Furnace Limit Cycling - Potential fix?

[Witch Doctor]

Originally Posted by rockfordroo

I think you're on the right track with improving air flow. Some folks have replaced all, or parts, of the corrugated ducting with smooth ducting as you find in a house. Anything you can do to improve air flow.

But just to be clear, in case you actually talk to a real furnace rep, the proper term is PLENUM, not pileum.

Thanks I was never good at spelling, by the way they use flex duct in just about all houses now, and commercial, but they do use 90 and 45 degree elbows. when you bend that duct 90 degrees it cuts the air flow down to the same as a 2 or 3" duct....

[Allistah]

Put it all back together after cleaning the blades on the blower. Cleaned both the blower blades and the blades for the combustion chamber.

Connected it again like in this picture and temps hung around 125F. I didn't have the front grate on and the side was open and the bedroom duct was being piped direct into the room as in this picture.


I. We'd to get a grate for the new hole and then some of the duct got damaged from all the jacking around with it so I need to get a piece with two clamps so I can put some new ducting on the end of the run at the furnace.

I'm not sure if the temp was so low because it was so cold in the room, the bedroom duct was bypassed, or because I cleaned the blades, or all of it.

Going to repair the end of the duct and install the front grate back on with the bedroom duct and see what it does.

The rear of the unit is open but it mounts flush up against the side wall of the RV so it's essentially closed. It has so much airspace around the top and sides this can't have any affect on it. The whole rear of the tin is very well vented.

Witch Doctor - what was your idea you had mentioned?




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[rockfordroo]

Originally Posted by Allistah

Put it all back together after cleaning the blades on the blower. Cleaned both the blower blades and the blades for the combustion chamber.

Connected it again like in this picture and temps hung around 125F. I didn't have the front grate on and the side was open and the bedroom duct was being piped direct into the room as in this picture.


I. We'd to get a grate for the new hole and then some of the duct got damaged from all the jacking around with it so I need to get a piece with two clamps so I can put some new ducting on the end of the run at the furnace.

I'm not sure if the temp was so low because it was so cold in the room, the bedroom duct was bypassed, or because I cleaned the blades, or all of it.

Going to repair the end of the duct and install the front grate back on with the bedroom duct and see what it does.

The rear of the unit is open but it mounts flush up against the side wall of the RV so it's essentially closed. It has so much airspace around the top and sides this can't have any affect on it. The whole rear of the tin is very well vented.

Witch Doctor - what was your idea you had mentioned?




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My suggestion is to do one change at a time and see how it goes. If you do it all at once and it doesn't work, it will be harder to figure out exactly which item caused the problem.

[Witch Doctor]

Originally Posted by Allistah

Put it all back together after cleaning the blades on the blower. Cleaned both the blower blades and the blades for the combustion chamber.

Connected it again like in this picture and temps hung around 125F. I didn't have the front grate on and the side was open and the bedroom duct was being piped direct into the room as in this picture.


I. We'd to get a grate for the new hole and then some of the duct got damaged from all the jacking around with it so I need to get a piece with two clamps so I can put some new ducting on the end of the run at the furnace.

I'm not sure if the temp was so low because it was so cold in the room, the bedroom duct was bypassed, or because I cleaned the blades, or all of it.

Going to repair the end of the duct and install the front grate back on with the bedroom duct and see what it does.

The rear of the unit is open but it mounts flush up against the side wall of the RV so it's essentially closed. It has so much airspace around the top and sides this can't have any affect on it. The whole rear of the tin is very well vented.

Witch Doctor - what was your idea you had mentioned?




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Well that is just great, the reason your temp was at 125 degrees only was because you now pulling R/A and and exhausting it so in other words you were moving more air over the exchanger. When you said them temps you were getting I knew it had to be airflow. Okay I'm not sure about how we can change the ducts don't know where the two from the other side heat.But if we can't change some stuff we can still make better even great. I would move the kitchen duct to the side by the vent. Go to lowes and buy at least 6 90 degree elbows, you will hook them to the cut out's then any sharp turn like going under the unit you would use two on each line you can cut that flex with a knife. Tape it at the joints and also get some zip ties and put them over the tape. You must have a rear or front Galley design. Also if you can cut the OSB board to make a sloping 90 if you can Like a big "S", no sharp turns it will reduce your air flow to half, that's why your going to buy the Galvanized 90's, By another register for the R/A that we will leave in so you will have 2 R/A's now, they come in all sizes even wood. You also want the biggest spacing of veins they have it will cut down on the air noise going through it. I would still take pictures you are well documented and send them to FR and to Suburban GM. Suburban has a man at the plant, his only job is to make sure these are installed correctly, per suburbans GM who I spoke with over the phone. keep your receipts you may get reimbursed. Then take pictures of the after to send with your letter to both. By the way you never told me were you lived in the Bay Area? I used to live in Martinez which is in the east bay, I raised my family there and that were either I worked out of and owned my Business there, maybe you heard of my company it was called BAY AIRE MECHANICAL. We worked all over the Bay Area. Design, Repair, installation and Maintenance. Well I think we are well on the way to having a great heating system anything else I can help with or you don't understand, either PM me our start a thread......:u s-flag-waving:

[Allistah]

I'm not sure there is enough room for a 90* bend of the hard stuff on the sides of this thing. You can see how there is barely 4" of space on either side of the furnace. It's exactly the same width as the surface it sits on. I'll take one of the duct mount pieces with me to Home Depot to see if I can figure something out.

Drives me nuts how all four of them take a direct 90* bend down right out of the furnace.









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[Allistah]

And btw, this little security screw pissed me off. Looks like they installed it so you couldn't get the blower blades out. I sat there with a pair of plyers and back it out enough so I could get the blades out easily. They also put some non-standard head on it so I could t remove it but it just slowed me down a bit.

[Witch Doctor]

You can cut the 90's down with tin snips it will fit, there about 3" on each end you can trim.

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[MNtraveler]

Sorry to be a latecomer and there are a lot of messages already. I did a thread search and don't see that anyone asked if you had checked all the ducts for an obstruction or clogging . . . such as a rodent nest. Don't ask how I know this. Just ignore if you already covered this. GL

[Allistah]

Don't think there are any obstructions but I'm going to double check.

I bought two 90* hard elbows. Will trim them up and see if I can get them to fit. Bought a grille for the new RA opening I cut as well. Will report back later today how this turns out.


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[Allistah]

Couldn't get the hard elbows installed. Not enough room. Will pick up on this tomorrow.

I pulled drawers out and the circuit breaker panel under the fridge to see some of the ducts. I was able to move one of them around a bit and was able to cut about 1.5 feet of excess duct out. The water pump was screened to the floor and the duct would around it so I unscrewed it from the floor and slid it over so the duct had more of a straight shot.

Another thing I noticed is that the fan blower goes faster with the generator on. I can hear it pick up speed once the extra power is there. I'm wondering if the batteries aren't what they used to be and is contributing to the problem.

I do not have a clamp amp meter. If the batteries aren't giving as much power as they used to, how do I test this? Just voltage or would current draw be less?


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[Allistah]

I'm thinking that it is still a duct problem. I think the blower moving a bit slower doesn't help but I think the batteries are at 12.8v if I remember right which is fine.


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[rockfordroo]

Is the 12.8 volts with the blower running? I suspect it's not. When the voltage gets too low (9.5VDC?), I believe the heater shuts itself down. So you may be fighting two issues. Maybe it's not shutting down on high temp but on low voltage.

[Allistah]

I'll measure it in the morning to see what the voltage is at the battery when the blower is running. I'll report back.


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[Witch Doctor]

Originally Posted by Allistah

I'll measure it in the morning to see what the voltage is at the battery when the blower is running. I'll report back.


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If your furnace starts your above the safety voltage. If your running it off a generator or plugged jn you will be running off the convertor anyway
Your batteries only come into play when you have no power hooked to the trailer.

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[northstar1960]

Originally Posted by Allistah

It's in the cabinet directly below the oven/stove.

I was thinking about taking it apart and cleaning the fins on the blower real well. There's barely anything on them but I'm grasping at straws at this point. There is a tiny bit of dust buildup on the blades. How sensitive are those types of blowers to being dirty and losing CFM?


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First off your trying to fix a piss poor design by suburban and bad install . those vents they take off the side of the furnace and the limit switch is right up against the back cover of the furnace . it's going to over heat . you will see if you leave that front cover off you'll get great air flow , much better then what you get at the registers . . one you can relocate the hi limit to a better location so it gets better air flow or re duct the furnace . there really would not be an issue if you left one or both of those 4" end covers off

[rockfordroo]

Originally Posted by northstar1960

First off your trying to fix a piss poor design by suburban and bad install . those vents they take off the side of the furnace and the limit switch is right up against the back cover of the furnace . it's going to over heat . you will see if you leave that front cover off you'll get great air flow , much better then what you get at the registers . . one you can relocate the hi limit to a better location so it gets better air flow or re duct the furnace . there really would not be an issue if you left one or both of those 4" end covers off

The problem with leaving a 4" end cover off is you'll short-circuit the furnace. The hot air will simply get pulled in as return air.

Of course, because one of the hot air registers is just "around the corner" from the Return Air (RA) grill, this is already a bit of a concern, but leaving the end cover off will definitely exacerbate the problem.

in my Mini Lite 2503S, my furnace is under my dinette bench. They put one hot air register and the RA grill on the outside wall of the dinette bench about 2 feet from each other. So the furnace simply pulls in the hot air from 2 feet away. They COULD have put the RA grill on the inside of the dinette bench just as easily and it would work much better. I'm expecting to rectify this situation by blocking off the existing RA grill and moving it to the inside of the dinette bench, but I haven't had a chance to do it yet.

My Roo 19 was similar, except the hot air register and the RA grill were both under the dinette bench, about a foot apart. You can see this in the pic below. This is even worse than my 2503S because the hot air was "trapped" under the dinette table and was simply pulled back into the RA grill. I know someone on the forum with a Roo 19 moved the hot air register to the end of the bench, so at least it discharged out into the living space better. I never got around to doing that.

[Allistah]

Did some more testing.

With the generator on providing full power, the limit switch did not trip. I removed excess ducting in a couple of places and did the best I could with the access I had available. I cleaned the blower blades well. I added another return air vent to make sure it's getting enough air. This was he temp reading from the various ducts:

Generator on with 13.5v available, limit switch never hit
(Degrees are all in F)
Intake 62
Living 188 (126 Rise - ~3' away)
Stairs 140 (78 Rise - ~14' away
Bath 135 (73 Rise - ~14' away but more turns)
Bedroom 146 (84 Rise - ~18' away but less turns)

With generator off:
13.3v before start (Generator just turned off)
13.0v after blower started
12.7v hit temp limit switch
Living area register was pushing out 198* air at the time.

This sucks. I just think that the furnace is not very forgiving, the install of the duct work is not the best, and I still think that 160* on the limit switch is really tight as a safety device limit temp.

Here is a pic of the water pump and how they routed the ducting around it.


Here is the return air grille installed with the original one on the front.






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[northstar1960]

Originally Posted by rockfordroo

The problem with leaving a 4" end cover off is you'll short-circuit the furnace. The hot air will simply get pulled in as return air.

Of course, because one of the hot air registers is just "around the corner" from the Return Air (RA) grill, this is already a bit of a concern, but leaving the end cover off will definitely exacerbate the problem.

in my Mini Lite 2503S, my furnace is under my dinette bench. They put one hot air register and the RA grill on the outside wall of the dinette bench about 2 feet from each other. So the furnace simply pulls in the hot air from 2 feet away. They COULD have put the RA grill on the inside of the dinette bench just as easily and it would work much better. I'm expecting to rectify this situation by blocking off the existing RA grill and moving it to the inside of the dinette bench, but I haven't had a chance to do it yet.

My Roo 19 was similar, except the hot air register and the RA grill were both under the dinette bench, about a foot apart. You can see this in the pic below. This is even worse than my 2503S because the hot air was "trapped" under the dinette table and was simply pulled back into the RA grill. I know someone on the forum with a Roo 19 moved the hot air register to the end of the bench, so at least it discharged out into the living space better. I never got around to doing that.

Think you mean short cycling not short circuit. my xlr 29 was setup bad hardly any hot air at the 4 ;piss poor vents . so i discontented the vents on the side of the furnace , blocked up the access holes in the furnace cabinet and now all the hot air is blown out the front vent that used to be cold air return . left the ducting in place and they now act as cold air return to the furnace . of course i sealed off between the furnace and the grill opening so no hot air is being sucked back in .
now short cycling the furnace will not hurt anything and most of the hot air will be pushed out into the room and not back in toward the furnace .
the way they builtthe furnace and the way it's installed leave the hi limit in a place where i gets very little air flow over it causing it to trip

[northstar1960]

Originally Posted by rockfordroo

The problem with leaving a 4" end cover off is you'll short-circuit the furnace. The hot air will simply get pulled in as return air.

Of course, because one of the hot air registers is just "around the corner" from the Return Air (RA) grill, this is already a bit of a concern, but leaving the end cover off will definitely exacerbate the problem.

in my Mini Lite 2503S, my furnace is under my dinette bench. They put one hot air register and the RA grill on the outside wall of the dinette bench about 2 feet from each other. So the furnace simply pulls in the hot air from 2 feet away. They COULD have put the RA grill on the inside of the dinette bench just as easily and it would work much better. I'm expecting to rectify this situation by blocking off the existing RA grill and moving it to the inside of the dinette bench, but I haven't had a chance to do it yet.

My Roo 19 was similar, except the hot air register and the RA grill were both under the dinette bench, about a foot apart. You can see this in the pic below. This is even worse than my 2503S because the hot air was "trapped" under the dinette table and was simply pulled back into the RA grill. I know someone on the forum with a Roo 19 moved the hot air register to the end of the bench, so at least it discharged out into the living space better. I never got around to doing that.

the set up you have would be easy to alter for better performance . i would again block off where the vents attach to the furnace . leaving them laying on the floor under the bench . take the end off the furnace and duct it straight out the the end of the bench . it would work twice as good and heat you up twice as fast

[rockfordroo]

Originally Posted by northstar1960

Think you mean short cycling not short circuit. my xlr 29 was setup bad hardly any hot air at the 4 ;piss poor vents . so i discontented the vents on the side of the furnace , blocked up the access holes in the furnace cabinet and now all the hot air is blown out the front vent that used to be cold air return . left the ducting in place and they now act as cold air return to the furnace . of course i sealed off between the furnace and the grill opening so no hot air is being sucked back in .
now short cycling the furnace will not hurt anything and most of the hot air will be pushed out into the room and not back in toward the furnace .
the way they builtthe furnace and the way it's installed leave the hi limit in a place where i gets very little air flow over it causing it to trip

Originally Posted by northstar1960

the set up you have would be easy to alter for better performance . i would again block off where the vents attach to the furnace . leaving them laying on the floor under the bench . take the end off the furnace and duct it straight out the the end of the bench . it would work twice as good and heat you up twice as fast

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.