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golfnut65 11-18-2008 01:25 PM

Honeywell RTH8500 spare contacts?

I am wondering if anyone knows if there is a spare set of contacts on the Honeywell RTH8500 thermostat. I have an air to air exchanger that I would like to turn on only when the furnace kicks in.....instead of running constantly.

dac122 11-18-2008 02:06 PM


Originally Posted by golfnut65 (Post 186845)
I have an air to air exchanger...

Do you mean an HRV or ERV unit?


Originally Posted by golfnut65 (Post 186845)
...that I would like to turn on only when the furnace kicks in.....instead of running constantly.

Do you mean whenever the fan runs, or just heat and cool?

golfnut65 11-18-2008 02:31 PM

Fan on

I would like it to come on in either mode.....when heating or if the furnace fan comes on. I only run the air exchanger in the winter, so probably only when the heat comes on.....but if the heat kicks in, the furnace fan runs as well.

dac122 11-18-2008 02:42 PM

Then I think it could be energized or switched from either the tstat or air handler depending on what you have. If you just want to know if its possible to hire someone else then I think so, but if you want to diy we're gonna need a whole lot more info such as: what exactly is the unit, and how is it switched now; what type of air handler do you have and how is it controlled now; and anything else you can think of?

golfnut65 11-18-2008 02:48 PM

more details
It is a Lennox furnace....and a Lennox HRV unit. The HRV has it's own humidstat. So, the HRV will continue to run to try and satisfy the humidity in the house even though the furnace is not running. I think it would make more sense to have the two interact and only kick in when it calls for heat.....and more energy efficient. After all, the HRV mixes with the return air.

kennzz05 11-18-2008 04:47 PM

seems like unit should be wired the other way around so that the exchanger runs the furnace fan as needed not the other way round the
easy answer is wire it to the fan terminal in the airhandler,however dont have enough info to say that will even work.the xcange. would have to derive its low voltage from the furnace to begin with. assumeing its controlled by low voltage

golfnut65 11-18-2008 04:53 PM

Xchange running

That does make sense doing it that way. I'll have to check into the voltage to see if they can even be used as common together. I was just hoping that the thermostat would have an empty set of dry contacts to break the xchanger an auxillary set.

beenthere 11-19-2008 04:41 AM

Are you trying to humidify, or dehumidify, withe HRV.
HRV's don't add humidity in the winter. Unless its extremely humid outside.

golfnut65 11-21-2008 11:14 AM

I figured it out
I finally have it working how I want. Now the furnace fan will run when the HRV is on. I use it to dehumidify the house so condensation does not form on the windows.

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