Looking to change out from wood furnace to a wood boiler. Here's the deal I have a propane backup furnace now and was hoping to still use it to blow across a water to air hx. However the more research I do I'm learning I won't be able to utilize my storage to as low of a temp as other heat emitters. Does someone have experience to how low I could draw my storage down and still keep the heat comfortable?
water to air hx temps
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I have a 500 gallon tank and when the top of the tank gets much below 145F in 20 degree weather the blower will continue to run and can't raise the house temp to meet set point of the thermostat. Some may be able to go lower but it really depends on the design of the heat exchanger. My system works best between 150F and 185F.
Our WTA HX is in the output duct of our propane furnace. Prior to having storage online when temps into the HX got much below 145-150F (end of a burn or fire going out) the supply water temp would drop rapidly (the HX rapidly sucks heat from the small water volume). Then the fan would be on continuously blowing air that wasn't warm. particularly for the longest duct runs. Now with storage, there were days when 130F water into the HX worked. BUT, this is highly dependent on the outside temp. For days below freezing, we need +150 to keep the place consistently warm. But you're correct, the guys with radiant emitters or under floor can use lower temps than forced air. For us air conditioning requires ducts and plumbing for radiant is not in the cards for our old complex home. We did our system in stages. We've never had radiant, but if I had to choose between plumbing a radiant system vs storage, I think most would do storage. Then later, once you've worked the kinks are out, add radiant. Best of all worlds, radiant and storage. Next best, WTA and storage. We're happy. But the addition of storage transformed the operations of our boiler. I waited to long for that addition. Very big deal.
brant2000
Feeling the Heat
It is most likely true that you may struggle to maintain supply air temps, as your HX's supply water temperature drops. The best way to accommodate this is by using deeper (3 or 4 row) heating coils, which will slightly increase the resulting pressure drop - reducing the amount of supply air. Alternately, you really want to try to implement a strategy to minimize mixing in your storage to promote a thermocline to develop, and ensure that the warmest water can be delivered to the HX.
Another point - I use a system like this with a circulator providing flow to my HX constantly (regardless of a call for heat/blower being on). I've found this to be adequate to provide enough heat through natural convection, to heat my house down to moderately low temps (25-30F).
Another point - I use a system like this with a circulator providing flow to my HX constantly (regardless of a call for heat/blower being on). I've found this to be adequate to provide enough heat through natural convection, to heat my house down to moderately low temps (25-30F).
Thanks for the input. How's the best way to size a WTA hx? Just what my heat loss is when its being supplied by 180° water or leave a little cushion for cooler water? Also is there a recommended place to buy?
Plumedic, check out this link for a place to purchase a w/a hx. https://www.hearth.com/talk/threads...-coils-for-hybrid-hydronic-air-systems.21941/
Mike
Mike
Great memory on that thread Mike. Those are the guys I used and they nailed it based on my desired temp drop as the CFM rating of our propane furnace blower. I did not assume max temp, I think I used 170F in with a 30F drop given the blower's CFM. Told them the cross sectional size of the plenum and they computed the depth to do the rest. All thumbs up.
They were very helpful in helping configure mine the same way Tennman. How has yours functioned for you?
Make sure the duct work is insulated also. It's the temperature at the register that is important.
Mike, I have an AZEL sensor on my HX in and out stubs. When supply water is ~170F, the exit is very close to 140-145F exit. Given that the HX CFM is influenced by lots of things, like air filter condition, duct geometry.... etc. I'm amazed how well the guys spec'd the HX.
Bob, Part of our next upgrade is duct upgrades. We have a un-insulated sheet metal trunk with some of that insulated round hose type stuff. I'm sure we're getting lots of energy loss between the HX and the vents. Wish it was radiant, but AC is more important in our latitude (especially now to end-Sept). So, ditto the comment on insulated duct work.
Bob, Part of our next upgrade is duct upgrades. We have a un-insulated sheet metal trunk with some of that insulated round hose type stuff. I'm sure we're getting lots of energy loss between the HX and the vents. Wish it was radiant, but AC is more important in our latitude (especially now to end-Sept). So, ditto the comment on insulated duct work.
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