furnace cycles twice

[ponch37300]

Well just my luck Robert. After I posted that all was well it started double cycling again! I tested it again on the second cycle and there was not 24V between W and C and I couldn't see a flame in the burner and there wasn't hot air comming out of the vents. So I guess this time it was just the fan that came on during the second cycle.

So any advice on how to track this one down? My first thought is the sensor that tells the fan to come on and off isn't working? Is there a seperate sensor that tells the fan to come on and one to tell it to shut off or is that just one sensor for both? If I remember right these sensors are pretty cheap so if you think this might have something to do with it I might just order them and switch them out to see what happens.

Thanks, Scott

Update. Right after I posted this the furnace came on for the first cycle and I went to check that just for fun. No 24V between W and C and no 24V on the two browns coming out of the molex plug. So now only the blower is coming on during the first cycle, sometimes. Maybe this has something to do with why it cycles twice and so often. I never checked if there was voltage during the first cycle, guess I didn't even notice that it was cold air coming out. Was just worried about the abnormal 2nd cycle. So now I need to figure out why the burner is only coming on when it wants to. Any advice on tracking down why a burner is being lazy and only working when it feels like it. Do I have a union burner?!

[Robert]

Hi Scott,

When it does this on first cycle, is it the circulation blower that is running or the small booster motor or both ?


Do you have a flashing bulb on control board inside control box when this happens ?

If so, have many blinks per group ?


Thanks,
Robert

[ponch37300]

I believe that the time I "caught" it the booster came on first like normal. Then the flame forgot to come on. Then the blower came on. Then I watched it for about the next 6 times it came on and they were normal, the burner came on.

There were no light codes giving off, stays green the whole time.

I'm trying to get to the furnace everytime it comes on so I can be there when the burner doesn't come on and try and see what's going on. If I understand it right the 2 brown wires that go from the molex plug to the burner are what tell the gas valve to open? And then there are some other wires that go down there to ignite the gas? So the 2 browns should have 24V during the burner cycle, right after the booster runs for a little while? I'm trying to get there when it doesn't light so I can double check the browns and see if they have voltage or not when the burner doesn't come on. Would be nice if I could watch the whole cycle when it doesn't light so I can see exactly what's happening, if it even tries to light or if there is nothing at all.

Thanks, Scott

[Robert]

Hi Scott,

T-Stat calls for heat:

W and C = 24vac

G and C = 24vac

2 black wires to booster motor = 120vac

After 30/45 seconds give or take:

2 blue wires to pressure switch = 0vac (closed)

Then:

2 white wires to hot surface ignitor = 120vac for 30 seconds or so

Then:

2 brown wires to gas valve = 24vac for about 6 seconds

Then:

IF remote sensor senses flame, gas valve remains open for circuit. IF no flame sensed, gas valve closes.



Thanks,
Robert

[ponch37300]

Thanks for that explanation Robert.

Had a new twist in this thing today. Wasn't home from about 330-11 tonight and had thermostat set at 67. Got home and trailer was cold. Looked at thermostat and temp was 60 degrees in here. Looked at the furnace and the indicator light was blinking twice, cumbustion air switch stuck closed. Started banging lightly on a few things and when I hit the booster motor the furnace kicked on and we have heat now! Not sure exactly where the cumbustion air switch is on this furnace or exactly what part it plays. Do you think this switch is bad and would that be causing the other problem of cycling twice?

Also I was doing some testing after reading your explaination on how the furnace works and you said W and C should be 24V and G and C 24V. I have 24 between W and C but G and C is nothing at any time that I could see. The G wire just goes to a ground screw. I do have 24V between W and G.

Any advice on where to go from here with the new issue of the cumbustion air switch? If this could be the cause of the furnace cycling maybe I should just replace this and see what happens?

Thanks, Scott

[Robert]

Hi Scott,

Disregard the W and G part, was very tired and had too many questions and answers inside my brain still.

For yours, just the W and C test is needed.


That switch is the pressure switch with the two blue wires.

It will/should read 24vac and after booster motor runs for 30 seconds or so, it should close and read 0vac.


Yes, that could be the whole preoblem. IF it does not close, NOTHING will work except the blower motor and the booster motor.


Thanks,
Robert

[ponch37300]

Hello Robert, Well I think we found the problem. Where is the smily for fingers crossed! No problem on the G and C, I figured it was just a typo or something.

How does this pressure switch work? I'm thinking taht the board sends 24V to the switch threw one of the blue wires(the middle wire in the molex plug) and when the switch is closed it sends 24V back to the board threw the other blue wire. So when I test for 24V I should be testing the return wire and common right? That is what will tell me if the switch is opening and closing? I ohmed out the switch and was getting 30-50 ohms, according to a youtube video I found I should be getting 0 ohms when the booster motor is running. Here is the video http://www.youtube.com/watch?v=P3-Ki-eEWo8 . In the video he mentions that there are a couple other things that can cause the switch not to open.

So from these readings and the flashing light code last night I would be placing my money on this. Would you agree with my "wanna be furnace repair guy" diagnoses? I priced out the pressure switch from the parts page on here and it's 39 bucks so if you tell me that I'm on the right track I will probably just order it and hope for the best and if not start looking at something else!

Thanks, Scott

[Robert]

Hi Scott,

Quickest test is the one I gave you. Place meter leads across the switch and should read 24vac IF it has power to it.

Then after 30 seconds or less/more, IF/when it closes, you will suddenly read 0vac.


IF it does not close, you will continue to read 24vac.


Yes, I believe the switch is not working. Check the tubes connected to it for loose, cracked, kinked or clogged.

If ok, be sure booster motor is running strong and smooth.

Also check air pipe to booster motor for blockage or separation.


IF air pipe, booster motor and pressure switch tubes/hoses all seem ok, then switch is probably bad.


Thanks,
Robert

[ponch37300]

Hi Robert,

I checked the tubes, they seem fine. I blew/sucked on them and could here the switch clicking open/close. The booster sounded a little loud so I took it apart and cleaned everything, had some leaves in the cage. Put everything back together and it runs smoother now. Don't think the air pipe is clogged.

Now for the electrical, I tested a bunch of times and here is what I came up with. Turned temp on thermo up to turn furnace on. With the fluke on the two blue wires I get 24V when the furnace starts it's cycle. But only for about 2-3 seconds, then down around 0V while the booster is running. So I'm assuming the switch is open for a second and then is stuck closed the rest of the time? If I understand you correctly I should be getting 24V for the 30 seconds or so that the booster is running, which I"m not, I'm only getting 24V for about 2 seconds at the very beggining of the cycle. But this is happening even when the burner comes on so I'm a little confused.

I did manage to catch the furnace one time when the burner failed to come on. Had the fluke on the two blue wires and had 24V for about 2 seconds at the start of the cycle. Then went to 0V while the booster was running and then kinda jumped all over, had 0, 12, 24, and just basically jumping fast for about 3-5 seconds and then no burner when it should have came on.

So I guess I'm going to lean towards this pressure switch and order one Monday unless you have any other ideas after reading this. If so please let me know.

Thanks, Scott

[Robert]

Hi Scott,

The 30 seconds is basicly a max. time, usually is less, but also more than 2-3 seconds.

Anyway, only way to know exactly is test to see when it reaches water column pressure to close contacts.

With what you described, sounds like it is opening and closing.

That would either be due to bad switch or not reaching and holding water column pressure.

Most often the pressure switch.

The booster motor should run and switch stay closed for entire burner cycle.


Thanks,
Robert

[ponch37300]

Thanks Robert,

I'm going to go ahead and order the switch and hope for the best. I think that I've tested out to the best of my ability, with much help from you, and think the pressure switch is the best choice.

Thanks for your help and hopefully replacing the switch will end this thread!

Scott

[Robert]

Hi Scott,

Look forward to the results, hopefully they'll be positive ones.


Thanks,
Robert

[ponch37300]

I just ordered a replacement from this site. After I get it and install it I will do the same tests on it and hopefully I will have 24V for longer than 2 seconds. Will post results when I have them and I to am hoping for some positives.

Thanks again, Scott

[Robert]

Hi Scott,

I won't close unless it gets correct water column pressure,so main thing is that when it goes to 0vac, that it remain there entire cycle.

Burner and blower and booster motor should run entire cycle as well.


Thanks,
Robert

[ponch37300]

I won't close unless it gets correct water column pressure,so main thing is that when it goes to 0vac, that it remain there entire cycle.

That really helps, the one time that I was metering it when the burner failed to come on I had 24V then went to 0V and then when the burner should have came on the meter jumped all over.

I have a good mechanical background but new to this furnace stuff so guess it takes me a little to understand everything!

Thanks again and will post results at the end of the week when I get it installed.

And if your ever in Wisconsin I owe you a beer or ten for all the help!