Water/air exchanger from boiler to furnace

Old 04-15-17, 04:29 AM
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Water/air exchanger from boiler to furnace

I will be installing a water/air exchanger from an outdoor coal boiler into my furnace plenum. There are a number of things I need to consider when doing this.

The boiler is a bit oversized so one concern is short cycling of the boiler. To make this more complicated I only want to heat a portion of my house meaning I'll be shutting down registers causing potential excess pressure within the duct work.

There are a number of things I need to consider and want to run this by a few people here.

My furnace blower is the permanent split capacitor (PSC) type and from what I've read closing registers will cause this blower to work harder, stress the unit, and slow it down.

I believe I can lower speed and be ok as long as I take measurements and stay within a static pressure of appr 0.5 inches water column.

Once I find that I can work apply heat to my water/air exchanger. I could measure the temperature rise across the heat exchanger to make sure it's within manufacturer's specs. AI could verify I'm ok by measuring supply and return temperatures on my boiler to ensure I'm at a Delta T of 20.

If I'm not pulling enough heat off the exchanger I could either manually slow the rate of water flow to the exchanger and/or lower my boiler temperature (not a bad thing). I could also pull more heat by installing a radiator or two if needed.

There will be no oil fire so I won't have to worry about flue gasses.

Not sure if I'm missing anything. Any other measurements anyone can think of I may want to consider? Any other thoughts?

Thanks in advance
Old 04-15-17, 10:22 AM
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Welcome to the forums.

I'm not the boiler pro.... just some observations.

There will be no oil fire so I won't have to worry about flue gasses.
How about the flue gasses from coal ?

First obvious question to me is what do you mean by boiler short cycling ?
How does a coal boiler short cycle ?
Once it's stoked with coal... it's all or nothing.

Since you are dealing with a semi unknown heating source.... you may need some type of interim storage tank.
Old 04-15-17, 04:03 PM
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Thank you.

The coal boiler is somewhat like a pellet boiler (I believe). There is an automatic feed mechanism and a combustion motor which only runs when the aquastat senses a need for heat (there's also an "outfire" timer to run combustion air just enough to keep the fire going every 30 mins for 2 mins).

Pulling too much heat off the exchanger quickly would satisfy the room temp too fast causing the thermostat in the room to tell the zone valve to shut down quickly. Boiler temps would then rise fast and the boiler would only run for short periods of time.

The coal flue gasses will be outside where the boiler is. The furnace will not be burning oil which is inside. Instead the furnace will on be used to circulate hot air. Many people become alarmed when others want to make adjustments to their furnace fan although after thinking about it I guess the lowering the furnace fan would not have an effect on oil flue gasses either.

After thinking about it I don't believe changing aquastat settings would have an effect on delta T return boiler temps. Decreasing the flow of water to the exchanger may be the option. I suppose opening or closing furnace return ducts could also achieve some of this as long as I maintained my static pressure within manufactures specs or appr. 0.5 w.c within the furnace supply/return.

The newer variable speed ECM smart pumps are supposed to monitor delta t readings and adjust flow of water accordingly. I didn't think of that.

I guess I'm thinking out loud. I like to think things through to see if they make sense instead of just sticking a heat exchanger if the plenum and hoping for the best.
Old 04-24-17, 05:05 AM
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Closing down some of the registers will not put a strain on the blower motor but will actually cut down on the amperage draw (work load). The higher the resistance to air flow the lower the strain on the motor. The motors overload when there is not enough restriction on the air flow. As far the outdoor boiler is concerned, if the coal is fed by an automatic feed device , it should be able to somewhat control the heat output. Is this outdoor unit a manual model or an automatic model? If you have not yet purchased the unit a slightly smaller unit may be a better choice. One last thing, is this a hard coal or soft coal unit. It make a large difference in the operation. Soft coal units are very dirty and need a lot of attention and cleaning. Post the name of the unit and model for better direct answers

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