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We replaced a 20-year-old furnace with a new Lennox furnace and added air conditioning. The new system is still controlled by the 20-year-old Honeywell Chronotherm III thermostat. The furnace had (and still has) a fresh air intake with a Columbia fresh air damper controlled by a timer. The timer also turns on the whole-house exhaust fan, and USED to turn on the furnace fan. However, on the new Lennox furnace, the installer said the timer could not be used to turn on the furnace fan. So, we no longer have fresh air brought into the house based on the timer — I have to turn the timer on manually and turn the fan on via the old thermostat.

Can the timer be connected to the furnace fan without damage or risk? If so, how?

A far better option would be to replace the current thermostat and timer with a smart thermostat to control everything. However, ecobee support said:

Essentially, the damper and exhaust for the air intake is similar to a ventilator if there is no timer involved. If it required some sort of trigger from the HVAC unit in order to run, the thermostat will not be able to detect which parts that are needed and will result in it not engaging properly.

That's why there is no way for the damper to be connected directly to the ecobee, as far as I know as the ecobee won't be able to communicate with the timer or the damper .

Hypothetically, if there is no timer involved and the damper doesn't require any trigger from the HVAC unit, it is possible to have it connected to ecobee as an accessory (ventilator in this case). You can view this article Troubleshooting accessory installations for more info.

However, this is hypothetical as it all depends on the functionality of the intake and how it responds to separate voltage. Since it's only in theory, I would stick with my former answer that it's not possible for the ecobee to control the damper.

Is it possible to use a new smart thermostat to control everything? (I prefer Honeywell or ecobee.) If so, how would I wire it to the Lennox furnace control board? (I assume I would remove the Honeywell timer.)

THANKS!

Fresh air damper and Honeywell timer Thermostat wiring and Lexxox model numbers Lennox furnace wiring

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  • Did the installer say why the timer cannot be used to turn the fan on? It sure looks like the timer has the wires (though disconnected) needed to trigger the hvac fan.. – negacao Feb 21 at 23:23
  • No, the installer didn't say why. The installer is well-known in the area but not very good—they had to send five different crews to do the installation. It could be that they just didn't know how to connect the timer to trigger the hvac fan. Or perhaps doing so would void the Lennox warranty. No idea, sorry! – Andre Feb 22 at 4:33

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