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Just recently the fan setting on my Honeywell RTH8500D doesn't turn on the air handler. This is a recent problem. I usually run the air handler 24/7 during "pollen season" to filter the air in the house and had no problem simply setting the tstat fan setting to "on". The air handler is a Saving Products model HV-140H.

The only recent development is that it's been really hot here in western Washington and the A/C has been running nearly continuously. But when it does reach setpoint and the outdoor unit shuts off, the air handler shuts off too, even though the fan is set to "on" on the t-stat. The air handler runs normally when there is a call for cooling.

There is another air handler (2nd floor) with it's own t-stat and the fan setting operates normally, like the other one used to do.

Any ideas why the fan setting is suddenly inoperative on the 1st floor tstat?

AH with cover removedIced over evap coil.

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  • Since it used to work and works with a call for cooling the battery would be my first guess as on some stats the “fan on” is a separate relay and a low battery can cause this.
    – Ed Beal
    Aug 1, 2022 at 16:50
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    @EdBeal thanks for the idea. I pulled the tstat off it's housing and turns out there were never any batteries in it since it's install about 15 years ago. I went ahead and installed 3 AAA cells anyway and it made no difference. So obviously the tstat was powered by the AH. I do hear a lite click on the tstat when I set fan to "on", so I think it's working correctly. I'm more concerned with the AH, there was tons of condensation seeping out of the seams in the ductwork. And no, the condensate drain isn't blocked, I checked. Just hope it didn't hit the electronics. Aug 1, 2022 at 18:23
  • there was tons of condensation seeping out of the seams in the ductwork is your drain pan overflowing ? Is the heat exchange coil frozen ?
    – Traveler
    Aug 1, 2022 at 18:30
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    @Ruskes the AH has these slanted metal mesh curtains (for lack of a better word) that supposedly are to catch the condensate from the evaporator coils, but they are not very effective. I'm going to pull the covers now and see what I find. Aug 1, 2022 at 18:33
  • Metal mesh is washable, so service them, wash and de-grease.
    – Traveler
    Aug 1, 2022 at 18:40

2 Answers 2

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The 8500D is a pretty straightforward thermostat - not a lot to go wrong there. I would start looking at your wiring terminations for loose or broken connections.

The thermostat is one of those types that has a wallplate that accepts the wire connections and the thermostat itself slots into pins on the wallplate. This can lead to broken connections when pulling the thermo off the wall to change batteries, etc.

Also check the air handler end, specifically the 'G' terminal. If that became disconnected, the fan would still run automatically when a call for heat or cooling is received, but not when the call goes back to fan only.

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  • Thanks Chris: I"ll pull the cover off the AH and inspect. The tstat has never been pulled out of its wallplate since it was installed 15 years ago (see my comment / resp to Ed Beal, above). Aug 1, 2022 at 18:26
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You have two separate problems.

Ice on the heat exchange coil

Clearly that should not be.

Possible causes, Fan is not working, or AC is cooling too much.

Fan only mode not working on one thermostat

Model Honeywell RTH8500D

According to they manual you must have working batteries installed for proper operation.

Since Fan is working in Cool mode, it is not the Fan

Fan only is also working from second thermostat.

Clearly something wrong with one thermostat.

Carefully compare wiring and make sure the batteries are new and installed properly.

Make sure the settings on the thermostat are in accordance to the manual manual

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    Wouldn't a low refrigerant charge also cause icing of the indoor coils & possibly the refrigerant lines?
    – SteveSh
    Aug 2, 2022 at 1:33
  • Yes it would cause that
    – Traveler
    Aug 2, 2022 at 2:24
  • actually the fan could run in cool but not constant mode if it or a capacitor or whatever was failing; they often run at 25% power when "on on" and 75-100% power when "auto on". WD40 might reduce the friction enough to get that 25% working again.
    – dandavis
    Aug 2, 2022 at 18:36
  • @dandavis are you talking about adjustable speed fan ?
    – Traveler
    Aug 2, 2022 at 18:41
  • adjustable if you're comfortable swapping jumpers on the control board. the defaults i've seen give non-call-mode fan a lower speed, which might not be enough all-the-sudden. other fan problems would probably stop it from ever spinning...
    – dandavis
    Aug 2, 2022 at 18:43

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