LP furnace not working, but all other LP appliances are..

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New member
Sep 6, 2005
Long Island N.Y.
Hello, my name is Tommy. I have recently obtained a 1978 Sunline 18' towable trailer. The problem I am having is with my furnace. If I use the trailer's LP tanks the furnace will not work, but if I connect a rubber hose to the furnace and then to a LP tank the furnace works. All of the other LP appliances in the trailer work fine with the trailer's tanks (stove, fridge, hot water heater). I am very new to RVing and I am boggled by this. If I disconnect the trailer's line from the furnace while the propane is on I can here the gas so I know it isnt a kinked line. Can it be a bad regulator on my tanks? What should my LP pressure be?? Someone please help!! The winter is coming and I'm excited to go camping.
Pressure in the tanks is often hundreds of pounds per square inch

Pressure in the lines to the furnance is measured in INCHES OF WATER (that is the pressure needed to hold up a collum of water I think about 4 or 5 inches high,,, I once figured the pressure needed to push water 250 feet up, it was less than the pressure in a Propane tank so you do the math... I hope you had a regulator hooked to that gas tank you tested with.

To test the gas pressure you need a special meter, You can make one (Instructions in the Woodall's RV Owner's Manual, I'll not violate their  copyright by copying  them, plus I haven't the manual handy just now) or you can buy one.  Hook it up the the distribution manafold and check with a burner lit on the stove.  If it's low, either adjust or get a new regulator.

Sounds like a clogged outlet on the manafold or a clogged hose between the manifold and the furnance. Less likely but possible is the wrong kind of furnance (vey less likely) which requires someone with expierence (another RVer perhaps) look at it with the mark 1 eyeball

Furnance burners need to be cleaned and maintained at least once a season, There is a spider which likes to live in them
To set or check the propane regulator you need 1/ to know exactly what you are doing. 2/ a manometer capable of reading 11 inches of water.  As I recall the correct regulator pressure for propane measured with a manometer is 11 inches of water.  I don't recommend trying to repair gas appliances or systems unless you have some experience in this area.
The regulator on your tanks controls the whole system, so it is likely it is OK (since the other appliances run fine).  However, the furnace is the largest volume user on your system and it is possible that an insuficient quantity of LP is getting through either the regulator or the LP lines. The most likely culprit would be some oil/water sludge lodged somewhere in the LP line that branches off to the furnace.  You might still get some LP hiss out of the line, but it might not run the furnace. An oily sludge can accumulate in the LP tanks and if a bit of it gets into the lines, it can clog a valve or block a low point in the line.  Since your water heater apparently works OK, I'm guessing the regulator flow is sufficient and the problem is in the branch of the line to the furnace.

The correct pressure is 11 water-colum inches, measured with a manometer.  I'm not sure what flow rate your furnace might need, but a call to Suburban or Atwood (depending on what brand you have) could probably give you that info.
I double checked the pressure levels and it is 11 inches of water, not the lower number I thought

So for the guy who said "I think it's 11 inches" yes, it is

A thought .... Check that pressure at the manafold, and also check it at the furnance when it tries to kick in (you will need to do some adapting to put a "T" fitting in there)

I suspect you are going to see a serious pressure drop when the gas valve opens... Thus indicating a clog somewhere in the line

If you can't clear the clog, replace the line
Thank you very much for the replys. First off just to let you know i did have a regulator on the tank when i conncected my own hose. it was a regulator from a old Bar b Que. I guess the cheapest and easiest thing to do is try to clean out the lines. I will try that this weekend. however im not thinking that this is the problem, when i would losen the fitting the hiss was loud and the i felt a decent amout of pressure. Like one of the responses said the furnace requires the most amount of preasure to work. A? bad regulator may not allow the 11" of waterpress. the regulator looks like it is original, or at least darn old... if i blow out the lines and it still doesnt work is it saf to say that it is the regulator??? Also i did not write this in my first post but the pilot light does not stay lite either, when i release the pilot primer button. it fades out slowly, but when i use the seperate tank with the regulator off my BBQ the pilot remains lite and the furnace works great... (sorry if this is confusing).... Nights here in NY are starting to get chilly and im iching to do a lil traveling. please write back soon...p.s. Are all LP systems on RV's 11 inches of water press???? ???? ??? :-\
Yes, all RV propane systems use 11 water colum inches of pressure. So do BBQ grills, cookers and such.  All regulators will be set for 11 inches of pressure, with the exception of a few specially calibrated for LP fireplaces (and those will be well marked as non-standard).

As Ron says, a regulator is not real expensive, so go ahead and replace it if in any doubt.  Since yours is quite old, replacing it now will save you a headache later anyway. I suggest an automatic changeover regulator if you have twin tanks on the trailer - they are very convenient in the middle of a cold night and also allow one tank to be easily removed for filling while the other continues to function.

Your furnace requires a large VOLUME of LP as well as the 11 inches of pressure. Flow volume and pressure are different and it is possible to have pressure without adequate flow volume.  The symptom will typically be a furnace that lights but quickly goes out, as you indicate happens to your pilot light.  Enough gas leaks by the blockage to fill the pipe line beyond it and that gas will be at the required pressure, but as soon as the valve at the furnace opens the gas in the line moves into the pilot and burner tube.  Because of the blockage, additional gas does not quickly flow into the pipe line to replace what has burned, the pressure drops at the furnace end and soon the available gas supply runs out or becomes too think to burn.

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