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sheppard1977 11-06-2008 12:44 AM

Lennox Furnace G27M Series Ė Unreliable operation: pressure switch or else?
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I own a Lennox G27M3-75A-2 two stage furnace and have the following question. The furnace has been installed in 2000 and has been running fine for many years. But lately this furnace has given me trouble on and off over the last year or so. I live in Alaska and have to figure out this latest issue with winter already here!

Iíve had several HVAC technicians repair several problems over the last year. The main control board has been switched out and replaced with a Lennox 83M00 Sure Light replacement kit last Nov 2007 because the furnace was misfiring and the board had black spots on the back and was therefore diagnosed to be faulty. The flame sensor has been cleaned already several times to resolve some problems.

I recognize this furnace contains two circuit boards: the main board and another board that controls the two stage mode switching (refer to 4th picture for P/N). The furnace has been permanently set in one stage mode. I am far from an expert here, but you get to know your furnace real well when youíre sick of paying over $200 for every visit.

This time my furnace throws the following problem at me. Here is the scenario: the furnace works fine and provides heat. At some point in time the call for heat has been satisfied and the thermostat switches the furnace off. But somehow the exhaust blower (combustion air blower) sometimes stays on, leaving the two pressure switches energized (closed), setting the furnace up for failure next time there is a call for heat. Because when there is a next call for heat, the furnace recognized that the pressure switches are stuck closed and therefore throws an error code (LED#1 Off, LED#2 Slow Flash). This situation happens sometimes (once or twice a week), but that would be enough to freeze my home when I am away!

By now the main board has been replaced (Nov 2007), and I just replaced both pressure switched myself, although nothing appeared to be wrong with the ones I took out. I tested them by sucking on them and my meter showed the relays close and open fine.

The furnace has not failed on me yet, but I do not trust it. I have to know what causes this and I am not so sure itís the pressure switches alone. I believe my exhaust pipe is clean and free from debris, but have not checked this. I will try to attach a copy of the manual to this post.

1. Does someone recognizes this situation?
2. Do you believe two new pressure switches solve my problem?
3. The manual tells me that one of the causes can be that a pressure switch is stuck close. Isnít the pressure switch stuck close because the exhaust fan is on? What switches the exhaust fan? The main board? Can the relay be stuck closed that switches the exhaust fan? Last weekend the failure appeared again and I tapped on some relays on the main board and the exhaust fan switched off!
4. Can it this be a problem with the main control board again? I checked the back and it does not seem to have black spots (see 3rd picture). I have not been able to check the other board, the two stage control board.

Your help will be greatly appreciated!

Thank you,

beenthere 11-06-2008 07:19 AM

The pressure switches aren't causing the problem.
Something else is.
Could be teh sure light board.
Or your thermostat. Or a problem with the thermostat wire.

sheppard1977 11-06-2008 11:19 PM

Thanks for pointing me in the right direction.

1. Which board contains the relay that switches the exhaust blower motor? The Sure Light board or the two stage board? I have already bought a new Sure Light board ($115 deal on eBay!) and will install if the problems keep continuing.

2. How can I prevent the Sure Light board from being fried in the future? Is there such a thing as a surge protector for your furnace to protect my main control board against power surges, spikes etc.?

3. What can be wrong with the thermostat or thermostat wire? You might have a good point there! How can a wrong thermostat hurt your furnace. My thermostat is a Honeywell RTH230B.

Please advise on the questions above.


beenthere 11-06-2008 11:36 PM

The sure light board.

Whole house surge protector.

Some thermostats use triacs. If they bleed to much voltage, the furnace boards start to do strange things.
EG: run the combustion blower for no reason.

At 12 volts, it may just run the combustion blower, at 20 volts the furnace will go in to a heat run.

sheppard1977 11-07-2008 01:07 AM


Originally Posted by beenthere (Post 181546)
Some thermostats use triacs. If they bleed to much voltage, the furnace boards start to do strange things.
EG: run the combustion blower for no reason.

Do you think my Honeywell RTH230B thermostat has a triac in it?

beenthere 11-07-2008 04:52 AM

Can't tell for sure.
Probably doesn't.

mrauckman 03-03-2011 11:23 PM

You might have a spider web in the hose going to the pressure switch. You need to clean it out every season. The error light only tells you that you are not getting the closed circiut from the pressure switch not the switch is bad......Could also be a flue problem.

You can use a GFI as a surge protecter, the new standards for them will provide surge protection. Easy enough to wire. Don't forget the garage door opener needs protection too.

A whole house surge protection is easy and cheap. It just replaces two breakers in the panel and you still have the surge protector as two breakers.

You need whole house and indivual surge protection. The whole house will protect for what comes down the line from the power lines. The indivual ones will protect against induction lighting. It seems kind of over kill, but I have replaced three furnace boards untill I over killed the surge protection. I have a house in the rural area with long exposed power lines from the rural co-op. In the city you still dodn't know what the power company will do, or lighting, or a neightbor's equipment. Surge protectors on the surge protector..

Ground everything that is metal in the house, sheet metal, water lines, gas lines, I beam, garage door frames. Also you need to add and additional ground rod at lest six foot away from the existing one and bonded to the existing rod. Can't get enough grounding. You will be happy when thenext electrical storm comes.

You can isolate the 24 vote circuit from the outside condenser by adding an isolation relay and seperate transformes. They are baseicly a relay or contactor switch with it's oen transformer.......

HIC 01-14-2013 05:42 PM

Lennox Furnace G27M
I have experienced the same problem with the constant running of the exhaust chamber blower motor. I had the Surelight board replaced for another problem this summer and have found out that by tapping on the relays of this board I can cause the unit to work properly for a while and then bingo - it acts like a relay hangs up and the motor runs constantly. I spent close to $500 to have the board replaced and they only gave me a 90 day warranty. This unit worked flawlessly for 12 years but I didn't even get 6 months out of the replacement board.

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