My house used to have just Forced Air heat, but a year and a half ago, I had central air installed. When it was installed I had questions about the wiring they weren't addressed to my satisfaction.


  • The forced air heat and the central air use separate duct sets. So, they have separate blower fans.
  • There is one thermostat (Honeywell PRO 4000 5-2) controlling both systems.
  • It has a setting for fan on/auto, and I would like to use the fan-only setting.
  • These systems are on different circuits.
  • The Inside AC unit is “ICP”FXM4X3600AL.
  • The furnace is HEIL model 5000.


  1. Is it safe for the fan to be wired?
  2. Should only one be wired? (Which) Could both safely be wired to fan?
  3. Should there be two thermostats?
  4. If the breaker pops for one does the power for the fan going through cause issues to the blower of the one whose breaker is popped?

Thermostat wiring:

thermostat pinout added after question was answered

  • 1
    OK, why in the world did they put an entirely separate air handler in for cooling to begin with? (Never mind the lunacy of ductwork in vented attics to begin with....) Sep 9, 2018 at 4:34
  • @ThreePhaseEel I am a software engineer who has a past in satellite communications. I have family in carpentry, but A/C and heating are not my wheel house. When I was originally provided quotes I was told that since heat rises and cool air falls that A/Cs go in the attic and Furnaces go in the basement. That this was the efficient way things work. It made sense to me. One of my neighbors told me having two seperate systems makes them both work better and one of my old coworkers said that his configuration was the A/C was in the attic. I assumed it was normal.
    – DasScooter
    Sep 9, 2018 at 5:07
  • The ducts for the heat are almost floor level and the house is a split level. The A/C vents are in the ceiling. Given the configuration, I don’t know how they’d share an air handler.
    – DasScooter
    Sep 9, 2018 at 5:09
  • @BrockAdams there is only one configured thermostat installed by the company that did the A/C install. I have another thermostat There previously, but they supplied the thermostat that they installed. The furnace does have a fan pin for the wiring that appears to be usable for a fan only mode. The Inside AC unit is “ICP”FXM4X3600AL and the furnace is HEIL model 5000. Installed thermostat is Honeywell PRO 4000 5-2
    – DasScooter
    Sep 9, 2018 at 5:43

1 Answer 1


You can certainly connect and use the fan-only wire for the AC.

Whether you can also drive the heating unit's fan-only input depends a great deal on your configuration and is probably not a good idea.

A breaker popping for one fan is not a concern for the thermostat system as the controls operate via 24V relays. As long as the appropriate 24V transformer is still powered, the other fan/system will operate normally. No additional harm will accrue to the dead fan other than what caused the breaker to trip in the first place.

First you must determine your exact control configuration, especially if you have one 24V transformer or two.

  1. Provide a picture of your thermostat's base-plate (wiring terminals).
  2. You should have 1 to three power terminals, typically labeled: R, Rc, and Rh. Is there one wire connected to these or two wires?
  3. Is there one Common (C) terminal or 2? With 0, 1, or 2 wires?
  4. Verify you only have one fan-relay terminal, typically labeled G.

The biggest problem with one thermostat for 2 separate residential systems is usually the power supplies. (2 different 24 VAC systems tied together often leads to one 0 VAC system and possibly smoke or a blown fuse.)

Your thermostat is designed to keep these 2 supplies separate; trying to drive both fans at once can tie them back together (and release the aforementioned smoke).

If you have 2 or more power wires and/or 2 or more common wires,

then you have 2 separate 24V transformers (this is the normal configuration) and driving both fan wires will do one of three things:

  • Not drive one fan at all (usually heating) due to the thermostat applying Rc power to an Rh circuit, both un-closed, for the heater's fan relay. This is the most likely scenario.
  • Immediately blow one or both transformers and possibly one or both control modules.
  • Appear to work (transformers roughly paralleled) but transformer life will be shortened due to current loops and slight phase differences.

In this scenario, you would need to bust out your Electronics-engineer skills and add a low voltage relay to safely drive the heater-fan relay (and not blow any transformers). That's beyond the scope of this question and there might be some commercially available solution an HVAC pro could hook you up with, maybe.

If you have only one power wire, and zero or one common wire(s),

Then the last installer might have driven everything off one transformer . And it might be a third transformer installed just for that purpose.

In that case, it might be okay to connect both fan wires to the same thermostat G terminal, see below.

The next question becomes whether the thermostat and transformer can supply enough current for the fan relays on both systems.

The thermostat can probably handle it, but the transformer might not supply enough current.

It would be prudent to:

  1. Locate the 24V transformer(s). Each air handler should have one, and one or both should be disconnected (in this 2-wire scenario).

    There may also be a third transformer installed to meet the demands of the combined system. In this case, you're probably good to go.

  2. If labeled/possible, note the current or wattage the transformer is rated for.

  3. Test the furnace fan control. Unplug the thermostat and jumper the power wire to the heater's fan(only) wire.
    Does the heater fan come on right away and the air stay cool after 10 minutes?
    ~~~ Do not attempt to run both fans just yet. ~~~

  4. If the fan failed to blow cool air in step 3, then your heater may not really have a fan-only mode.

  5. If you can, measure the current across the jumper in step 3 for both the heater fan relay and the AC fan relay (separately). The total of both currents should be less than 95% of the transformer's rated capacity.

  6. If the conditions of step 3 and 5 are met, then you should be able to connect both fan wires to the appropriate (usually G) terminal of the thermostat. There is a small chance that it will burn out the thermostat, but it probably won't.

  • Looks like both provide a set of wires to the thermostat, but its not wired with a common. When the fan switch is on the heat fan does not come on. Ill have to look later so see how the heat fan is set up, but I know in the past it was able to come on. Ive had it on for a few hours in the past without the heat. I was thinking of how I want the whole system configured but theres not a thermostat for that which has the pinout I would need. It would take a bit of electrical engineering and thats way out of the scope of this. Thank you for the detailed response.
    – DasScooter
    Sep 9, 2018 at 14:27
  • @DasScooter, what you have/show is a 2-transformer scenario. The commons were omitted precisely to keep you from blowing up transformers, although usually the AC common is brought through (but you don't need it for anything but powering the thermostat). So yes you need a relay, or a second thermostat (not recommended), or just a separate low-voltage switch, properly configured. Sep 9, 2018 at 18:08

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