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Old 04-14-2013, 12:03 PM   #1
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Lennox oil furnace short cycleing


Older Lennox oil furnace with Heat pump secondary is short cycling. I'm in the NE, and just starting to come out of heating season. The heat pump comes on above 42, and has the past couple of days (still run oil most of the time though). However, when its to cold, the oil kicks in, and is short cycling. I had a local tech out, did the cleaning/nozzle replacement, and it did not short cycle. Next call for heat, short cycle.
I can watch the thermostat on the unit (rotary guy with 4 wires) rise to the kick out temp, the fan comes on, t-stat goes down, oil fire comes on, temp rises, etc....until main house T-stat is satisfied.
Just took that main unit thermostat out, and it looks clean, nothing jumps out as dirty/broken.
Is there something else that could have started to cause this? Everything worked fine all heat season, this just started recently.
Thanks.


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Old 04-14-2013, 12:36 PM   #2
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Lennox oil furnace short cycleing


The indoor coil may be dirty, causing low air flow so the furnace gets too hot.

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Old 04-14-2013, 07:34 PM   #3
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Lennox oil furnace short cycleing


I'll dig to try and clean the indoor coil. Should it be hard to get to? Is there a good online drawing reference or resource for these older units so I'm not taking apart things un-necessarily?
Thanks
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Old 04-14-2013, 08:52 PM   #4
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Lennox oil furnace short cycleing


Not always easy to get to on a furnace, specially on a oil furnace. Post pics. Can tell you more then.
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Old 04-15-2013, 09:09 AM   #5
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Lennox oil furnace short cycleing


So I still plan to clean the indoor coils, but I listened to it run last night, and it didn't short cycle.
What I noticed was, that when it was short cycling, the circulation blower was not running, it would only come on when the unit went hot, then blower on, then burner on (blower went off), etc....
when it doesn't short cycle, the blower is on during combustion (which is what I'd expect).
So, is there a control board or relay or something somewhere that controls the blower speed or cycle? Is it easily replaced? Is it easily tested? I think this may be my culprit.
Thanks
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Old 04-15-2013, 04:32 PM   #6
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Lennox oil furnace short cycleing


The rotary thermostat as you called it. is a fan limit control. Its contacts for the fan may be wearing out.
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Old 04-15-2013, 04:40 PM   #7
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Thanks.
Is that what controls the fans normal operation?
When combustion happens, there's usually a slight delay (2-5sec) before the fan comes on normally. But it runs the entire time there's combustion, and slight amount of time after the house thermostat is satisfied.
When it was short cycling, the fan was only coming on when the temp was to high, but it always came on.
Are there other controls for the fan somewhere?
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Old 04-15-2013, 05:00 PM   #8
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Lennox oil furnace short cycleing


Not sure on that unit. Don't think it has a timer for the fan on it.
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Old 04-15-2013, 07:43 PM   #9
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Lennox oil furnace short cycleing


most of the older lennox oil furnaces we worked on ...had only fan and limit control....ben sr no timer for fan..
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Old 04-15-2013, 09:51 PM   #10
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Lennox oil furnace short cycleing


Ok, so a fan control and limit control.
Where is the fan control then? The on unit Thermostat being the limit control. Or are they one and the same?
Thanks for all the help
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Old 04-16-2013, 04:11 AM   #11
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Lennox oil furnace short cycleing


Rectangular gry box is the fan and limit control. Should have 3 pointers in it. Fisrt one an off, second one fan on, third one burner off.
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Old 04-16-2013, 09:10 PM   #12
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Lennox oil furnace short cycleing


fan and limit control in same box on old style units....ben sr
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Old 04-23-2013, 08:13 AM   #13
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Any thoughts why it would do it sometimes and not others?
Sometimes the fan comes right on when the burner is on, and no short cycle. Sometimes, it doesn't the first time, does 1 short cycle, and the fan stays on when the burner comes on again.
Its very strange.
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Old 04-23-2013, 02:07 PM   #14
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Could be the nozzle is clogging and not always putting as much oil in as it should. Could be the contacts are carboned in the fan limit. Can't say for sure from here.
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Old 04-23-2013, 07:15 PM   #15
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Can I get at and clean those contacts? Or is that something that's just replaced?
When I have a spare minute, I'll have to take things apart and see if I can tell anything more specific.
Thanks for the help.

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