped in blankets.  As they (used to) say, no one gets fired for buying IBM.  At the end of the day, the new system has to work.  I think the plenum HX approach gets one started, but by far isn't appearing to be the best long term solution.  I am hoping the dog eats the electric bill for running the blower motors so long, before we get it.  Plus, it is almost impossible to think about running the system in a prolonged power outage, without a generator, and the air handler has to be hooked up to the backup, rather than just a circulator.
> 
> So I am thinking about alternatives, and Dan Holohan's love of radiant heating comes to mind.   For our converted barn, we have major volume, with 13 ft ceilings in the 35 ft half of it, and conventional 8 ft ceilings in the other half/bedroom area.  Everything I have read says that convective heat is worst in tall ceilings.  Radiative heat is the key here.  Ideally, I would put in a second floor on top of the existing one with radiant heat underneath - but things have been a little busy, and I am still waiting for the numerous blisters to clear up after soldering so many joints on the main heating system. So forget a new floor for a few years.  But Holohan got me thinking, that I have plenty of wall space and ceiling space available.  Instead of this somewhat ridiculous little HX stuffed into the plenum of the oil burner, why not put up expansive panels of radiant heat surfaces?  It makes no difference if they are on the floor, the walls or the ceilings.
> 
> ...



none of us ever said that air was an ideal way of transferring a lot of heat to a large cubic voume in a cold climate... I knew that even before I dropped the better part of a grand on a ridiculously big water-air HX.  

but if forced air is what you've got (like me) and if you don't have a bottomless wallet to change the whole system while you shift to wood gasification+storage, then a properly sized water-->air HX is not a stupid thing to consider.  and, to my mind, better to "blow the dough" once, and hopefully only once, on something that'll work really well than to try to cheap out on something that is affordable up front but ineffective in the long run

and/ or if, like me, you have existing floors that you want to keep (in my case, 180 year old random width old growth spruce planking 2+ inches thick) that are incompatible with meeting your whole heat load with radiant, then air can be part of the solution

between figuring out my ancient house that was scarily close to falling in on itself when I bought it, and looking at "state of the art" new houses that are so tight that they need air+air HX units to maintain indoor air quality while they heat with radiant floors that are slow to respond to changes in desired temperature, I increasingly think that a "hybrid" approach--- radiant for the "base load" of keeping the chill out, plus a timed air system (which can respond faster than radiant panels) would be the cat's meow.  

I suppose, as usual, that the Europeans are ahead of us on this, as their wall-hung panel radiators combine  radiant and convection.


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## mtfallsmikey (Nov 18, 2008)

A 110K BTU furnace would probably be rated in the ballpark of 1200-1400 cfm, sans AC/hydrocoils, etc. Duct sizing can be critical here too. As well as the GPM you are running thru the coil. I agree...my HX was a stopgap measure as well, the original plan was to do BB with TRV's, but cost and time came into play. I'm also looking at redoing my floors next year, only want to pull the base trim once.


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## Medman (Nov 18, 2008)

Radiant wall panels would be a good approach for me as well, especially considering the amount of accessible wall space I have.  However, I would have to run the supply/return piping for the zones in the attic, and the thought of spending any more time up there (after a week of replacing ducts and insulating) makes my back hurt.  Maybe in a year...or two...

I am interested to see how well the new air handler/backup electric furnace works.  With my old electric furnace, 70 k BTUs of electric heat could not maintain 68 degrees in January, even though my heat loss calcs come in under 50 k BTUs worst case.  When I got in the attic to add some additional returns, I found that at some point in the past one of the renovation contractors had crushed a section of the return trunk.  The damage was not visible because the duct was covered in insulation, however the seams were split and the return was basically pulling in cold attic air.  The furnace was trying to heat this below freezing air and could not maintain temp. 
Now that I have replaced all the return ducts and sealed all joints tight, the new furnace should be able to keep the temp. up using only electric.  The cost for electric will still be the same, but the temp should be higher.  With the plenum coil installed, I should be able to move the required 50k BTUs into the house without issue.
Also, the new air handler has a variable speed DC fan, which uses about 1/10 the electricity of the old single speed AC blower.  This should help reduce costs too.  The bonus is that I can adjust the speed to account for higher backpressure from the coil.  I am using a differential air pressure gauge to measure the pressure on the ducts, but I haven't connected it yet.


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## Donl (Nov 18, 2008)

I managed to get our water to air HX connected as counterflow  today.  Of course when I made the change It developed a good leak at the HX connector.  I screwed up one of those rubber sealing rings. I had some spare connectors so a borrowed a ring and all worked out OK.


This is all unscientific but I tried to maintain the operating parameters as closely as I could.


Concurrentflow:

HX Input water temp =  167 degrees F.     
HX input air temp = 68 degrees f.
HX output air temp = 139 degrees F.


Counterflow:

HX Input water temp = 167 degrees F.
HX Input air temp = 68 degrees F.
HX Output air temp = 150 degrees F

Changing to counterflow increased the temperature by 11 degrees F. It would appear that it was worth making this change. I would be interested in finding out if others reach the same conclusion.

Don


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## Tony H (Nov 18, 2008)

It appears the counterflow increase is reason to rejoice ! No longer do I have to worry that when the temps dip low I might have to kick on the ga$ TO WARM UP!
THANKS DON  for doing the test ,an 11% gain is quite a bit.  Now I will have to switch mine and get the extra heat before it gets really cold.


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## rickh1001 (Nov 18, 2008)

DonL

Thanks from me as well.  That is a good enough improvement to definitely make it worthwhile.  Looks like I will be breaking out the torch again - even before all the blisters healed from my last soldering expedition. 

Also, following on my semi-rant yesterday regarding radiant heat being more suited to our systems.  Doing the initial install of the entire new system has been such a massive undertaking (or at least seemed so at times), I never really thought too much about the heat emitters, and figured the plenum HX's that my dealer recommended would be good enough - and they are.  The system is operating, and putting out decent heat, and is simple, with an automatic backup.  However, now that I have had a chance to catch my breath a little on this project, I can see a Phase 2 coming up.  For example, on the other house where I ran the underground PEX to, I could easily put in some lengths of finned baseboard in one or two of the main rooms, and pull some more heat out in addition to the plenum HX.  In the barn, as I get the money, I can add a couple of attractive radiant panels in the rooms where I need a little more heat.  The real beauty of a hydronic system is the total flexibility to put together a system tailored to each individual problem.  With hot air, there were no options.  Now with the new system working, I am having some real fun thinking of improved ways to move heat around to where I need it.  I am even thinking if I could make my own radiant panel by making a cast gypcrete type thing in a frame.  Put a poster over it, or have someone paint it, and it could become art, and put it on the wall while in reality it is a radiator.  I know I am going over the edge, as I can see all kinds of objects in the room that could serve as radiators!  Heck, one could even weld up some stainless steel tubing into a sculpture almost (I couldn't), and use it as a fancy radiator.  Who knew hydronic heating could be so much fun.  Now I can see the source of Holohan's enthusiasm in his books.  

I really like this new system - it was worth all the hassles and expense for sure!


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## Vtgent49 (Nov 19, 2008)

I agree that the radiant floor system may be nice, comfy, etc, and possibly a good first choice. I haven't yet seen a comparison to a radiant panel (euro style with non-electric controls) to the usual floor set-up. I suspect the floor doesn't match-up efficiency wise (except for those who love to be barefoot in Jan, which is not on my list of goals for saving this planet...)

I spent some time last Spring in a hotel in London, with new radiant panels, non-elec dial controls, steam heat generated a few blocks away. This was a retrofit to a 1820 4 story town house, actually a solid block of housing. Effortless, comfy heat it seemed to me. The US is way behind it seems.

So, I'd encourage you to persue an alternative to the Hot air HX. Leave it as a back-up. And please share your findings.

Al


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## Duetech (Nov 19, 2008)

boilerman said:
			
		

> I just completed installing the last of 3 plenum heat exchangers, into oil hot air furnaces that serviced our converted barn/home (two furnaces - one at each end), and a smaller farmhouse 50 ft away.  Although the weather has just started turning colder (it is about 30F today), it just seems to me they are not putting out the heat equivalent to the oil burners they are replacing.  I am using 16X18" units for the two furnaces in the barn.  These are "rated" at about 129,000 btu/h.  I measured the air temp coming up at the register at about 140F, when the boiler system was heated to about 175F.  As oil burners, I used to put in a 0.7 gal/min nozzle, or about 97,000 btu/h, and they would run for fairly short periods then shut off.  However, the heat exchangers seem to run forever, even with only a 30F outdoor temp, as though they are not really putting out that much heat.  I think they won't be able to handle it when it hit 0F or below.  I certainly don't believe they are putting out more heat than the oil burners did.  I haven't measured the exact supply/return temps on each of the zones yet - but both lines are very hot.  I am running 1" copper lines, and have should at least 8 gal/min as calculated by several approaches.  The supply and return line temps don't feel that different (this is very unscientific at this point, and I don't expect my hands to accurately detect 20F differentials), but it seems the exchangers are not extracting as much heat as they should, rather than it being a supply problem.  I feel I am not getting anywhere near the same heat out of the heat exchangers that I did with the oil burners, that theoretically should be putting out less heat.
> 
> I know this is a complex question, but in general, what experiences have others had on using these plenum heat exchangers of this general size?
> 
> ...



I installed a 20"x20" air exchanger in my oil furnace plenum in 2006. All tolled my electric bill went up $10 per month as per kwh usage in comparison to 2005 but that includes the boiler with forced draft, 2 circs (one runs 24-7) and the blower from the oil furnace. My blower for the oil furnace does run longer but my home is warmer than when it was "oil only" or oil/wood furnace supplemented. I have much the same temperature no matter what room I go into. The hardest winter temps we have had in the last three years was last year with -20*f with 40 mph. winds and the system worked without oil supplementation and maintained an avg. of 71*f. The up side, if the warmer house is not enough, is not spending money on 181 gal. +/- @ ?? gal. per month on oil for heat and $65 +/- on propane per month for dhw.
My oil furnace plenum  fits my air heat exchanger like a glove with no blow by on the sides. Hopefully yours does as well. If not there is a huge transfer problem there.
After reading this string that your question generated I now know I have a way of making my heat exchanger more efficient as my system is plumbed concurrent. That in mind my system will run better than previously stated and if you plumb your system to counter-flow yours should too!


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## rickh1001 (Nov 19, 2008)

I replumbed one of the plenum HX,s from con-current to counterflow.  Looks like I picked up about +6F.  Originally, even with the storage up to 180F, I could only get a max of about 140F out of the floor register.  After switching, with similar supply water temp, I am getting 146F.  Other conditions are pretty similar, so it looks like there was a small, but definite increase in the temp at the register.  

Thanks pybyr!  

Two more HX's to go.  I'll check them before and after as well.


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## pybyr (Nov 19, 2008)

boilerman said:
			
		

> I replumbed one of the plenum HX,s from con-current to counterflow.  Looks like I picked up about +6F.  Originally, even with the storage up to 180F, I could only get a max of about 140F out of the floor register.  After switching, with similar supply water temp, I am getting 146F.  Other conditions are pretty similar, so it looks like there was a small, but definite increase in the temp at the register.
> 
> Thanks pybyr!
> 
> Two more HX's to go.  I'll check them before and after as well.



glad it turned out to help.  

and even though 6 degrees sounds small when you think of it relative to 140, when you look at it from the standpoint of % of the temperature increase through the heat exchanger (let's assume 60 degree return air, so you had an 80 degree gain through the HX, now increasing to an 86 degree gain) you've picked up between 10-15% in performance from a simple reversal of the pipes

and it should also help you extract max feasible BTUs from the water when your storage is getting cooler


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## Tony H (Nov 25, 2008)

pybyr said:
			
		

> boilerman said:
> 
> 
> 
> ...



That's a nice gain , I plan to do mine this weekend and will check the before and after as well.


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## Duetech (Nov 25, 2008)

Coupled with "Tweaking my EKO" and by re-plumbing my air exchanger I'm getting a warmer house for less energy spent.


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## mwk1000 (Nov 25, 2008)

Based on what others are getting I am looking forward to the switch. I have a newer York 2 stage and the return temperatures have never been very high. It is designed to have long fan run times. I have the 16N model I included a page from the manual showing how these newer systems are designed. My register temps are in the 90* range most of the time. The unit is only designed to raise the temp by 60* at best on high blower speed. The reality is that stage one is on medium blower speed ( 1400 CFM ) so we actually see about a 65->95 = 30* rise. 

My wife and I have never gotten used to the "feel" of this style heat since it is never actually hot. Exit temperatures like others are seeing sound like good news to me !


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