I have a bryant model 350mavo36080 series B input 80,000 output 74,000
The furnace runs all day when it is cold but the house never gets above 60 degrees
The temperature in the plenum is 84 degrees so the furnace is only raising the temp by 24 degrees
All 3 burners are working
The air being blown out of the registers in the house is not very warm

The exhaust outside the house is 60 degrees so I am not losing the heat there
It is a side instalation and the temp on the outside of the furnace, just above the place where the flames are is 156 degrees that is the surface temp on the outside of the furnace

What can cause the plenum temp to be so low when all the burners are burning and no heat is lost ou the exhaust

Are the burners actually staying lit all the time, or are they cycling on and off?

Do you have natural gas or propane and what are the weatheer conditions in your area--- typical for your area or extreme?

On all the time
Natural gas

It has been below zero at night

You may have inadequate gas pressure being supplied by your gas utility due to high demand or other problems with the system ---- such as a frozen regulator or gas meter.

Do you have ot5her gas appliances? If so, what types? Have you noticed any low flame conditions or problems with those other appliances?

Around here, it's cold when it gets to thirty degrees!

The range and water heater work just fine

I can see good flames in there

<<What can cause the plenum temp to be so low when all the burners are burning and no heat is lost ou the exhaust
>>


That is a very good question --- and the answer is pretty much nothing.


That implies that you have inadequate gas pressure to the furnace or perhaps a defect in the gas valve resulting in gas pressure to the burner manifold that is too low.


So you need to have someone measure the gas pressure at the furnace, typically about 3.5" water column, and the burner manifold pressure.

I have seen very rarely furnaces that had the wrong size burner orifices installed, so that should be checked at the same time.

One furnace had burner orifices 'way too small installed at the factory. The homeowner had spent several years in a very cold house, and the heat exchanger was so cold the combustion gasses were condensing in the furnace and completely plugged the heat exchanger with rust.

How old is the furnace and how long has this been a problem?

Have you checked to make sure your return air duct is not pulling in outside air?

Hmmmm. Good question T-Top! I suppose if you are sucking in enough of that zero degree outside air you aren't going to get much heat into the house!

It has relatively short return ducts and long heat ducts, the return being in the middle of the house and the heat being on the outside walls

Furnace is 1994 or 1995

When I look at the flames in the burner box I see that at the top, some flame is not going into the tube for the flame and the burner box is hot on top so above that is where I get the 156 degree temperatur on the outside of the furnace. The furnace has not been working very good for a long time already and has been repaired by my furnace guy many times. Old parts are piled on top so I have a record of what was changed in parts. I plan on trying to clean the heat exchanger and run it with the exhaust pipe of for a while to see if it may be restricted. I already am taking combustion air into it directly at the furnace. The burner box has gotten hot enough to burn the plastic combustion intake pipe right through. The furnace can heat the house when the temp is above 10 degrees only. I am using some electric heaters and the temp will be up in the 30 degrees on Sunday. I am wondering if the heat exchanger could have something restricting the inside of it. What I really don't understand is where the heat is going because it isn't going out the exhaust and it isn't going into the heat ducts. The flames are there and it is very hot on the burner box so where is that heat going. It isn't out the exhaust tube because that is only 60 degrees.