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Carrier 58MXA080-8-1-20 inadequate return air supply

2 posts in this topic

I had this unit installed about 4 years ago. It went out last night and the technician found a crack in the pressure switch hose end. It was quickly repaired. Then the technician noted that the unit was giving a trouble code of 31. He determined that the return air duct was inadequate for the furnace size. The unit has apparently been short cycling for the past four years.

He then tried to tell me that it really didn't hurt the unit and probably only cost me an extra dollar or two per month in energy costs. For the record the technician was from the company that sold and installed the furnace.

I'm an Electrical Engineer so I'm not buying that but I have no HVAC experience and I'm not well equipped to argue the point. It seems to me that an inadequate air supply will, at the very least mean that the natural gas combustion would not be complete and more natural gas would be required to provide the same heat to the house.

Can you give me any details on the possible ramifications of this problems? If memory serves I think this unit also provides forced air for my central air unit. Could this have impacted my AC cost as well as my heating costs?

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Fault code 31 indicates "pressure switch failed to close" which is exactly what it should be given the pressure switch hose was cracked (not typical by the way on a 4 year old furnace) -

As for inadequate return air, it has absolutely nothing to do with the combustion process. Combustion air is brought in through the pvc pipe into a sealed combustion chamber then exhausted through another pvc pipe.

However inadequate return air will cause a furnace to cycle on the hi limit control(causes stress on the heat exchanger). Your furnace is rated for 40-70F temperature rise, meaning if the return(room air) temp is 70F then the supply air can be as high as 140F (recommended rise is 55F). Take a temperature measurement a few feet downstream from the supply pleneum and subtract the room air temp. Allow the furnace to come up to temp for 5 minutes or so before taking the reading.

On new equipment start-up this would have been a required set-up check. Keep us posted...

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