Fan Cycles on and off, Gas Furnace

I live near Nashville, this problem only happens when it's below 30 degress outside, when it's above 30 degrees it runs fine.

We have an outside type Gas Furnace AC system, the unit is self contained and sits outside the house.

The furnace starts, I can hear the outside component fire up, builds up heat then the fan kicks on, it runs for about 45 seconds then stops. The outside part is still running producing heat but the blower stops, it then kicks in in less than a minute and runs another 45 seconds or so. It repeats this cycle until the inside temp is brought back to the right temp.

Any ideas what could be wrong?

I can't find the brand name, what I do have is it was made by a company named Inter-City products in LaVergne TN, model numbers PGA/G4P/G6P

 

More than likely it has a fan switch that needs to be set properly.


On older furnaces, the fan switch is that dial thingy that switches the fan on and off, and could be adjusted to turn the fan on and off at a temperature that can be selected. If it's set too low, the fan will switch on early and cool off the heat exchanger enough to shut off the fan, as it's designed to do. When the heat exchanger again heats up, the fan turns on again.

 

Blowers are often (or maybe even always?) controlled by a "fan limit switch" exposed to the temperature of the heat exchanger. If some or even all of the air being heated by your furnace is actually being drawn from outside the house, a low temperature outside would cool the heat exchanger more quickly and stop the blower until the exchanger is again brought up to a useable temperature.

If all of the air that passes through your heat exchanger is supposed to come from inside the house, you might have an outside leak in your cold-air return line going to the blower. But if some or all of the air supplied to the blower is supposed to come from outside the house, there is probably little you can do to stop the cycling. One possibility would be to slow the blower just a little, and there might be a simple way to do that. However, adjusting blower speed and limit settings for best efficiency can be a little tricky, with mistakes possibly being costly as far as energy is concerned.

 

Since the problem is reported to occur only when it gets cold there, may I suggest that it could possibly be an anticipator setting problem?... It would be so if your thermostat is not of the digital type but rather electromechanical.

What perhaps is happening here is that the thermostat is cycling the unit ON and OFF too often, on its way to satisfying a call for heat. Improper anticipator settings are known to do that, and the problem is more noticeable in cold weather.

Please post the thermostat's MFR and Model Number.

This unit may also be cycling on "high limit", which could happen if airflow has become somewhat restrictive (dirty filter, insufficient return - return grilles shut closed)

 

Like SP has pointed out, I am saying the fan limit is set too low, and/or you are drawing lot of cold air from some where.

Where are the return duct at? Is it sealed?

 

Raising the fan limit could end the cycling, but that would also send more heating dollars out the chimney before the blower starts.

 

Can you have any bad leaks in the cold air return??? That could suck in that colder outside air??? That for sure will make the blower control act like that. Also some unit like that have the Klixon fan control in them . You cant set them for on and off and they do go out a lot.

 

Fan Switch-Limit Switch are 2 seperate switches. Of course there Fan/Limit combination switches....not fan limit..
Sorry just a pet peeve

 

Hi, thanks for all the ideas. The cold air return comes from inside the house. The return is in the main TV room on the wall, goes down vertically to the crawl space then over 5 feet passes through foundation block to the outside unit. I'll crawl under the house this week and see if there are any leaks.

The thermostat on the wall is a digital one I got from HD, seems to work fine, I have the same for the upstaris system too... One of those $50.00 programable types.

Also, I did have an HVAC guy do the regular maintaince in the fall, mentioned this issue to him, he said that something had a crack in it. I can't recall what he said it was, but he said there are two of them, and in order to propoerly diagnose he has to catch it when it's doing the cycling on and off so he replaces the right one. And it's about $400.00

Does this info help?

 

A Crack?? Sure hope it's not the heat exchanger! Do you have a CO Detector in your home?

I would take a temp reading near the return grill in the living space, and then a reading just before it goes into the blower itself outside to see what the temp drop is between the two spots.