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Old 10-27-2015, 12:40 PM   #1
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Furnace Short Cycling Burn

Maybe someone here can give me some ideas of what may be happening and/or suggestions on how to troubleshoot the problem.

The problem I'm experiencing is a very short furnace burn cycle (in the order of 15 - 30 seconds) after the inside temperature reaches the thermostat setting.

We have a 2015 361REQS Eagle Premier 5vr with an Atwood Excalibur 8900 III Series 40,000 BTU Hydro Flame Gas Furnace (Model # 8940) and a Coleman/Mach Single Stage Heat/Cool Digital Wall Thermostat.
We were plugged into 50A shore power with full LP tanks.

The furnace brought the inside temp to setting (71 degrees) with no problem.
Here's what happened after the inside temp became stable at 71 Deg.

1. Thermostat dropped to 70 Deg.
2. Furnace blower came on for the normal duration prior to furnace igniting.
3. Furnace ignited.
4. Thermostat increased to 72 Deg. within seconds (~15 - 30 seconds)
5. Furnace flame / burner shut off.
6. Furnace blower continued for normal duration.

It then took a few minutes (e.g., 10 - 15min) for the thermostat to drop to 70 Deg. and the cycle repeated.

Outside temps were in the 40s.

I've already had the thermostat replaced once.
I've also put insulation inside the wall around where the thermostat is mounted to the wall.

Any ideas or thoughts on what may be happening here and/or the cause for the furnace's short burn cycle?

Thanks in advance for any suggestions and/or ideas.
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Old 10-27-2015, 01:21 PM   #2
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According to your description, I'd say the thermostat is at fault. If it believes the room temperature has dropped, it is calling for heat. The furnace is doing it's best to respond to the irrational input from the thermostat.


A common problem in houses is cold air in the wall behind the thermostat. It causes all kinds of overtemp/cycling issues.


My suggestion is to remove the thermostat from the wall and neatly cover the hole with duct tape. Then reinstall the thermostat. Also look for any potential drafts that could be confusing it.
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Old 10-27-2015, 01:41 PM   #3
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I have something of the same issue. It IS the digital thermostat that is the issue. Unfortunately, I don't think it's faulty!! I replaced it because the old thermostat was not digital and it was forever coming on and going off. The trailer would cycle between warm and too cold with the old thermostat. As soon as your blower kicks on, it moves air and changes a very sensitive thermostat. I'm going to try Mike837go's suggestion with the duct tape but I'm not too optimistic. My thermostat hangs on an interior wall which I would think wouldn't be very cold. The sensitivity of the thermostat is the issue. An older analogue thermostat will fix your current problem but you probably will not like the temperature swings that come with that thermostat.
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Old 10-27-2015, 01:52 PM   #4
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Plus the whole technique of heating a building using hot air is quite prone to wide/erratic cycling issues. Ask me about my last house with the oversized hot air furnace...


Like Ela1984 says, a mechanical thermostat will dampen the cycling in favor of very wide swings in temperature.


Try getting rid of any drafts near the thermostat. Especially, make sure none of the registers blow in the general direction of the thermostat.
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Old 10-28-2015, 10:48 AM   #5
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My Dometic digital thermostat has the ability to have a 1 degree or 2 degree temperature differential that can be made in the set up.
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Old 01-18-2016, 06:38 PM   #6
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Could be a flame sensor/thermocouple if it has one. If it is dirty that will cause a furnace to recycle. It would be about the diameter of a pencil and a couple inches long and in direct contact with the flame. Steel wool to clean and that should take care of it.
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Old 01-18-2016, 07:26 PM   #7
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Quote:
Originally Posted by mike837go View Post
Plus the whole technique of heating a building using hot air is quite prone to wide/erratic cycling issues. Ask me about my last house with the oversized hot air furnace...


Like Ela1984 says, a mechanical thermostat will dampen the cycling in favor of very wide swings in temperature.


Try getting rid of any drafts near the thermostat. Especially, make sure none of the registers blow in the general direction of the thermostat.
Good advice. I have seen so many thermostats with gaping holes behind them with air rushing by the thermostat. Foam works well.
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Old 01-18-2016, 08:18 PM   #8
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Thanks for the replies everyone.
As always, some good suggestions have been provided.

The trailer is put away in winter storage now. I plan to do some more troubleshooting in the spring.

We're hoping to take a trip to the Jayco factory this summer, if nothing is resolved before then, maybe I'll reach out to them for some help and advice.

I'll post any updates and if I ever find a resolution.
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Old 01-18-2016, 11:23 PM   #9
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look up "heat anticipation" for your particular furnace
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Old 01-19-2016, 06:08 AM   #10
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Quote:
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look up "heat anticipation" for your particular furnace
Hi CharlieD,

Thanks for the suggestion.

I have a digital Coleman Mach Thermostat, which I don't believe has an adjustable heat anticipatory.

I do think it's a problem with the thermostat or the location of it.
Because once the furnace burn starts, the thermostat does a 2 degree increase within seconds and shuts down the furnace.

In the spring I plan to setup a room thermometer next to the thermostat to see if the room thermometer's temperature reading is consistent with the thermostat.
I'm hoping that will help start to isolate the cause.

Thanks again for your reply.
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