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Topic: Suburban SF-30 furnace limit switch cycles

Posted By: oldbeek on 01/20/12 10:03pm

My SF-30 hi limit switch cycles on and off. I am told by several tecks and Suburban that it NOT suppposed to cycle. With all the sophistocated test equipment that NASA can afford, I have tested my SF-30 heater. Fan rpm ok, duct area ok, inlet gas ok. temp rise ok, duct outlet pressure ok, voltage ok, inlet air area ok. limit sw on and off ok, tested that on 2 diffrent switches.
My question now is: Does your hi limit switch cycle on and off??? Set thermostat at 90 degree and listen at your furnace outlet to see if you can hear it cycle on and off. I also used a temp probe in the fire box outlet.
This could be an intresting survey. Will it cycle if you cover the bedroom heater duct to force more heat to the living room area?


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Posted By: N3YMY on 01/20/12 10:19pm

oldbeek wrote:

My question now is: Does your hi limit switch cycle on and off??? Set thermostat at 90 degree and listen at your furnace outlet to see if you can hear it cycle on and off. I also used a temp probe in the fire box outlet.


How old your furnace?

It may simply be the limit switch needs replaced.

These this are known for getting flaky as they get older...


Posted By: oldbeek on 01/20/12 11:31pm

Tried a new limit switch. Same problem. Took both switches to a test lab and tested them. They both opened at 170f and closed at 150f. Original limit switch had a product # crosss referenced the number and the Mfg spec were 170 open 150 close. These limit switches allow the fire box exhaust (on side of trailer) to get up to 405 degree. That is plenty hot. That equates to a duct outlet temp of 159f
I am asking readers just to try their furnace and see if it cycles. Also I think all of us have laid a pillow over the bedroom duct to push more heat to the living room. Does that cause the limit switch to cycle???


Posted By: trailrider on 01/21/12 12:01am

My old 2002 Komfort with a SF35 would cycle on the limit switch and my 2003 Komfort with a SF30 cycles on the limit switch. I'm in the process now of figuring out how to stop it. I think the main problem is collapsed floor ducting. The ducts are sized exactly what the Suburban instructions say is the minimum duct size. But when they sandwich the ductwork between the wood floor and the insulation it collapses the duct giving it a bowed effect that reduces the area below the minimum required. Haven't figured out a simple way to fix it yet without cutting into the underbelly.

* This post was edited 03/30/12 08:17am by trailrider *


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Posted By: oldbeek on 01/21/12 12:51am

Don't start tearing things apart until you see what it does with all ducts disconnected. Mine will cycle with all ducts and openings removed. That is why I am asking for others to test their heaters.


Posted By: enblethen on 01/21/12 07:42am

No, it should not cycle.
Have you cleaned and checked the combustion chamber. Could have build up that creates a hot spt.
Check electrical connections on the cut out. Excessive resistance can cause heat.


Bud
USAF Retired
Suzuki XL7 pushing Pace Arrow




Posted By: dougrainer on 01/21/12 07:53am

Suburban AND Atwood state that the limit should NOT cycle in normal operation, BUT, it has been MY experiance (32 years as an RV tech and Master certified) that they DO cycle. There are variables that may cause it to cycle.
1. WHAT is the outside temp and how warm is it inside the RV. The colder the outside temp, the less likely it will cycle the switch. That brings in that colder intake air to help cool the chamber.
2. As it gets warmer in the RV, that causes the case temp to rise as the return air is warmer.
3. The BEST test is to remove the firnace and just run on the bench and see if it cycles as it does in the RV. This bypasses the internal ducting to see if you have inadequate flow of air. MOST all cycle problems are cause by the inadequate flow of the ducted air.
4. No matter WHO makes the RV, the amount of ducts and HOW and WHERE they are run will cause static (case) pressure and that will cause the furnace chamber to heat up faster. OEM's rarely install the CORRECT amount of ducting for the BTU rating of the furnace. If you can add 1 or 2 extra duct lines to the RV and that will probably stop the cycle problem.
5. Engineers and ROCKET Scientist's are a real pain Doug


Posted By: Chris Bryant on 01/21/12 08:20am

I don't have a spec for the blower speed, but per your pm:
Quote:

Gas 11.6 in wc, voltage 12.4v fan speed 2000rpm, Temp rise 159f outlet 58 return=101tr, Outlet pressure .25in WC. Outlet duct area >50sq in, return >50 sq in, Jet verified 52, burner stainless steel ok, Air path in heater all open and clean, fan squirl cage removed and cleaned. Limit switch replaced. Old and new verified open at 170f close at 150f board is Dinasour fan 50. The heater will cycle on limit sw 6 times and turn off for 60 min and then restart. Everything is perfect but the limit sw cycles.


I would drop the LP pressure to a max of 11"- 11.6 is enough over to cause problems. I would also do what Bud said- make sure the chamber is clean and not cracked. If worse came to worst, I would install a smaller orifice, because constant cycling on the limit switch can crack the chamber.


-- Chris Bryant
My RV Service Blog
The RV.net Blog


Posted By: oldbeek on 01/21/12 01:33pm

The chamber is clean and ok. The 2 stage gas valve is supposed to regulate the 11.6 main line pressure to approx. 10.2 to 10.5. Will have to pull heater again and do that test on the bench. The gas valve pressure tap on my suburban water heater is accesable. Outlet pressure from the valve on the water heater is 10.2, jet restricted and 9.1 jet flowing. The regulators are set to match the precise BTU of the heater. Thanks for the lead.


Posted By: trailrider on 01/21/12 01:50pm

I just confirmed that my main plenum duct is significantly collapsed. If it wasn't collapsed it would measure 28 x 2 which is 56 square inches. This is the minimum required area for the duct according to the documentation furnished with the furnace. It is collapsed in the center to less than an inch. I stuck the camera in the plenum under the furnace and took these pictures. This plenum tees into the duct for the floor registers. You can see the opening for that duct in the corners.









I got this picture from Google that shows what it looks like in a typical trailer before they put the floor down.



And here is a video of the Keystone assembly line. At about 1:00 shows what happens to the ductwork when the workers "carefully" lay the floor down.

http://youtu.be/SZ1UYHgk7m0

* This post was edited 01/21/12 02:34pm by trailrider *


Posted By: Learjet on 01/21/12 02:30pm

oldbeek wrote:

I am asking readers just to try their furnace and see if it cycles. Also I think all of us have laid a pillow over the bedroom duct to push more heat to the living room. Does that cause the limit switch to cycle???


The only time my heater cycled was when the cover came loose and it was re-circulating hot air. Putting a pillow over a vent could be causing your problem by restricting airflow and allowing the temperature to rise to high. Even NASA knows this

* This post was edited 01/22/12 09:03am by Learjet *


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Posted By: doughere on 01/21/12 04:26pm

A limit switch is a SAFETY device; it is designed to prevent damage in abnormal circumstaces. It should NOT be operating under normal conditions. If it is, something in the system operation or installation is not correct.

Regards,
Doug


Posted By: durangomike on 03/30/12 10:24am

SF-30F
What is the difference between a SF-30F and SF-25F burner?

What is the difference between a SF-30F and SF-25F orifice?


Posted By: Chris Bryant on 03/30/12 11:08am

durangomike wrote:

SF-30F
What is the difference between a SF-30F and SF-25F burner?

What is the difference between a SF-30F and SF-25F orifice?


My breakdowns show a different orifice as being the only difference- burner, blower motor and wheels are the same.


Posted By: durangomike on 03/30/12 11:13am

http://www.socaldist.net/files/44721510.pdf


This show two different parts numbers...


Posted By: Chris Bryant on 03/30/12 11:40am

That's for units built between 2001 and 2004, with serial numbers between 012106952 and 041206276. Units built before or after that are different.


Posted By: smkettner on 03/30/12 11:41am

I had same issue as trailrider. I propped it open by inserting two pices of aluminum stock.

Also the furnace duct that connects to the main fore/aft ducting protrudes 3/4 across within the main duct restricting airflow. I have not yet worked out a solution. I need to get a cutter or drill down 3' of ducting to open the restriction.

Much better after the first primary fix. Discovered the issue by removing the furnace and inspecting the ducting with a mirror.


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