Two Stage Thermostat Keeps Running Second Stage Forced Air Even When Temp Diff Isn't Reached

dblagent007

I have an Aprilaire 8870 two stage thermostat set to run radiant floor heating as the first stage and forced air heating as the second stage. The radiant floor heating is set to turn on when the temperature is 1 degree below set point and the forced air is set to turn on at 2 degrees below set point.

The problem I have is that the forced air comes on after the radiant has been on a while and this happens even though the temperature doesn't drop to 2 degrees below the set point.

Any ideas? The only thing I can think of is that the thermostat is programmed to turn on both heating stages if the temperature hasn't reached the set point after a certain amount of time, but that seems to make setting the set point for the second stage pointless.

ratio

I don't know that stat in particular, but your guess is correct for a number of other stats, 2nd stage will come in both from a temperature droop and a timer—if setpoint isn't made after x minutes bring on 2nd stage.

It's also possible that the stat is fibbing about the temperature a little & you actually are 2° below setpoint.

pecmsg

Cool air circulating in the wall cavity behind the stat forcing 2nd stage on?

Let the stat hang away from the wall far a day or 2 and see if it continues!

Jamie Hall

dblagent007 said:

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Any ideas? The only thing I can think of is that the thermostat is programmed to turn on both heating stages if the temperature hasn't reached the set point after a certain amount of time, but that seems to make setting the set point for the second stage pointless.

That's basically it. The problem is that the thermostat thinks it is controlling a furnace -- hot air -- which responds to a temperature change command very quickly, or, if it doesn't respond, needs more power. So... when it doesn't respond, it says "OK, hit the throttle" and your second state comes on.

Radiant heat -- your first stage -- responds very slowly to a call for heat. Indeed, it may take hours -- literally -- to move from one temperature to another. With radiant heat, you are basically looking at a "set it and forget it" situation, which should be controlled by a floor sensor or outdoor reset -- or both. No setbacks.

SuperTech

I agree with everything stated above. The thermostat doesn't function the way you want it to because of your setup. Radiant, forced air and a single digital thermostat can't work together in that configuration due to the nature of the way each system heats, one slow and steady, the other with quick response.
Digital thermostats can usually be setup with a setting for cycles per hour, with radiant flooring is usually 1-2 CPH and furnaces are around 5-9 CPH if I remember correctly.

dblagent007

Thanks for your thoughts. The thermostat manual has a page describing dip switch settings for different heating setups. It shows the same dip switch setting for a two stage furnace and a first stage radiant, second stage forced air (link).
I guess I'll just disconnect the second stage furnace wire so it only heats with radiant.

ChrisJ

I don't know if it'll help or not but my Honeywell Prestige allows me to change between "comfort" which turns on the next stage after running a certain amount of time or I can change it to something temperature difference from the setpoint.

It allows this for cooling and heating and they can be different. It also allows different cycles per hour for each and I think I can do different cycles per hour per stage as well but I could be wrong.