Suburban (SF42) furnace igniting then clicking off & on every few minutes

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Dougie Brown

Dougie Brown

Well-known member
Joined
Nov 16, 2007
Posts
960
Location
UK
Last night, I left the furnace set up for a cool night. It cut in & out correctly during the night but at around 6:00am, I became aware that the fan had been running for quite a while, and continued to do so. At 7:00 am, I found the energy to get up and check the temperature only to find that it was blowing (very) cold air.

I removed the furnace today, disconnected every terminal and cleaned them with switch cleaner (it failed to fire up completely a couple of years ago and this cured it). Cleaned everything out, refitted it, and now it fires up fine, runs for maybe 3-4 minutes then the heater cuts off. Fan continues and does not turn off (as when venting after reaching temperature). Heater fires up again after around 2 minutes, and the cycle repeats indefinitely.  Everything works correctly from the inside thermostat; when it reaches temperature, it powers down, when it calls for heat, it powers up and starts.  It's only when running after calling for heat that it ignites intermittently like this.

Any ideas?

Dougie.
 
I've read Gary's excellent troubleshooting guide and from what I can see, the furnace may be overheating and the High Limit Cut-off is shutting off the burner.  That would leave the fan running to cool the furnace plenum, though it eventually allows the burner to come back on once a safe temperature is reached after several minutes.

The switch is at the back (I cleaned all terminals earlier including these) and can't be accessed until the furnace is removed, but then of course I can't test it.

The only thing different happening after removing and cleaning everything is that the burner will reignite by itself as described above. This morning prior to removing the furnace, the burner stayed off and the fan stayed on indefinitely.

:eek:

Dougie.
 
The high limit switch is a "normally closed" type, so you can put a jumper wire across the terminals to bypass it.  If the furnace stops cycling, you have found the reason.  However, you can't just leave the jumper on, because the furnace may in fact be overheating. If it is, bypassing the high limit could be fatal if left in place very long!

The limit switch itself is a simple thermostat, so very rugged and reliable. Switch failure is rare, but not inconceivable. In most cases, though, the furnace burner chamber really is overheating.
 
Thanks Gary. If the chamber is overheating, what could be the likely causes? I've cleaned out everything and there are no nests anywhere (I have cages on the inlet/outlet ports).  The main duct flap is working freely and all the vents are open in the coach.

I'll try jumping the switch later today in any case, but things are generally never that simple. :/

Dougie.
 
The most common cause of LP furnace overheating is blockage in the heater ducts. By that I mean the house interior circulation air, not the external exhaust & intake ports. A crushed duct, dust bunnies, etc.  I've seen furnaces that had such more ducting, e.g. loops and bend that reduce flow badly, that they are barely adequate when new, so it may not take much to cause inadequate flow.  A big furnace like your 42k btu model has to transfer a lot of heat into the ducts every minute to avoid overheating the burner chamber.
 
I got a replacement limit switch ($15) while we were out visiting so I?ll hopefully have time shortly to fit & try it before we move off tomorrow. *see below Figured it was easier to replace & try than jump the old one.

What?s your Recommended method of cleaning ducting?

Dougie.
 
Update

The new limit switch has made no difference. Burner starts and runs fine for 2-3 minutes, then cuts out, then cuts in again. Rinse and repeat.

My ducting in the coach is relatively in a straight line apart from one off to the side behind the passenger seat. I'll try rodding it through starting at the back & running forwards.

Is there any other thing I should be considering?

Dougie.
 
Try disconnecting the output duct(s) if there is space for the furnace to simply blow the hot air out.  If that stops the tripping, you know you have duct work to do. If not, look elsewhere.  Also make sure the circulation air inlet (return air) is not blocked by anything. Most RV firnaces simply suck the air in directly at the furnace, but a few have ducted return air as well.

Can you see the burner chamber well enough to make sure there is no hole in the outer casing?  If it has somehow burned through at some point, that can cause the casing to heat enough to trip the limit switch.

Do place a jumper wire across the limit switch to bypass it for a test run. Just to make sure that you are chasing the right problem. It would be a shame to spend a lot of time on duct work when the problem is a flaky circuit board!
 
I will jumper the switch - point taken.  No holes in the burner chamber as I inspected it thoroughly the three times it's been out this week.

Regarding output ducts, are these the two 4"(ish) holes on each side blanked with removable round plates? If so, they're not in use. All the output is out the back straight onto the large hinged flap which opens and closes freely. I did wonder what the two pairs of side outlets were for?

Dougie.
 
Most furnace casings have multiple outlet ports and the installer uncovers as many as needed to suit the particular installation needs.  The furnace has installation specs for a minimum amount of air intake and output via those ports, but RV makers sometimes ignore them, either by design or sloppy installation.
Air intake is usually direct into the furnace cabinet but output is typically through some sort of duct, even if it is more or less direct to a grill near the furnace.
The Suburban furnace service manual has a basic picture of the circulation flow abd volume requirements.
http://bryantrv.com/docs2/docs/subntnew.pdf
http://bryantrv.com/docs2/docs/suburbantech.pdf

 
Thanks Gary. I jumped the (new) limit switch and it did not cut out; reconnected, and it did. I then cleaned out the ducting with a long rod & duster, followed by a long vacuum extension which did all the ducting except one way off to one side. As we're without shore power for a few days, I tried the furnace without the generator and the blower was noticeably slower. The burner cut out as before. However, I ran the generator and trie it again, and the burner did NOT cut out for the 10 minutes I let it run. 30 seconds after I turned the generator off, it cut out.  Presumably this means the blower - even on shore power - isn't blowing adequately, either due to the blower (I did replace the sail switch a few days ago) or perhaps the batteries being suspect?  They were just on 12V without the genny and 14.2V reducing to 13.8V after some charging.

Dougie.
 
Could be weak batteries, but the furnace won't light if the fan speed is too low (that's what the sail switch is for). However, if your ducting & circulation air flow is marginal to begin with, a slightly slow fan may be enough to let it overheat. You've got a BIG  furnace (the 42k btu model), so that burner is really hot.
 
House batteries were totally flat this morning. Mrs Dougie?s cpap machine must have killed them overnight which never happened before so I?ll have them load-tested in the morning. Furnace ran perfectly again for 2 hours with the genny.

Dougie.
 
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