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....).
Answer:
My older Honeywell cells dont restrict air by much, as you can see right thru them, but the mesh pre filter may do it. My setup is pretty restrictive, as the return funnels into the cleaner then back into the plenum, about 80% of total. If you are satisfied with the air flow out of your ducts upstairs, I still think cranking down the gas burners a little may do the trick, so the plenum doesnt get so hot as to trip the limit switch. As you mentioned, straightening out the air cleaner path would also help air flow over the plenum.