Natural Gas Furnace Cycling on and off.

JackW

Our natural gas furnace is cycling on and off. I timed it this morning, if fired normally, ran for 5 minutes then the flame shut off but the fan motor continued to run. About a minute later the flame came back on, ran for 30 seconds then went off again. Stayed off for a minute or so, fired back up, and ran for four minutes, then back off. Any help will be greatly appreciated.

Jamie Hall

Furnace? As in hot air? Sounds like it is cycling off on a high limit -- so if it hasn't been doing that before, the first thing to do is to replace the filter, and with a good one (not a cheap one) of the sort specifically intended for the system. Then see what happens. If it keeps doing that, chances are something else is restricting the air flow.

JackW

@Jamie Hall thanks, that's a start.

Zman

What make and model furnace?
How big is the house?
What type of thermostat are you using?
Is the t-stat located on an interior wall close to the return air grill?

ewang

A good test to see if its the thermostat or furnace is to crank the thermostat up 10 degrees so it is for sure being told to run for an extended amount of time.

Assuming the thermostat is not satisfied, if the furnace shuts off, then there is an issue with the furnace.

Usual suspects:
-plugged air filter results in high limit trips = replace filter
-plugged condensate drain line - clean condensate line

JackW

@Zman , the furnace is a Bryant Plus 90, model #WD45102N14. It was installed in 1987, had the igniter module replace about 10-12 years ago. The thermostat is a Honeywell Home thermostat, nothing fancy. I've changed the air filters, thermostat batteries and made sure the flue is open.

I took 'ewangs' advice and turned the furnace up 10 degrees above room temperature, it ran for about eight minutes before the flame went out, the fan continued to run.

We don't use whole-house air conditioning, replaced it with Mitsubishi Splits, but the A frame is still under the furnace.

JackW

@ewang. Thanks for the advice. I turned the thermostat up 10 degrees above room temperature, it ran for about 8 minutes then the flame went out, the fan continued to run.
I'll check the condensate line, thanks.

Zman

Do you have way to measure the discharge air temp? It sounds like it is hitting the high limit.
Have you tried increasing the fan speed?

JackW

I just noticed that there is a green light that blinks on and off on the ignition module after the flames go out, the fan continues to run.

Zman

You should be able to look up the meaning of the flashing lights in the I&O manual.

JackW

@Zman, the ignition module was replaced about 10-12 years ago, but I'll take a look at the manual.

I set the thermostat to 68 degrees this morning and everything ran fine, then I bumped it up to 70 degrees, the flame went out after a short amount of time but the furnace fan never shut off. Could the fan control be bad?

JackW

So, I took the covers off my furnace, set the thermostat to 70, and watched what the Fan Control switch did. The fan control is set at 90-120. The furnace fired and the fan came on at the 120 setting. The dial slowly rotated up to the limit setting, the flame shut off, the dial rotated back to around 150, the flame came back on, and the process repeated itself. I let it repeat this sequence three times. The fan continued to run the whole time but did shut-off when it reached the low limit. What's causing the furnace to reach the high-end limit?

pecmsg

Lack of air flow. Could be several things
Plugged Filter
Dirty squirrel cage and or evaporator coil
Restricted ducts
Blocked vents

On and ON

Might want to get it serviced and and inspected before it gets colder!

mikeapolis

70% furnace from the 1980's?

JUGHNE

Does the 90 indicate a condensing furnace...with PVC exhaust?

And it has a old school fan limit rotating dial control?

mikeapolis

Whoops I missed where you said what year it is. That is a really old furnace. Condensing furnaces have come a long way since then. Perhaps it's time for a new system.

unclejohn

So it's a downflow furnace. The A frame should be easy to see and clean in a DF system. Either the coil or secondary Heat Exchanger dirty. Maybe.

JackW

@pecmsg. I've checked all those things and it's still doing it.
Thanks.

JackW

@mikeapolis Yes, it was installed in 1987.

JackW

@JUGHNE Yes, it has a PVC exhaust and old school fan limit rotating dial. That's what I'm watching as it turns it goes to the high limit, the burners shut off, it rotates back to about 150 degrees, and fires up again. Just keep repeating this sequence.

JackW

@unclejohn Yes, it's a downflow furnace. Not sure how to clean those parts, will have to look into that, thanks.

pecmsg

JackW said:

@pecmsg. I've checked all those things and it's still doing it. Thanks.

So you performed a Total External Static Pressure test?

You also preformed a combustion Analysis?

if no then ......You don’t Know! 

Time to call for service. 

Also consider a Low Level CO detector!!!

pecmsg

JackW said:

@JUGHNE Yes, it has a PVC exhaust and old school fan limit rotating dial. That's what I'm watching as it turns it goes to the high limit, the burners shut off, it rotates back to about 150 degrees, and fires up again. Just keep repeating this sequence.

How is this possible. 

Your high limit is 150*

The PVC high limit is 140*

JUGHNE

I believe the exhaust gases would be cooled in the condensing heat exchanger to get below the 140 PVC safe point.

I do now recall early 90+ furnaces with the fan/limit dial control, the controls changed pretty quickly after that time.

JUGHNE

Also for the HO, there is a very similar posting with your problem.
He had a bad fan motor capacitor causing the motor to run slow...probably overheating also.

pecmsg

JUGHNE said:

I believe the exhaust gases would be cooled in the condensing heat exchanger to get below the 140 PVC safe point. I do now recall early 90+ furnaces with the fan/limit dial control, the controls changed pretty quickly after that time.

So again how can the flue gas temperature be below the high limit? 

I don’t know!

JUGHNE

The 65-70 degree RA hits the cond HXC first getting a little warm up. The flue gases get more of a cool down as they condense.

The RA then passes thru the main HXC for serious heat up.
There is just not enough air passing to keep that temp down.

Maybe I have to "cipher" on this more.

JackW

@pecmsg The reason I come to this site is for the expert help, which is very graciously given and greatly appreciated, What's your purpose here?

icy78

Pecmsg is right. All heating work calls for combustion analysis. 

On a condensing furnace, the flue gas temp is usually around 105-125, due to the removal of latent heat.
   The limit may have cycled for years? Maybe you just happened to be there and heard it?
Basically it trips when the heat produced cannot be delivered fast enough.
It can also trip because of a heat exchanger fault. Usually its airflow related and the filter is the first suspect. Especially if you've just "upgraded" the filter due to covid concerns. 
Or a gradual problem such as the A coil collecting dirt. It really needs to come out anyhow. Why pay for the horsepower to push air thru that if it's not used?
Sometimes, if sheetrock work has been recently done, the filter will look clean  but be loaded with white powder.

pecmsg

JackW said:

@pecmsg The reason I come to this site is for the expert help, which is very graciously given and greatly appreciated, What's your purpose here?

You didn't answer my questions

Was the TESP Tested?
Was a combustion analyses performed?

JackW said:

@pecmsg. I've checked all those things and it's still doing it.
Thanks.

^This is not testing^

Unless you've had extensive training and invested a lot of money in tools you've gone as far as you can. Its time to leave it to trained, qualified professionals!

pecmsg

pecmsg said:

JackW said:

@pecmsg The reason I come to this site is for the expert help, which is very graciously given and greatly appreciated, What's your purpose here?

You didn't answer my questions

Was the TESP Tested?
Was a combustion analyses performed?

JackW said:

@pecmsg. I've checked all those things and it's still doing it.
Thanks.

^This is not testing^

Unless you've had extensive training and invested a lot of money in tools you've gone as far as you can. Its time to leave it to trained, qualified professionals!

Now if you don't want to accept this advice and continue to operate the furnace at the least, invest in a low level CO detector. Not the UL approved big box variety that alarm too late but a low level that alerts below 15 PPM

JackW

@pecmsg don't know what makes you think I don't want to accept the advice of all the great technicians on this site. Check out my post in the Radiant Heating section and you will find that I do.

unclejohn

I think since the furnace was installed in 1987 that it is something that is age related other then design related.

pecmsg

JackW said:

@pecmsg don't know what makes you think I don't want to accept the advice of all the great technicians on this site. Check out my post in the Radiant Heating section and you will find that I do.

So you WONT answer my questions?

JackW

@unclejohn plan on getting it through this winter then am going to have it replaced, thanks.

JackW

Want to thank everyone for the help and suggestions. It turned out to be a defective fan control.

hcpatel78

Great to see everyone helping here. It's really amazing help. Even reading these discussions add so much knowledge.

Thank you all