[davel318]

This is an interesting one. My Carrier gas furnace will run a complete heat cycle (thermostat set to 70, furnace will heat to 71, and then stop as expected). Right after it stops, the red LED on the circut board will turn off, making it appear that there is no power going to the circuit board anymore.

Some time later, when the heat should come on again, suprisingly, it does (and the LED is then on). Again, a complete heat cycle will occur. And, again, when that cycle ends, the LED will turn off.

Every couple days, when the heat should come on again, it does not. I typically have to turn the thermostat off and on, turn the power switch (the one that looks like a light switch that is next to the furnace) off and on, and have to press in the furnace door switch several times. Some combination of those things will eventually cycle the furnace on again; I have not figured out an exact sequence that causes it to come on again.

I know that the LED light is supposed to stay on solid when the furnace is idle, so I can't understand how the LED can turn off after a heat cycle (indicating no power to the board), and then the thermostat can call for another heat cycle. I think the LED stays off until the thermostat causes that next heat cycle to start, but can't be 100% certain as I have not stared at the LED waiting to see if it somehow comes back on prior to the next heat cycle.

It's as if power is going to the board in some manner since the thermostat is causing a new heat cycle to occur. Could a bad furnace door switch cause this intermittent oddness? I know that the circuit board would not get power if the door switch was broken. The door switch is obviously not broken though since the heat will come on after the LED goes off, but maybe somehow the switch is having intermittent problems??

Any ideas are appreciated.

 

[user_12345a]

It has to be caught when the problem occurs and checked with a meter.

The blower access panels switch needs to be taped or bypassed so power isn't killed when the panel isn't removed.

Line voltage going to the board, transformer voltage output, limits, voltage between w and c need to be checked with power left on and the same call for heat.

If the board is getting all the correct inputs for heat and just sitting there and not responding, it needs to be replaced.

 

[ClimateCreator]

Jump the door switch out permanently as in take the two wires off of it and securely connect them together. Check every terminal screw for tightness and loose or broken wires at every connection. Push all wires into the plugs don't just push the plug. If all else fails it'll be the board most likely.

CC

 

[surferdude2]

The leds works on DC voltage, as do the relays on the board. If the board is getting a constant AC feed and the led goes off, there is a failure of the on-board rectifier power supply that creates that dc voltage. It sounds like while it's in the run cycle, the fault is somehow mitigated, like being latched by a relay, and when the cycle ends, the led loses its source until the board cool off and the possible thermal intermittent connection heals itself due to contraction.

Probably going to need a new board unless you are capable of troubleshooting the problem down to the discrete component level.

 

[davel318]

Thank you for the replies.

Starting today, once the heat cycle finishes, the LED light turns off, and the next heat cycle will not start on its own. So, it's gotten worse as I can't get 2 heat cycles to work without touching the furnace door switch.

To try to get it to start heating again today, I took the furnace door off, turned the power switch off, pushed the furnace door switch (front and back side), turned the power switch on, pressed the furnace door switch (front only) and it came on after a couple tries.

If I want to tempoarily bypass the door switch as noted by ClimateCreator's response, how do I safely connect the two wires that are currently going into the furnace door switch?

 

[surferdude2]

If I want to tempoarily bypass the door switch as noted by ClimateCreator's response, how do I safely connect the two wires that are currently going into the furnace door switch?

It tell me that since you have to ask, it isn't a good idea for you to do that. Stay safe, call a repairman. These intermittent type of problems are baffling for competent servicemen but they have enough insight to cut to the chase and do it safely.

 

[Yodaman]

If I want to tempoarily bypass the door switch as noted by ClimateCreator's response, how do I safely connect the two wires that are currently going into the furnace door switch?

For starters, depress the switch and put a piece of duct tape over it to hold it closed. Then use a meter to check that voltage is passing through.

 

[user_12345a]

bypassing or taping the door switch temporarily is okay as long as you don't do something really stupid like leave the access panel off or stick your hand into the blower wheel while the unit has power.

now, if you're not familiar and comfortable with working on line voltage circuits live, best to call someone out.

 

[davel318]

OK. I'm starting small. I took the furance door off. With the power off, I pushed the wire in on each of the conections to the door switch (as suggested above - yes I know the suggestion was more than that). The wire seemed to be connected in good, so I don't know if that helped, but obviosuly couldn't hurt.

I turned the power switch on (door switch not pressed in yet), and then used a Klein Tools voltage tester on the black wire coming out of the junction box, going into the door switch. The tester turned red indicating there is voltage to the switch (I know it's supposed to about 120v, but I havent moved on to a multimeter; just wanted to quickly see if the door switch was getting power from the junction box).

I then touched the wire coming out of the door switch with the voltage tester, and it showed green (no voltage), but I had not yet pressed the door switch, so I wouldn't expect voltage. To see if pushing the wires in might have helped, I pressed the door switch and the furnace came on, so I put the door on.

Tomorrow, when I need heat again, I'll press the door switch and see if there is voltage on the wire coming out of the switch (if pressing the door switch does not turn on the furnace as I expect).

 

[user_12345a]

Like I said before, you have to catch the problem when it occurs and not reset power to the system.

Removing the access panel and then pressing the switch in resets it. The switch needs to be taped or bypassed so you can remove the panel without momentarily cutting power.

You also need to test with a multi-meter, not a non-contact tester which is only for safety.

 

[davel318]

user_12345a - thanks for the info. The reason that I was thinking I could press the door switch in and test out the problem is that most of the time that i push the door switch in after the LED light has gone out (from the last heat cycle), pressing the door switch does not turn the furnace back on. But, I hear what you are saying about testing after the LED goes out without pressing the door switch again.

 

[ClimateCreator]

It tell me that since you have to ask, it isn't a good idea for you to do that. Stay safe, call a repairman. These intermittent type of problems are baffling for competent servicemen but they have enough insight to cut to the chase and do it safely.

I'd second this....

CC

 

[davel318]

Yes. I have a multimeter, but I need to learn how to properly use it before proceeding, so I generally agree with you. Having said that, I found this out:

I went downstairs yesterday morning and the furnace LED was off (as usual). I took off the furnace top panel (not the panel with the door switch). I put the non-contact voltage tester on the black wire that goes from the junction box to the furnace door switch. It showed no voltage. So, I think that tells me that there is no power getting past the junction box.

The circuit breaker is fine, so that's not the issue. So, that made me wonder about the power "light" switch since the power goes from circuit breaker to power "light" switch to junction box.

I pressed my hand firmly against the light switch cover (it's still all closed up; I didn't open it of course). That started the furnace. The next time the heat cycle finished and the LED went off, I did it again, and it started the furnace again. So, I am guessing one of 2 things:
(1) The power "light" switch is bad
(2) There is a loose wire in the power "light" switch or in the junction box

Does that sound right to you?

 

[surferdude2]

That sounds like a perfect "Aha" moment! Replace that wall switch with a spec grade (or at least a commercial grade) one and hope for the best. Inspect the wiring while doing so, in case it has overheated at the old switch connection points, etc.

Sounds like you may have gotten lucky! Consider getting a lottery ticket today!

 

[davel318]

Can you tell me which switch is the one I need? I found a bunch of commercial grade ones here (I filtered on commercial switch, single pole). My existing power switch looks just like a standard light switch.

https://www.platt.com/platt-electric-supply/Commercial-Switches/15-Amp-Toggle/search.aspx?q=furnace+switch&sectionid=8&groupid=95&CatID=1039&xtype=Commercial+Grade&xpoles=1&SubCatID=4259

 

[surferdude2]

Any of those you show will be fine. You will find those upper grade switches at your local big box builders supply stores. The $5 commercial grade ones they carry will do fine. No point in going $25 spec grade. What you had that failed can be had for $1.29 or less.

 

[coolair111]

Choose the commercial switch because these switches are capable of withstanding the high voltage and do not budge easily. Invest in a good one instead of replacing again and again.

 

[davel318]

I'm going to Lowe's on Wed as I'll be near one of their stores. I'll see if I can find a commerical grade switch there.

Once I get the switch, I'll take the light switch plate off of the furnace "light switch" and see if I can tell if it's just a loose wire. If I see no loose wire, I'll put the new switch in. I have had another light switch in a bathrool in my house go bad once, though in that case, the light it was tied to wouldn't turn on at all.

Thanks for the help.

 

[surferdude2]

Put it in regardless, the present switch isn't likely motor H.P. rated and should not have been supplied for that application nor passed by the final inspection.

 

[user_12345a]

Even the largest furnaces don't draw more than 8 amps - a basic light switch will do fine as long as it's not complete junk.

Light switches are usually rated for up to 15 amps.

Switches go bad due to the arch which forms between contacts when large and or inductive loads are turned on. In your application, that's not an issue - it stays on. It may be switched on with the blower coming on right away a few times over 20+ years.

Don't worry about getting a commercial grade one.