suburban furnace tripping on high temp

[xtal_01]

Hello,

I have a Bounder RV with a year old Suburban SF-35Q furnace ... about a weeks run time on it.

A few weeks ago I replaced the propane regulator.

At the time, I noticed the unit was tripping out ... I suspected on high temp.

I was also having an ignition problem (would not lit after ignition).

So, I bough a Dinosaur board. This fixed the ignition problem.

Now I can say for certain, the safety circuit is tripping (board has an indicator).

I tested the sail switch ... seems fine.

I replaced the high temp switch ... we are still tripping out.

So, I pulled the ducting off the rear port on the furnace .... ran it with no line ducting on it ... just dumped the warm air into the basement ... worked. Ran for an hour with no trip.

So, I put hard piping (house type galvanized steel) onto the furnace ... just some straight and 4 bends ... this is the line that comes across the RV and into the washroom. there is a small Tee to bleed air below ... I left inline as it was.

Trips out again on high temp!

Tried pulling two of the duct lines off the plenum coming off the top of the furnace ... still trips out

Tried opening one of the other side ports on the furnace ... still trips out.

I did put a manometer on the regular ...

old ... 12" wc static ... 10 - 11 while running (just bleed some off as it is in the shop and not on the furnace).

New 15" WC static ... 13" when the furnace is on.

motor is a constant 6.5 amps

It even tripped with the outside cover off so I now it is not the return air.

OK, I am stumped ... I have been at this for days standing in the snow.

I did everything I can think off ... I checked with the factory. They said it should not be tripping the high temp switch.

I am getting desperate ... suppose to be leaving on a trip this weekend.

Any suggestions ?????

Thanks .... Mike

[dougrainer]

You have done some things that will help fix your problem. BUT, almost 100% of the time when you have trips of a hi limit it is the ducting. Either NOT enough of the 4 inch knock outs used or the ducting has been smashed or restricted. This can mean in the floor if you have floor ducts or anywhere where the 4 inch tube ducting flows to. ADEQUATE ducting is critical and OEM's do not always supply the correct amount. Doug

[xtal_01]

My problem with that is it was when flowing only 10.5 (10 - 11) ... maybe only 10 ... under the min requirements of the furnace.

You are right ..... it might solve the problem ... but two wrongs don't make a right.

I would rather fix the problem correctly if I can.

Mike

[DFord]

As I recall from your posts, the problem didn't occur until you put on the new regulator and boosted the pressure higher. I'd go be to the lower pressure. Good luck.

[xtal_01]

I am using the gauge port on the regulator.

I do not believe I have a pressure problem. My "thinking" is that my old regulator may have been giving low pressure (below 10.5 at the furnace).

This may have masked a problem that may have been there for years.

I got the part number for the switch from the factory.

For an SF-35Q, I got a 232504.

I believe it is set to 160 deg F.

The problem appeared with the factory switch ... my second switch and I just changed it out again ... we well see what happens.

I need to go out and get another thermometer ... my IR died last night (said the burner was over 1000 ... and snow (and rv walls ... thermostat said 32) was 3.4 deg F.

I should have said, I am an instrument tech ... worked at a refinery and a nuclear rod plant.

This one just has me scratching my head.

I am just dressing up to go re-install the furnace (falling to single digit cold tonight).

I will time the off / on cycles .... my best guess today with the new duct is about 5 maybe 8 m in on .... less than a minute off.

I can't help but thing it must be an air flow problem. My thinking .... with just an open hole on the back of the furnace (middle port so you can see part of the high temp switch) it runs and does not over temp.

Even putting on just 6 ft of straight hard pipe and 4 elbows (just aimed it outside so there would be no other restriction) it over temped!

I just made a plug for the other holes (never pushed out). I am thinking maybe letting some air out from the hole right next to the temp switch on the other side.

I also just relocated the board so it no longer blocks any of the air intake into the blower.

I will let you guys know how I make out!

I really appreciate all the advice and input!!!!!!!!!

Mike

[dougrainer]

There are 2 appliances that have built in regulators on the system.
1. The Oven/Range has a separate regulator that drops the 11.5 to 10 as that is the operating pressure of this appliance
2. I have NEVER in 40 years had to worry about what a Furnace valve /reg may do. ALL I do is verify I have 11.5 minimum to the LP furnace. Since Suburban states it has its own reg that drops to 10 to 10.5, we will believe this. This is the 2nd appliance then.
3. Where are you taking the LP pressure on your manometer? And is this the same place when you have the furnace running?
4. How hard is it? Well, if you are not trained to work on RV systems and furnaces, then it may seem difficult. You seem to have an unusual problem and this is where years of experience can come in.
5. You still not have posted 2 answers. 1. How hot does the furnace casing get?(IR thermometer) 2. How long does it take before it trips the hi limit? also, how long before it resets?
6. Are you positive you have the correct Hi limit for your newer model furnace? You stated you replaced it, but do you have the correct part number and temp range? Doug

[xtal_01]

WOW ... learned something again!

I have never been on anything but natural gas.

In this case, the valve does have an active regulator (according to the factory and the manual).

The valve actually has a green tag on the side saying among other things "REG 10.5"

I guess would be that since these furnaces are made for RV's, they are not assured the exact pressure of the system ... where is with a house, you have a large well built regular.

Here is a listing for the valve on Amazon ... it has a few good pictures of what it looks like.

https://www.amazon.com/Suburban-SB161122-161122-Gas-Valve/dp/B003G9HW08

How hard can it get to make a furnace work correctly in an RV #?#?#?#?#?#

Thanks ..... Mike

[DFord]

On residential equipment, the regulator in the valve is disabled on propane. Converting from natural gas to propane involves not just changing the orifices but blocking the regulator in the gas valve.

[xtal_01]

OK ... maybe I am wrong but is this not a regulator on the bottom of my valve? I think this is why the factory said it makes no difference if I put in 11 - 14 inches of propane ... they regulate it just before the valve down to 10" - 10.5"

Now ... on thing that did not cause the problem but might be adding toward it ....

Suburban use to mount their boards on a separate piece of metal not in the way of the intake.

On my new furnace it is mounted over the fan intake.

On top of this, the new Dinosour board is larger than the factory board, taking up even more of the intake!

Wondering if I use parts off my old furnace and move the board, would this help?

Any thoughts?

https://imgur.com/a/wBlGa0F

[Old-Biscuit]

Furnace gas valve is just that a gas valve that is solenoid operated to OPEN the valve and allow flow based on orifice size and SYSTEM Pressure.
It is NOT a regulating valve....OPEN or CLOSED is all it does.

LP Regulator is called a regulator because it regulates the system pressure/flow based on DEMAND.

High system pressure causes overfiring.

[DFord]

Lower the gas pressure to 11" WC.

[xtal_01]

Well yes and now on the motor .... I assume it ran correctly but maybe I just never noticed that the burner came on and off while the furnace was running. I really only noticed it after I changed the regulator. So I am wondering if a low pressure regulator was masking a "real" problem.

But maybe there was a problem with the motor from day one (there is only about a week running on the furnace ... last winter).

Here is want the factory said about the regulator pressure when I wrote them: "The LP pressure should have no effect on the Limit Switch opening. Of course the LP has influence on the flame but the gas valve in the furnace is a regulator that brings that pressure down to 10.0-10.5 inches so whether your LP pressure from your tank is at 11 inches or 14 inches, the gas valve should bring it down to an operating pressure (10.0-10.5) that feeds to the orifice."

Manual says 11" min ... 14" max

So I have touched the regulator ... but maybe I should?

I am working on opening up ducting even more ... don't know if I can, not much room for more ducting in this are under the floor.

Thanks .... Mike

[Old-Biscuit]

Propane System Pressure 13"WC with furnace running??????

Too High/Overfiring

[dougrainer]

How long does it take for the Hi Limit to trip? Have you checked the INTAKE from outside to make sure there is no restriction? A partial restriction would cause lack of some air to the burner, but usually this will cause a rich condition and black sooting. I would suspect a bad motor, not running at correct RPM's, but enough to make the sail switch close. With inadequate volume of air, the furnace burner casing will run hotter. The reason I suspect the motor is you stated that until now, the furnace ran correctly even before you attempted the ducting modifications. Doug

[xtal_01]

I did replace the switch ... but it could be a second bad one.

I ordered two to I can change that out this morning.

I am thinking of trying to open up the port next on the far side of the unit (second closest to the high heat switch).

Maybe this will give enough air flow.

maybe a floor register right off the unit and try moving the pipe that goes across to another port.

I worked on it till 2 last night ... running out to look at it again ... trying to beat the snow and 9 deg temps (way tooooo cold here in VT) heading our way tomorrow.

Thanks ... Mike