Grand Design furnace short cycling problem

[velran]

Is 11 WC static or when the furnace is operating? Still could be a restriction somewhere that allows full pressure when static, but reduces when drawing a large quantity of LP.

It's 11WC static for the most part. If I turn on the water heater (for example) it drops a bit and stabelized.
The problem apears to be that the furnace is getting too hot. Could that be due to low gas flow? I would think it is because over gas flow.. no?!

[orbiker]

Hey everyone,

AirExcel sent us a new furnace and even then it kept happening. That eliminated being an issue with the furnace. Grand Design sent another mobile RV repair company for a second opinion. After almost three weeks of back and forth where everyone thinks and admits this is an issue, the tech and Grand Design decided that this is normal. They are saying that the furnace rests a couple of times until it reaches the desired thermostat temperature. Rests meaning that the furnace runs for about four minutes and then the flame goes off for about a minute. The hi-limit switch makes contact again, the igniter makes a ticking sound and the flame comes back ON (blower is ON the entire time). These stops of flame ON flame OFF will continue until the thermostat reaches the desired temperature.


I have multiple techs telling me that a furnace should continually run until the thermostat cuts it off.

Does anyone else with a Suburban SF 25Q furnace experience the same thing? We have Imagine 23BHE so if you have this model or 21BHE I would greatly appriciate if you let us know if your RV does the same thing.

The flame can not stay on all the time. The system would melt down.

Even a gas clothes dryer heats to desired temp and cycles. It cools down to a lower temp and cycles back on.

The clue you told us is; (blower is ON the entire time)

Does the heat coming out of the vent in the RV continue to blow warm air, or does stop for too long and starts blowing cold air? You might want to use a digital temp probe to see the true temperature. KEN

[mcl1jw]

The flame can not stay on all the time. The system would melt down.

Even a gas clothes dryer heats to desired temp and cycles. It cools down to a lower temp and cycles back on.

The clue you told us is; (blower is ON the entire time)

Does the heat coming out of the vent in the RV continue to blow warm air, or does stop for too long and starts blowing cold air? You might want to use a digital temp probe to see the true temperature. KEN

Ken,
Your comment about the fan constantly running jogged my memory I had with my home furnace recently. The thermostat worked but a valve or switch turned out to be the problem. I'm not a heating person but here goes. The tstat says send heat. Switch says OK and sends heat. Temp gets to correct temp and switch stops sending heat but the fan doesn't stop and eventually it's sending cool air as the furnance no longer heats the air. Now the tstat is confused because the fan is still running so it doesn't tell the furnace to send hot air. There is a spring in the switch/valve that was failing. Live in northern NY and woke up to some pretty cool mornings.

[velran]

This thing is driving me crazy!

I recorded another video bench testing the OLD furnace completely outside the trailer with a separate gas line and a new battery (and bypassing the thermostat) and it worked for 42 minutes straight without the flame going off. I turned it off otherwise it would probably continue indefinitely.

Knowing that it's working as it should have while disconnected completely from the trailer, I went back to the new furnace (the one installed in the RV) and bypassed the electrical in my trailer by connecting the new battery and it failed: flame runs for about 3.5 minutes, off for 1 minute and turns on for another 3.5 and this cycle repeats (fan is on all the time unless system is turned off by the thermostat). I then connected the trailer electrical back and bypassed the propane AND it did the same thing! WHAT?!

For those of you who would think at this point that this is an airflow issue, I have over 400 sq.inch of return (minimum required by AirExcel is 56 sq.inch) and I have all six duct openings utilized so much better than the successful test I did with only two open (see video) AND the old and the new trailer failed the bench test while connected to the trailer electrical and propane.

It is a new RV we bought four months ago. We used it only once and my wife doesn't want to put the kids there until we know it's safe (after all we run this furnace 6 hour during cold nights) - I think she has a point knowing it could be a safety issue. One tech told us that we are abusing the hi-limit switch allowing it to run this way. The hi-limit switch is a safety mechanism not intended to be on and off 12 times an hour and over 70 times over night.

For the past three weeks AirExcel and Grand Design worked diligently to help us find the problem but they are now pretty much telling us that this is normal. I disagree.

Here is the video:
https://www.dropbox.com/s/57kcg20lko...rnace.mov?dl=0

[openrangeowners]

Excellent job bench testing .... watched your video. Makes me wonder if you have uncovered 2 defective furnaces with the old one now working as it should with your bench setup but not in the camper. Maybe you have two furnaces with defective control boards? The following may be of help ....
https://www.dinosaurelectronics.com/Fan_50_PIN.html
and for reference - you can find them much cheaper that Amazon ....
https://www.amazon.com/Dinosaur-Elec.../dp/B0031NNFLY

The furnace is cycling because of the following:
1) Sail switch problem (can test with DVM) - bench test - not likely broken on 2 units though
2) Limit switch problem (can test with DVM) - bench test - failure usually results in the system not lighting at all since it takes time for the limit switch to reset.
3) Faulty thermostat (you eliminated that issue already with bench testing)
4) Faulty control board (given you see the same problem in both, there likely was a bad batch of control boards that cause this condition). Since you already understand the operation of the furnace, replacing the control board (the Dinosaur Electronics ones are considered better than OEM) would be what I did next. If you can keep both furnaces (assuming you can) then you have plenty of backup parts in case something fails in the future.

Given the fact that you are getting intermittent failures with a successful test of the old one indicates probably a bad solder joint on the control board(s) - just moving them around might change things - had a similar problem with the 12V distribution board in a previous camper that turned out to be a bad solder joint on a board - I would loose all 12V power at times. Finally replaced the distribution board and all worked fine.

Personally I would rather repair it myself than have an RV dealer try to fix it. The $82 board from Amazon is where I would go next - would put it in the old one and reinstall and see if that fixes it. If not, then go back to Suburban and GD and make them send an RV tech to fix it.

[velran]

@openrangeowners, thank you for sharing these resources!

Grand Design sent a tech here which concluded that the furnace is running just fine and said that the flame goes off and on is just the furnace resting unit it's getting to the desired thermostat temperature. Every trailer owner I talk with told me that thier furnace runs continuously until the desired room temp is reached.

We went through 3 boards. The board on the original furnace, a replacement one (this is the first thing we replaced), and the third one on the new furnace. Do you think it's worth getting a four board? By the way, here is a drawing of my ductwork. I'm again leaning taoward airflow and questionig the initial two bench tests I did becasue the back cover of the furnace was open while running (here is an image of that back cover I'm referring to: https://www.dropbox.com/s/i1ypabsmwp...0open.jpg?dl=0)

Here is a (poor) drawing of my ductwork layout:
https://www.dropbox.com/s/105themuxb...awing.png?dl=0

[openrangeowners]

Did not realize you had already tried control boards - at this point sounds like you have eliminated most things. Can you connect a probe to the limit switch or control board to see if the limit switch is actually activating. If so, then you know what is ultimately triggering the short-cycling. At that point, do what you can to fix the duct work and flow, then replace the limit switch since it does tend to cycle easier/faster after each cycle. Also take note of where it is located and make sure there are no obstructions near it internally that might cause heat to build up in that area. Sometimes there are baffles that direct airflow over the limit switch to keep it from falsely tripping. If it is a limit switch, then there is likely a problem with airflow. I don't think combustion is an issue unless the gas pressure is adjusted too high, causing the furnace to overfire.

[velran]

Did not realize you had already tried control boards - at this point sounds like you have eliminated most things. Can you connect a probe to the limit switch or control board to see if the limit switch is actually activating. If so, then you know what is ultimately triggering the short-cycling. At that point, do what you can to fix the duct work and flow, then replace the limit switch since it does tend to cycle easier/faster after each cycle. Also take note of where it is located and make sure there are no obstructions near it internally that might cause heat to build up in that area. Sometimes there are baffles that direct airflow over the limit switch to keep it from falsely tripping. If it is a limit switch, then there is likely a problem with airflow. I don't think combustion is an issue unless the gas pressure is adjusted too high, causing the furnace to overfire.

Yes, I can confirm that the hi-limit switch opens the circuit and the gas flow is 11" WC measured at the firepit outlet.

[orbiker]

Reading in RV Net and the short cycling problem is talked about a lot. It's a great older forum for RV'ers.

One guy had this problem.
A blank, unused hole plug ( four inch round) for the outlet air distribution openings. This item was wedged directly in between the blower outlet and heat exchange where you've drawn the blue outline. Sloppy assembly. You wouldn't see it if not looking for it.

KEN

[Biery-Bullock]

Sorry to chime in so late, but I had a similar problem in our 21BHE, and solved it by changing the anticipator setting on the thermostat. My notes show it was originally set at 1.2, and I changed it to 0.7. I won't claim this is an optimal setting, but the furnace is cycling more like ours at home (staying on a little longer and coming on less frequently).