HVAC help

[motobrewer]

I'm having an issue with my furnace. For the most part, it works fine. However, my blower fan is cycling on and off pretty frequently for short periods of time. I'm talking like, on 15-20 seconds, off for 2 minutes, on 15-20 seconds, etc.

Normal operation when the thermostat calls for heat is this:

Furnace turns on. I have a lennox pulse, so it's pretty apparent when it fires.
1-2 min later, fan turns on.
Thermostat satisfied, furnace turns off
2 min later, fan turns off.

I believe that is pretty normal. But during periods when the furnace is off, the fan continuously cycles as described above. Note, I just installed a brand new programmable thermostat yesterday (it was doing it with the old stat as well).

It does not cycle when not in heat mode. I don't remember if it cycles under cooling mode. But it doesn't cycle when mode is "off".

Another interesting piece - when the thermostat set back the setpoint for night time, it stopped cycling. Setpoint was at 70, house was at 70, and it was cycling. When the thermo dropped it to 65 for the night, it stopped cycling.

I'm really stumped here.

 

[dataz722]

I would put the old thermo back on and see if it still does it. It is probably some setting or something that is screwed up in the new one.

 

[sonex]

try putting the old stat back on if it fixes the problem the new stat maybe either defective or not compatible with your system. just what I would do. maybe there is a service tech that will chime in.

edit: dataz you beat me to it, great minds must think alike.

 

[motobrewer]

oh, sorry, that was misleading. the reason i replaced the thermo was because I thought it was the culprit, lol. it was doing it with the old one.

 

[sonex]

Is your furnace a modern furnace with a brain? I have a geothermal heat pump that went nuts and the mother board had to be replaced.

 

[motobrewer]

it's not very recent. i mean it has a control unit but it's just a regular gas furnace, not a heat pump.

 

[motobrewer]

hmmm, any other ideas?

 

[wildwest450]

Mine is 10 years old, but it has a panel with a diagnostic light on it that will flash a code when something is wrong.


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[Jacob_Marley]

The fan continuously cycling would, I would think, be the furnaces attempt to reduce the temp in the bonnet.
So ... why un-intended heat in the bonnet after the cycle has turned off?

In an older furnace with a standing pilot that is on all the time ... if that is the only heat going on in the unit when the fan is cycling, I’d suspect the fan & limit control.

Anyhow ... in any furnace, if the bonnet continues to be hot enough to kick the fan on for other reasons ... that would be of concern.
One of the suspects “could” be a stuck gas valve (although I think that would cause the furnace to constantly run the fan, but what the hell do I know, I’m not an hvac guy).

That said, I’m not sure if the valve can be stuck open just little bit ... providing just a bit of uncalled-for flame when it’s not supposed to be there ... just enough to make the fan want to cool things down again and again.

I mention it BECAUSE, on the outside chance it does have something to do with a stuck gas valve ... that is dangerous.

You might be able to spot such an “inappropriate flame” at the furnace during a period when there should be no flame. If you spot actual flame when no flame is supposed to be present ... I’d call repair immediately.
This might be the less likely of the scenarios though.

Like I say ... the other suspect I would think is a malfunctioning fan-limit switch on the furnace.
In the furnace you should be able to visually see the switch dial rotate (toward the “fan on” position) as the furnace heats up ... then, after the house is heated, the wall thermostat would tell the furnace that temp has been reached ... the flame is killed ... the fan-limit switch’s dial starts rotating back down toward the pin that will turn the fan back off. If for some reason, the fan limit switch is not functioning properly ... that could be the problem ... a worn heat sensing spring or somesuch.
If visually you don’t see the smooth, proper operation and travel of the fan limit switch back and forth ... that may be your culprit.
(Do a google *image* search for "fan limit switch" to see one of these little devils)

Not sure how the fact that it has the problem at 70* but not at the lower set point during the night fits in ... maybe the lower ambient temperature in the house is enough to re-seat the fan-limit switch back down to the “off”-pin position?

Anyhow ... observing for inappropriate flame .. and also, observing the fan limit switch’s travel might be something you could try.

(I'd still google the fan limit switch images ... but just for good measure, here is one of the results that shows where such a switch might be located ...
http://factoidz.com/images/user/furnace-limit-switch-location.jpg )

 

[motobrewer]

Mine is 10 years old, but it has a panel with a diagnostic light on it that will flash a code when something is wrong.


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don't recall seeing one but I'll check

In the furnace you should be able to visually see the switch dial rotate (toward the “fan on” position) as the furnace heats up ... then, after the house is heated, the wall thermostat would tell the furnace that temp has been reached ... the flame is killed ... the fan-limit switch’s dial starts rotating back down toward the pin that will turn the fan back off. If for some reason, the fan limit switch is not functioning properly ... that could be the problem ... a worn heat sensing spring or somesuch.
If visually you don’t see the smooth, proper operation and travel of the fan limit switch back and forth ... that may be your culprit.
(Do a google *image* search for "fan limit switch" to see one of these little devils)

Not sure how the fact that it has the problem at 70* but not at the lower set point during the night fits in ... maybe the lower ambient temperature in the house is enough to re-seat the fan-limit switch back down to the “off”-pin?

Anyhow ... observing for inappropriate flame .. and also, observing the fan limit switch’s travel might be something you could try.

i'll check into that, thanks. this furnace doesn't have a pilot, it has an ignition. i'll certainly check out the fan limit switch.

 

[motobrewer]

ok, so i took a look at the limit switch. didn't really learn much, I mean, I can watch it rotate and cycle my fan every 2 minutes, lol. I still don't know if the switch is good or bad. Obviously I could try just replacing it and see if I still have the same issue. But if I assume it's good, then either the heat exchanger is still hot for whatever reason, or heat is still being applied somehow. If the gas valve was leaking, I'd assume that it would cycle even when the system is off. So i don't think that's the case.

One thing I did was, as soon as the furnace was off after a cycle, i dropped the setpoint like 10 degrees. It was still cycling. So, my current theory is, bad switch, or the heat exchanger isn't being cooled completely after a cycle. which...might be caused by a bad limit switch?

 

[racin_ny]

When the fan runs like that is the air coming out of the registers hot or cold? If it is cold it's probably not removing residual heat.

 

[motobrewer]

i'd have to check i guess. the fan doesn't really run long enough, if the heat exchanger was indeed still hot, i doubt that hot air would even make it to the registers. maybe the one in the bathroom....