furnace cycling twice when thermostat calls for heat

Furnace made by York, replaced thermostat with programmable without a mercury switch. Therm. will call for heat, vent blower will come on, Igniter will glow after about 30 secs. and gas will lite then fan blower will come on. Gas will go out but all the while the therm. is calling for heat. igniter will glow again, gas will lite and blower fan will come back on. Therm. will click off and 'heat on' visible on therm. will go out. Gas goes out fan runs until temp is low enough and will shut off. Furnace will not run again for another 15-20 min or longer and start the same cycle over again. Replaced flame sensor, made sure furnace was grounded properly. Could the control board be bad 50e47-150. Would appreciate any thoughts on this or is this just the nature of the beast?

 

The furnace seems to be doing what it is expected to do...though you don't say how long (minutes) do the burners fire before going out?

This could be something as simple as a lack of proper setting/programming of the thermostat

Need more info about the furnace & thermostat.
Furnace: how old? full M/N?
Thst: MFR and full M/N?

did you have this problem before replacing the t-stat?

 

The furnace is 14 yrs old made by york model PMU-LD16N095B converted to propane.
I had an old style round honeywell thermostat and changed it to a RTH221 honeywell programmable. The gas will burn for appx. 1min 25-30 sec and go out, the blower fan will come on about halfway the the first cycle, therm still has heat on showing. Less than a min later the gas will refire this time it usually stays on for about 40 secs and go out. I noticed when it was colder out on the second cycle the gas would stay on longer and the blower would come on while the gas was still burning, but today its warmer out the gas has been going out before the fan kicks on, but the blower fan will come on a few secs after the gas goes out. It was much worse with the old therm. running every few min. I have used the furnace more this year cause I have always burned wood.

 

Cycles of a minute or so in duration are not normal.

This sounds like the limit switch is tripping.
It could be something as simple as a restricted airflow. Possible reasons for the restricted airflow:
* dirty air-filter (when was the last time you replaced it?)
* a filter of too high an efficiency (have you upgraded? - increase blower speed)
* return registers that have accidentally been shut closed (inspect re-open as needed)

Try removing the air filter (just for test purposes) and see if the burners now remain ON or still short-cycle.

Does it feel like there's much less air coming off the registers than in previous years? if so, I'd look at the fan-wheel and see if it spins freely (does is squeaks?)...maybe the bearings are going bad.

Lennox's website only has available tech manuals of their most recent units. They don't have yours available online, so pics will help here so I can see the various components and hopefully come up with a couple of suggestions of things to check.

 

After watching the limit switch, the blower fan would kick on at appx. 140 deg. The filter is clean and have plenty of air movement coming out the registers, but the limit switch kept moving towards its shutoff temp of around 200 deg. it turned a little further all of a sudden the thing bounced back and the gas shut off. The limit switch has to be the problem. Can I clean it or does it just get weak over the years? Sure do appreciate all your help and time you spent. Thank you!!!

 

So you have a Honeywell L4064 fan-limit switch I see.
The fan setting (ON at 140F) is too high and could be the reason for your trouble.



Notice the pins (point levers) on the bottom part of the dial.
Tell me where are they at.

Move the middle one (Fan-ON) from its current setting down to 100-110F and the right one (Limit) to 190-200F.
If you leave the left one at 80-90F you'll be O.K. there too.

The fan is kicking ON too late (140F as per your last report) to stop the heat build-up inside the plenum and is reaching the limit setpoint way to quickly for the fan to prevent it from happening.
let me know how it worked out.

 

One thing that caught my eye was the fact it has been converted to propane. I'm wondering if the correct conversion was made for for propane, as it sure sounds like it is overfiring.

 

good point!
I missed out on that one when reading the first posting

 

I moved the fan on pin to about 110 deg the fan off is set around 80 deg. The furnace run without cycling twice. The limit switch got close to the 200 Deg setting but did not kick off. I would say the gas was on for about 4-5 min. and went out only when therm. no longer called for heat. This is the first time that I can recall the furnace run without recycling. I believe it is fixed. Thank you once again for your help!

 

O.K.
Glad to read the babe is working as it should.

Having said that, I believe your blower speed (winter speed) should be moved a notch up. You should have a multi-speed blower there and I bet in winter you're running at the lowest of them all.

Why is the plenum temperature getting this high?
possibly you unit is grossly oversized. They tend do this (high plenum temperatures) if too much oversizing exists. You can work yourself out of this problem by increasing the blower speed (though it appears is no longer a problem after re-setting the limit)

Your call...