American Standard Freedom80 Single Stage ADD080C936F2 Blowing cold air

My gas furnace is blowing cold air.
The burner are not staying on longer then 30 seconds.
Help!

 

Do you have a blinking error code visible through a small window in the furnace door?

Probably blinking a flame sensor code.

 

It was blinking 4 times earlier, but then I turned the electricity off at the fuse box for a while and now it has finally brought the home up to temp.

While it was heating it was blinking continuously fast and now it is blinking continuously slow.

Do you know how I can get a manual?
Thanks,

 

Open limit.

Is the air filter clean?

 

It is doing it again...Both air filters are clean.

 

What kind of filters are you using? MERV rating? Size?

Many filters on the market are very dense and greatly reduce airflow.

Closing too many vents and blocking returns can cause high limit trips.

 

No filter in the furnace, both the filter are 3M 800 MPR 20 x20 x1 in the cold air returns in the ceiling of two halls in the house. None of the vents are closed or blocked

 

You have enough filter surface area for sure; however, allergen filters are notoriously restrictive to airflow.

Try something that says "dust and pollen" - should be MERV 6 or 7

When the furnace does operate, how warm is the supply air?

 

it is usually moderately warm not hot coming out of the vents. I have had the furnace off all night and when I turned in on this morning it still has the 4 blink pattern.

 

If the error code does indeed indicate that the high temp limit is tripping, check the temperature rise of the furnace. Search this board for instructions.

If the temp rise is within range, the limit might be faulty -> tripping prematurely. (that, or something strange is happening with respect to even airflow over the heat exchanger)

 

I called the furnace repairman. After checking the coil, duct and blower, he said that the sensor is faulty (tripping at 112) and will need to replace the circuit board at a cost of $325 for board and labor.

 

If the "sensor" (presumably the high temperature limit switch) is bad, why replace the circuit board?


I don't think I ever saw a high temperature limit switch shutting off at an excessively low temperature, That combined with the recommendation to replace the circuit board suggests the dignosis is wrong.

IS the high temperature limit switch opening and shutting off the burner? The temperature the limit switch should open at is usually on the limit switch ("L140" indicates a limit switch that opens at 140 degrees F)

You can remove the limit switch easily and use a themometer to measure the temperature the limit switch is measuring to double check what you are being told.

Also, I'd consider temporarily removing the filters to see if the added air flow solves the problem. If so, I'd seriously consider chucking the fancy filters and installing $1 Home Depot specials.

 

He is telling me the switch is on the board. He says the switch is opening and shutting off the burners. I can see the switch on the furnace but he is telling me their is another high limit switch attached to the board and it is tripping and shutting the burners off at 120.

 

There's probably no limit on the board itself, but there might be an aux limit somewhere else.

Normally I advise against jumping safeties, but in your case - might as well try bypassing the high limit for a couple of minutes to rule that it out. DO NOT operate the furnace for more than 5 minutes with the limit bypassed.