My furnace is a Lennox G71MPP. I wasn't sure if I should start a new topic but my issue is very similar to those described here so decided to start here first.
My furnace is having trouble with the vent calibration sequence on heating startup. It often errors out of the pressure switch calibration sequence with code E228 (unable to perform successful pressure switch calibration). I had a technician out last winter but they weren't able to identify the problem and while they were here, the furnace happened to complete a calibration cycle successfully.
My furnace was installed in 2010. The complete model number is G71MPP-60C-090-3.
Here's things I've tried:
- Ensured that inlet and outlet vent pipes are clear.
- Ensured vent pipes are sloped and supported properly.
- Cleaned out condensate trap.
- Ensured that pressure switches are closing (by measuring resistance across switch terminals with ohm meter).
- Ensured that inducer blades spin.
- Removed air inducer motor and made sure that it is clear.
- Removed cold header box and made sure it is clear of debris and moisture.
- Removed cover on burner box assembly and made sure it was clear of debris.
- Replaced dual combustion air pressure switches (part number 36W77).
- Ensured differential pressure matched specification of pressure switches (using differential manometer).
- Tested voltages and resistance on inducer motor input and they seem good. (AC voltage ranged from about 40VAC to 90VAC. Resistance across black and white leads is about 15 ohms. Resistance between black/white and green is infinite.)
What I found using my ohm meter and manometer is that the differential pressure in the cold header box seems to be high. The furnace has a 0.25"H2O low-pressure switch and 1.22"H2O high-pressure switch. During calibration, when the inducer first powers up, the pressure jumps to 0.37"H2O. At this point, the low-pressure switch is already closed and the high-pressure switch is open. Then the inducer speeds up and the pressure goes to about 1.40"H2O. The high-pressure switch is now closed. The inducer starts stepping down in speed and eventually the pressure reaches 1.25"H2O and the high-pressure switch opens. Then the inducer jumps up to a fast speed where the pressure is 2.48"H2O. It starts stepping the inducer motor down in speed, presumably trying to find the point where the high-pressure switch opens. It steps down to a low of about 1.40"H2O but since the high-pressure switch is designed to open at 1.22"H2O, it never opens. At this point the calibration fails with code E228.
The problem seems to be that the calibration sequence is never able to find the inducer speed at which the high-pressure switch opens. The inducer steps down in speed until the differential pressure is 1.40"H2O but that is not low enough to open the high-pressure switch. My guess is that the inducer is operating at too high an RPM or there is something causing the differential pressure to be too high. My first inclination is that the second scenario is happening. That is, something is causing the differential pressure to be too high. What other steps can I do to find the cause of this problem? Could there be debris inside the cold header box that is obstructing air flow in the channel?
When I removed the cold header box, it was very clean. There wasn't any debris or moisture. Just a very light coating of fine dust. Of course, I wasn't able to look inside the channel glued to the back of the box so I don't know if the channel was clear. When I replaced the pressure switches, I opened one of the old ones and it was completely clean and dry inside. When I took the cover off the burner box, it had light dust on all the surfaces and one wasp carcass inside. Still, it seemed relatively clean inside.
I could have a technician come over again but I'm afraid they would go through their same diagnostics and come up blank again.
I'm tempted to replace the cold header box but it would be nice to eliminate other possibilities first.
Any help would be much appreciated.
My furnace is having trouble with the vent calibration sequence on heating startup. It often errors out of the pressure switch calibration sequence with code E228 (unable to perform successful pressure switch calibration). I had a technician out last winter but they weren't able to identify the problem and while they were here, the furnace happened to complete a calibration cycle successfully.
My furnace was installed in 2010. The complete model number is G71MPP-60C-090-3.
Here's things I've tried:
- Ensured that inlet and outlet vent pipes are clear.
- Ensured vent pipes are sloped and supported properly.
- Cleaned out condensate trap.
- Ensured that pressure switches are closing (by measuring resistance across switch terminals with ohm meter).
- Ensured that inducer blades spin.
- Removed air inducer motor and made sure that it is clear.
- Removed cold header box and made sure it is clear of debris and moisture.
- Removed cover on burner box assembly and made sure it was clear of debris.
- Replaced dual combustion air pressure switches (part number 36W77).
- Ensured differential pressure matched specification of pressure switches (using differential manometer).
- Tested voltages and resistance on inducer motor input and they seem good. (AC voltage ranged from about 40VAC to 90VAC. Resistance across black and white leads is about 15 ohms. Resistance between black/white and green is infinite.)
What I found using my ohm meter and manometer is that the differential pressure in the cold header box seems to be high. The furnace has a 0.25"H2O low-pressure switch and 1.22"H2O high-pressure switch. During calibration, when the inducer first powers up, the pressure jumps to 0.37"H2O. At this point, the low-pressure switch is already closed and the high-pressure switch is open. Then the inducer speeds up and the pressure goes to about 1.40"H2O. The high-pressure switch is now closed. The inducer starts stepping down in speed and eventually the pressure reaches 1.25"H2O and the high-pressure switch opens. Then the inducer jumps up to a fast speed where the pressure is 2.48"H2O. It starts stepping the inducer motor down in speed, presumably trying to find the point where the high-pressure switch opens. It steps down to a low of about 1.40"H2O but since the high-pressure switch is designed to open at 1.22"H2O, it never opens. At this point the calibration fails with code E228.
The problem seems to be that the calibration sequence is never able to find the inducer speed at which the high-pressure switch opens. The inducer steps down in speed until the differential pressure is 1.40"H2O but that is not low enough to open the high-pressure switch. My guess is that the inducer is operating at too high an RPM or there is something causing the differential pressure to be too high. My first inclination is that the second scenario is happening. That is, something is causing the differential pressure to be too high. What other steps can I do to find the cause of this problem? Could there be debris inside the cold header box that is obstructing air flow in the channel?
When I removed the cold header box, it was very clean. There wasn't any debris or moisture. Just a very light coating of fine dust. Of course, I wasn't able to look inside the channel glued to the back of the box so I don't know if the channel was clear. When I replaced the pressure switches, I opened one of the old ones and it was completely clean and dry inside. When I took the cover off the burner box, it had light dust on all the surfaces and one wasp carcass inside. Still, it seemed relatively clean inside.
I could have a technician come over again but I'm afraid they would go through their same diagnostics and come up blank again.
I'm tempted to replace the cold header box but it would be nice to eliminate other possibilities first.
Any help would be much appreciated.
