New thermostat can turn blower on but not off

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New thermostat can turn blower on but not off

Postby fahmad42 » Sun Sep 16, 2012 1:06 am

On 9/13 I replaced an old Rite-Temp thermostat with a Honeywell RTH8580WF thermostat (just came out a few weeks ago). We have a 2.5 ton single stage AC (electric) and radiator heat with a boiler/water pump (natural gas), both older systems that have been working without fail for the entire time we have been in the house.

While I haven't actually turned on the heat part yet, the AC seemed to be controlled perfectly by the new thermostat until today. This evening I noticed air from the vents when I thought the AC was supposed to be off (we keep our fan set to Auto), then realized after playing with the thermostat it will not turn off the bloewr no matter what I do. The compressor is acting appropriate, only kicking in when the temperature/settings dictate, but even when I have the Fan set to 'Auto' and actually turn off the system from the thermostat the blower won't turn off. I turned off the AC blower at the breakers and that shut it off, I then turned on the breaker and the blower came right back on.

So I turned it off again at the breaker, popped the thermostat off the wall (sticks onto a wall-mounted plate where the wires are connected) and rechecked the green wire (going to G), took it out and screwed it back in to it's spot. I turned the blower power back on at the breaker and voila! The blower did not turn on. I then went to the thermostat again (still set to system off with fan auto), and put the fan to on. The fan powered on immediately, but when I switched the fan back to auto, the fan stayed on, even with the system set to off. I tried turning off the power at the breaker again, then turning it back on and the blower stayed off again (this time I didn't mess with the thermostat wires).

Instead of switching on the fan on the thermostat this time, I tried lowering the set temp so the AC would try to kick on the whole system. It kicked on the blower/compressor the way it should, but when I dialed the temp back up, the compressor turned off but the blower again stayed on.

So for some reason, it has the ability to turn the blower on, but has lost the ability to turn it off, even though I am 100% positive yesterday the fan was working correctly in the Auto mode.

I have no idea what the problem is, but it clearly is new with our new thermostat. I've already reached the limit of my ability, so would appreciate any insights.
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- New thermostat can turn blower on but not off

Postby Freon » Mon Sep 17, 2012 7:51 pm

Would you post the make and model number of the air handler. It's possible, depending on the blower control, you have an older, dial type fan control. It would be in a 3"x5" metal box. The cover pops off easily and there's a dial inside.

When you did the thermostat changeover, did you turn off all power to the air handler and boiler?
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