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Old 01-08-2017, 03:40 PM   #1
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Furnace Limit Cycling - Potential fix?

Hey all --

We have a Suburban SF-30FQ furnace in our 36' 5er Shockwave trailer. The furnace started cycling off the limit about 3-5 minutes after starting it. Then after a couple of times, it would go into lockout mode. Installing a jumper in place of the limit switch fixes the problem. So I bought a new limit switch for this furnace and it happens to be a L160-40 switch. Today I spent about 4 hours in the trailer making sure that the heat exchange was clean, ducts are clean, etc. I blew off the outside of the heat exchanger with a blower and it was already clean as well.

I have pulled out the unit and there was barely any dust on the blower blades. The unit seemed pretty clean overall. I also put a vacuum hose on the exhaust and it pulled out about a teaspoon of silt/dust/particles from inside the heat exchanger so thats pretty clean and clear.

So I drilled a tiny hole in the front cover right next to the limit switch. I wanted to see with the jumper in there how hot was it getting after about 20-25 minutes of running if the limit switch didn't trip. It seemed to top out around 163 or so.

I feel like 160F is pretty low for a limit safety switch value for a furnace. I also feel like they put the limit value so close to the normal operating temperature that any change in temp upwards would cause the limit switch to be hit and then it would go into the limit cycling issue that so many of us are having.

Here is another thing about this limit switch. It's mounted on a metal bracket that is attached directly to the heat exchanger. The limit switch is to trip if the air temp gets above the limit switch value right? It seems like heat from the hot heat exchanger would cause the limit switch to prematurely trip as the heat would directly transfer from the heat exchanger, into the bracket, into the switch. With the value at only 160 degrees, this would be pretty easy to reach.

I put two tiny metal washers under each screw on the limit switch so that the whole thing wasn't touching the metal bracket that was attached to the heat exchanger. Now the switch is really only evaluating the air temp and not being as influenced by direct transfer of heat through the bracket.

I drilled a tiny hole in the front cover right next to the limit switch. I wanted to see with the jumper in there how hot was it getting after about 20-25 minutes of running if the limit switch was bypassed. What is "normal" for this furnace? It seemed to top out around 163 or so.

I feel like 160F is pretty low for a limit safety switch value for a furnace. I also feel like they put the limit value so close to the normal operating temperature when the unit is brand new and sparkling clean that any change in temp upwards after a year or two would cause the limit switch to be hit pretty easy and then it would go into the limit cycling issue that so many of us are having.

Here is what I don't know.. When we bought the trailer brand new, I have no idea if it was cycling off the limit switch or not. I just payed any attention to it.

Now that I know that I have cleaned out the heat exchanger, made sure the ducts are clear, the outside of the heat exchanger is clean, the unit overall is pretty clean, I think that if I installed a L170-40F switch, it would still be safe and not cause the cycling off the limit switch issue which is bad for the heat exchanger.

Sorry for the long post, I spent a lot of time observing this unit to learn it and see exactly how it was operating so that I can make a good decision on a fix. I almost feel like the company is either being overly protective with the limit being very close to the normal operating temp or they put it so close so that any amount of age would generate a lot of service calls. I'd like to think the former, but you never know.

At any rate, I think I'm going to install a L170-40F and see how it goes.

Any input would be appreciated.

Thanks!
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Old 01-08-2017, 04:26 PM   #2
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Quote:
Originally Posted by Allistah View Post
Hey all --



We have a Suburban SF-30FQ furnace in our 36' 5er Shockwave trailer. The furnace started cycling off the limit about 3-5 minutes after starting it. Then after a couple of times, it would go into lockout mode. Installing a jumper in place of the limit switch fixes the problem. So I bought a new limit switch for this furnace and it happens to be a L160-40 switch. Today I spent about 4 hours in the trailer making sure that the heat exchange was clean, ducts are clean, etc. I blew off the outside of the heat exchanger with a blower and it was already clean as well.



I have pulled out the unit and there was barely any dust on the blower blades. The unit seemed pretty clean overall. I also put a vacuum hose on the exhaust and it pulled out about a teaspoon of silt/dust/particles from inside the heat exchanger so thats pretty clean and clear.



So I drilled a tiny hole in the front cover right next to the limit switch. I wanted to see with the jumper in there how hot was it getting after about 20-25 minutes of running if the limit switch didn't trip. It seemed to top out around 163 or so.



I feel like 160F is pretty low for a limit safety switch value for a furnace. I also feel like they put the limit value so close to the normal operating temperature that any change in temp upwards would cause the limit switch to be hit and then it would go into the limit cycling issue that so many of us are having.



Here is another thing about this limit switch. It's mounted on a metal bracket that is attached directly to the heat exchanger. The limit switch is to trip if the air temp gets above the limit switch value right? It seems like heat from the hot heat exchanger would cause the limit switch to prematurely trip as the heat would directly transfer from the heat exchanger, into the bracket, into the switch. With the value at only 160 degrees, this would be pretty easy to reach.



I put two tiny metal washers under each screw on the limit switch so that the whole thing wasn't touching the metal bracket that was attached to the heat exchanger. Now the switch is really only evaluating the air temp and not being as influenced by direct transfer of heat through the bracket.



I drilled a tiny hole in the front cover right next to the limit switch. I wanted to see with the jumper in there how hot was it getting after about 20-25 minutes of running if the limit switch was bypassed. What is "normal" for this furnace? It seemed to top out around 163 or so.



I feel like 160F is pretty low for a limit safety switch value for a furnace. I also feel like they put the limit value so close to the normal operating temperature when the unit is brand new and sparkling clean that any change in temp upwards after a year or two would cause the limit switch to be hit pretty easy and then it would go into the limit cycling issue that so many of us are having.



Here is what I don't know.. When we bought the trailer brand new, I have no idea if it was cycling off the limit switch or not. I just payed any attention to it.



Now that I know that I have cleaned out the heat exchanger, made sure the ducts are clear, the outside of the heat exchanger is clean, the unit overall is pretty clean, I think that if I installed a L170-40F switch, it would still be safe and not cause the cycling off the limit switch issue which is bad for the heat exchanger.



Sorry for the long post, I spent a lot of time observing this unit to learn it and see exactly how it was operating so that I can make a good decision on a fix. I almost feel like the company is either being overly protective with the limit being very close to the normal operating temp or they put it so close so that any amount of age would generate a lot of service calls. I'd like to think the former, but you never know.



At any rate, I think I'm going to install a L170-40F and see how it goes.



Any input would be appreciated.



Thanks!

Suggest be safe someone mandates the 160F. That may not be the max allowed but then again it may be. I'd start with the mfg. engineering section, next you can check NFPA and ANSI rules, some agency must rule on such an important safety device and if discovered modified could null fire insurance.
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Old 01-09-2017, 01:27 AM   #3
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I measured the intake opening space around the furnace and it is 69 sq inches of room. The label on the furnace says it needs 55 sq in. Was thinking of opening it up a bit more to see if it helps. Thoughts?
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Old 01-09-2017, 03:59 AM   #4
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Originally Posted by Allistah View Post
I measured the intake opening space around the furnace and it is 69 sq inches of room. The label on the furnace says it needs 55 sq in. Was thinking of opening it up a bit more to see if it helps. Thoughts?
I have done HVAC for a living, do not change the limit that the manufacturer installed, Have you measured the exact temp of the heat exchanger when it trips? there are a few factors that can make it trip the 1) and most important the air flow over the heat exchanger and the amount of runs coming off (Your Duct Work) I would bet that if you add a duct your problem will be over, to prove that just open 1 of the round take offs at the plenum itself. #2) if your blower is not putting out enough CFM that could cause the the 165 degree heat. That would worry me more then the limit.I haven't read the specs of your heater but there is what is called a temp. rise, the difference between the R/A vs supply air. 165 just sounds high to me. You could in reality cause damage by increasing your limit. If the limit that was tested and designed by Suburban was the problem we all would be having that problem. Do not close off any registers or block them off you need to exhaust that heated air. Hope this helps you a little before you start messing with your limit and do not put spacers in that is definitely a no no.... PS: I put two tiny metal washers under each screw on the limit switch so that the whole thing wasn't touching the metal bracket that was attached to the heat exchanger. Now the switch is really only evaluating the air temp and not being as influenced by direct transfer of heat through the bracket. VERY DANGEROUS YOUR CHANGING THE DESIGN>
s:
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Old 01-09-2017, 09:03 AM   #5
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Ok point taken on the limit. Here is what is interesting. The sticker on this unit says there should be a temperature rise of 100-130F. It also goes on to say that the air coming out of the registers should be no more than 200F. If the air is 70F, and it raises it by 100F, you're already over the limit switch at 170F and well under the recommendation for the output of 200F. How do you have a temp rise of 100-130 and still stay under 160F for the limit switch?
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Old 01-09-2017, 09:07 AM   #6
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Take a look at the label on the top of the unit for those numbers. Sort of confusing - maybe someone could explain how the 160F limit with up to a 130F temp rise is easily achievable.

Thanks for the replies, I appreciate. I have been thinking how I can add another register to just fix it but I already have four on this unit.
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Old 01-09-2017, 09:08 AM   #7
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Have you tried calling Suburban tech support??
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Old 01-09-2017, 09:10 AM   #8
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Going to be calling them today..
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Old 01-09-2017, 11:24 AM   #9
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Attachment 127757

Take a look at the label on the top of the unit for those numbers. Sort of confusing - maybe someone could explain how the 160F limit with up to a 130F temp rise is easily achievable.

Thanks for the replies, I appreciate. I have been thinking how I can add another register to just fix it but I already have four on this unit.
It depends where they mounted the limit more then likely it not reading you heat exchanger but the outside housing. Your just plain and simple not exhausting enough air. The temp rise will work with the static pressure. I doubt it is designed duct work is at that. It say's .20, your blower is capable of exhausting 350 cfm. if you have 4 -4" ducts coming off you are only producing in the neighborhood of 240 cfm, so the unit is heating up. You will never hit 200 degrees I'm telling you that. You would be lucky to get 60 or 70 temp rise measure the return air temp and the supply temp at the register.....Also call Suburban as KY DAN recommended....
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Old 01-09-2017, 11:26 AM   #10
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Thanks. I'll be calling them today and reporting back.
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