DIY Home Improvement, Remodeling & Repair Forum > Appliances > HVAC > A/C and Furnace ON at the same time?

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Old 05-18-2009, 05:45 AM  
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Default A/C and Furnace ON at the same time?

Hi folks,

I have a Carrier A/C (Model 38BRC048370) and Gas Furnace (Model 58STA090) using a Basic Digital Thermostat by Honeywell (Model 69-1490).

Since the last couple of weeks I noticed that the place does not cool as much and the Room temperature (82F) never reached the temperature that I set on the Thermostat (76F). The air that came out from the vents seemed to be cool at first, later it seemed to get warmer.

When I checked the A/C unit outdoor a week ago, the unit did not seem to turn on and it seemed that the capacitor and contactor were bad (contactor had an open coil), and the 24V fuse on the furnace circuit had blown, so I replaced all those 3 parts.

Now, I do see that the A/C unit outside DOES turn on (both compressor and condensor fan) but it seems to stay ON!! If I manually shut off the system on the Thermostat, the A/C unit outside does turn off.

However, I have the same original problem..... The room inside initially seems to have cool air coming out from the vents. Later the air seems to be warmer and the A/C unit outside seems to be fighting to reach the Set temperature (It is constantly ON). The thermostat shows 82F even though I set it to as low as 68F.

Is it possible the A/C and Gas Furnace are ON, AT THE SAME TIME!!?
The status LED on the control circuit in the Furnace box in the attic shows a constant RED light. How can I tell if the Gas Furnace is also engaging at the same time as the A/C? What could be the problem?
Could it be a bad control board?

Also, when I turn the Thermostat switch from "Cool" to either "Off" or "Heat" the LCD display on the Thermostat goes blank.

Thanks in advance for your help!

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Old 05-18-2009, 07:44 AM  
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Obajaj, You might be missing something with your system. I know little about how AC and heating systems interact, but I do remember having a house that had the AC coils mounted in the ductwork just above the heater. It was a heating system from the early 60's, with the AC retrofitted in the 80's. I had to follow a pencil written direction sheet on how to shut off the heat, to include turning off the gas and isolating the thermostatic controlled gas solenoid, and resetting a control system for the fan motor mounted on the supply duct. To the best I can remember, I had to push a knob and turn it until it tripped a lever inside, then in the fall, I had to do the process in reverse to get the heat back on. The Heater and AC were two different manufacturers, and I am not sure who had the design on the round black control unit. Good luck to ya!
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Old 05-18-2009, 01:11 PM  
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I did a Google search and figured out the problem!

What was happening was that because the thermostat connector pins were so close to each other, when I last plugged in the thermostat, 2 pins (presumably the A/C and furnace control signals) were pinched together into the same pinhole connector and were shorted.
Everytime the A/C was turned on by the thermostat, I think it triggered the furnace as well, and so I had a mixture of hot and cool air, the room temperature thus never reaching the set temperature and making the A/C fight to stay on, trying to reach the seat temperature.

Anyway, I found the 2 pins shorted, (1 of them had got bent in the process). I unbent and straightened the pin and plugged in my thermostat and the system seems to be working fine now (It seemed to be cooling fine, with the vent producing cool air strictly). I left home for work this morning, but I will test it again, later this evening and also test the heating independent of cooling..... I'm fairly sure I have fixed the problem though....
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