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 > suburban furnace tripping on high temp

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DFord

Near St Louis, MO

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Posted: 12/18/19 09:31am Link  |  Quote  |  Print  |  Notify Moderator

Lower the gas pressure to 11" WC.


Don Ford
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Old-Biscuit

Verde Valley

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Posted: 12/18/19 11:44am Link  |  Quote  |  Print  |  Notify Moderator

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.


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xtal_01

Vermont

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Posted: 12/18/19 12:03pm Link  |  Quote  |  Print  |  Notify Moderator

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

DFord

Near St Louis, MO

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Posted: 12/18/19 04:01pm Link  |  Quote  |  Print  |  Notify Moderator

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

Vermont

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Posted: 12/18/19 04:31pm Link  |  Quote  |  Print  |  Notify Moderator

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

dougrainer

Carrolton, Texas

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Posted: 12/18/19 05:06pm Link  |  Quote  |  Print  |  Notify Moderator

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

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Posted: 12/18/19 05:27pm Link  |  Quote  |  Print  |  Notify Moderator

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

DFord

Near St Louis, MO

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Posted: 12/18/19 05:41pm Link  |  Quote  |  Print  |  Notify Moderator

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

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Posted: 12/18/19 05:53pm Link  |  Quote  |  Print  |  Notify Moderator

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

dougrainer

Carrolton, Texas

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Posted: 12/19/19 05:59am Link  |  Quote  |  Print  |  Notify Moderator

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

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