[johnmeyer]

Summary: Lennox G24M furnace. Normal startup sequence, but 5-20 seconds after flames start, the gas shuts off. This cycle repeats. It is NOT the flame sensor (more below).

Flame sensor is fine based on testing using micro-ampere meter. (I cleaned it anyway.) Other tests (see below) completely rules out the flame sensor.

I measured the voltage across the pressure switch. I get 24 VAC just after the inducer motor starts, then 3-5 seconds later, after inducer gets to full speed, that goes 0 VAC indicating the pressure switch has closed.

So, all normal so far in the startup sequence.

However, 5-20 seconds after the flames start, the flames stop and, simultaneously, the voltage across the pressure switch goes from 0 to 24 VAC indicating the switch has opened. After 2-3 seconds, with the inducer motor still running, the pressure switch closes again, and after the 40 second purge, the furnace ignites, runs for a few seconds, and then shuts down. The blower starts shortly after the first cycle and continues to run after that, so I have both the inducer fan and the furnace fan running.

I get no error codes.

To make sure it is the pressure switch which is causing the furnace to shut down, I bypassed the pressure switch (shorting across the leads) just after it had closed. When I did this the furnace stayed lit and ran until I removed the jumper wire. It then instantly shut down. Therefore, it is not the flame sensor.

The flames are being sucked into the furnace, as they should be. Neither rollout switch is being activated, and the high temp switch did not open during any of these cycles (I tested these).

The key to the whole thing is why does the pressure change only after the flames start? I would think that a plugged exhaust vent or cracked heat exchanger would cause the pressure switch to stay open during the cold air purge and the furnace would not start at all.

I bought and installed a new 0.40" pressure switch. The old one was spec'd at 0.46". After installation of the new pressure switch, the furnace ran for 20-30 seconds instead of 4-10 seconds.

But now it gets interesting.

I let the furnace short cycle for half a dozen cycles and each time it ran a little longer, and eventually I was getting over two minutes of operation per cycle.

I was unable to get to the vent on the roof of the two-story house and was unable to disassemble the metal vent underneath the cap. This is the upstairs furnace and it is in the attic. It was installed in early 1995 but everything today looks as new as the day it was installed. The burners and the burner orifices are pristine as is all the sheet metal. The length of the flu from the furnace to the roof vent is less than eight feet total.

The only thing I can think of is that there is something in the roof vent or in the heat exchanger that built up during the seven months the furnace isn't used (we don't need heat here from April until late November). However, that doesn't make sense to me because if there was a blockage, the pressure switch should never close and the furnace should never start in the first place.

So here's the $64,000 question (reference to quiz show that was popular when I grew up in the 1950s): What would cause the pressure to significantly change only a few seconds after the furnace starts heating?

Any ideas would be greatly appreciated.

Thanks!

 

[eggs]

Summary: Lennox G24M furnace. Normal startup sequence, but 5-20 seconds after flames start, the gas shuts off. This cycle repeats. It is NOT the flame sensor (more below).

Flame sensor is fine based on testing using micro-ampere meter. (I cleaned it anyway.) Other tests (see below) completely rules out the flame sensor.

I measured the voltage across the pressure switch. I get 24 VAC just after the inducer motor starts, then 3-5 seconds later, after inducer gets to full speed, that goes 0 VAC indicating the pressure switch has closed.

So, all normal so far in the startup sequence.

However, 5-20 seconds after the flames start, the flames stop and, simultaneously, the voltage across the pressure switch goes from 0 to 24 VAC indicating the switch has opened. After 2-3 seconds, with the inducer motor still running, the pressure switch closes again, and after the 40 second purge, the furnace ignites, runs for a few seconds, and then shuts down. The blower starts shortly after the first cycle and continues to run after that, so I have both the inducer fan and the furnace fan running.

I get no error codes.

To make sure it is the pressure switch which is causing the furnace to shut down, I bypassed the pressure switch (shorting across the leads) just after it had closed. When I did this the furnace stayed lit and ran until I removed the jumper wire. It then instantly shut down. Therefore, it is not the flame sensor.

The flames are being sucked into the furnace, as they should be. Neither rollout switch is being activated, and the high temp switch did not open during any of these cycles (I tested these).

The key to the whole thing is why does the pressure change only after the flames start? I would think that a plugged exhaust vent or cracked heat exchanger would cause the pressure switch to stay open during the cold air purge and the furnace would not start at all.

I bought and installed a new 0.40" pressure switch. The old one was spec'd at 0.46". After installation of the new pressure switch, the furnace ran for 20-30 seconds instead of 4-10 seconds.

But now it gets interesting.

I let the furnace short cycle for half a dozen cycles and each time it ran a little longer, and eventually I was getting over two minutes of operation per cycle.

I was unable to get to the vent on the roof of the two-story house and was unable to disassemble the metal vent underneath the cap. This is the upstairs furnace and it is in the attic. It was installed in early 1995 but everything today looks as new as the day it was installed. The burners and the burner orifices are pristine as is all the sheet metal. The length of the flu from the furnace to the roof vent is less than eight feet total.

The only thing I can think of is that there is something in the roof vent or in the heat exchanger that built up during the seven months the furnace isn't used (we don't need heat here from April until late November). However, that doesn't make sense to me because if there was a blockage, the pressure switch should never close and the furnace should never start in the first place.

So here's the $64,000 question (reference to quiz show that was popular when I grew up in the 1950s): What would cause the pressure to significantly change only a few seconds after the furnace starts heating?

Any ideas would be greatly appreciated.

Thanks!

For $64,000 - water. The water is building up in the secondary heat exchanger causing your problem. Seen it before. There is a drain trap and tubing. Take pictures FIRST, then remove hoses, blow them out (mouth pressure is fine). Then blow into where the tubes go into the trap and exchanger, NEVER blow into the sir switch, that will damage it. There is a trap more than likely in the blower compartment, take it out - washing, blow it out. If water is unable to leave the secondary exchanger, then pressure builds up and air flow is reduced which will in effect cause exactly what you see. Good Luck

 

[johnmeyer]

For $64,000 - water. The water is building up in the secondary heat exchanger causing your problem. Seen it before. ... Good Luck

Thanks for the reply. Unfortunately this is not a 90% HE furnace, so it does not have a secondary heat exchanger. As I said in the first line of my OP, this is a 1994 Lennox GM24M.

 

[eggs]

Thanks for the reply. Unfortunately this is not a 90% HE furnace, so it does not have a secondary heat exchanger. As I said in the first line of my OP, this is a 1994 Lennox GM24M.

Sorry, I missed that - it was 1 am and could not sleep when I responded. On a 80%'r, a few things can cause your issue. You already eliminated some. This leaves improper air switch, not set to the correct pressure, venter motor issues or the hose that measures the air pressure is in the wrong place / bad or plugged. Remove the venter motor assemble carefully and inspect the blower wheel, make sure the pathway to the chimney is clear. I have already seen a bird in one of them. If you have a good manometer, you can tee off and measure the air pressure, not quite sure the exact rating with out doing more research. The hose location can be found on page 6, figure 2 at:

https://www.heatingandairparts.com/documents/G24%20SERVICE.pdf

Low gas pressure could also cause issues. You should make sure it is at 3.5" at manifold or what the name plate shows. The last thing that is know to cause this is a bad control board. Even though you have 1-5 micro amps, it does not see it, then locks out. Seen this about 10% of the time. Hope this helps.

 

[Virtual_Bomber]

If it has it, check the capacitor on the inducer motor. The motor itself could be weak because it must be moving the air fast enough to constantly be holding that vacuum pressure on that switch.

Like the guy above me said, check that wheel and the stiffness of the motor. Don't forget to clean the hose out real good that hooks up to the pressure switch as well.

And the pressure changes because the air is less dense as it heats up when the flames ignite.