Re: OT: RV furnace help needed

A furnace is a consumers friend when the outside
temperature gets colder. Unfortunately though, cold is an
enemy of LP gas. The BTU capacity of LP per volume
decreases as the outside temperature gets colder. Therefore,
based on how full the LP tanks are, the ambient temperature
outside and how many BTUs the furnace is, there may not be
enough gas to sustain ignition on the furnace.
Using the charts below, lets say that a 40,000 BTU furnace
wont fire up, and we also know that the 65 lb. LP bottle on
the RV is 40% full and it is 0 degrees F. outside. Ones first
thought might be that the burner or valve is bad. However, if
we use the chart, the vaporization capacity of the tank in
these conditions is only 38,500 BTUs. The furnace is not
going to perform very well because there is insufficient BTU
capacity in the tank.
If you were to put an insulated fire resistant blanket over the
tanks and a 75 watt light bulb under that, you would probably
raise the temperature of the bottles 10-20 degrees and
almost double the BTU capacity of the tank. This in turn
would allow the furnace to operate properly. So keep in mind
that a furnace problem is not always a component problem.

I THINK what they're saying is that the furnace just won't heat the
space very well, not that it won't ignite the propane.

The following is based on my experience with the furnace in my '78
motorhome. In that unit you had to manually light the furnace so you
could easily observe its behavior.

As I recall, the starting sequence of events were (let's say we've
just turned the furnace on and thermostat above ambient temp):

1. The furnace turns on the fan. If the sail switch is pushed far
enough, goto 2 else unit spins the fan for say 20-30 seconds then
shuts off.

2. Propane turned on to pilot circuit. In the old furnace this is
where I attached a wooden match to a foot long tool and reached in and
lit the pilot. In current furnaces, this is when the peizo (sp?)
starter is activated and starts snapping a spark to light the pilot.

This continues for a fixed period of time until one of two things
happens, either the thermocouple gets hot enough to tell the system
the pilot light is burning (at which point the furnace turns propane
on to the main burner) or the time period expires and the unit shuts
off propane to the pilot curcuit, stops the peizo and turns off the
fan.

The thermocouple is a sensor that is positioned in the pilot flame
that either opens or closes a circuit when it gets hot. If, per your
information above, propane generates fewer btu's at lower
temperatures, then perhaps the pilot is lighting but not able to
generate enough heat for the thermocouple to tell the system the
pilot's burning.

This may be what mine is doing as well, it's just too damn cold out
there in the snow to even think about going out there and playing with
it. I suppose I could put a full propane bottle in the garage, let it
warm up then hook it to the RV & see if the furnace then worked....

brrrrr

-Joe
.

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