davidpwhite
May 14, 2014, 08:37 AM
We have a 1999 Carrier 58 MVP 100.
About 2 months ago, the furnace in our rental house stopped working rather suddenly. A tech from a local firm came out for a diagnostic call and said he found the following:
Low fire pressure switch stuck closed.
Inducer outside valid speed range.
Pressure switch calibration fault.
This was summed-up as Failing pressure switch and draft inducer. The tech then powered the unit off and back on and it began working just fine. He said this sometimes happens - the computer needs to be rebooted. I then got an "estimate" (no real details provided, just a number) to replace the draft inducer assembly and the hi/low switch assembly for $1900 along with a suggestion to just get a new furnace. I was not really very confident in all of this because:
a) I could not be there to actually see what the tech did while he was there. So I really have no idea how troubleshooting was performed or what was actually tested.
b) A tech from the same outfit came to do a preventive service check in 2009 and at that time said the surface ignitor should be replaced. We never did this and the furnace seemed to keep working just fine for many years until now.
c) After the tech had left, I saw that the condensate pump (which I had replaced a year or two ago with the same brand/model as originally installed in 1999) had somehow slipped off one of its mounting bolts, was at an angle, and was very full of water. Once I leveled and properly supported the pump and turned on the power, the pump immediately ran and water was removed from the tank. I know that a non-working pump will cause the furnace to NOT run so I would have thought the tech would have at least checked this and apparently did not.
After the diagnostic service call, the furnace kept working well for maybe another month and a half, then failed again and would not start working after power was removed and restored. It has been not working since then. When power is applied and the thermostat calls for heat, the furnace starts running, goes for a few minutes and shuts down. It tries this cycle multiple times.
I have received 2 proposals to replace the current furnace and I decided that since I was basically considering the current furnace as scrap, I might as well see if I could check things out. What harm could I really do?
So yesterday, I spent several hours with a neighbor and we tried to walk the troubleshooting steps. We also have the original install manual with wiring diagrams, etc. First, I watched the furnace go through its power-on and checked the error code. The blower motor ran for 1 minute (as indicated by the NOTE bottom of page 1 in the troubleshoot manual), followed by the inducer running for about 15 seconds. I did not see the ignitor get red. Then the furnace shut down and I got code 32. This is called "Low Pressure Switch Fault" and this is at least somewhat consistent with what the tech said. I tried this again and again with the same results. I then put the unit into component test mode (page 8 in the manual) and it followed the designated sequence exactly. I was able to see the ignitor get red visually. No fault codes followed the component test. Finally, I tried out emergency heat mode on page 2 in the manual. The motors did run at high speed but I still did not see the ignitors get red and there was no heat.
I had already removed the thermostat wire from step 21 on the Start Here List (page 6). I then started following the troubleshooting guide at the top of page 5 and ended up on page 23 for the error code 32. I do steps 1, 2, and get NO for step 3. So I follow step 5 and jumper R and W/W1. Then steps 6, 7, and 11 because the same fault occurs again. The fault is not show in 5 seconds so I move on to step 17. I am not totally sure about what step 17 means. But I keep fingers on both pressure switches and feel/hear nothing happen. So I conclude that the low pressure switch does not "make and break". Step 18 is true because it takes about 45 seconds before the fault. And step 50 is true because the inducer motor does come on. This takes me back to step 19 and I monitor the DC voltage between the board C and the common on the low pressure switch during the entire start-to-fault cycle. I see slight variations + and - from 0. Nothing close to 24 volts DC. I do the same in step 20 and see no 24 volts DC. This leads me to step (lucky) 13 - replace the control board.
I wonder why I am not seeing 24 volts DC even when I unplug the section 1 and section 2 wires and probe them directly during the start-to-fault cycle. I don't really understand these in detail but I can confirm that 110 volts AC is coming the connection block on the far right of the control board. And it seems like this should be routed to the transformer under the control board which looks like it feeds the 24 volts DC output to the control board directly at the section 1 and 2 connectors. So this makes me suspicious of the transformer.
About 2 months ago, the furnace in our rental house stopped working rather suddenly. A tech from a local firm came out for a diagnostic call and said he found the following:
Low fire pressure switch stuck closed.
Inducer outside valid speed range.
Pressure switch calibration fault.
This was summed-up as Failing pressure switch and draft inducer. The tech then powered the unit off and back on and it began working just fine. He said this sometimes happens - the computer needs to be rebooted. I then got an "estimate" (no real details provided, just a number) to replace the draft inducer assembly and the hi/low switch assembly for $1900 along with a suggestion to just get a new furnace. I was not really very confident in all of this because:
a) I could not be there to actually see what the tech did while he was there. So I really have no idea how troubleshooting was performed or what was actually tested.
b) A tech from the same outfit came to do a preventive service check in 2009 and at that time said the surface ignitor should be replaced. We never did this and the furnace seemed to keep working just fine for many years until now.
c) After the tech had left, I saw that the condensate pump (which I had replaced a year or two ago with the same brand/model as originally installed in 1999) had somehow slipped off one of its mounting bolts, was at an angle, and was very full of water. Once I leveled and properly supported the pump and turned on the power, the pump immediately ran and water was removed from the tank. I know that a non-working pump will cause the furnace to NOT run so I would have thought the tech would have at least checked this and apparently did not.
After the diagnostic service call, the furnace kept working well for maybe another month and a half, then failed again and would not start working after power was removed and restored. It has been not working since then. When power is applied and the thermostat calls for heat, the furnace starts running, goes for a few minutes and shuts down. It tries this cycle multiple times.
I have received 2 proposals to replace the current furnace and I decided that since I was basically considering the current furnace as scrap, I might as well see if I could check things out. What harm could I really do?
So yesterday, I spent several hours with a neighbor and we tried to walk the troubleshooting steps. We also have the original install manual with wiring diagrams, etc. First, I watched the furnace go through its power-on and checked the error code. The blower motor ran for 1 minute (as indicated by the NOTE bottom of page 1 in the troubleshoot manual), followed by the inducer running for about 15 seconds. I did not see the ignitor get red. Then the furnace shut down and I got code 32. This is called "Low Pressure Switch Fault" and this is at least somewhat consistent with what the tech said. I tried this again and again with the same results. I then put the unit into component test mode (page 8 in the manual) and it followed the designated sequence exactly. I was able to see the ignitor get red visually. No fault codes followed the component test. Finally, I tried out emergency heat mode on page 2 in the manual. The motors did run at high speed but I still did not see the ignitors get red and there was no heat.
I had already removed the thermostat wire from step 21 on the Start Here List (page 6). I then started following the troubleshooting guide at the top of page 5 and ended up on page 23 for the error code 32. I do steps 1, 2, and get NO for step 3. So I follow step 5 and jumper R and W/W1. Then steps 6, 7, and 11 because the same fault occurs again. The fault is not show in 5 seconds so I move on to step 17. I am not totally sure about what step 17 means. But I keep fingers on both pressure switches and feel/hear nothing happen. So I conclude that the low pressure switch does not "make and break". Step 18 is true because it takes about 45 seconds before the fault. And step 50 is true because the inducer motor does come on. This takes me back to step 19 and I monitor the DC voltage between the board C and the common on the low pressure switch during the entire start-to-fault cycle. I see slight variations + and - from 0. Nothing close to 24 volts DC. I do the same in step 20 and see no 24 volts DC. This leads me to step (lucky) 13 - replace the control board.
I wonder why I am not seeing 24 volts DC even when I unplug the section 1 and section 2 wires and probe them directly during the start-to-fault cycle. I don't really understand these in detail but I can confirm that 110 volts AC is coming the connection block on the far right of the control board. And it seems like this should be routed to the transformer under the control board which looks like it feeds the 24 volts DC output to the control board directly at the section 1 and 2 connectors. So this makes me suspicious of the transformer.