Furnace Repair

Our house is heated and cooled by central air. We have a combination unit made by Carrier sitting in the mechanical equipment closet provided by the architect along with the water heater. The furnace part of the unit is gas fired. It is only ten years old.

During those ten years we have already had to service the unit twice beyond the usual cleaning and filter replacement. In both cases it was the flame sensor and the repair was simple. It just needed to have the powdery carbon buildup wiped off.

This summer we had a lightning storm. I am pretty sure we were not struck directly but it was close enough to take out the cable modem. Later I discovered it had also taken out the HVAC controller. The blower motor refused to energize even though the compressor was pumping freon through the A-coil. Plenty of cold. Just no airflow.

When we had the HVAC system replaced, we did not upgrade the thermostat. We kept the old mechanical device which is nothing more than a big spring with limit switches mounted to it. The spring contracts and expands with temperature, moving the limit swiches, and energizing the start relays for blower and compressor.

The thermostat was not effected. It still controlled the airconditioning compressor outside as the temperature changed. For a quick fix, I hot wired the power to the blower motor to a switchable 110volt outlet. When I wanted to cool the house all I had to do was switch on the blower motor. The same outlet also provided power for the 24vac circuit to the thermostat. So by turning it off I disabled both the blower and compressor. That allowed me time to find a new HVAC controller. Something I would need in order to use the furnace.

As usual, I waited until the last minute. Waited for it to get cold before taking action. An exact and direct replacement was not available but I did find a conversion kit. One week and 250 dollars later I received the kit.

I took my time reading the instructions cover to cover, making notes, and finally had the new controller installed and connected. Self test seemed to work fine. I finished connecting the thermostat wires, closed up the unit, turned on the thermostat, and set it to a high temperature to start the furnace. The furnace came on normally. Everything seemed to be working fine. Five minutes later the flame went out and the furnace shut down.

I knew it had to be the flame sensor which was serviced and replaced but I still could not make the furnace work. It would start up with the blower motor on. Error code started out as 13 and advanced to 33 after a few minutes. A 33 error code is a limit circuit fault and has a long list of possible causes.

I went back to the controller self test and was horrified to find it no longer worked. After a short delay, the blower would come on and stay on. It stayed on for over three hours with nothing else happening as I searched for answers.

Finally I started to inspect the wiring. Lots of wires and all connections made with spade lugs pushed onto spade terminals. One of the spade lugs had been pulled free of the over temperature limit switch. I restored the connection and the furnace worked fine.

I do my own repair because it is easier than finding a technician I can trust. Besides, when you have more time than money DIY becomes a necessity.

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