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Furnace short cycles in early morning, then runs fine

  • Last Updated:
  • Apr 15th, 2019 8:17 pm
[OP]
Deal Addict
Apr 18, 2013
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Canada
So I got home after work, turned on the heat and it ran perfectly for almost 35 minutes. Very strange.
Deal Addict
Jan 28, 2007
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SW Ontario
newlyborn wrote:
Apr 8th, 2019 4:02 pm
BTW these furnaces are well known for the defective secondary heat exchanger. A bulletin was issued by Carrier long back.
We have almost the same furnace, and had to have ours replaced at only 5 years into its life. If your not sure if it's been changed, have it inspected and checked, definitely not something to put off.

And yes, these are a royal PITA to work on anything.
I'd rather be outdoors camping, kayaking, and mountain biking ...
Jr. Member
Nov 2, 2015
128 posts
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Oakville
Good idea. If you kept your old thermostat when replacing to the Nest try to hook it back up to see if the problem persists . If not, new Nest
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Aug 12, 2007
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gbill2004 wrote:
Apr 8th, 2019 8:12 am
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?
  • Can you check to see if you have an outside sensor? also is the thermostat set to manual heat / or auto heat /cool? ( Make sure the outside sensor is in a dry spot. maybe cover it with PVC tape when its dry . Setup thermostat mode to Heat only )
  • Can you verify what you mean by short cycle? is it like runs for 30 seconds or runs for 2 minutes? ( does the furnace fire up actually or does it just go thy a startup routine? a startup routine lasts around a minute and a fireup would last atleast a few minutes )
  • Do you have a Humidifier installed? is it setup to run the furnace fan or does it run when furnace is on and humidity is required. ( dont know what options Nest has for this.)
  • What is the actual temperature in the house early morning and what is the required temperature at the time?
  • Most likely its a failure of the actual furnace inducer motor. ( based on the age of the furnace. check the motor by spinning it and see how freely it spins and if it makes any sort of sound.) also if you can check for codes when it attempts to start up )






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[OP]
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Apr 18, 2013
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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
Is this it?
6986CE3E-C2CE-45E7-9496-4FF71B020E25.jpeg
[OP]
Deal Addict
Apr 18, 2013
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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.
You are just a fountain of knowledge! The short cycling occurred again tonight so I checked the codes as you suggested. According to the legend in the pic below it looks like a dirty flame sensor:
208B1C82-46DF-4BCB-9E3A-7121C4D74BEB.jpeg
Jr. Member
Nov 2, 2015
128 posts
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Oakville
gbill2004 wrote:
Apr 8th, 2019 8:18 pm
Is this it?
6986CE3E-C2CE-45E7-9496-4FF71B020E25.jpeg
That would be the culprit, but do note there are other possible issues according to the error code chart. The flame sensor is just the easiest to DIY
[OP]
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JivesHVAC wrote:
Apr 9th, 2019 8:33 am
That would be the culprit, but do note there are other possible issues according to the error code chart. The flame sensor is just the easiest to DIY
True, but the flame sensor seems to be the most likely based on the symptoms i.e. short cycling.

Not sure how I will remove that screw as it does not look like I can use a regular screw driver for that. And I think I will definitely need to remove that white tube to access it.

I'm hesitant to pay a local company to do the cleaning because I'm consider they will find other issues that may not really need to be done. I don't mind paying the $100, I just don't want to be scammed. My GF's brother, who is pretty mechanically proficient might be able to help me with cleaning the heat sensor...

And to the other poster mentioning the secondary heat exchanger being an issue on my furnace, I called Bryant this morning and they said my model number was not affected by the secondary heat exchanger bulletin. They did confirm that both heat exchangers have a 25 year parts warranty.
[OP]
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Canada
Oh and by the way, it did not short cycle this morning, but did short cycle a bit last night, but then got going normally after a couple tries. Seems to be just inconsistent and short cycle periodically.
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Dec 5, 2009
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Have you ruled out the thermostat yet? That should be step 1.
Jr. Member
Nov 2, 2015
128 posts
73 upvotes
Oakville
gbill2004 wrote:
Apr 9th, 2019 8:52 am
True, but the flame sensor seems to be the most likely based on the symptoms i.e. short cycling.

Not sure how I will remove that screw as it does not look like I can use a regular screw driver for that. And I think I will definitely need to remove that white tube to access it.

I'm hesitant to pay a local company to do the cleaning because I'm consider they will find other issues that may not really need to be done. I don't mind paying the $100, I just don't want to be scammed. My GF's brother, who is pretty mechanically proficient might be able to help me with cleaning the heat sensor...

And to the other poster mentioning the secondary heat exchanger being an issue on my furnace, I called Bryant this morning and they said my model number was not affected by the secondary heat exchanger bulletin. They did confirm that both heat exchangers have a 25 year parts warranty.
It's a red hex head screw driver you'll need it looks like. Prob can pick one up for like $3 if you don't have one. Yes remove the white tube if it's easy and doesn't vent to the outdoors. I don't think most companies are out to get a customer and shut their equipment down. That only happens if an actual problem exists and its possibly a matter of life and death.
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Dec 17, 2007
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Alliston, ON
Does your nest have a C-wire hooked up? It could be the nest trying to cycle the furnace to charge the internal battery. Throw an old thermostat on it for a night and see if you have the same problem. If you do, then dig into the flame sensor. If you don't, then pull in a new thermostat cable so you can use the 5th wire for the C terminal
[OP]
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fdl wrote:
Apr 9th, 2019 9:07 am
Have you ruled out the thermostat yet? That should be step 1.
Wouldn't the error codes I posted suggest that it is not the thermostat?
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gbill2004 wrote:
Apr 12th, 2019 9:06 am
Wouldn't the error codes I posted suggest that it is not the thermostat?
Im not sure but I would just rule that out to be sure. The reason it sounds like a thermostat issue is you had said sometimes it runs fine all day. But in early morning it shorts.
[OP]
Deal Addict
Apr 18, 2013
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Canada
fdl wrote:
Apr 12th, 2019 10:11 am
Im not sure but I would just rule that out to be sure. The reason it sounds like a thermostat issue is you had said sometimes it runs fine all day. But in early morning it shorts.
It's been short cycling in the evenings now as well. I think I'll try and take a stab at cleaning the flame sensor myself and if I can't, I'll hire a company I'm fairly comfortable with who does annual maintenance/tune-up for $100, and as part of that they include cleaning the flame sensor.

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