Home & Garden

Furnace short cycles in early morning, then runs fine

  • Last Updated:
  • Apr 15th, 2019 8:17 pm
[OP]
Deal Addict
Apr 18, 2013
2440 posts
950 upvotes
Canada

Furnace short cycles in early morning, then runs fine

I have a 16 year old Bryant furnace that has been very reliable. I've been in the house 3 years now. I've also had a Nest thermostat for about 3 years as well, which has mostly been reliable.

The last 2 days, the furnace has been short cycling early in the morning around 4:00am; turning on, then shutting off after a minute, and continues to do this; a minute on, then off, then re-starts for a minute, then off and the cycle continues. Then around 5:00am, it starts running fine and runs fine the rest of the day. For example, at 6:00am yesterday it ran for 25 minutes nice and smooth until it reached the required temperature (while short cycling around 3-4am). Same thing this morning at around 5:30am...ran for 30 minutes until it reached the required temperature (but again this morning was short cycling around 3:30-4am). The problem is early morning at 3:00-4:00am or so.

Any idea what's going on?

Update code 34 seems to suggest flame sensor:
16D4C828-5EBD-4F01-BB9A-4950D0A3820E.jpeg
Last edited by gbill2004 on Apr 12th, 2019 11:35 am, edited 1 time in total.
33 replies
Jr. Member
Nov 2, 2015
125 posts
69 upvotes
Oakville
I know it might be a pain to do this so early, but if you are able to be awake when the problem arises you'll be able to get the error code (flashing lights in the bottom compartment of the furnace). You can match the error code flashing to the legend to see what's going on. Example 3 flashes = failed ignition or 4 flashes = too many attempted startups. You'll find the error chart on the backside of the bottom cabinet door once removed. Off the bat I'd say dirty flame sensor, but could be other problems. Check what error code you're getting and go from there.
Deal Addict
Jul 3, 2017
3860 posts
2763 upvotes
My furnace does this sort of short-cycle sometimes first thing in the morning when the house gets very cold overnight (we use bedroom room heaters). I think it has a sensor that detects when the inrush of cold recycled air makes the heat exchanger too cold, and stops the fan for a moment until it heats up again. But the gas burner stays on, only the fan pauses.
[OP]
Deal Addict
Apr 18, 2013
2440 posts
950 upvotes
Canada
JivesHVAC wrote:
Apr 8th, 2019 8:58 am
I know it might be a pain to do this so early, but if you are able to be awake when the problem arises you'll be able to get the error code (flashing lights in the bottom compartment of the furnace). You can match the error code flashing to the legend to see what's going on. Example 3 flashes = failed ignition or 4 flashes = too many attempted startups. You'll find the error chart on the backside of the bottom cabinet door once removed. Off the bat I'd say dirty flame sensor, but could be other problems. Check what error code you're getting and go from there.
Thanks. I'm usually up around 3:00am so next time this happens I'll see if I can find the codes/flashing lights. If it was a dirty flame sensor, would that make sense that it causes short cycling first thing in the morning, then after an hour or so, it works fine?

I have not had the furnace serviced since I bought the house almost 3 years ago, so maybe I'll look into getting that done. I assume that cleaning the flame sensor is part of a regular inspection?
[OP]
Deal Addict
Apr 18, 2013
2440 posts
950 upvotes
Canada
Exp315 wrote:
Apr 8th, 2019 9:47 am
My furnace does this sort of short-cycle sometimes first thing in the morning when the house gets very cold overnight (we use bedroom room heaters). I think it has a sensor that detects when the inrush of cold recycled air makes the heat exchanger too cold, and stops the fan for a moment until it heats up again. But the gas burner stays on, only the fan pauses.
Interesting. Not very cold her in Winnipeg at night lately. The only change I have recently done is that I changed the air filter on April 1. Same filter a usual though...
Jr. Member
Nov 2, 2015
125 posts
69 upvotes
Oakville
gbill2004 wrote:
Apr 8th, 2019 11:36 am
Thanks. I'm usually up around 3:00am so next time this happens I'll see if I can find the codes/flashing lights. If it was a dirty flame sensor, would that make sense that it causes short cycling first thing in the morning, then after an hour or so, it works fine?

I have not had the furnace serviced since I bought the house almost 3 years ago, so maybe I'll look into getting that done. I assume that cleaning the flame sensor is part of a regular inspection?

The flame sensor can build up with soot or dirt from the flame burning during every heating cycle which in turn doesn't send a signal to the board saying everythings okay. The sensor should be cleaned once a year and it is included in an inspection, yes. Actually it's quite simple to do yourself as it's normally 1 screw and a clean and scrub. The time of day should have nothing to do with it though. Even with the flame sensor being dirty it can still eventually generate enough electricity to send the ok to the board which may be why it starts running again.

You'll see the burners come on, run for between 3-15 seconds and turn off, then the cycle will repeat itself to try again. It'll do this 3 times before locking your furnace out if it fails 3 times in a row.
[OP]
Deal Addict
Apr 18, 2013
2440 posts
950 upvotes
Canada
JivesHVAC wrote:
Apr 8th, 2019 11:49 am
The flame sensor can build up with soot or dirt from the flame burning during every heating cycle which in turn doesn't send a signal to the board saying everythings okay. The sensor should be cleaned once a year and it is included in an inspection, yes. Actually it's quite simple to do yourself as it's normally 1 screw and a clean and scrub. The time of day should have nothing to do with it though. Even with the flame sensor being dirty it can still eventually generate enough electricity to send the ok to the board which may be why it starts running again.

You'll see the burners come on, run for between 3-15 seconds and turn off, then the cycle will repeat itself to try again. It'll do this 3 times before locking your furnace out if it fails 3 times in a row.
Thanks again. By "locking" my furnace...do you mean forever, or just a period of time?

If I posted a picture of the inside of my furnace, would you be able o show me where the flame sensor is, so I can do it myself?
Jr. Member
Nov 2, 2015
125 posts
69 upvotes
Oakville
gbill2004 wrote:
Apr 8th, 2019 11:54 am
Thanks again. By "locking" my furnace...do you mean forever, or just a period of time?

If I posted a picture of the inside of my furnace, would you be able o show me where the flame sensor is, so I can do it myself?
You would be locked out until you reset the furnace (switch off, then on).. but it will just go through the cycle again

I can certainly try based on the picture!
[OP]
Deal Addict
Apr 18, 2013
2440 posts
950 upvotes
Canada
JivesHVAC wrote:
Apr 8th, 2019 12:20 pm
You would be locked out until you reset the furnace (switch off, then on).. but it will just go through the cycle again

I can certainly try based on the picture!
Here's the pic:
62D5250F-091C-42CE-945A-0129E4949F9D.jpeg
Jr. Member
Nov 2, 2015
125 posts
69 upvotes
Oakville
gbill2004 wrote:
Apr 8th, 2019 12:37 pm
Here's the pic:

62D5250F-091C-42CE-945A-0129E4949F9D.jpeg
Ah my arch nemesis .. this one is a pain in the ass to access.. depending on where that intake pipe goes

Ok so you have two white pipes going out the left side of the cabinet. The top pipe is the fresh air intake. Just behind that is the flame sensor, you can see a single lone wire on that side which connects the flame sensor to the board. You'll have to look behind that white pipe to see which type of screw is used to hold it in place. You can't mess up how it goes in or comes out as it's fixed. If the screw goes into where you found it then it's okay. Once the screw is removed it will slide downwards.

To make this easier (maybe), the top intake pipe.. does it go all the way to the outside like the exhaust? Or does it just exit the cabinet and 90 downwards? If it just exists and 90s downwards there is 1 screw holding it in place inside the cabinet. Remove that screw and pull the pipe out. This is safe to do as this is just the fresh air pipe and once done just put it back
[OP]
Deal Addict
Apr 18, 2013
2440 posts
950 upvotes
Canada
JivesHVAC wrote:
Apr 8th, 2019 12:49 pm
Ah my arch nemesis .. this one is a pain in the ass to access.. depending on where that intake pipe goes

Ok so you have two white pipes going out the left side of the cabinet. The top pipe is the fresh air intake. Just behind that is the flame sensor, you can see a single lone wire on that side which connects the flame sensor to the board. You'll have to look behind that white pipe to see which type of screw is used to hold it in place. You can't mess up how it goes in or comes out as it's fixed. If the screw goes into where you found it then it's okay. Once the screw is removed it will slide downwards.

To make this easier (maybe), the top intake pipe.. does it go all the way to the outside like the exhaust? Or does it just exit the cabinet and 90 downwards? If it just exists and 90s downwards there is 1 screw holding it in place inside the cabinet. Remove that screw and pull the pipe out. This is safe to do as this is just the fresh air pipe and once done just put it back
Thanks again. I'm somewhat handy, but I've never worked on a furnace before so I'll try your instructions when I get home from work tonight and will see if I'm confident in doing this myself. If no, I'll have to bite the bullet and pay a company $100 to do this for me...
Jr. Member
Nov 2, 2015
125 posts
69 upvotes
Oakville
gbill2004 wrote:
Apr 8th, 2019 1:07 pm
Thanks again. I'm somewhat handy, but I've never worked on a furnace before so I'll try your instructions when I get home from work tonight and will see if I'm confident in doing this myself. If no, I'll have to bite the bullet and pay a company $100 to do this for me...
I guarantee if you watch any youtube video about cleaning a flame sensor you'll understand. They're all the same just in different locations. But mind you this may not be your problem. The symptoms resemble that of a dirty flame sensor but it could be something totally different. Try it and see, check error codes if problem persists, and at that point you may need to pay someone $99 to have it looked at .
Deal Fanatic
Dec 5, 2009
5100 posts
2879 upvotes
I’d bet on it being a nest issue. Can you view the logs for the next?
Deal Addict
Jun 16, 2009
1682 posts
394 upvotes
Woodbridge
Hope you are well skilled to disconnect the intake box without breaking anything and reach flame sensor which is behind the intake vent. Certainly not an easy to do job.

BTW these furnaces are well known for the defective secondary heat exchanger. A bulletin was issued by Carrier long back.
gbill2004 wrote:
Apr 8th, 2019 12:37 pm
Here's the pic:

62D5250F-091C-42CE-945A-0129E4949F9D.jpeg
With greater power comes greater difficulty in factorizing the polynomial.
Deal Addict
Feb 4, 2015
4435 posts
1360 upvotes
Canada, Eh!!
As others have mentioned it is probably flame sensor... however if easy to do I would also suggest to try a simple "dumb" tstat to rule out smart tstat issue.

Can sometimes find at hw stores for less then $20

Top