# Carrier 58MXA080-8-1-20 Inadequate return air supply



## owlfan12000 (Nov 3, 2006)

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?

John M


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## glennjanie (Nov 3, 2006)

Welcome to the Forum Owlfan:
Any heating system needs a free flow of air to transfer heat from the heat exchanger to the air in the house and the A/C needs even more air to do the job. The return air will not normally affect the combustion of the gas, unless it is drawing air from the room the furnace is in. That would create a negative pressure and make the gas burners go bonkers. 
In my opinion the heat exchanger will overheat if there is insufficient air flow, causing it to burn out in a short time. Low flow will also put an undue strain on the blower motor causing it to use more electricity and burn out quicker. I would recommend an insulated 8" round duct picking up air from say, 2 bedrooms on the far end of the house to the return air plenum to create extra relief.
Glenn


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## Aceinstaller (Nov 15, 2006)

its too bad that the tech that went to your house just said that without offering a solution and a quote to solve the return air issue.

call around for some quotes on solving your return air issue and get this problem fixed right away.  youll see a signifigant amount of energy savings, much more cooling capacity, and longer equiptment life.


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## j&krenovation (Nov 26, 2006)

Its possible you could have a massive buildup of dust in the return, especially if you smoke alot in the house.  Id shove a vacuum in there just to get all the effeciency you can out of it without starting a major project.


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