# Lennox Furnace G27M Series – Unreliable operation: pressure switch or else?



## sheppard1977 (Nov 6, 2008)

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!

I’ve 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 you’re 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 it’s 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. Isn’t 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,
Rob


----------



## beenthere (Oct 11, 2008)

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 (Nov 6, 2008)

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.

Thanks!
Rob


----------



## beenthere (Oct 11, 2008)

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 (Nov 6, 2008)

beenthere said:


> 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 (Oct 11, 2008)

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


----------



## mrauckman (Mar 4, 2011)

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 (Jan 14, 2013)

*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.


----------



## DennisSoCal (2 mo ago)

sheppard1977 said:


> 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!
> 
> I’ve 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 know this post is old, but I'm going to respond in case others find this post and have the same issue. 
First - that exhaust blower is the first sign of life when the furnace is getting started to output heat. It runs first, so it's possible you could hear it run when the heat was just cycled on (but before you get heat from the vents). To be sure your exhaust blower is running when it shouldn't, turn the thermostat to OFF.
On your specific furnace, the "exhaust blower" is directly powered by an electronics board. When that board wants to activate the exhaust blower, it activates a relay on that electronics board, and that relay sends power to the exhaust blower. When the relay is deactivated, the exhaust blower will stop running no matter what. The most likely problem is a sticking relay on that electronics board. It could be some other component on that board too, but the problem IS on that electronics board. Your model of furnace likely has two electronics boards - a system control electronics board and a "Surefire" electronics board. Follow the white/red/black (3 wires) wiring from your exhaust blower (after turning off power to the furnace) and look where those wires attach to one of the electronics boards. That will be the electronics board were your issue is. The two most common failures on these electronics boards is relays and electrolytic capacitors. Look at the capacitors that look like little upright cans with thin plastic labels shrunk onto them - do any of them have bulging tops? They should be flat on top. Any bulging means the capacitor is failing or is already bad. If you feel safe doing it, you could wait until the exhaust blower is staying on again when the heat is definitely off, then, find the relay(s) right next to where the exhaust blower and tap firmly with the back (plastic end) of a screw driver. Don't touch anything with your bare hand or a metal tool or you risk electrocution/shock. If tapping on a relay stops the exhaust blower, you need to replace that relay. Relays create little electrical "arcs" (sparks) when they are triggered on and off, and that eventually causes pits to form in the contacts. Those pits can cause the contacts to stick together intermittently (or permanently).

Sometimes, you can find new exact part number relays on eBay or an electronics supplier (Mouser electronics, DigiKey) and if you find the one you need, just unsolder the old one and put the new one in it's place. If you don't feel confident doing that, find someone that can do it for you (TV repair shop?). If you're replacing a relay, look closely at any electrolytic capacitors (the kind that look like a little metal can) if yours are more than 10 years old and try to find replacements. Don't ever use used relays or capacitors unless you're desperate and can't find new ones. You can't tell their condition from the outside, and old used ones can have just as much issue as yours.


----------

