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loum · Dec 24, 2007, 02:43 PM

I have a Bryant model 376C, induced combustion, gas fired, downflow/horizontal furnace with an overheating problem. Unit is approx 18 yrs old. Limit switch blew, replaced it, and replaced dirty filters and slightly increased blower motor speed. New limit switch also kicks in. Air/return intake is excellent but exiting hot air out of room vents inconsistent. 11 ducts in all. Some room vents have good air flow while others have much less. Any ideas as to why furnace overheats?

hvac1000 · Dec 24, 2007, 03:54 PM

All the vents need to be open.
All return airs need to be open.
If this is a switch located at the burners you could be experiencing flame roll out. If that is the case check the chimney connector/chimney or B vent to be sure it is clear of all obstructions.

Sometimes you can see the roll out or feel the heat in the burner area. Wtch that area while someone activates the furnace. The burner should lite and the flames/heat should not enter the area where the gas valve is located right in front of the burners.

loum · Dec 24, 2007, 04:17 PM

The area of the burners and gas valve is fine. Flames are nice clear blue. Whil awaiting the new limit swich, I bypassed the wiring for it to keep the furnace going. The limit switch sensor fits into ar area where the forced air and heat go past it

acetc · Dec 24, 2007, 05:28 PM

If you have air conditioning, you may want to check the evaporator coil, it may be plugged if the filters were very dirty, the coil may also be. The easiest way to look at the coil is to remove the blower assembly and with a flash lite look through the furnace heat exchanger at the entering side of the coil. Merry Christmas , Mike

loum · Dec 24, 2007, 06:27 PM

Thanks. No air conditioning on this unit. I've been checking those things that you twist on top of the vent pipes to alllow more/less hot air to flow through to the rooms. So far , all of them fully open. Can anything ever clog these vents?

AC Doc · Dec 24, 2007, 06:52 PM

You may want to look at the Blower wheel and see if the curved blades need to be cleaned. If they need to be cleaned remove the whole blower assembly by turning off the power and unwiring it from the furnace. There should be several screws or nuts that hold assembly in. Remove the assembly and clean with something like greased lighting. Try and keep the motor as dry as possible. Once fully clean and dry reinstall and turn it back on. If this does not fix the problem it sounds like it may just be a bad temp sensor.

hvac1000 · Dec 24, 2007, 08:40 PM

Vent pipes/duct runs can become plugged up especially if the kids remove the floor grill and stash toy in them. I also had a customers kids store food in there because he was on a diet. They could never figure out where the smell was coming from. When I took them up stairs they could not believe all the food stuffed in the duct. The duct had a elbo and that allowed it to stay there until you put more food in. Then the food would go down the duct. We had to open both lines from the furnace to the upstairs and clean them out. I thought you would get a kick out of that one.

NOTE: be very careful jumping a safety device. The safety devices usually go bad for a reason and with it jumpered you have lost the safety factor. Have a safe and Merry Christmas.