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Old 09-20-2021, 08:54 PM   #1
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Suburban Furnance 42Q - oversized?

Hello,
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.
Few questions:
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.
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Old 09-20-2021, 09:16 PM   #2
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Suburban SF-42Q is rated at 40,000 btu and 430 CFM


Short cycling....operating on high temp limit switch
*Faulty limit switch (195*F for your model)
*high LP System Pressure (11"WC/0.4psi with 50% of appliances in service)
*Obstructed discharge (ducts/floor registers)
*Obstructed Return air flow (items stored in return path block return air)
*Obstructed Exhaust port (nests/debris)
*Dirty/sooted heat exchanger internal



Limit switch measures Air temp/not heat exchanger metal temp
*IR temp gun can not measure air temp

FOUR 4' ducts are Minimum requirement
Flexible ducts should run directly to registers and stretched tight....no excessive ducting

Airflow out of heat exchanger should be lower then the Limit Switch set point
Limit switch is doing as designed....shutting down to overheating
*Burner damaged/high LP pressure/blocked airflow etc
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Old 09-20-2021, 10:21 PM   #3
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Quote:
Originally Posted by Old-Biscuit View Post
Suburban SF-42Q is rated at 40,000 btu and 430 CFM


Short cycling....operating on high temp limit switch
*Faulty limit switch (195*F for your model)
*high LP System Pressure (11"WC/0.4psi with 50% of appliances in service)
*Obstructed discharge (ducts/floor registers)
*Obstructed Return air flow (items stored in return path block return air)
*Obstructed Exhaust port (nests/debris)
*Dirty/sooted heat exchanger internal



Limit switch measures Air temp/not heat exchanger metal temp
*IR temp gun can not measure air temp

FOUR 4' ducts are Minimum requirement
Flexible ducts should run directly to registers and stretched tight....no excessive ducting

Airflow out of heat exchanger should be lower then the Limit Switch set point
Limit switch is doing as designed....shutting down to overheating
*Burner damaged/high LP pressure/blocked airflow etc
Thank you Old-biscuit! I was hoping you would comment on my post
Out of all the items you suggested is there one or two that you have seen most often, and would start with first? I measured the temp off the metal of the limit switch but must not be very accurate if the switch activated at 195F and my thermo says 300, I also just measure the temp of the duct immediately exiting the collar and its over 200F so yes the limit switch is functioning as designed. I live in so. calif not many bugs here I don't see any obvious obstructions but will run inspection camera into the outside intake/exhaust ports and see if any obstructions.
Also I'll try running it again early in the AM when it's cooler here, it was 85F this afternoon when I was testing. With an extra duct and typical ambient temp maybe will remain below the high temp limit.
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Old 09-20-2021, 10:34 PM   #4
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One more question... could a handheld Anemometer be used to measure and calculate air flow out of the ducts? There seems to be good air flow I don't suspect any kinks but this is just my observation I don't have a reference to compare it to.
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Old 09-20-2021, 11:58 PM   #5
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Obstructed airflow...from/to furnace
LP Pressure too high causing overfiring

Those would be by top 2 items
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Old 09-25-2021, 09:25 PM   #6
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Quote:
Originally Posted by bobca1 View Post
Hello,
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.
Few questions:
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.
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Old 09-26-2021, 08:01 AM   #7
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Your subject line mentions the possibility that your furnace may be oversized.
In my haste to finish telling you what I had done I left out an option that might be better for you. You could install a SF35Q furnace. Your four ducts if they flow well enough might be enough to keep the SF35Q from cycling. Four 4" unobstructed ducts have a an area of 50 sq in. A fifth duct might still be required.

I think the external dimensions of the SF35Q is the same as the SF42Q and should mount in the same position as your current furnace.
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Old 09-26-2021, 08:15 AM   #8
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First thing to do is see what is shutting down the furnace. Is it the high limit switch or even the thermostat. Had a neighbor that thought his furnace was on the fritz and shutting down. Turns out he had a furnace outlet blowing directly on the thermostat. It was simple enough to direct the vent away from the thermostat.

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Old 09-27-2021, 09:23 PM   #9
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Modification appears to have successfully solved my short cycling issue. The furnace had 4 ducts attached to the furnace. I popped out one more blank at the top of the back panel added another duct into the bedroom. Fortunately there was room (barely) to fit another duct from the furnace through the basement, and up under the bed. Tested twice today ran furnace for 10+ minutes each time with no cycling. Previously it cycled about every 3-4 min. Measuring temp at ducts where they connect to furnace now are 180F vs the over 200F before. Real test will be in couple months when I head to mountains and it's below freezing, right now it's a "chilly" 65F outside and working well. Just need to cut a hole for the 2nd register now in the paneling under the bed. Bedroom will be warmer now Click image for larger version

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Old 09-27-2021, 09:56 PM   #10
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Quote:
Originally Posted by yeloduster View Post
Your subject line mentions the possibility that your furnace may be oversized.

In my haste to finish telling you what I had done I left out an option that might be better for you. You could install a SF35Q furnace. Your four ducts if they flow well enough might be enough to keep the SF35Q from cycling. Four 4" unobstructed ducts have a an area of 50 sq in. A fifth duct might still be required.



I think the external dimensions of the SF35Q is the same as the SF42Q and should mount in the same position as your current furnace.
Awesome info thank you for researching and the calculations! Indeed my test after simply popping out the blank on the back cover was sending hot air out that circled back into the intake that must be what still triggered the limit. The good news, after adding one more 4" duct it's no longer cycling! The temp on the duct is still hot I measured 180 so it's really close to the limit. I'm hoping that when I run it when it's actually cold, that reduces the temps further. I was testing before when it was 80F in the MH, today I was able to run when it was 70F inside and 60F outside. Fortunately I was able to add one more duct, but I can't add any more the other side it completely blocked by the bathroom drawer, I would have to eliminate the drawer to add ducts on the "top" side. So fingers crossed that I will get by with 5. Makes me wonder though if winnebago messed up installed a 45Q when it should have been a 35. I'll have to check with some other Forza owners.
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Old 09-27-2021, 10:05 PM   #11
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First thing to do is see what is shutting down the furnace. Is it the high limit switch or even the thermostat. Had a neighbor that thought his furnace was on the fritz and shutting down. Turns out he had a furnace outlet blowing directly on the thermostat. It was simple enough to direct the vent away from the thermostat.

Ken
In my case, it is definitely high limit, I tested this by TEMPORARILY bypassing the high limit switch with a wire jumper. The hi limit is normally closed and opens when triggered. When I did that furnace ran continuously. Unfortunately not as simple as the thermostat it I wish it was!
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Old 09-28-2021, 08:12 AM   #12
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Awesome info thank you for researching and the calculations! Indeed my test after simply popping out the blank on the back cover was sending hot air out that circled back into the intake that must be what still triggered the limit. The good news, after adding one more 4" duct it's no longer cycling! The temp on the duct is still hot I measured 180 so it's really close to the limit. I'm hoping that when I run it when it's actually cold, that reduces the temps further. I was testing before when it was 80F in the MH, today I was able to run when it was 70F inside and 60F outside. Fortunately I was able to add one more duct, but I can't add any more the other side it completely blocked by the bathroom drawer, I would have to eliminate the drawer to add ducts on the "top" side. So fingers crossed that I will get by with 5. Makes me wonder though if Winnebago messed up installed a 45Q when it should have been a 35. I'll have to check with some other Forza owners.
It sounds like you have eliminated or greatly reduced the high cycle symptom. Good work! I looked at the brochure for your MH and it specifies a 40K BTU furnace. I think Winnebago in your case and Forest River in my case did not follow the installation instructions from Suburban. Possibly Suburban over spec'd the total area needed by a little bit to give the installer a little wiggle room and that is why your 5 ducts seem to be sufficient.

I am going to install another 4" duct today. It is gonna be a real PITA. Out of the furnace compartment, Under a sliding door, into the water heater compartment, up over the water heater to a wall register in the bedroom. I'll have to either cut the door shorter or flatten the duct hose. i'll make it work.

I have very little faith that this will solve the high cycling problem in my case but it should warm the bedroom adequately.
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