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Discussion Starter · #1 ·
I have a Lennox G20 Q3/4. The heater would come on, blow for a while, shut off, then come on five minutes later. Three different technicians said we needed either a new control board or a new entire system. I bought a BCC3-2 control board, and I installed it a week ago, and it did not take care of the problem. Now the heater does the same thing except the wait time is only a minute now instead of five.
The technicians said the fan control relay switch on the board was out. Apparently, that did not fix the problem. Here is another factor: before we started having this problem, there was a second fan switch attached to the furnace to control the blower for the downstairs. This would be turned on when the wood stove became hot enough. These wires were cut this last summer.
 

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First you need to see if when it shuts off there is still a signal from the thermostat calling for heat. Your thermostat could be short cycling. Should have 24 volts from W to C on the BCC board while it is burning/calling for heat. That is a OLD furnace and hard to work on and has about 6-12 expensive problems that will occur soon. I am a Lennox tech and know that furnace all too well. Not one I would spend much money on unless you are moving in the Spring.
 

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Do the burners shut down when this happens or it's just the blower cycling? For the former you should jump R and W at the board.

I know that this furnace has some kind of proving switch for the damper.

Heat exchangers known to crack and you need to check it before putting more money into it.


g20 was the last of the lennox natural draft furnaces. Kind of an adulterated G8 or 12, lennox engineers taking a low efficiency design and "fixing" it to meet the 78% afue minimum. Was the lowest end model offered by lennox in the 90s.
 

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Discussion Starter · #4 ·
Lennox G20

The furnace will turn on, then the fan blower. After five minutes, the furnace flames do shut off. When the fan is switched from the auto to run (so it just remains on) the flames stay on--the furnace does not shut down. We have had the thing inspected for carbon monoxide leaks, and none were found.
 

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Did you buy a brand new BCC board, sealed in a bag or box and not from E-Bay where it may have been used?
 

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Had similar issue with Lennox 80 series gas furnace. A pressure sensor on the backside of the furnace went. I had to slide the blower out to get at it. The sensor was one of two wired in series. Relaced both for about $15 each online.
 

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The furnace will turn on, then the fan blower. After five minutes, the furnace flames do shut off. When the fan is switched from the auto to run (so it just remains on) the flames stay on--the furnace does not shut down. We have had the thing inspected for carbon monoxide leaks, and none were found.
It sounds like one of your safety limits may be opening up. Do you know if your blower runs faster than the heat mode when it's in the "on" position? Could have an airflow problem and running the blower at a faster speed is keeping the limit cooler so it's not opening up.
 

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Good point.

You need to put a meter across the limit control which is above the burners and see if it opens after 5 minutes or whenever the flames cut out. If it opens you will get 24 volts AC across it. When closed you get 0.
 
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