Suburban SF 42 furnace help

[GBill]

My furnace does not shut down and continues to heat after reaching its set point. This happens after it has cycled approx. 8/10 times. In other words it is hot. I have shut it down at the thermostat but a couple time that approach did not work. Hmmmm Installed a new thermostat, it is a 5 button, but not go. I am about to pull the furnace but with scant knowledge of how it operates hate to have to shot gun the repair. Bad board or relay or something else? Anyone with knowledge or wanting to make a educated guess will be greatly appreciated. Thanks

[wingnut60]

Probably can't help you much, but am having problems with my same furnace.
After having 3 mobile techs out to do the same repair over the last 6 years:
Could be the safety switch;
Could be the sail switch;
Could be the circuit board;
Could be all three.
See if you can find help on YouTube.com---probably some videos there to help you.
If you manage to pull it out, you can't verify operation with out having it hooked up to 12volt and propane--this isn't a repair for the novice, I don't think. Also, carefully inspect the firebox metal for age-related holes/rust.
But, if you do get it fixed yourself, please post some notes back how you did it and what was wrong.
Best of luck and results...
Joe

[GBill]

Joe you are right.....this is a bugger. Looked at how to get it out today. Not too easy. Looks like it slides out the side on this model. But the gas line is solid all the way across the basement to the other side and then down to the black pipe under the belly. So if I can't disconnect the line at the furnace it will be quite difficult to remove. If I don't hear from anyone else looks like a relay and board and hope for a good result. Good idea to research YouTube. Throwing darts at an anomaly......lots of fun.

[billr]

I recently had an issue with our furnace in the mobile suite. It would short cycle. Run for mayby 7-10 min but no where near target temp. Furn shuts down About 2 min later fires back up and runs 2 min and shuts down. Continues this

I have Dometic two zone with remote sensors two heat pumps.

Tried a spare Dometic thermostat no fix.

Disconnected the two blue wires from the furnace at the furnace and connected them together. Furnace started right up. Ran until I disconnected them. This eliminates the temp control from thermostat some remote sensors. I now suspect the remote sensor.

As a temp fix I ran new wires from the blue wires to furnace upstairs and installed a simple non powered thermostat on the wall in hall to bath.

Works perfect. I may just leave this in place as perm fix. I like the heat control way better. The sensor by back window drove the heat too hot night in bed.

Your issue may be caused by the temp control in trailer. If you fire the furnace by connecting the wires and it stops when you disconnect them the furnace is fine.

Note the furnace goes through normal start up and shutdown with cool down using the manual control.

[GBill]

Billr thanks. I will try to get at the blue wires later today. As Joe suggested I checked YouTube and found on SF-42 video which did show the blue wires. If I can get the furnace out I'll replace the relay to start with. Thanks

[billr]

Try to bypass the control syst before you yank the furnace. If it runs normal when you connect and disconnect the two blue wires then furnace is ok.

The wires are coming out the top front oh side as you look at it from outside

The wires connected to them are not likely blue. Just keep track. Shouldn't matter though. Just a switch

In your case if the furnace stops with a normal shutdown then furnace is good.

Should take 5 min to check.

[GBill]

I'm on it. Going to pull the exterior furnace cover and see if I can get to the wires. You are right removing the furnace if not needed....yikes.

[GBill]

Well the good and bad. This furnace does not have a separate relay. It is part of the control board. This is good in that I won't have to decide which to replace. Remember everything on the furnace works perfectly until it is good and hot and then it simply keeps running. The bad is I pulled the furnace and probably could have replaced the board without doing that although would have to disconnect the gas line and cut some wires in order to finagle the board out. Now the question is should I use a Suburban board or a Dinosaur ? Dinosaur has a new universal replacement board #521099 which I located at Amazon for $79 or the Dinosaur Fan50PlusPins $109. Both have 3 yr warranty. Anyone have a thought as to which might be better ?

[billr]

Sorry no help on the parts choice.

Did you try to isolate the trailer temp control ?

[wingnut60]

Altho I haven't used one myself, the Dinosaur boards are reported to be solid replacements--I am not aware of anyone who has gotten a bad one.
And congrats on figuring out how to remove it--not an easy task.
Joe

[GBill]

Billr yes to your question and as always things worked as intended. And Joe yes pulling the furnace is a pain in the neck and back. Ha Ha. I'll contact Dinosaur tomorrow for proper fitment. From pictures I've studied where the wires attach is a little different. Thanks

[MooseSuite]

Our furnace just quit working on a freezing nite a week ago, just as we were leaving south, the next day.
Out of warranty, by 5 months. The board on the SF 42 is obsolete, replaced to another model by Suburban.as I read on the net. Lots of good things on the Dino board, quality and price.
Is it difficult to change the board on a MS? Interested to hear more.

[GBill]

Being difficult depends on the furnace location. Yes Suburban has a new board out and so does Dinosaur with the six pin plug set up. I bought the dinosaur for $96 delivered. Now if you have the large outside panel on the outside which the intake air intake and exhaust pipe attach to you can remove it and from there you will see the board to the left attached by a stamped metal bracket which is attached to the motor by two screws. Further if you set is similar to mine you will need to disconnect the gas line (13/16 open end wrench) and move out of the way. There is also a clamp which holds the line in place which needs to be unscrewed. Ofset screw drive will do the trick. From there you may or may not have to cut 4 wires which attach to plugs which connect to the board. I pulled the furnace but in retrospect probably could have left it in place maybe. If you need anything else let me know.

[MooseSuite]

Thanks
Can you provide the Dino board order number. 520820 Amazon or Dinasor?
6 pin plug not sure why you need to cut wires?
This has an LED lite?
We have the large opening by the door so sounds pretty simple.

[GBill]

The Dinosaur board is a Fan50plus pins. I purchased it at pdxrv.com 503-305-6962 and they got it to me in two days. Best price I found. The wire may need to be cut because they feed in from the left side of the furnace and you may or may not have enough slack to pull the board forward enough in order to unplug the farthest rear plug locations. If you get into it you will see that these wires (4) are already connected to the house wire with butt connectors so cutting them and installing a new connector is not hurting anything. Moving the furnace forward is a bear because you have to disconnect the ducts. The two duct on the side restrict forward movement with out being disconnected but the rear ducts (3) are held by one screw on each side of the furnace back and will all come off together. My unit has 5 but the furnace can accommodate up to 7 ducts. The reconnect is where it becomes interesting so if you have to pull the ducts you may want to have some duct tape available and I also added a 18 inch tie wrap/zip tie for added security to each duct. Hope you can slide the board out with minimal disassembly. PS The Dinosaur board does have a built in fuse to protect components. Nice add on vs the Suburban. Good Luck

[MooseSuite]

Ok, thanks for your help.
Fused and a three yr warranty.

[MooseSuite]

So, the extended warranty will repair 100 $ deaductible.

They will authorize a new suburban board plus installation.

So do I go thru warranty or get the upgraded Dino board and install it myself.

Might be a stupid question.

[hitchup]

We have the 30SF twin furnaces. A few months ago, cleaned and tested them and all was working. But then it turned cold again and the downstairs one decided it wasn't going to work. Tracked down a Mobile Tech to come look at it and he came out this week.
Because it was still under the 2-yr warranty, he could only use Suburban parts instead of the Dino board. It also needed a switch in the rear that went out.
It cost us $140 for Mobile Service, the rest was paid 100%.
He would have used a Dino board if we were paying for it.

[MooseSuite]

Did you notice the time that was charged? Was the furnace removed, for bench testing?

What's the warranty on the new suburban board? I read that the new suburban control board can be used on HW heaters as well. Not sure if Fan 50 works the same two devices.

Thanks

[hitchup]

Moosesuite: Invoice charge of $142.50 was labor, but he didn't specify hours worked. 2 Parts were listed at N/C. $140 was service call, which he had to drive over an hour from TX to get to us in OK.
According to our dealer, when the 3 Dometic AC's went out in July within the same week, the original warranty would still be in effect. So at the end of 2 yrs from Suite purchase date, if the replacements went out, we paid 100%. So would assume Dometic had same warranty 2-yr with WH or Furnaces.
David was here watching him. I only know he went in through front to replace 1 part, but had to open the rear, through the basement for change out 2nd part.
He did quiz him on what we would have paid if it was out of warranty. Figure about $450. A new unit would have been around $600, but I did a Google search and it was closer to $499.

[MooseSuite]

Thanks Hitch.

The limit switch is at the rear of the furnace, furthest from the exhaust pipe. $10 part. Didn't realize you could remove the back furnace cover plate to access replacing.

Amazing how one service call ant two parts equals the price a new furnace.

I guess it's like that with most electronics, ie microwaves etc.

[rvndave]

You can't access the stat wires from outside. From inside there should be four wires that connect to the furnace. Two of these wires will be the 12 volt power supply the other two will be the control wires that connect to the thermostat. I would disconnect the control wires at the thermostat if the furnace turns off after the cool down cycle I would replace the thermostat. If the furnace continues to run I would disconnect the control wires at the furnace. If this turns the furnace off I would look for a short in the control wires. If the furnace continues to heat after the control wares are disconnected I would check the control wires for a short inside the furnace. If no short is found replace the furnace control board.

[rvndave]

Altho I haven't used one myself, the Dinosaur boards are reported to be solid replacements--I am not aware of anyone who has gotten a bad one.
And congrats on figuring out how to remove it--not an easy task.
Joe
I installed the dinosaur board on my in laws rig it failed after a few years.

[stanleyz]

So, Gbill, did the board solve your problem. I have the same issue with a 42 but its complicated by my heat pump on the roof air which also has the same issue. It is intermittent but I'm sure I'll have to repair it eventually. Just wondered how you came out.