Hot air furnace cycling

HT in Pa_3

I've seen many times where a furnace will cycle on and off, when the burner goes off on high temp. limit. Today I watched a gas furnace blower cycle on and off when the fan-limit control kept shutting off the blower at the low temp setting, before the t-stat was satisfied! Could the fan-limit control be bad? Or, could the motor need slowed down? The homeowner says that this cycling just started. Anyone have any ideas. I'd sure appreciate your comments. Thank you.

HT in PA

nicholas bonham-carter

short-cycling furnace

have you measured the temperature at which the limit switch cuts off and on the blower? temporarily shorting out the limit switch might establish whether the blower motor itself was electrically intermittent.

have they closed off any ducts in an effort to save fuel, and changed the air flow volume through the furnace?

could the thermostat anticipator be faulty?  has the furniture layout changed which now may affect the way the thermostat senses temperature?--nbc

Jean-David Beyer

Air filter clogged?

Is this recent? My did (who was cheap in most ways) did not change the air filter in his hot air furnace every year because they were too expensive. This had a forced air blower to help circulate the hot air. There were two filters, each about 1 foot by two feet about an inch thick. They got a layer of dust on them between 1.8 inch and 1/4 inch thick. He would vacuum the dust off and reuse them. These were disposable filters. The furnace did not cycle because it was coal fired. He thought the layer of dust increased the effectiveness of the filter. Well, maybe, but then a sheet of aluminum blocking the duct would have been more effective still.



I can only guess how much better it would have worked with new filters every coupla munce.

HT in Pa_3

Thank's for your comments. Here's some more info.

I haven't checked any temps yet. I will short cycle out the limit switch to check out whether the blower motor will actually continue to run. I don't think the problem is with the motor, because the fan-limit switch always goes to the off(cold) setting before the blower shuts off. Also, all ducts are open and the blower door has been off while troubleshooting. I'm not sure how the anticipator setting could effect the operation of the blower motor. I'm also going to check the gas pressure; read on an old post that that could possibly be the problem. The gas comes from a private well. Thanks again for your comments.

HT

Tim McElwain

What is the

make and number of the Fan/Limit switch? Is there an auxiliary limit on this unit?



Is the fan controlled by an EFT instead of a Fan/Limit Switch?



The fan should come on after the initial call for heat and then run during the entire call for heat staying on after the thermostat is satisfied until the plenum cools.Put your multi-meter on the wires to the fan motor and see if power is interrupted when this happens if so the fan switch may be faulty. Measure temperatures with a thermometer while doing this.

furnacefigher15

How cold was the house?

If the return air was really cold, like 45 degrees or less, this is normal, and will go away once the return air climbs to over 50 to 55 degrees. It all depends on how high the fan on setting is, and what the temp rise is.



Many of the fan switch on settings are around 110 or so. If you have say a 55 degree rise, 45 + 55 is only 100.



There's a good chance the problem solved itself already.

HT in Pa_3

update

I checked the gas pressure at the manifold today; is only 1.5" w.c. The data tag states it should be 3.5" w.c. Could that be the problem? I couldn't see a test port to test inlet pressure. The owner said that the gas co. recently was working at the gas well, which is on the owner's property. He will contact them about the pressure. To answer your questions: The fan-limit control is a Honeywell L4064B. I don't think there's an auxiliary limit switch on this unit (it's about 25 years old). The fan IS controlled by the fan-limit switch, and yes, the fan does come on after the initial call for heat, but shuts off after about 1/2 minute, while the burner continues to run. I can see the temp. change by watching the fan-limit switch with the cover off. Also, the house is cold, in the 40's. Did I mention that no one is currently living in the house? Thank you all for your "heating help".

HT

furnacefigher15

House in the forties!

That's the whole problem.



LOOK AT MY LAST POST!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

HT in Pa_3

Yes, In the 40's

I did read your post, and understand that this may be the problem, but what do you think about the manifold pressure? Could that also be a problem? I found an old post that talked about too low of a gas pressure may cause the fan to cycle.

Tim McElwain

You have a combination of two things:

1. Very cold return air which will rapidly cool the bi-metal on the fan control causing the fan to shut off.



2. The gas pressure means your BTU is 1/3 of what it needs to be so that is also a factor.



Get the gas pressure back up and I think once the unit gets up and running and gets some heat in the house the fan switch will be fine.



You can check inlet gas pressure anywhere on the inlet side of the furnace that has a tee, just insert a bushing reduced to 1/8" and use a 1/8" nipple into the bushing and hook up your test gauge. Check the pressure static (nothing running) and with a full load. There should not be a drop in pressure. The 1.5" outlet pressure is very low just make sure this is not a two stage gas valve. If the inlet pressure is good then try adjusting the servo regulator which I assume is built into the gas valve. Make sure there is not anything blocking the outlet sensing port on the gas valve. What are the Make & numbers on the gas valve?

HT in Pa_3

Tim

Thank you for your explanation. The well tender came out to the property today and found the regulator at the house was bad. After installing a new regulator, he said the inlet pressure is now up where it should be! I think the furnace will work better now. I'm sorry I don't know the make and numbers on the gas valve, but the gas valve is one stage, and the pressure adjustment IS built into the gas valve. Thanks again for your help.