furnace is going off on high limit

[msdalem1]

My unicell furnace is overheating. This causes it to shut down. The repairman says "The furnace is going off on high limit. The heat exchanger is getting cherry red and is a fire hazard. The vent and combustion air are too small for the size of the furnace. The furnace is too big for the duct work and should be replaced."
He said its a 120 btu and my house should only need a 75 btu.

The furnace is 11 years old. Do I really need a new one? Can't something else be done? I have had the igniter changed, but that only solved the problem for about 3 weeks.

[Hube]

Before I give you any advice on this problem, one thing puzzles me; "you say the unit is 11 years old, and the duct, etc is too small." When did this "overheating on limit" start to occur. If it was because of the unit being too big and the ducts too small this would have happened right from the start 11 years ago..... An oversised furnace will have a "short cycle" effect, and will cause the unit to cycle for short, but oftener times.... (its best to have a unit the proper size, but slightly under is better than over.... Normally "limiting out" on a proper sized unit is caused by dirty filters, improper air speed, obstructions in the air flow, dampers closed,etc. Again I ask ,when did this first occur.? Post back with this info and we will try to advise you further.Thanks. Hube

[msdalem1]

This happened a couple of years ago for the first time, and bigger cold air return ducting was installed. It has run fine since, until last month. The high limit switch would shut down, but I could get it running again by shutting the thermostat down to 50 then turning it back on. A repairman installed a new igniter, and changed piping outside to eliminate drafts. This worked until last night. Now it won't work at all. Even shutting the power switch off has no effect.Also forgot to mention.. I pulled out a filter that I hadn't changed in a while and it fell apart inside while the furnace was running. I told repairmen this.

[Hube]

The best thing to do right NOW would be to get that Damaged old Filter out of there. It is probably obstructing the AIR FLOW and this is what is causing the Limit to kick off the unit.Also, it seems like this furnace needs some TLC (maintenance work) such as cleaning the fan impeller blades, duct work vacuuming,etc. Let us know how you make out. Hube

[msdalem1]

After being off all night, I turned the thermostat up to try it, and it is now working again. Thanks for your help. I will get someone in to do the maintenance work.

[bbgru1]

i have seen this same thing before if the unit is to big it can cause this to happen short cycle is the same as going off on limit you can have a tech shut off one of the burners and and cap off a cell but you need to check out the footage of the house to a btu chart to see if you are over sized

[Super]

quote:Originally posted by msdalem1

This happened a couple of years ago for the first time, and bigger cold air return ducting was installed. It has run fine since, until last month. The high limit switch would shut down, but I could get it running again by shutting the thermostat down to 50 then turning it back on. A repairman installed a new igniter, and changed piping outside to eliminate drafts. This worked until last night. Now it won't work at all. Even shutting the power switch off has no effect.Also forgot to mention.. I pulled out a filter that I hadn't changed in a while and it fell apart inside while the furnace was running. I told repairmen this.

The evaporator coil can plug with dirt and block the air flow and overheating occures.

[Super]

quote:Originally posted by bbgru1

i have seen this same thing before if the unit is to big it can cause this to happen short cycle is the same as going off on limit you can have a tech shut off one of the burners and and cap off a cell but you need to check out the footage of the house to a btu chart to see if you are over sized

If you block a burner on a 80% furnace, the flue may condense water back into the furnace due to the lowered flue temperature.