How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site. Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask gotoman for ac Your Own Question
gotoman for ac
gotoman for ac, HVAC Technician
Category: HVAC
Satisfied Customers: 5215
Experience:  51 years of experience install design and repair of all makes and models of AC
Type Your HVAC Question Here...
gotoman for ac is online now
A new question is answered every 9 seconds

I have an M3RL Series Downflow furnace (Model M3RL080A BWT)

This answer was rated:

I have an M3RL Series Downflow furnace (Model M3RL080A BWT) It will run normally for a while then the burners and main blower will shut down for 30 to 50 seconds while the exhaust blower continues to run, then the burners and main blower restart and continues to heat. Sometimes this happens only once during the heat cycle, other times it may happen a second time until the thermostat finally signals the proper temp and shuts down. Any ideas what would cause this? We are entering our 3rd season with this furnace.
A few things that can cause this problem. Check for dirty filters. Next you may want increase fan speed,check the flame rod to see if it is dirty, Clean with sandpaper. Pull and clean limit switches.Russ
Customer: replied 6 years ago.
Changing the filter was Done as I was getting ready to switch from AC to heat, and at that tiime I also removed the drain plugs from the exhaust blower to drain and water collection from it. (I learned about that last year after we'd had some pretty violent storms and the exhaust blower wouldn't run) I did make a mistake, We are entering our 4th season with this furnace (I was originally thinking 3rd) SO everything is still pretty clean. I pulled both limit switches, they appeared clean and the bi-metal discs felt free (they had a wee bit of wiggle room) The Auxillary Limit switch is next to the main blower and had a fair amount of dust on it. I blew both limit switches off around any seams with a can of compressed air. The Flame Rod visually only had a little minor staining on it. Rather than use sandpaper, it polished it up with a scotch brite pad. ANd vaccuumed out the bugs and dust in the firebox. The shutdown cycle the furnace has been running is Flame goes out, exhaust blower stops, main blower stops and exhaust blower restarts, then the furnace runs until it either repeats, OR the thermostat shuts off. AND is doesn't run like this all the time...just MOST of the time.
So far the furnace has only cycled one time and ran as it is supposed to, BUT that is not unusual either, I need it to cycle a few more times so I can observe it for consistency.
Fan speed makes sense to me also, as if the air isn't being forced through the chamber fast enough, the heat would trip a limit switch until the chamber cools down. I do not know how to change the fan speed....or how to tell what speed it is set on. All I have is the install manual with wiring diagrams. While I know next to nothing about computer boards I can follow Wiring diagrams and schematics. But I also don't want to blindly switch wires around on the board and wind up buring out my board. My diagram shows a 4 speed motor with The wires at the motor being L=red, ML=orange, MH=blue, H=black.
The diagram shows on the circuit board red(L) attached to "Heat" terminal, orange(ML) = M2 terminal on the board, blue(MH) = M1, and Black (H) = cool terminal. With the M3 terminal left blank.
HOWEVER on the actual furnace I have red(L) attached to the M3 terminal, orange(ML) = M1 terminal, Blue(MH) = M3 and black(H) =cool and the M2 terminal is blank.
I am assuming that M1, M2, & M3 are each identical and really means nothing here, but on my unit the red and blue wires have been basically reversed. ?????

Blue is medium and black is high speed. How is the operation now. Russ

Customer: replied 6 years ago.
No change in operation. It just fired again and is doing the original problem. OK. How should the wires be attached to the board? On my board the black wire is attatched to the terminal labeled "Cool" Blue is attached to "Heat" Red to M3, and Orange to M1. Should I move the black wire to "Heat"?
Yes try that to see if it helps, Russ
Customer: replied 6 years ago.
I swapped the blue with the black wires Black on Heat, Blue on Cool Turned on the power NO led LED (Status light) So I killed the power again and switched them back. STILL no red LED. I now have NO heat at all.
Should not of effected anything to do with power to unit. Make sure your door is back on tight. There is a door switch that need to be closed. Russ
Customer: replied 6 years ago.
It is a manually operated SPST switch. The door does not need to be mounted for the furnace to run. You can't see the status light with the door on anyway. Status light is off. I have called local repair company. Earliest they can be here is 5 PM EST today.
Switching the two wires does not have any effect on board . They are fan speed only. All furnaces by code have spring loaded door switch to shut power off to furnace when door is open. Sometimes they do not make all the way. On limit switches this is a downflow unit there is an aux limit on fan housing that is manual reset. Let me know what he finds. Russ
Customer: replied 6 years ago.
Found it. First, there is NO spring loaded switch on this furnace. Where you would think such a switch would be is a factory installed, manual on/off switch. Furnace does run with the front cover off. While looking for the reset switch on the fan housing (Which I haven't found yet) I went back over the wiring around the circuit board and found a loose wire nut on a neutral wire that wasn't immediately noticeable. The wire was disconnected but still inside the nut. I only found it as I was pushing a harness out of the way. Furnace is now running.
I switched the wiring back (again) to try the higher motor speed. Black wire is now on Heat, Blue wire is now on cool. And then brought the house back up to temp in small stages so I could observe the furnace. Changing the motor speed doesn't seem to have fixed the original issue. While it did operate normally on the first firing, when I stepped up the thermostat the original problem sowed up again.
Great job. From what you described the problem could be had flame sensor,too much gas pressure or bad limit. Russ
Customer: replied 6 years ago.
OK. Furnace runs too long before the first shut down, so odds are it isn't the gas pressure or flame sensor. I've ordered a limit switch. I was leaning toward Limit Switch anyway before first contact with "Just Answer". I was used to a 35+ year old clunker furnace before buying this house, and while most of the systems are similar it does have it's differences so I wasn't sure. Thanks
There is a lot of difference in a 35 year old furnace and even 10 years newer. You are on the right track. Usually on problems like this I measure gas pressure is the first thing and then bonnet temperature. Russ
gotoman for ac and other HVAC Specialists are ready to help you
Customer: replied 6 years ago.
Still waiting for the new Limit switch to come in. I ordered it.....IT came in yesterday but they sent the wrong one. Numbers are correct, BUT the switch is 3 times the physical size and wont fit in the opening and I can't call them until tomorrow.
Thanks for feedback. Let me know how it turns out. Russ
Customer: replied 5 years ago.
Ordered correct switch on Monday, came in yesterday. Seems to be what was causing the problem. The few cycles that I was aware of happening (so I paid attention to the entire cycle) seemed to be running properly. Thanks for the help.

Great job I am impressed. Thanks for feedback, Glad you back online. Russ