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I have a traditional gas furnace and A/C system (York unit outside). The thermostat is the Honeywell T9 wifi-enabled. In the summer I want to be able to turn system to "off", turn fan to "fan", and have the air handler fan run continuously as it will circulate the cool basement air to the warm upstairs. But, when I set thermostat this way, not only does the handler fan come on in the basement, but the outside condenser unit and fan also come on. In other words, it goes into "cool" most even though system is set to "off". Ideas?

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    can we see the wiring of the T9
    – Traveler
    Commented Jun 28, 2022 at 18:00
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    The thermostats I've used don't do anything if Fan=ON when the main part of the system is OFF. Normally the process is "Fan=ON" + either Heat or Cool, and to force the AC to not run, use either Heat (with a low setting) or Cool (with a very high setting). Commented Jun 28, 2022 at 18:07
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    Honeywell T9 user manual says you can run the fan separately, if this isn't working there may be a wiring error. Can you edit your post and add a picture of the thermostat wiring? The icon to insert an image looks like mountains and a sun. Commented Jun 28, 2022 at 18:43
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    @manassehkatz-Moving2Codidact Adding to your comment, some do some don't mine will run the fan even if the "system" is set to off. But for tstats that don't, I like your suggestion to set it to "fan on, heat only with a very low setpoint" That's a good workaround for tstats that don't have this capability. Commented Jun 28, 2022 at 18:49
  • Anecdote time: Many years ago I had a customer (computer programming, not HVAC). They had a near 24-hour operation 5 days a week. They wanted fan running all the time and same temperature all the time. Got new HVAC, fancy thermostat, etc. Night warehouse staff reported fan wasn't running. HVAC people came in during the day, couldn't figure it out. I figured it out (because a thermostat is really just a computer these days...) - when thermostat was in Unoccupied Mode with Heat or Cool + Fan "ON", fan was really running as if it were AUTO. Daytime was occupied, Nighttime unoccupied but all Commented Jun 28, 2022 at 18:50

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I have a Honeywell tstat that looks like this. Sorry but I can't remember the exact model number. But it will run the fan (air handler) if set to fan on, even if it's set to "system off". If you can't find it, I'll be glad to try and find the model number.
Just recently, I've been running the system on fan only bc the evaporator coils were freezing over. Turned out, bc I was running the fan almost continuously to deal with pollen this time of year, the filter got fairly plugged up and there wasn't enough air flow. Changing the filter fixed. Clearly this last paragraph is an aside. Just wanted to mention that with the right t-stat, it's possible to run the air handler only

honeywell tstat

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    I figured it out. Where the 5 thermostat wires come into the furnace unit itself there was a jumper wire connecting R and G. This wire apparently forces the compressor to come on for cooling anytime the fan is on. When I disconnected that jumper wire it now works the way I want it to. Thanks for everyone’s input. Commented Jun 29, 2022 at 19:36
  • Sure it wasn't Y and G? Anytime you call for cooling or just fan you'd get both. With R and G the fan would run always until the fan or the transformer burnt out.
    – Mazura
    Commented Nov 24, 2022 at 0:35

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