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LP furnace not firing

Our 1993 front LP furnace is not firing up. When we turn the thermostat on, the blower comes on but no heat comes out. The propane tank is full and never run dry so there should be no air in the line. Stove and back furnace run fine. Any suggestions as what to look for?
Jim & Ann Chestnut
1994 U300 40'
Have a glorious day!

Re: LP furnace not firing

Reply #1 –
First measure the voltage.  If low the sail switch will not be activated by the fan (which will run slower due to lower voltage).  W/O the sail switch being activated, the gas will not be allowed to flow into the burner igniter.

The sail switch is an on/off device. (normally in the off position) It gets it's name from the "sail" or paddle that is attached to the switch mechanism. As the blower comes up to speed, it blows air onto the sail with enough force to push the switch closed, thus allowing electrical current to flow to the next component in line.

It's job is to determine if there is adequate air flow for proper combustion to take place. If the battery voltage is low or the fan does not come up to 75 per cent of it's design speed, the sail switch will not close.

Possible causes of this malfunction are: low battery, restricted return air inflow, restricted or inadequate outlet vents, restricted combustion air inlet or exhaust, faulty sail switch.
Peter & Beth Martin
No Forrest? What have you done?
MC# 15890 until Dec 2016; FMCA #F329677
Cincinnati, OH

Re: LP furnace not firing

Reply #2 –
Our furnace failed with similar symptoms. The fan blew, I could hear the propane valve operate, I could hear the spark, I could smell a whiff of propane, but it would not maintain a flame. FOT fixed it. Daubers had fouled the burner head and it would not support a flame. A new burner head fixed the problem.

Be sure to keep "dauber screens" on the furnaces and the water heater. We've had failures of both due to dauber nests inside the appliances.

A "gotcha" on our U295 is that the front furnace will not operate if the driver side storage bay door is open. There is a switch that opens when the bay door is open. It protects the door from being blasted with exhaust heat from the furnace.
J D Stevens
1997 U295 CAI 36' Build #5085
2002 Subaru Outback
Motorcade 16869
Bellville, TX

Re: LP furnace not firing

Reply #3 –
Two good posts.

Determine how far into the "run" process you get.

Basic question-- it it getting far enough into the run process that the gas valve is opening (can you smell propane outside by the furnace exhaust) and is the ignitor firing (clicking)?  If that happens, next is there fire (heat).  If so, for how long?  If heat is not sensed quickly enough, the thermocouple will shut down the unit.

Brett
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: LP furnace not firing

Reply #4 –
I took an air hose and blew out the exhaust and around the motor compartment. Started to check the voltage after turning the thermostat up and this time it fired up and stayed on. May have been something blocking the propane inlet. It is working for now. Thanks for all the suggestions.
Jim & Ann Chestnut
1994 U300 40'
Have a glorious day!

Re: LP furnace not firing

Reply #5 –
Another suggestion for anyone with this issue is to pull the vent cover and look down the exhaust tube with a good flashlight. On my '91 right after purchase neither furnace would fire. Both had a mud wasp nest about half way down the tube, which prevented adequate air flow to activate the sail switch. Cleaned out the mud nests, and both fired right up.

Ahaa, I just googled "dauber" and it is also called a mud wasp. Dauber was mentioned above by J D Stevens - awesome. ^.^d
Peter and Tammy Fleming
1991 U300 GV 40 - Sold, owned for 4 years
Downsized to Roadtrek Popular 210 class B

Re: LP furnace not firing

Reply #6 –
Quote from: prfleming –
Ahaa, I just googled "dauber" and it is also called a mud wasp. Dauber was mentioned above by J D Stevens - awesome. ^.^d
I don't recall hearing of a "mud wasp." In Oklahoma and Texas we have "mud daubers" or "dirt daubers," which are obviously known as "mud wasps" in other areas.

Whatever you call them, be sure to protect your motorhome from them. They tend to build nests in air intakes, air exhausts, and air passages. They will foul leveling systems, furnaces, and water heaters. They might foul refrigerators.

You can buy screens to protect furnaces and water heaters. I've fabricated a screen for the water heater vent on our U295.

Spiders fouled the orifice on our water heater and caused it to burn "dirty." The heat and fumes stained the area above the water heater vent on the coach. Check for fouling of passages by spider webs as well as dauber nests.
J D Stevens
1997 U295 CAI 36' Build #5085
2002 Subaru Outback
Motorcade 16869
Bellville, TX

Re: LP furnace not firing

Reply #7 –
One other cause of a sail switch not activating is bad motor bearings or lack of bearing lubrication.
If you hear any weird sounds from the fan motor when it comes on or cycles off, you probably have failing bearings preventing the fan from spinning fast enough to activate the sail switch. 
Change the motor.  Probably about $140 or more these days.
Regards,
JON TWORK KB8RSA
Full Time RVer (10+ Years) & Dedicated Boondocker
Retired, Unemployed, Homeless Transients
1996 Foretravel U270-36 w/24' Timberwolf Trailer
I firmly believe that tomorrow holds the possibility for new technologies, astounding discoveries, and a reprieve from my remaining obligations.
Welcome to WeRV2 (Under Construction)
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The Second Amendment is in place in case they ignore the others.

Re: LP furnace not firing

Reply #8 –
often, the probes have carbon build up- a little wire brush fixes that. Ernie
 

Re: LP furnace not firing

Reply #9 –
Another fix for a dry motor is to remove it, drill a small hole at end of casing and use a fine oil to lube it.  I did this a year ago and it saved the motor.  Kind of a PITA but it works.
Gary B