Discussion Forum - Northstar Chevelle Club

SShink · 3K+ Club

Furnace is not staying on-Any ideas?

Bobs_Place · Vice President

Sounds like the hi limit switch is shutting the burner off. Is the fan running a normal speed, if so, than an air flow problem. Recheck the filter, try running without the filter or pull the service door off than run the furnace. Can you see the fan, vacuum or blow it out without removing it? Hard to believe the dust could have plunged the ducts but could they have left something in the ducts as part of the cleaning procedure?

If the fan is running slow, check for a fan speed switch, put it on high.

Hope this helps.

Bob

SShink · 3K+ Club

Thanks Bob.

I turned the Heat off at the thermostat and turned the fan On for full time use, and determined it's the blower fan that runs about a minute then shuts off. After 10 min. or so it will restart and try again with the same result.

When the fan does run, it seems like it's at low speed (I confirmed it's spinning), so thinking it might be an ECM problem on the circuit board. Or, there is something blocking the fan airflow from the duct cleaning yesterday, and it's shutting itself down. I did remove the filter thinking there could be air flow blockage, and no change.

I'd inspect the blower fan itself, but it's mounted behind a panel that I'd have to cut off some of the screws that protrude through the floor of the furnace in order to get it out. I guess the engineers didn't think the blower would ever need to be serviced, or I'm missing an easier way to get to it.

The tech is supposed to show up sometime before 7 p.m., so hopefully it's noting serious.

SShink · 3K+ Club

Here's a pic showing where the pcb is mounted. The blower is on the other side of the panel the pcb is mounted from. I did look on the other side of the furnace for an access panel and it's a solid piece if sheet metal .

Chris R · 2K+ Club

The duct cleaning company should have removed that panel and cleaned out the blower fan before turning the furnace back on. That's what Centerpoints (my employer, I'm an Home Service Plus Tech) does when they are finished cleaning ducts. Otherwise your blowing a ton of that dust that got disrupted, up through the furnace which gets stuck on the A coil for the air conditioning. They also need to be mindfull that a bunch of dust and debris in the ducts didn't pile up on top of the A coil as well, both of which just end up blocking air flow which trips the hi limit. If the fan motor is only running for a minute and shutting off when thermostat is set to heat off-fan on (instead of auto). Then I'm suspecting something is going on with the fan motor itself.

When the heat is on, the fan motor quits working due to an internal issue with the fan motor itself then with no air flow since the fan stopped spinning, the hi limit opens, shutting off the burners. Ive worked on many of that model furnaces and its a PITA but not really too bad. There are some that are just like that with twice the amount of wires that are worse to gain access to the blower on. I would make sure the filter is clean & put it back in place, the residual amount of dust left behind from the duct cleaning process will easily sucked up into the furnace now and our duct contractor recommends changing the filter or at least checking it a week or two after the service to be sure.

I'm expect the tech will use a manometer to check the static pressure before & after the filter. On the top of furnace cabinet & after the A coil and see if there are any pressure drops. He may also check the temp rise on the furnace as well unless the blower fan issue becomes obvious after running several minutes. I always use an AC amp clamp to be sure but its usually pretty obvious the fan motor is bad in such cases, a clamp isn't necessary.

SShink · 3K+ Club

Thanks Chris! I ended up calling Centerpoint this morning, and the tech is supposed to show up in the next half hour.

What you said makes sense to me. The duct cleaning guy pointed out to me that normally they clean the blower fan blades, but this model of furnace didn't allow access, so he didn't. I'm guessing he didn't check the A coil either. It was a couple of very young guys that one of them sort of knew his stuff, but the other guy had no clue.

Anyway, I did check with our home warranty company and if Centerpoint determines the furnace needs replaced, they will replace it as part of our coverage.

It may not need a full furnace replacement if the blower motor is bad, but so I'm hoping if that's it the tech can has a replacement handy.

I'll update after his assessment.

SShink · 3K+ Club

Furnace is fixed!

Chris you were right. Bad blower motor. I think it's been going out for awhile.

Chris R · 2K+ Club

With the motor going bad right after you had the duct cleaning done was probably just a coincidence. At least now there is a new motor in there and he likely cleaned the blower wheel if it needed it when its out as well and any dust that collected in the blower "drum" housing got cleaned out too so at least that got cleaned in the process.

Bobs_Place · Vice President

Good to hear they got it going.

Did the furnace guy also check for a cracked burner?

Chris, what is the possibility of a crack developing in the burner/heat exchanger from the elevated temp when the fan was not working?

SShink · 3K+ Club

He did check the burner. The Hi temp limit was being hit from the blower fan only spinning slowly, so the burners were only running for about 30-60 seconds then they'd shut off. Seems o.k. from what he was telling me.

I did ask him if parts were still available for this 20 year old AirPro furnace, and he said the parts are still very common and available, including the PCB assembly, which I didn't figure would be available. The PCB even has old 'through-hole' component technology which isn't used much these days except for certain applications.

Chris R · 2K+ Club

Bobs_Place wrote:
Good to hear they got it going.
Did the furnace guy also check for a cracked burner?
Chris, what is the possibility of a crack developing in the burner/heat exchanger from the elevated temp when the fan was not working?

The possibility of a crack is there but I wouldn't directly associate it to the fan failing, the hi-limit is designed from the factory to open at a much lower temp to protect the exchanger from damage. If a crack occurs from a fan failing, it more then likely had been there already from things like age and/or rust.

Ive never had any problems getting a part for Air Flo brand furnaces either, Stan. Sometimes we may have to leave in the middle of a repair to pick up a part and come back & install it or order a part to be shipped to the customer then come back and install it once the customer gets it but never a problem with parts availability.