Another Furnace Question

[DTro]

My forced air furnace seems to be acting strangely. It cycles quite often. I just timed it and it was 8 minutes. After that, the gas shut off and the blower ran for about 1 minute and right as the blower was to turn off, the sequence started again.

Last year I replaced the flame sensor, but at that point it wasn't firing at all, this is a little different.

It seems to be keeping the house warm enough, but yet doesn't seem right to be cycling all the time like that.

The thermostat is in a good location (no draft) and is a programmable type that is about 9 years old. Also I tried a new filter too.

***Ok as I was typing this it cycled again (9 minutes total)

[Sprinter]

My first thought would be your High/Low limit switch may be bad. This would make the furnace think it is overheating even if it isnt. I believe that furnaces are wired to shut down if the max temp on the limit switch is exceeded. Then it cools down a bit and thermostat calls for heat immediately again because the house never actually heated up and the cycle starts again. Just a thought, NOT an expert by any stretch of the imagination. My other thought would be the Thermostat took a dive.

Can you turn your fan on constant and see if it still cycles the flame as often. This would tell you if the furnace is overheating because it will pull the heat out of the unit.

[DTro]

Thanks, I was kind of thinking the same thing. It's been on a constant 9 minute cycle since I first posted this.

It just now shut off. I'm guessing it has met the thermostats set point now.

[JBar]

I agree to me it sounds like the furnace is hitting it's high limit point. low air flow, could be furnace filter, pluged secondary heat exchanger, too many supply or return air vents closed down. 95% of the time this is the causes of short cycling.other considerations if it has always done this is incorrect btu sizing of the furnace usually too large, or incorrect sized duck work too small, possibly a defective limit switch or faulty heat anticipator in the thermostat which would be in the programing on a digital stat. hope it helps. good luck!

[BobT]

Quote:

other considerations if it has always done this is incorrect btu sizing of the furnace usually too large, or incorrect sized duck work too small, possibly a defective limit switch or faulty heat anticipator in the thermostat which would be in the programing on a digital stat. hope it helps. good luck!

I agree with checking for closed vents, dirty filters, etc. but this last statement shouldn't be valid on a system that's been in place and working fine for years. I could see it if this was the first year with this system but not now.

What about your fan speed? Does it seem normal or are you getting less air movement, assuming you've already checked the filters and vents, etc?

Some furnace blowers are belt drive and your belt could be getting worn down, and this will slow the blower rpm. I've also seen fans caked up with dust enough to restrict their ability to move air too. Some have an adjustable pulley for changing speed and it is possible the pulley's set screw came lose allowing it to open up and slow the fan rpm.

[Surface Tension]

As suggested, first thing I would do is change the filter if you haven't done that.

Next would be turn up the T-Stat. If your furnace continues to run then you can cross off anything furnace related and would then think T-Stat or heat anticipator.

[BobT]

Quote:

If your furnace burner continues to run then you can cross off anything furnace related and would then think T-Stat or heat anticipator.

[Surface Tension]

Bob, that should have been your first reply in this thread along with the filter.

It was mine though.

Instead you want to split hairs.

Very simple way to troubleshoot that everything on the furnace is working properly.

[BobT]

Not splitting hairs. The blower by itself isn't the best judge and I wanted to be sure it is understood not to judge by how much the blower is running vs the heater itself. In fact, in most forced air applications, it is desired that the blower continue running almost continuously once the outside air temperature drops below about freezing. One OEM manual I saw for a furnace recommended that optimum would be a continuous fan at outside temps below 40 degress F. Cycling the burner more often is better than not often enough as it will maintain temperature with less swing.

When we use PID control for heating systems on our equipment we try to achieve a point where the heaters only run enough to maintain temperature. We use electronic temperature controllers to achieve this optimum. For example, if it takes X continuous BTU to maintain a set point temperature, that's what we want to obtain from the heating system, nothing more and nothing less. Efficiency drops the more the heaters have to cycle due to wide hysteresis.

[Surface Tension]

Just ribbin ya Bob.

But yes your right depending on Darren's furnace the blower could run continuously.

Darren, when you do that test keep and eye on the burners. If the furnace goes past that 8 minute threshold and raises the temp then you lucked out.

8 minutes at these outdoor temps is NOT a short time.

My house is 2,300 sq f, with a 60k BTU forced air furnace. Right (15 degrees outside no wind)now my furnace will run 5 minutes and shut off. 1 degree swing in temps will feel to the body like cool and hot.

[Tom7227]

"hysteresis"

That's a new one. I didn't know a furnace could be that funny.

[DTro]

Well what clued me into something "different" is that there was no time inbetween cycles. On for 8 minutes, gas shuts off, fan continues to run for 1 minutes and re cycles gas on. Hadn't done that in the past.

Normally every year I close a few vents upstairs, so I opened them all up and it seems to be running fine now. Thanks

[pikechaser]

definitely sounds like an airflow prob..opening vents helped it out...May not be a bad idea to to check the filter, make sure the blower wheel is clean..a-coil..secondary heat exchanger if your furnace has one..if its been awhile, may be a good time to get that stuff cleaned.

[Eric Wettschreck]

Kinda tough for me to troubleshoot this over the internet without actually tossing a real eyeball on your system. However, it really sounds like you have either a return air problem or a T-stat problem. I lean toward the return air thing. Stupid question, is your fan direct driven or is the squirrel cage driven from the motor via a belt???? If so, is the belt tight and not slipping. You'd be amazed at the amount of air flow increase/decrease with a loose belt.

Knowing you had AC issues this summer.....don't know if they were ever resolved......I'm really thinking you have an air flow thing going on. Does the air flow coming from the registers feel like it should??? Or does it feel low? Possibly a plugged A-coil also.

Like I said dude, tough to pin this one down without looking at it but it sounds like an air flow thing.

[DTro]

Colder outside today....cycling every 10 minutes again

I guess I need to dig into this further.

[Surface Tension]

Run for 10 minutes, burner goes out, blower goes out, induction blower goes out and then starts right back up again?

[DTro]

Yes, that is what happens.

Hmmm....seems fine now. Seems like it does it more when I get home from work.

I have it programmed for a higher temp right when I get home....as I keep it pretty cool during the day when nobody is home. I wonder if that has something to do with it.

[delcecchi]

Yes, that is what happens.

Hmmm....seems fine now. Seems like it does it more when I get home from work.

I have it programmed for a higher temp right when I get home....as I keep it pretty cool during the day when nobody is home. I wonder if that has something to do with it.

Here is experiment. When it is acting normal, over ride and kick the thermostat up a few degrees, comparable to your setback. See if it happens.

I think maybe the furnace/thermostat combo might be designed to run for a maximum time then cut off. If the thermostat is still calling for heat it then turns back on again for another interval of time. That's what my old boiler used to do.

[Sprinter]

Cross your thermostat connection with a jumper wire (usually the R and G terminals on your furnace) and leave them crossed for 15 minutes. If at any time during that 15 minutes it turns off then you know that the furnace is probably overheating. If it stays on the whole time then the problem is in the thermostat.

[spearchucker]

Or at least it thinks it is overheating from a faulty limit switch. You would then need to determine if it is really overheating from lack of air flow.

[baitshopboy]

for a short cycle like that it could be numerous reasons. sounds like a a tstat problem and its just satisfying to fast. or a limit switch is bad.

[Chad711]

I realize that everyone want s to save few bucks, but please call a pro. There are a plethora ( havent used that in awhile) of ACTUAL reason a system can do what you are saying it is doing.

Water backing up in condensate, dropping out pressure switch, plugged intake/exhaust, anticipator setting, miswired tstat, air flow issues, gas pressure settings, flame sensor problems, circuit board failure, debris in inducer assy, loose wires, flux capacitor issues.

Find a reputable hvac guy and have you system thoroughly inspected, cleaned, etc.

[delcecchi]

I realize that everyone want s to save few bucks, but please call a pro. There are a plethora ( havent used that in awhile) of ACTUAL reason a system can do what you are saying it is doing.

Water backing up in condensate, dropping out pressure switch, plugged intake/exhaust, anticipator setting, miswired tstat, air flow issues, gas pressure settings, flame sensor problems, circuit board failure, debris in inducer assy, loose wires, flux capacitor issues.

Find a reputable hvac guy and have you system thoroughly inspected, cleaned, etc.

Flux capacitor? I thought those were only installed in Deloreans