Originally Posted by bobca1
I own a 2017 Winn. Forza 36G that has a suburban SF42Q that is short cycling about every 2-3 min. I suspected high limit switch so I jumped the switch (bypassing it) and confirmed that indeed the switch is causing the short cycle. Replaced it with new one (twice) and no difference still cycling. Checked the ducts no kinks and flow seems good. There are 4 ducts connected to the furnace, 2 on bottom, 2 on the back. The back cover has room for one more duct so I popped out the blank installed a new collar and left it open with no vent connected and ran with cabinet doors open. See photos showing modification. Still short cycling but improved now it runs about 5 minutes before cycling. This open vent collar is right over the high limit switch I thought maybe that was the issue not having enough air flow over the switch. I measured with IR themo and the switch gets up to about 300F then opens. That seems quite hot to me.
Has anyone tested temp of air vent directly exiting the furnace is 250-300F too hot?
Any experience with 42Q how many 4" ducts are typical, is 4 (or 5) enough?
Is it possible to reduce the flame in the burner so it doesn't burn as hot?
Any other ideas? I rarely use the furnace do depend on it occasionally in the winter when staying in mountains, one trip last year was about 15F outside the short cycling eventually caused the furnace to give up and shut down until I reset the thermostat.
Attachment 343722Attachment 343723Attachment 343724
According to the Suburban installation manual the SF42Q needs a minimum of 88 sq. in. of UNOBSTRUCTED
ducting if a minimum of 5 ducts are used. A round 4" duct is 12.56 sq in. Your 4 ducts added together are 12.56 x 4 equal 50.24 sq in. So the answer to your question of whether 4 (or 5) ducts are enough, The answer is NO!
Your furnace has 7 4" round ducting outlets, using your nomenclature the furnace has 2 outlets on the top, 2 on the bottom and 3 on the back. If only the round outlets are used you would need to use all 7 to achieve 88 sq in (12.56 x 7 equal 87.92 sq in).
In addition to the round outlets there are rectangular outlets on the sides of your furnace, one of 56 sq in and another of 72 sq in. According to the pictures your installation is using neither of these rectangular outlets.
Your experiment of removing one of the knockouts did help a little but that act is in violation of another installation MUSTS. No outlet duct is allowed within 18" of the cold air return. Your experiment placed an outlet IN the cold air return!
To keep your furnace from high cycling I think you will have to add 3 more 4" ducts that are routed to outlet registers that are located at least 18" from the cold air return.
My motorhome's SF35Q furnace required 56 sq in of outlet. It used a 56 sq in rectangular outlet from the furnace that dumped into a plenum of equal size. The outlet of the plenum fed into an under floor ducting system that had floor registers. The problem in my case was that the outlet of the plenum was very small. Hardly any air blew out of the floor registers. Not only did my furnace high cycle it didn't warm the MH very well. I was able to route 2 4" ducts to wall registers in the living area of the motorhome. This helped a great deal and the living area of the motorhome is now well heated. However, the total ducting is still less that the minimum required according to the installation instructions and the furnace still high cycles.
There are multiple problems associated with high cycling. Two of the problems are the electro-mechanical high cycle switch is not designed for continuous cycling. As the high cycle switch ages due to the continuous cycling it occasionally hangs open long enough that the electronics of the furnace to time out and turn off the furnace leaving you cold in the middle of the night. In addition the ignition system is required to cycle more than its design.
There is no way other than redesigning the motorhome to improve the total ducting of my furnace enough to eliminated high cycling. I decided that I would help the limit switch. I purchased a digital temperature controller from Amazon for ~$15 and installed it so that its internal relay contact is in series with the limit switch. I set it so the digital temperature relay controls the furnace plenum about 10F below where the limit switch opens. By doing this I eliminate wear on the limit switch. The anticipated life of the digital temperature controller relay is thousands of cycles and will probably never wear out.
I hope the ignition system is robust enough to handle the multiple cycles!
My bedroom is still much cooler that the living area. When the bedroom temps in the low 60's, the living area is ~10F warmer. This wastes propane heating a part of the motorhome that is not used.
I have decided that it is possible to add a duct that will go to the bedroom. This will help the temperature differential but will probably not fix the high cycling. I will still be about one 4" duct to few to do that!
If you are interested I can supply information on how to add my modification.