Manufactured home furnace short cycling

josephny

I have a CMP80 (LP) controlled by an Ecobee with Honeywell fan limit switch and I think it is short-cycling.

It is now 15 *F out and the tstat setpoint is 70.

It appears that both the burner and fan stay on for ~10 minutes, shut off for about 2 minutes, then come back up.

The Ecobee is set for minimum runtime of 20 minutes with a 1.5* swing.

I've watched the rotating disc of the limit switch, and it rotates. Not sure if the limit switch is cutting off the furnace.

This first graph (I like collecting data) shows the furnace's behavior when the Ecobee is set to 70*F.

The "boiler temperature" is just a sensor I have a few inches from the exhaust pipe -- it's used (at best) to show when the boiler is making heat. The "Furnace Running" line is based on when the Ecobee is calling for heat. So, the Ecobee is not continuously calling for heat.






This second graph shows the behavior after the Ecobee is set to 75* (at 6:05am).




Note the setpoint is never reached (or gotten within 2*) and yet the furnace goes off. I don't know if for some reason the Ecobee is telling it to shut down, or a control/safety-device on the furnace is shutting it down. It is never running for more than ~10 minutes.

Do I have a problem? Is it short-cycling? What should check, test, adjust to make the furnace run longer?

EBEBRATT-Ed

Probably tripping the high limit.

Are your air filters clean?

All return and supply ducts open and not blocked by boxes or furniture.

Try and take/record the temperature of the discharge air coming off the furnace. If it is a downflow this may be difficult to access, you could take a temp at a supply register.


Watch the high limit control and see what temp it seems to be tripping at, you might be able to here it click.

Maybe take the door off the blower compartment and check for obstructions.

HVACNUT

Whats the supply air temperature at the bonnet?
What's the return temperature? 
The furnace has a rating for heat rise listed on the sticker in the burner compartment. Is it within range?

josephny

WOW! I am a total idiot!

Simultaneously with this problem, I have been troubleshooting a regular WIFI disconnect/disassociation of the Ecobee.

I just put 2 and 2 together, checked the logs of the wifi again the on/off data of the furnace and sure enough they correspond.

Seems the transformer for the ecobee was wired into the burner power, and, for reasons I do not understand, every 10 minutes there was an interruption in the power, causing the ecobee to reboot.

I just rewired it (you can see it on the graph from 8-9am) and it's been running for an hour.

Thank you for the guidance.

HVACNUT

Is GMP80 a Goodman furnace? The wires weren't connected to R,W,C at the board?

SuperTech

HVACNUT said:

Is GMP80 a Goodman furnace? The wires weren't connected to R,W,C at the board?

That sounds like a Miller/Nordyne mobile home furnace to me. Although I don't remember seeing one with a fan and limit control. They usually have those 3 cheap bi- metal switches to do the job of a fan and limit control. 

mattmia2

My first suggestion was to jump r-w and see if it runs continuously.

HVACNUT said:

Whats the supply air temperature at the bonnet?

Do people still say "bonnet"? I've seen it in old literature and my uncle that is now in his 80's called it that but I thought it was replaced with "supply plenum" long ago.

SuperTech

mattmia2 said:

My first suggestion was to jump r-w and see if it runs continuously.

Whats the supply air temperature at the bonnet?

Do people still say "bonnet"? I've seen it in old literature and my uncle that is now in his 80's called it that but I thought it was replaced with "supply plenum" long ago.

People use the term all the time in the UK. They call the hood of a car the bonnet.

EBEBRATT-Ed

GMP80 is a Miller I think.

josephny

Yes, this is a miller.

Here is a picture of the limit switch (I hope I'm using the correct name for it):

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EBEBRATT-Ed

Watch the dial on the limit control when the burner is firing. In the picture there is a small notch or pointer over the 100 deg on the dial. The dial should move as the furnace heats and cools. See what temp it rises to with the burner running. The limit is set to turn the burner off if you get up to 180ish or so.

With the burner on you should be in the 120s maybe 130.

HVACNUT

Like @EBEBRATT-Ed said, the indicator for the dial is at 9 o'clock (the notch on the black part). See if the dial turns to the 200 spot while running. Do not turn the dial by hand. It shouldn't if everything is correct. 

To clarify, the burner and the fan both shut down at the same time, or just the burner? If the fan shuts off with the burner, that's an issue for sure.

pecmsg

Place the boiler temperature probe in the supply air plenum.

mattmia2

(I think the op solved this when they got the smart thermostat constant power).

josephny

Yes, I (OP) figured out that the power to the thermostat was cycling because I had it wired to the burner.  

I don’t know why that power was cycling but now that the transformer that powers the tstat is connected to a constant electric supply I believe the system is working well.