Aux/Emergency Heat comes on in Cool Mode!


Old 07-21-04, 06:54 PM
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Question Aux/Emergency Heat comes on in Cool Mode!

Last week, I awoke in the middle of the night and accused my wife (wrongly, it turns out) of fiddling with the thermostat. I stumbled out into the hall and tried to turn it down but realized that it was at a pretty low temp already. I felt the air coming out of the register and found that it was HOT even though the thermostat was in cool mode. The fuel-oil furnace that functions as our emergency/aux heat was on!

My first inclination, which I followed, was to replace the thermostat. After doing some more research, this seems dumb, but honestly I had been yearning for digital/programmable thermostat in the depths of my heart anyway and this was the perfect excuse. Sort of like the whole WMD thing.

After installing my new fancy thermostat, I'm faced with the same problem. I followed the W1 (ask for heat) wire on the thermostat into the equipment room and found that it is connected both to the oil furnace and to the heat pump. I disconnected the three wires and measured the voltage too see if the thermostat was, indeed, asking for heat or not. I found that the thermostat was not asking for heat, but that the white wire that connected W1 on the thermostat to the heat pump was high (24vac) in cool mode. I suspect that this was tricking(?) the oil furnace to thinking that the thermostat was asking for heat.

My basic question is whether heat pumps have any sort of failover mode where they would trigger the ask for heat line or whether there's something else that I should examine/consider. I have to admit that I noticed the compressor was running louder this spring than last year, but I didn't think much of it at the time. Thanks in advance!
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Old 07-22-04, 01:58 PM
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first you sure you got the right tstat for heatpump with back up oil?? and then what is you switch over tstat set at to go from heatpump to oil?

I followed the W1 (ask for heat) wire on the thermostat into the equipment room and found that it is connected both to the oil furnace and to the heat pump.
You have to run one or the other??????

Old 07-22-04, 02:24 PM
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Pro Diagnosed

The temp went up today and I broke down and called a service pro. It turns out that I did get the right thermostat, and actually did manage to wire it correctly, but the control board on the Carrier had gone bad. I watched him follow the signal from the yellow wire to the compressor unit and he found that it was going into the control board, but not out the other side.

He spent a good amount of time trying to figure out if it was some other switches that might have failed, but in the end it seemed like it must have been a problem in the board. I have an identical unit right next to the problem one and he sort of compared the behaviors to rule out other components.

I'm not sure that I understand why the 24v signal was coming back down to flip on the heat signal, but maybe it was the malfunctioning control board?

I think my setup is pretty standard in that the heat pump runs until the temp hits 35-degrees and then the aux heat should flip on. Neither should have been triggered though as the t-stat was in cool mode....
Old 07-23-04, 06:24 AM
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Another Update

I just thought I'd post again and let you know that the service person was out again today to install a new control board, and before he did so, he realized that he forgot to test the outdoor temp sensor that was a part of the unit. It turns out this was bad instead of the control board.

This makes a lot of sense because the white wire coming out of this temp sensor was coupled with the call for heat from the t-stat and the oil furnace. Since the t-stat didn't have it's own outdoor temp sensor, in normal heat mode it would simply call for heat and this sensor in the Carrier unit was in charge of determining whether the heat pump could handle the call (temp over 35-degrees outside), or whether to pass the call along to the oil heater.

When the sensor failed, I guess it shorted to common and ended up calling for heat all the time. The guy is replacing that part now so I'll tell you if it turns out to be more complex, but I doubt that it will. I'm pretty happy because that part is definitely going to cost less than the control board!
Old 07-24-04, 08:38 AM
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You've got yourself a good honest tech.
Old 07-25-04, 08:52 AM
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matt is right, be sure to recommend them to others, and let the service manager know you are happy with their service.
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