Outdoor Furnace ignites but blower doesn't run

Problems related to residential installations.

Outdoor Furnace ignites but blower doesn't run

Postby bmkoz559 » Sun Nov 20, 2016 12:59 am

Here's the background- It's an older Payne furnace / AC unit from the late 80s maybe. No, digital controls with error codes. The issue is that the blower motor never starts when heat is called for by the thermostat. This is the sequence of events that I observe...

1 switch thermostat to heat and set temperature
2 induction / draft blower starts running
3 I can hear the "tick" and see the tiny arc from the ignitor striking the small hood for the gas
4 within 30-45 seconds of turning on the thermostat, the flame ignites and everything seems good so far
5 the draft blower is still running and things are heating up
6 after about 1-2 minutes the flames shut down and the entire unit is locked from what I assume is a heat-activated switch

At this point I cannot go to the thermostat and switch the blower from "auto" to "on"; it's completely unresponsive. I've figured out that if I let it cool off for a couple of hours it "unlocks" and I can run the blower from the thermostat. Also, I have been getting heat into my house by switching to heat and because the unit is on the roof I can hear the draft blower start up and eventually the flames ignite. I count about 45 seconds then switch the fan to "on" and run it like this until the temperature reaches my thermostat setting and the furnace shuts off on its own, though, the blower will still continue to run if I didn't notice that the heat shut off and switch it back to "auto".

I have done the following on my own so - removed the ignitor / hood assembly and scrubbed the surface with steel wool. Removed the tube that leads from the gas valve to the hood and blew both pieces out with an air compressor. I figured this wasn't the problem but, I had it open and now notice the flames ignite sooner than they used to after doing this. I tested the limit switch with an Ohm meter and while it would open and close it felt sticky and didn't always want to return to its rest position easily. So, I replaced it; it's a Cam-Stat 3" adjustable. Basically it didn't help. I tried it in the middle range, and then high and low. On the lower range I think it opens too soon and I can't even start the blower manually by switching the thermostat to on. I haven't looked at that pressure switch with a diaphragm built into it yet and I can't find any other heat activated switches on this thing. There is a diamond shaped device that has a nickel sized round face which is situated above the flames where they shoot into the heat exchanger. I disconnected that and put the Ohm meter on it. There's a loose button on the back of it and I can put pressure on that buttom and watch the resistance values change. I believe it to be a "blow-back" sensor.

In summary, my furnace ignites but after burning for over a minute, maybe close to two, the blower doesn't start to run and the flames shut down. The blower motor DOES work because before I switch the thermostat to heat I can turn the fan to "on" and it blows air through the entire house.

Please advise as to what I can look into next. Thank you.
bmkoz559
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- Outdoor Furnace ignites but blower doesn't run

Postby Freon » Sun Nov 20, 2016 11:14 am

From everything you describe it does appear the blower start/stop/limit switch(s) are not working correctly. Since you can set the FAN to ON and the blower runs, that shows us the blower motor is okay. The furnace ignites and the main burner runs as you'd expect. The unintended shutdown of the main burner is most likely due to the high limit safety.

Now it is very possible there is an auxilary limit control. It could the device you think is the "blow-back" sensor. The button is what you press to reset that safety. Look for another similar device. Check the wiring schematic for the furnace that should be on the back side of the blower compartment door. If there is an auxiliary limit safety, that could be the problem. It's tripping the system off too early.

Also double check your wiring of the new Cam-stat. I am not familiar with this switch but the similar Honeywell limit switches had a dial and as the furnace heated, the dial would move letting you see the switch was responding to the increased heat in the heat exchanger. Possibly you can listen carefully and hear a "click" as the furnace heats and the limit switch turns the fan on.

Also check the Cam-Stat and see if it controls the fan by timing and not temperature. If it's a timing issue you can reduce the time before the fan starts to where it doesn't trip the limit switch and/or the auxiliary limit switch. There may be a heater element in the cam-stat that is supposed to warm the switch so the fan turns on. Check all wiring.
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- Outdoor Furnace ignites but blower doesn't run

Postby bmkoz559 » Mon Nov 21, 2016 1:20 am

I will look deeper for another heat switch. The schematic is partially weathered but I'll see if it helps lead me to another control. The Cam-Stat switch in this unit has no external features other than the N.C. , N.O. and common tabs. The old one was hooked up normally closed so I attached the new one the same. The heat-sensitive spring steel element basically pulls the switch when it's hot and pushes it back when it cools.

As far as that sensor I called a blow-back I did think that possibly it could be part of the issue. Can I test it by applying some heat with a flame or heat gun and see if it triggers somehow? Now, one time that I was watching the ignition sequence with the cover open I never saw that button on the back of it pop up or otherwise move.

I'll see what I come up with tomorrow. Thanks again.
bmkoz559
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- Outdoor Furnace ignites but blower doesn't run

Postby Freon » Mon Nov 21, 2016 2:22 pm

You can TEMPORARILY jump the sensor taking it out of thr circuit. Again, I do not know if this is a flame rollout safety or another high limit safety. But by TEMPORAIRLY taking it outof the circuit you can see if the furnace behaves as it should. Look closely at that button sensor for any manufacturer's information or part number. That's one way to determine what its purpose is. You will also need this information to buy a new one if that's the problem.

Most fan switches like yours have 3 settings- the Fan ON temperature, the Fan OFF temperature and the maximum allowable temperature. Set the Fan ON very low around 90 so it will start the fan more quickly. The Fan Off would need to be around 80 to be sure the fan does stop. If you keep your house at 85 (thermostat set temperature) then the fan off will never stop the fan. The high limit will be determined by the manufacturer's data sheet.
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- Outdoor Furnace ignites but blower doesn't run

Postby bmkoz559 » Mon Nov 21, 2016 7:02 pm

- Following the diagram I located the 3 limit switches. I did some testing, bypassed each one individually and checked conductivity through each one and in the end I can test that High-Limit circuit at the board and it's a complete circuit. The diagram lists an electronic timer which appears to be integrated into the board. On the board itself there is a POT labled "off delay" but there isn't one for the "on delay." There are 2 relays and a contactor which all have conductivity through them in the right points. The blower runs when switched to "on" so I'm going to say that they work.

This unit does not have an external fan switch listed in the diagram nor did I locate one in the unit. I've seen photos of the adjustable dials. Not in this machine though.

I have a couple of photos I'd share but there isn't a way to upload, or is there? Any other thoughts?
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- Outdoor Furnace ignites but blower doesn't run

Postby Freon » Wed Nov 23, 2016 4:29 pm

You need to jump each limit switch and then run the furnace. Continuity isn't an issue. The issue is the limit switch is tired and is opening (turning off the furnace) at the not specified temperature and not allowing the fan to come on. By TEMPORARILY jumping each switch one at a time, you will locate the one that has the false temperature sensor.
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