Questions about HX in forced air duct.

I don't know about size, as I purchased mine based on the BTU output at the given CFM of my furnace. I run the circ only when the furnace calls for heat, as you will easily overheat your house otherwise by gravity flow of air in the ducts. In fact, I run my variable speed furnace fan on low speed all the time and only have the circ turn on when there is a demand call. This works very well as it keeps air moving in the house all the time and reduces cold spots. The fan only ever runs on high if the water temp drops too low or I increase the tstat setting.

This setup works well and is comparable to gas heat when the water temp in my system is above a certain point (140F on design days, practically about 125F for me). Below that there is not enough usable heat to keep the house warm; this is one of the limitations of using the water-air hx. I have 500 gallons of storage and by design I can store about 1 million btus in the tank with my operating conditions. Radiant heat in the house would give me 1.5 million BTUs in the same tank.

Medman said:

I don't know about size, as I purchased mine based on the BTU output at the given CFM of my furnace. I run the circ only when the furnace calls for heat, as you will easily overheat your house otherwise by gravity flow of air in the ducts. In fact, I run my variable speed furnace fan on low speed all the time and only have the circ turn on when there is a demand call. This works very well as it keeps air moving in the house all the time and reduces cold spots. The fan only ever runs on high if the water temp drops too low or I increase the tstat setting.

This setup works well and is comparable to gas heat when the water temp in my system is above a certain point (140F on design days, practically about 125F for me). Below that there is not enough usable heat to keep the house warm; this is one of the limitations of using the water-air hx. I have 500 gallons of storage and by design I can store about 1 million btus in the tank with my operating conditions. Radiant heat in the house would give me 1.5 million BTUs in the same tank.

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Just asking a stupid question, when you say 1 mil BTU's vs. 1.5 mil BTU's - I assume you mean usable BTU's?

JohnDolz said:

Just asking a stupid question, when you say 1 mil BTU's vs. 1.5 mil BTU's - I assume you mean usable BTU's?

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Yes, usable BTU's.

Medman said:

Yes, usable BTU's.

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Thanks, that is what I thought. I am learning lots of stuff here so I don't want to make any assumptions.

I think I would oversize the HX, pretty well for sure.

Then once you get it up & running, if you think it's moving too much heat from the boiler to the house you could always reduce the flow through it. Whereas you can't do the opposite with one that is undersized - you can only get so much heat out of it.

Constant pressure pumps like the Alpha are pretty handy for that - it can be throttled down and it will just slow itself down to match the increased head you showed it. Whereas a constant speed pump like a 15-58 will keep trying to pump the same amount of water.

Agree with all Medman said. Back when sizing my HX folks here suggested a 30F delta for water in vs water out at required gpm and furnace CFM. The gpm (or btus/min) was computed based on a knock down from the boiler capacity. The HX vendor nailed it and it still amazes me how I consistently see 169F in and ~139ish out. I'd be concerned how to manage or determine cfm to two HXs. You know your blower rating, but I'd think you need to know how your cfm is split to properly size each HX. Probably a real HVAC guy can figure this out. Seems like it would be better to place the HX before the split to have one HX. But I'm not an HVAC guru. Best wishes.

Because I was in a hurry I didn't put data lines into the trench for the undergound to our boiler barn. So no way to control the pump based on the fan. So pump ran from Thanksgiving to some time in April. We were saving so much vs propane I ignored it. Last year ran Cat5 across the ground to control the pump based on fan ON (over summer ran underground). Much more efficient all the way around. If you need, I have post somewhere listing the two RIB relay components I used to control the pump based on the thermostat signal to turn on the fan. Possibly better ways to control the pump, but I'm a DIY'er controls rookie. BTW, some have installed an old water heater right before the HX to minimize time required to get hot water into the HX after the fan comes on. I haven't done this. When our fan first comes on it feels like air from a heat pump; not hot, but not cold. Lasts maybe 10 seconds then gets hot, so I'm calling it good enough. Changed my fine mesh, allergy reducing furnace filters this morning. Blocked filters can really hurt efficiency on our hot air systems.

For a lot of reasons I'd work hard to have one HX.

Running pump continuously will kill your tank stratification. You will have the same temp top to bottom because you are constantly mixing the tank. When you are calling for heat you have a nice delta T coming off your water so the bottom of the tank will typically be 30 degrees lower than the top. When you aren't calling for heat and only the pump is running you have only line loss coming off your water, so now you would be returning water to the bottom of tank that is significantly warmer than bottom of tank.

I have 1 heat exchanger, 1 trunk with zone dampers for the first floor and basement. When I get a call for heat it starts primary/secondary pumps. I have a aqua stat at my HX with a time delay. If the water isn't 130 after 3 minutes it shuts off the pumps and the furnace ingrates the gas burner.

gg

I don't want to discourage you from your plan, as i have hooked three boilers here on my farm up that way, but I will caution you that you won't get as much heat as you expect out of it. Now these are drafty old farmhouses without as much insulation as I would like, and I am a farmer with plenty of wood but not a lot of extra money, so I make it work. I assume your house has been constructed more recently than 1850, and has better insulation, so your actual experience may vary.

Think of it this way: Your current forced hot air furnace sends air around a heat exchanger heated directly by flame. I don't know the actual temperature of the outside of that heat exchanger, but lets say 400 degrees. With your hot water heat exchanger in the plenum you are instead in the 150 to170 degree range. So the output air from the ducts is warm and comfortable, but not as hot as the house is designed for. It seems typical for me to get a change in air temperature of ten to 15 degrees in these setups.

In addition you are restricting the airflow with the heat exchanger so the blower fan on your furnace is not moving quite as many CFM's of air as the house is designed for.

So when you add both of those factors together I have found that the blower just pretty much runs non-stop. In fact I have found it more comfortable to just run the blower continuously for the cold part of the heating season. Yes i could adjust the blower speed, but I like the noise level where it is.

In shoulder heating periods you can get quite a bit of heat even without the blower running, so the ability to shut the circulator down at a certain temperature would work well.

One additional factor to throw in: Typical operation is to set the oil/gas furnace thermostat to 5 degrees lower than the wood thermostat. When you start blowing air heated by the oil/gas through the water HX you are increasing the temperature of the water, so a system that shuts down the circulator while the oil/gas burner is operating would be ideal, not that I actually do that myself.

Now in your case where you can build it custom sized I would definitely make it as large as possible. Think about how much space the fins and tubes will block in the airflow path and make the HX that much larger. My initial thought is to double the surface area of the HX compared to the duct, so a 20" by 20" duct would get a 28" by 28" HX.

Now when I rebuilt the attached formerly unheated shed on the house into living space I did a radiant slab and absolutely love it. It complements the hot air in the rest of the house. I really see adapting forced hot air with a hot water HX to be more of a stepping stone, or perhaps one piece of the puzzle that should be complemented by something like underfloor radiant or hot water baseboard rather than a single complete solution.