AC turning on while the thermostat is set to "heat"!!


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Old 03-15-10, 02:12 PM
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AC turning on while the thermostat is set to "heat"!!

I don't know much about the installed systems because we moved into the house a few months ago.

Honeywell digital thermostat. It's set to heat, fan on auto. There is a switch on the furnace which also seems to be to turn the blower fan on and off. It seems to override the thermostat because if I have the switch "on", the fan will blow regardless of what the thermostat is set to. As of last night I had this switch on to constantly circulate the air, and I have not noticed any problems.

My current issue is that with this setup, at 11:30 at night, I noticed cold air coming from my vents!!!!! I checked the furnace and the AC was on!!!! It was snowing earlier in the day and I certainly did not need AC that night!!!! I flipped the "fan switch" off on the furnace and the AC thankfully turned off.

What is giong on? I thought that the AC should NEVER turn on if the thermostat is set to "heat". Is this likely a problem with the thermostat? Incorrectly installed "fan switch" on the furnace?

The thermostat was reading the temperature in the room as 20C and the nighttime setting is 18C so it seems like the thermostat forgot it was set to "heat"

Any thoughts on troubleshooting?

I'm learning FAR too much DIY stuff by living in this house (previous two houses were not nearly as much work)

Thanks in advance!
 
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Old 03-15-10, 02:59 PM
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Is your unit a Heat Pump? or just straight AC
 
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Old 03-15-10, 03:32 PM
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if the stat is set for system...heat/auto...fan.it shouldn't see a cooling call the only way cool,and heat operarte is in auto/auto nd that toggle on the furnace is to kill the 115V and the entire fan,furnace,system...your condenser is seperate line 208V powered and only other wires are the low voltage call from the stat.put all the cooling temp settings to as high as they go on the timed schedule then work the heat.if it continues break one of the wires out at the condenser on the low voltage ...pull the contactor wire and tape it off.
 
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Old 03-15-10, 03:34 PM
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Originally Posted by LukeP
Is your unit a Heat Pump? or just straight AC
Just straight AC with a box outside the garage.
 
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Old 03-15-10, 03:44 PM
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Originally Posted by sminker
nd that toggle on the furnace is to kill the 115V and the entire fan.
The fan still runs when there is a heat demand and the toggle switch is off (ie when the burner is active). The toggle does not kill the 115V. There is a seperate wall switch for that.

Originally Posted by sminker
it shouldn't see a cooling call the only way cool,and heat operarte is in auto/auto
There is no auto/auto setting. Only auto / on and cool/off/heat

Originally Posted by sminker
put all the cooling temp settings to as high as they go on the timed schedule then work the heat
I'm a little confused by this? Sorry for being dense (maybe I need another coffee)
 
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Old 03-15-10, 06:03 PM
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Possible nick in the wiring and wet conditions can cause a short powering Y and energizing the cooling. Lift Y off of your thermostat and see if cooling still energizes . If so it can be miss wiring or a short . let us now
 
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Old 03-17-10, 08:38 AM
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Originally Posted by LukeP
Possible nick in the wiring and wet conditions can cause a short powering Y and energizing the cooling. Lift Y off of your thermostat and see if cooling still energizes . If so it can be miss wiring or a short . let us now
Monday night I inspected the wires at the thermostat and the furnace for any bare or nicked wires, and all looked to be OK. The AC did not kick on.

Tuesday night as I'm taking out the garbage, I walk by the AC unit outside and it's running (at 8pm!) %$#%!!

Disconneted The Y wire at the thermostat as suggested and it turns off in 30 seconds.

Crap. This means there is a break somewhere in the wire which isn't at the thermostat or furnace? (ie in the wall somewhere?)
 
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Old 03-17-10, 01:36 PM
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Originally Posted by The Rusty Gear
Monday night I inspected the wires at the thermostat and the furnace for any bare or nicked wires, and all looked to be OK. The AC did not kick on.

Tuesday night as I'm taking out the garbage, I walk by the AC unit outside and it's running (at 8pm!) %$#%!!

Disconneted The Y wire at the thermostat as suggested and it turns off in 30 seconds.

Crap. This means there is a break somewhere in the wire which isn't at the thermostat or furnace? (ie in the wall somewhere?)
This means there probably is not s nick in the wire if it still ran with Y disconnected then it would be a nick . Looks like your thermostat is energizing your unit. After lifting Y off the stat it stops. I would replace stat.
Is this a mercury type thermostat . ( you can see the mercury in the glass bulbs) If so put a level on the stat to verify its level if not level it and check operations.
 

Last edited by LukeP; 03-17-10 at 01:40 PM. Reason: additonal info
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Old 03-17-10, 01:49 PM
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Originally Posted by LukeP
This means there probably is not s nick in the wire if it still ran with Y disconnected then it would be a nick . Looks like your thermostat is energizing your unit. After lifting Y off the stat it stops. I would replace stat.
Is this a mercury type thermostat . ( you can see the mercury in the glass bulbs) If so put a level on the stat to verify its level if not level it and check operations.
I don't think it is a mercury stat - It is digital. Someone else suggested it may be dead backup batteries? ? ? That didn't make much sense to me so I will try a new thermostat.

Thanks Luke P!

The people at home depot are starting to recognize me because I go there so often! (never had so many troubles with my previous two homes!)
 
 

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