Post in 'The Boiler Room - Wood Boilers and Furnaces' started by hartkem, Jun 26, 2012.
60' long firewood wall with plenty more to split
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Tonight I made some good progress on my WAHX . I finally received my second exchanger and began constructing the A coil to fit inside the furnace plenum. I bought some sheet metal and closed in the sides and bent a piece for the top for strength. All the panels were riveted on. I am mocking up the plumbing for the two exchangers. I believe the best way would be to connect the top two and bottom two pipes. The supply water would then enter the bottom and exit the top. I have the top two pipes connected in the first picture so you can see what I plan to do. Is there a better way to connect the WAHX?
That's a heckuva nice boiler shed. just a little bigger and it'd fit a lazyboy and a fridge for "tending the fire"
I also like the A coil setup. you'll get some serious btu's out of that, with little flow restriction. I normally run the hot to the top of the coil, but in this case it may not matter at all. if this is an unpressurized system, in the bottom and out the top will help any air move thru the system quickly and easily.
My system is pressurized. Is it better to run hot to the top vs the bottom? I didn't get any instructions on which is the correct way
Search "counterflow" here on hearth. Trying to remember the particulars but I think on a horizontal single HX install, they say you want the hottest water to enter the port that has already been heated by the 1st row and 2nd rows of copper on your HX.
Is it too late to flip one of your HXs so they both match? My gut instinct is the you would want to heat the inside rows of the A coil first and then the output ports on the "outer" rows (outside of your A coil) last. The theory here I believe is the partial cooled water heats the air first (ie. outside rows of your 3 coil HXs), then the middle row, then the inside rows (inside the A) would need the hottest water since the air is already heated at that point. Wait for one of the pros to comment on this, I don't remember how much difference in reality this will make. I'm sure yours would do the job as is, but if it's not too much re-work, I think it would be worth it. For what it's worth, I am very impressed with how my single HX does with low water temps (way less than 120), but I am used to "heat pump heat" which is only lukewarm. My fan runs more than a gas furnace but we find it much more comfortable than the old high efficiency gas furnace our last home had (too much hot-cold-hot-cold). Your A coil design should be even better.
No matter how you flip the HXs they won't match. I read about couterflow and basically you want the hot water to enter the top and leave the bottom. This is simple to understand when you have a single HX laying horizontal, but in my case is the top considered the narrow part of the (A) or is it the outer tubes? Im thinking the top is the outer tubes so I would need to cross the piping manifolds to make it flow correctly.
Yes think counterflow the whole way. The way you have the red arrows has the water moving counter to the air flow which gives the best delta or difference in temp = most BTUs for least amount of flow.
P.S. I'm an ASE certified master mechanic also, glad there are others here.
Spent about 2 hours last night soldering the manifolds on the WAHXs. Then began cutting into the duct to slide it in. This thing is heavy. A/C doesn't seem to be affected much. I am going to finish the plumbing and then remove it since its been so hot here and I dont want to compromise the A/C at all. I wish I had a way to measure air flow at a duct before and after.
I stand corrected on your counterflow. I was thinking your airflow was flowing the opposite way across your A. Does your air handler have different speeds? Mine actually had 4 but the HVAC installer only set it up to run on high. My research on the web found it should run faster for cooling than heating. I changed it to the slowest speed the first year with the W/A HX (too save electricity) and it worked fine. In fact I think I ran it last summer on the low setting. However, being so hot in the midwest this summer I changed it back to high the first time we broke 90* in March. I've never removed my HX but I did put 2 valves to prevent any chance of flow (thermosiphoning) during the cooling season. I suppose I am losing a little bit of efficiency by having to cool the HX but it can't be much.
I belive mine can be change by moving jumper wires. I haven't looked into it yet. Rkusek, where are you at in Nebraska. I go up there for work all the time.
Just west of Ft Calhoun about 15 miles north of Omaha. Residential boilers or hydronic heat period is almost unheard of around here.
Nice looking setup. I've never moved into a house with the furnace blower rates set right. I always have to kick them down to med-low or low. never had a place with AC though. now: all in-floor, and a wood stove. getting an effecta 35KW (maybe a 60?) as soon as the shop is close to construction.
keep up the good work. I like the counter flow plumbing.
I know this is an older thread, but I wanted to chime in. Early on it was mentioned to 'hartkem' to add a second coil to the plenum to lower his delta T. Putting in the A-coil layout will have just the opposite effect as desired unfortunately. In order to lower the ^T you should have STACKED the coils horizontally, one on top of the other. Run the hot water to the top-most coil and extract it from the lowest. Having them in series instead of parallel would also have added head to your system, lowering your water flow rates, which is also desired if you are obtaining greater ^T.
Of course your air resistance would have been much greater.
Hopefully 'hartkem' will pay us a visit to let us know how it's working. I'd like to know if one coil is air-locked and cold - there is no air release or diverting valve to force it out of air lock. Bummer to go thru all that and end up with 1 hot and 1 stone cold coil.
The A coil design seems to work just fine. I am able to heat my house using water down to 135 degrees. Surely the A coil design transfers more heat than just one coil.
One could tell if both WAHX are working, look at his duct setup, lots of exposed copper to put your hand on to tell if there is hot water there.
Also with double the surface area and therefore half the airflow I'd think you's get a much lower return temp. Yes stacking the coils would give the greatest delta but airflow would be a problem. I believe the slower moving air combined with twice the surface of the copper/aluminum would contribute significantly to the delta. Ideally we'd all have a serpentene coil in the counterflow, but the pipe would have to be 3/4 or larger, and costly. Hartkem's setup is the best compromise for real-world IMO.
With all this talk of an air locked coil, I went downstairs and felt the copper. Both coils are definitely working. The large supply pipes and the small copper pipes weaving through the aluminum fins are very hot to the touch on both coils. I have a thermostatic diverting valve further back on the return pipe and a spirovent on the supply side. I don't think there is any air trapped in it but I could be wrong.
I forgot to add that I now have 2 rows of firewood 6' tall 80' long.
There is an interesting thought - does slower or faster moving air transfer more heat? Obviously there is a 'sweet spot' in the air velocity. I was only thinking of the water aspect of the coil when I posted earlier. More to learn.......
Regardless, it apparently is working great, which is good to hear. I'm curious what delta exists in this setup and how it would compare with a single coil.
Funny you ask cause I was actually trying to measure it earlier but my non contact infrared gun was giving me readings all over the place. I don't have a good way to measure it right now.
I've found copper and IR thermometers to not get along so well. If you rpughed up a spot and shot it with some flat black paint I think you could get some accurate readings though.
I may have to try that but then my copper job won't look as pretty . I did measure my vent temp last night on a register upstairs. I had 120F air from the register with 160 degree water in my storage tank. Not to bad I don't think.
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