New Central AC Causing Furnace Problems
We had central air conditioning installed into our existing forced-air heating system a few months ago, along with a Honeywell electrostatic air cleaner. The furnace is a Day & Night 125,000 BTU downblower and is less than 2 years old. The new cooling coil unit in the furnace's supply outlet is sized for 4-ton cooling. Now that the seasons are changing and the furnace is beginning to run on heat mode, and I've noticed a new problem -- the furnace shuts down and restarts itself a number of times before the thermostat finishes calling for heat. Each time it shuts down, the furnace gives a status code of "33", indicating "Limit or flame roll-out switch is open," and lists a number of possible problems, including a dirty filter or restricted duct system. My guess is that the new cooling coil set has restricted the furnace air flow enough to cause this. But why? The furnace is designed to have air conditioning added to it and should be able to push air through a 4-ton coil set and not have a problem, right? Might the installer have done something wrong? And is it ok to tolerate this on/off operation until it is fixed or should we shut the whole system off? I ask here because I want to be as informed as I can before I speak to the installer about fixing it (after all, they missed it the first time....). Thanks for any and all help
Answer:
You dont mention if it is gas or oil. It's possible that the contractor re-located your safety and fan control thermostat that senses plenum temp, and put it too close to the plenum. Or, if the fan speed is too slow, the airflow cant remove the heat fast enough, the plenum heat builds up and the safety shuts down the furnace. You could increase fan speed for a first try. Is the heated air especially high in temp? Another alternative is to reduce the amount of fuel (lower GPH nozzle for oil, crack the supply valve closed a bit for gas) Good luck.