Heating Problem

Greetings Forum Chums,

Wintering in my 1985 Pace Arrow while my house is being built....put hay bales around the perimeter...it got down to 5 degrees the other night..stayed warm inside...nothing froze underneath ....last couple of days the furnace comes on as it should...heats normally...then all of a sudden, it stops working....the fan runs.... but only cold air....recycling the thermostat fixes it.....until the next time...ANY IDEAS??

On my furnace, the fan will continue to run until all the hot air is expelled from the furnace. If you let it continue to run, will it eventually shut off?

No I am afraid....just keeps running

Check your battery voltage.. You may be low..

Also check all registers make sure none are blocked or clogged  Clean as needeed

When the T-Stat calls for heat FIRST the blower blows.. When it comes up to speed a device (Suspect as well) Called a SAIL SWITCH (It is a normally open momentary type (Spring loaded) Switch with a "Sail" attached the force of the wind from the blower closes it) closes. this tells the control comptuer to fire the burner up.

Next check for ignition.. I've seen furnaces try 3 times and lock out  Due to excessive spark gap or other issues.

volts good....took off the thermostat....jumped the wires....Heat working....I will see if it quits after a bit....sometimes it ran for a considerable length of time before it began to blow cold air

I suggest you read my article on Furnace Troubleshooting in the RVForum Library. It explains the sequence of operation in layman's terms and gives hints about potential problems.
http://www.rvforum.net/miscfiles/Furnace_Trouble-2.pdf

It sounds as though you have an overheat condition and the High Limit switch shuts the burner off. The fan continues to run to cool it down. However, it will not re-light until you cycle the thermostat off & on again, which causes the furnace circuit board to reset and begin a new cycle. The advice John gave address some of the reasons for an overheat condition.

Is it possible the propane is not vaporizing because of the cold weather?

wackymac:

Is it possible the propane is not vaporizing because of the cold weather?

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Not likely at 5 degrees.  Propane stops gassing properly at about -40.

Frank.

Thanks all....ordered a limit switch....coming on Tues....will see what that and a good tune up does...will post back!

Rather than throwing money at it by replacing parts that haven't been checked, use a multimeter to determine if an actual part is the problem. You can determine if the sail switch is the culprit, or if it's the high temp limit, or something else.

Propane stops gassing properly at about -40.

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Pure propane completely stops gassing at -44, so by -40 it is effectively producing zero vapor while the furnace consumes it at the gulp. +5 would be fine in the lab, but in the real world there are other factors in play that effect the vaporization and delivery of the LP.  For example, the large metal tank surface that is radiating what little heat is within, the venturi effect in the regulator, and gas line temperature. Even wind can be a factor, increasing the heat loss and chilling the system further.  An ambient temperature above -44 is not the whole story by any means.

And it is rather unlikely that any LPG tank contains pure propane.

Frank B said:

wackymac:

Not likely at 5 degrees.  Propane stops gassing properly at about -40.

Frank.

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True but Butane is sometimes sold in place of propane. THis is a great "Deal" (Read that rip off) for the seller because Butane is less expensive but he charges for Propane and pockets the added  profit.

Butane stopps gassing long about the freezing point of water.

Gary:

+5 would be fine in the lab, but in the real world there are other factors in play that effect the vaporization and delivery of the LP.

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Agreed.  However, my comments had to do with the likelihood that a 'frozen' propane delivery system  would cause furnace issues at +5F.  Would you not be inclined to check other issues first as being more likely?

For example, the large metal tank surface that is radiating what little heat is within, the venturi effect in the regulator, and gas line temperature.

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Perhaps my understanding is wrong here, but would not a rapidly decompressing propane system  get colder than the +5 ambient temperature, and therefore tend to absorb heat from the 'warmer' air around it?

Even wind can be a factor, increasing the heat loss and chilling the system further.

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The so-called wind chill effect?  I always understood that while wind chill will cool things down faster, wind chill will never take an object below ambient temperature.  Would not a +5 degree wind actually warm an exposed propane system that may be approaching -20 or less?

As to the regulator, I believe you are right.  It should likely get even colder than the tank or the line, as that is the point at which the decompression is taking place.  However, I have no direct experience with that.

We do have direct experience with  winter camping -- years ago when I was younger.  We have been out camping in the Rockies west of Calgary in the dead of winter at -25C (-13F) and had no problem with propane -- even with the 20K btu furnace running almost steadily.  We did experience a sluggish furnace  at -32C (-25F) because of rapid depressurization, but putting an electric heater blowing on the propane tanks brought the propane pressure back up again.  Then we went home.    There ARE limits to what is considered 'fun'.  ;D

I just didn't see +5 as being a likely contributing factor to the OP's furnace problems, and thought I would share my experience with him.

Frank.

just as an update, today the furnace worked flawlessly....the temp was 40 and not 5....it cycled on and off as it should.....that being said....the furnace was working in a constant on mode when it was 5 out....even with the steps I took to insulate underneath...I am sure the R value is meager, forcing the furnace to work harder, more to the point longer......perhaps stressing a 32 year old limit switch

Flyingpiper59 said:

just as an update, today the furnace worked flawlessly....the temp was 40 and not 5....it cycled on and off as it should.....that being said....the furnace was working in a constant on mode when it was 5 out....even with the steps I took to insulate underneath...I am sure the R value is meager, forcing the furnace to work harder, more to the point longer......perhaps stressing a 32 year old limit switch

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You very well could be using the gas faster than it boils to a vapor at 5 degrees. Look to see if there is frost on the tank or regulator when it gets cold. If it is you may have to put a heat lamp in the bay to help it a long. As the LP boils off the tank will get colder and colder, and the pressure will drop. If it drops too far your furnace with soot up, NOT good when this happens. A larger tank maybe in order for you this winter, or maybe a heat tape.

its a 150 gal...no sweating...

Temps as low as the 20's can definitely cause problems with gas flow.  In my case the regulator on the tank was freezing up when the temp got below 28.  Put a low wattage lamp under the cover with the regulator(200 gal tank)and never had a problem even down into the teens.