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Heating & Air Conditioning

Selecting a Thermostat for a Solar Application

16 years ago

here is what I am wanting to do......

I am going to install a special heating coil into my plenum that circulates solar hot water and need to control a circulation pump and the furnace fan. I am looking for information on how to do this.

The logic of the control that is needed is that when the temperature in the hot water storage tank is more than a setable amount, let say X, and we get a heat call I'd need to turn on the 110v a/c circulation pump and the furnace fan, however, if the temperature in the tank is not X then I need to fall back to normal furnace operation.

I have a standard Honeywell thermostat (a/c & heat) with the standard 110v/24v transformer. The 4 wires in the thermostat are: red, white, yellow and green.

The transformer is mounted on the a/c box on the furnace and the red and white wires come off there. There are two other wires, one going into the furnace (I believe from the thermostat) and the other goes from the furnace out to the a/c condenser.

I found a 24Volt control (Johnson Control A419) that can sense the temperature in the tank and turn on a power strip which would be connected to the 110V a/c pump. This would give me the control that is needed however it would not give me the fall-back logic when the temperature is not X to fall-back to normal furnace operation and I am doubtful that control of the furnace fan would operate properly although it might.

I have come a long way in this process and need help getting the wiring and control right. The hole in my knowledge is in how the 24V control of the furnace is done, to control turning on/off the fan, handling the heat call and the manual/auto control of the fan. I have a circuit diagram of how this looks and could start to guess at what I need to do to get the controls right but thought it might be short work for a HVAC person.

Chris

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Comment (1)

don21
16 years ago
I have solar domestic water heating - The circulation pump is controlled by a differential controller. It only runs the pump when the solar panel(s) have made the water hot enough that it needs to circulate - Usually when it's 10 or 15 degrees hotter than the water already in the tank

Even on a good day with 100% sun, the water is only circulating perhaps 20% of the time . . . . and even then, it only circulates for a minute or two before it shuts off and waits for the panel to produce more heat

If you run the circulation pump whenever the furnace blower is running, you're not going to get much heat gain from your solar panels . . . . unless your roof is literally covered with high efficiency panels

I have a single 4 X 10 panel on the roof ($1200) and while it produces lots of hot water in a full day with sun, it wouldn't provide hardly any heat if used in a furnace application - I would need another dozen or more $1200 panels to even hope to halfway heat the house with solar

Don
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