Furnace Frustrations

Messages
3,104
Location
Chicago Area
I've been camped out at Mom's off and on since the Polar Vortex event and noticed something going on with her Tempstar T9MPV furnace installed in 2008. The filter is clean (verified by the furnace guys) and the batteries in the thermostat are good (verified by my DMM). I change those things plus the humidifier pad religiously over the Thanksgiving weekend so I didn't think those were causing a problem. Since the extreme cold weather ended it's been acting erratically - the draft inducer starts up, I see the igniter light up, gas comes on, and so far all is good. The blower doesn't kick in when it's supposed to, and after about 4 minutes the burners shut off and the blower kicks into high mode to dissipate the heat. The system then sits for 5 minutes and tries again. It looks like the high limit switch is kicking on and, at least in that case, the system is working as it should to keep the house from burning down. Called the warranty company Mom has through the local utility. First visit the guy says that the draft inducer had water in it and that was affecting the system. Claims to have drained it out and the furnace seemed to work fine for about 8 hours then started the sequence I outlined at the start. Called the warranty company again, same guy gets sent out, now he decides it's the limit switch and replaces it. This seems to solve the problem for another day, then the sequence starts again. Call the warranty company again, this time the HVAC company sends out someone different. This time they decided that the furnace is too big for the house (which it may be) and the electronics were not set up properly for the thermostat so they fixed that. System seemed to have worked fine for a day and now it's really erratic - things like the draft inducer motor running for 45 minutes at a time, burners cycling on and off, blower cycling on and off but at the high-speed it did when it was sensing the high-heat condition and not in the manner it would be if the system was running properly. Contractor is coming out again to check on the gas pressure and some other items but I get the sense that he's going to throw in the towel after that and the warranty company will probably not do any more if he declares the thing non-repairable. I'm obviously no expert but I'm wondering if the control board is at fault here? It's weird to me that the system worked fine for 10 years and is acting up now, so while I can't argue things like the furnace is too large, I have to wonder why we didn't see this before now. Any suggestions?
 
Messages
25,053
Location
ON, Canada eh?
Disconnect the fresh air intake pipe (not the exhaust pipe) and crack a window on the same floor as the furnace. See if that fixes the problem. It could be a drafting problem on the air intake. I had this issue with my furnace and had to end up upgrading to a 3" inlet pipe all the way to the furnace and then reducing down just before the inlet on the Furnace to make the connection to the smaller size. For whatever reason it wouldn't work with the standard size pipe and we proved this because as soon as you would disconnect the fresh air inlet pipe coming from the outside and allow it to use the air from within the house the problem would go away.
 
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Messages
361
Location
Wisconsin
The intake pipe has nothing to do with it. The furnance successfully completes the ignition sequence and stays burning, meaning the draft inducer was able to close the pressure switch. That eliminates all intake, exhaust, igniter, gas valve, and switch problems. Let the furnace complete the sequence with the front cover off and your test lead hooked to the HEAT blower control terminal. Then, when the main blower fails to run, is the main board controlling it? Or, perhaps since the blower will run on high, try temporarily moving the hi speed wire to the HEAT terminal and see if the blower will run that way. Keep in mind the board won't command the blower on until it sees confirmation from the flame sensor. But since you say it will run for 4 minutes before cutting out, I doubt the flame sensor is the problem...
 
Messages
361
Location
Wisconsin
His fires up normally but overheats and hits the high limit because the main blower doesn't come on (the key piece of information, here. Not the same as yours. Most furnace problems can be described as "firing up, then shutting down". This is akin to telling someone with car trouble that "one time my car wouldn't start just like yours. Turned out my fuel pump was bad so that's what I would check first", even though it could be a host of other problems...
 
Messages
1,453
Location
USA
Mine furnace is having the same issues. I was told by a friend of mine to remove and clean the flame sensor with a scotch brite pad. If this doesn't fix it then you"ll have to replace them.
 
Messages
736
Location
Indiana
The most recent behavior definitely sounds flaky. Is it possible the control board or system has a loose or bad ground connection? While I can't attribute the described behavior to a grounding problem, I have seen some strange things because of it.
 
Messages
361
Location
Wisconsin
Originally Posted by StevieC
Mine was also high limiting. But whatever. You seem to be the expert here... smirk2
No, it wasn't, if the fix involved the intake pipe... You must be confusing the limit switch with the pressure switch. The pressure switch is in series with the gas valve, and will close the gas valve and and turn shut the furnace down if it opens due to low draft flow (or a leak simulating low flow). It will open if there's an intake problem. Likely what you were experiencing. This isn't even close to related. Im not much for confrontation, but I've got a low tolerance for BS internet tech. When followed, it's a big waste of people's time and money.
 
Messages
3,646
Location
Worst Case, Ontario
Attack of the parts changers! There's no money is diagnoses, but plenty of money in swapping parts and "throwing in the towel" and suggesting a new system. Turning a billed repair into a replacement is the ultimate double dip revenue generator!
 
Messages
36,408
Location
ME
Originally Posted by maxdustington
Attack of the parts changers! There's no money is diagnoses, but plenty of money in swapping parts and "throwing in the towel" and suggesting a new system. Turning a billed repair into a replacement is the ultimate double dip revenue generator!
Since it's under warranty I would let them handle it up to and including this new system. We don't have to be furnace experts if we contract with someone who is. I would let this play out. OT when I was a kid we leased our hot water heater from the gas company. Got a letter in the mail, out of the blue, saying we could buy it for $200. We ignore it. Two months later the thing died, and we got a new one free. Leased, too, of course. LOL
 
Messages
751
Location
Huntington WV
I had problems a few years ago much like the OP situation. Long long story short was the gas pressure changed somehow and the increased pressure cracked the heat exchanger and began to sometimes trip the rollout switch. reset the gas pressure and new heat exchanger and all has been well.
 
Messages
25,053
Location
ON, Canada eh?
Originally Posted by emmett442
Originally Posted by StevieC
Mine was also high limiting. But whatever. You seem to be the expert here... smirk2
No, it wasn't, if the fix involved the intake pipe... You must be confusing the limit switch with the pressure switch. The pressure switch is in series with the gas valve, and will close the gas valve and and turn shut the furnace down if it opens due to low draft flow (or a leak simulating low flow). It will open if there's an intake problem. Likely what you were experiencing. This isn't even close to related. Im not much for confrontation, but I've got a low tolerance for BS internet tech. When followed, it's a big waste of people's time and money.
This guy had the same problem. He has restriction in his air intake. The furnace would fire up and then shut down thereafter tripping the limit switch...
Quote
Yep, we has a clogged intake last year. The furnace would start up and then shut off after 1 minute. Turns out it was just leaves and debris in the air intake https://www.reddit.com/r/DIY/comments/2j0zq9/bad_furnace_limit_switch/
Now his was debris but it proves that intake air problems can cause issues with high temperature limit.
 
Messages
361
Location
Wisconsin
Originally Posted by StevieC
Originally Posted by emmett442
Originally Posted by StevieC
Mine was also high limiting. But whatever. You seem to be the expert here... smirk2
No, it wasn't, if the fix involved the intake pipe... You must be confusing the limit switch with the pressure switch. The pressure switch is in series with the gas valve, and will close the gas valve and and turn shut the furnace down if it opens due to low draft flow (or a leak simulating low flow). It will open if there's an intake problem. Likely what you were experiencing. This isn't even close to related. Im not much for confrontation, but I've got a low tolerance for BS internet tech. When followed, it's a big waste of people's time and money.
This guy had the same problem. He has restriction in his air intake. The furnace would fire up and then shut down thereafter tripping the limit switch...
Quote
Yep, we has a clogged intake last year. The furnace would start up and then shut off after 1 minute. Turns out it was just leaves and debris in the air intake https://www.reddit.com/r/DIY/comments/2j0zq9/bad_furnace_limit_switch/
Now his was debris but it proves that intake air problems can cause issues with high temperature limit.
No, it doesn't. Return air doesn't equal intake air. Low air flow OVER the HEAT EXCHANGER can cause the limit switch to trip. Low flow THROUGH the HEAT EXCHANGER, which is where the intake pipe comes into play, causes a pressure switch problem. There are two areas of airflow concern in the furnance. Airflow THROUGH the heat exchanger, and air flow OVER the heat exchanger. Two segregated systems working, controlled by separate parameters. If you have poor/no combustion due to an intake problem, how can you overheat the heat exchanger? Conversely, the main blower which blows warm air throughout your home, works by removing heat from the heat exchanger and distributing it throughout your home. If there's no airflow over the exchanger, yet the burners are still lot, the exchanger will overheat. Make sense?
 
Messages
25,053
Location
ON, Canada eh?
He clearly says leaves in the air intake. This isn't return air vents... How would he get leaves in the return air vent?
 
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Messages
361
Location
Wisconsin
Leaves? As far as I can tell, the OP never came back to finish the discussion. No mention of leaves being a solution to a high limit trip. His last post is him stating he will try a new filter and doesn't know how else to diagnose anything. He was going to hire a tech and never came back. Whatever, stubbornness prevails. either way, when/if you do educate yourself on the topic, you'll realize how silly the intake suggestion is.
 
Messages
25,053
Location
ON, Canada eh?
I said that it was leaves in his case of the Reddit thread but it proves that an air intake restriction which was my problem tripping the high limit switch can happen and mine was corrected by a bigger air intake pipe required in my case. I will waive the white flag here because I don't wish to argue further. There is a group of folks here that can't seem to fathom the idea that it could be something outside the box and how dare anyone suggest something that might not have been seen before because they feel it can't be possible. smirk2
 
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Messages
361
Location
Wisconsin
Again, no mention in the thread that leaves were the cause of a high limit trip. It's not a matter of opinion or belief. It's a matter of what's possible and not. The entire design of your furnance revolves around ensuring the burners can't remain lit with an intake/exhaust problem. It's impossible. Can't reach an overheat condition without fire.
 
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