Initial Experiences with Plenum Heat Exchangers

[Donl]

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

 

[Tony H]

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.

 

[rickh1001]

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!

 

[Vtgent49]

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

 

[Duetech]

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?

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!

 

[rickh1001]

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.

 

[pybyr]

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

 

[Tony H]

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

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

 

[Duetech]

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

 

[mwk1000]

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 !