Furnace Limit Cycling - Potential fix?

[northstar1960]

Originally Posted by rockfordroo

I used short circuit because it relates to the flow path (circuit) of the air being inappropriate. Per Wiki, short cycling is "A short recurring period of time in which certain events (Air Conditioning) occur and reach completion or repeat themselves in a regular sequence." So the short circuit MAY result in short cycling, but not necessarily.

In any case, "short cycling" may not "hurt" anything, but if the furnace is pulling in hot air, then it heats that air hotter than it would "cold" air, thus the discharge temperature of the hot air goes up. This may, in fact, be what's causing the OPS's problem. It's short cycling (hitting the high temp limit) because of the short circuit in the flow path.

Your fix for the Roo 19 would cause the loss of hot air to the bathroom (the duct you see coming out of the left side of the furnace goes into the bathroom and comes out under the shower). Certainly do-able, but I for one (and more importantly, DW) would not want to lose that warm air in the bathroom. It's immaterial at this point anyway because I no longer have the Roo.

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In the HVAC world it's short cycle meaning the hot air temp is reached the furnace shuts down flame and quickly fires back up again because of cold air
anyway this is off subject . one little vent in the bathroom won't make or break the heat in the unit and is really a waste but to each their own . short circuit is an electrical term is not used or even similar to a furnace short cycling . again off subject . The op's problem is the hi limit is getting to hot because the air flow over the heat ex-changer is not even . the hi limit is stuck in a place where it really gets little air flow it's not in front of any of the vent outlets. it's really a piss poor design and the hi limit needs to be relocated or the unit needs to be re-ducted . taking off one of the 4" plates on the front will not cause over heating by pulling in all ready heated air . yes it will pull in some but not nearly enough to make it an issue. only been fixing design defects for 30yrs . sometimes the manufactures are aware of issues and sometimes not .
anyway OP good luck

[Allistah]

I've done everything I can do to fix this thing. The fact that it works with the generator running but hits the limit on batteries while the batteries are at 12.8v tells me that this thing is just barely on the cusp of hitting the limit still.

I think I'm going to increase the limit switch temp by 20 degrees and call it good. I removed the limit switch and put a jumper in it's place and measured what temp it would get up to and a 180F limit switch would fix this and be done with it. I can also tell that they chose a terrible location for the limit switch and it is transferring heat directly from the heat exchanger since thats what it is mounted to. Moving it off to the side by about 1" improved the time it took for the limit to be tripped.

It's unfortunate, but I've spent way too much time on this thing and I've done everything I can short of literally disassembling the trailer and reinstalling everything to make this work. I've spent probably 20 hours in this thing trying to fix it. The ducts are clear, increased the return air area, removed slats in the registers, removed excess ducting and tightened it up where I could so it wasn't bunched up, and cleaned the blades on the blower.

I just think they're ultra conservative on the limit switch so I'm going to up it a bit now that I know everything checks out.

I tell you, if we ever buy another trailer brand new, this is going to be the first thing that I check and they're fixing it.

[Allistah]

Originally Posted by northstar1960

The op's problem is the hi limit is getting to hot because the air flow over the heat ex-changer is not even . the hi limit is stuck in a place where it really gets little air flow it's not in front of any of the vent outlets. it's really a piss poor design and the hi limit needs to be relocated or the unit needs to be re-ducted

I moved the limit switch only 1" to the left so it was more out in the open and it really improved the time it took to hit the limit switch. Yeah, I just think for the location they chose for the limit, it's a very low value. I'm going to increase the limit by 20 degrees and move it over a bit like I said.

180 degrees inside the furnace as a safety limit seems well within reason. I tried calling Suburban again and all they do is say they won't talk to you and refer you to a service center. Sometimes I think they set these things up so tight on the tolerance knowing that any one thing steps out of line and it trips the limit and ultimately - a service call which generates dollars.

[Witch Doctor]

Originally Posted by Allistah

I moved the limit switch only 1" to the left so it was more out in the open and it really improved the time it took to hit the limit switch. Yeah, I just think for the location they chose for the limit, it's a very low value. I'm going to increase the limit by 20 degrees and move it over a bit like I said.

180 degrees inside the furnace as a safety limit seems well within reason. I tried calling Suburban again and all they do is say they won't talk to you and refer you to a service center. Sometimes I think they set these things up so tight on the tolerance knowing that any one thing steps out of line and it trips the limit and ultimately - a service call which generates dollars.

I sent you a PM. don't move it....You are risking your life. they are set and tested in that location. If your furnaces starts a fire god forbid you are liable. I don't care what anyone says, do not move it from that position.

[northstar1960]

Originally Posted by Allistah

I moved the limit switch only 1" to the left so it was more out in the open and it really improved the time it took to hit the limit switch. Yeah, I just think for the location they chose for the limit, it's a very low value. I'm going to increase the limit by 20 degrees and move it over a bit like I said.

180 degrees inside the furnace as a safety limit seems well within reason. I tried calling Suburban again and all they do is say they won't talk to you and refer you to a service center. Sometimes I think they set these things up so tight on the tolerance knowing that any one thing steps out of line and it trips the limit and ultimately - a service call which generates dollars.

SINCE You have done all the tests and know the temp differentials you'll be fine going with a slighter higher temp hi limit . . since the manufacture does not know how many of the duct ports will be used and the installer is a high school kid along with most of the designers no one is setting up/installing the furnaces properly . i'll bet they don't do static tests ,air flow ,delta T or any other for that matter . since it seems they are only using the side duct openings you don't get the air flow past the hi limit like you would if they incorporated the duct opening on the ends . but you know gotta shoehorn that thing in there . the only other way is to re duct the unit using the end ducts or move the furnace to another location where you can use the end ducts