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Please pardon this lengthy post from someone who may be out of their league here...
We have a Carrier 58 MVP 100 installed in our basement. It was purchased in 1999 and has been basically trouble free ever since. I have had to replace the condensate pump a couple years ago and did so with the same brand/model as originally used. The furnace is in upflow orientation with intake and exhaust entering via PVC on the right and condensate leaving on the left. The intake and exhaust pipes are covered with perforated disks outside the house and there are no blockages. There is a switch for a summer fan but that is not turned on. There is a Honeywell air cleaner on the right side where the return duct enters the area below the main blower fan. The whole thing is run from a Honeywell Chronotherm with an LCD screen (it is a simple unit and it is working - calling for heat when the setting requires it).
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 (which his company would be happy to sell me). I have to admit that 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) When I arrived and looked at the furnace 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. Before I "fixed" this, I did open a drain plug that sits below the pump level and branches off the condensate line to provide for draining into a 5 gallon bucket in case of a pump failure. When I did this only a couple tablespoons of condensate came out. So it seems pretty clear that the drain line had not backed-up.
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 with either a Lennox or a York 95% single speed unit for about $4000. While pondering these proposals and googling the internet, I found several forum posts with folks complaining of similar issues with their Carrier 58MVP furnaces. I also found this troubleshooting guide <http://www.graycoolingman.com/uploads/1/0/6/6/10667336/carrier_variable-speed_2-stage_troubleshooting.pdf>. 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.
If it is not abundantly clear by now, I am really a hack at all this. But 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 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. I admit that I know next to nothing about transformers but it seems like if one is fed 110 volts AC it has little choice but to produce 24 volts DC on the output side. So this makes me suspicious of the transformer.
To test this theory about the transformer, my friend and I rigged-up a Lithium battery pack and tested it to ensure it is sending 24 volts DC. We decided to try and supply the 24 volts DC at the section 1and 2 connectors on the control board thinking that if it is seeking 24 volts DC and not getting it at the low pressure switch then the switch could be operating physically but simply now no power to switch on/off as we think the control board requires in steps 19 and 20 of the troubleshooting guide. Anyhow, we apply the battery pack power and we get a solid green and a flashing yellow LED and nothing really happens. We reconnect the section 1 and 2 connectors and we end up with error 32 again. We try the battery pack again and again lowering the voltage thinking that at least initially lower voltage is desired and applying all 24 volts creates a problem. We try 1.5, 3.0, and 4.2 volts DC. All of these give either nothing or just a solid red (if I recall) LED. We reconnect the section 1 and 2 connectors and we end up with error 32 again. So it seems we didn't make matters any worse for our antics with the battery packs.
But we are still stumped about the transformer and wonder if it or the control board are the most immediate suspects (or perhaps we are totally wrong and don't recognize this). We removed the plastic covers from the 24 volt transformer and don't see anything that looks unpleasant - no burnt areas, decent looking solder joints, etc. So we just don't know what to make of this. Clearly these must fail sometimes because I can see sellers for this part on EBay and elsewhere. But do I need one and is that a likely culprit compared to the circuit card which is a much more expensive replacement part? Or something else all together?
Thanks for your time and patience with all of this. I appreciate any help you might give us.
We have a Carrier 58 MVP 100 installed in our basement. It was purchased in 1999 and has been basically trouble free ever since. I have had to replace the condensate pump a couple years ago and did so with the same brand/model as originally used. The furnace is in upflow orientation with intake and exhaust entering via PVC on the right and condensate leaving on the left. The intake and exhaust pipes are covered with perforated disks outside the house and there are no blockages. There is a switch for a summer fan but that is not turned on. There is a Honeywell air cleaner on the right side where the return duct enters the area below the main blower fan. The whole thing is run from a Honeywell Chronotherm with an LCD screen (it is a simple unit and it is working - calling for heat when the setting requires it).
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 (which his company would be happy to sell me). I have to admit that 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) When I arrived and looked at the furnace 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. Before I "fixed" this, I did open a drain plug that sits below the pump level and branches off the condensate line to provide for draining into a 5 gallon bucket in case of a pump failure. When I did this only a couple tablespoons of condensate came out. So it seems pretty clear that the drain line had not backed-up.
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 with either a Lennox or a York 95% single speed unit for about $4000. While pondering these proposals and googling the internet, I found several forum posts with folks complaining of similar issues with their Carrier 58MVP furnaces. I also found this troubleshooting guide <http://www.graycoolingman.com/uploads/1/0/6/6/10667336/carrier_variable-speed_2-stage_troubleshooting.pdf>. 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.
If it is not abundantly clear by now, I am really a hack at all this. But 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 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. I admit that I know next to nothing about transformers but it seems like if one is fed 110 volts AC it has little choice but to produce 24 volts DC on the output side. So this makes me suspicious of the transformer.
To test this theory about the transformer, my friend and I rigged-up a Lithium battery pack and tested it to ensure it is sending 24 volts DC. We decided to try and supply the 24 volts DC at the section 1and 2 connectors on the control board thinking that if it is seeking 24 volts DC and not getting it at the low pressure switch then the switch could be operating physically but simply now no power to switch on/off as we think the control board requires in steps 19 and 20 of the troubleshooting guide. Anyhow, we apply the battery pack power and we get a solid green and a flashing yellow LED and nothing really happens. We reconnect the section 1 and 2 connectors and we end up with error 32 again. We try the battery pack again and again lowering the voltage thinking that at least initially lower voltage is desired and applying all 24 volts creates a problem. We try 1.5, 3.0, and 4.2 volts DC. All of these give either nothing or just a solid red (if I recall) LED. We reconnect the section 1 and 2 connectors and we end up with error 32 again. So it seems we didn't make matters any worse for our antics with the battery packs.
But we are still stumped about the transformer and wonder if it or the control board are the most immediate suspects (or perhaps we are totally wrong and don't recognize this). We removed the plastic covers from the 24 volt transformer and don't see anything that looks unpleasant - no burnt areas, decent looking solder joints, etc. So we just don't know what to make of this. Clearly these must fail sometimes because I can see sellers for this part on EBay and elsewhere. But do I need one and is that a likely culprit compared to the circuit card which is a much more expensive replacement part? Or something else all together?
Thanks for your time and patience with all of this. I appreciate any help you might give us.
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