High altitude furnace operation

We usually spend some time at high altitudes (9000 ft) camping and usually need a little heat to get us through to morning.  In the 4 years that we have had our TT, we have had 3 occasions when the furnace would not light, even though battery or shore power was good and propane tanks were at least half full. In each case, the failure to light always occurs shortly before sunrise.  Or to put it another way, the furnace cycles pretty well during the night but around 6 -7 AM, the blower comes on but the furnace doesn't ignite and the blower stops after a short bit.  If I wait a couple of hours and do nothing other than raise and then lower the thermostat lever, the blower comes on and the furnace quickly ignites. 

I suspect this has something to do with either the altitude at which we are camping or a drop in propane gas pressure due to cooling.  But the refrigerator and stove both function normally so I question the gas pressue theory and it only got down to about 35 degrees.  On this last trip, we camped 4 days.  I did not have a problem the first evening but did the second.  So I switched to a full propane tank and did not have a problem the 3 evening but did on the 4th evening. Both propane tanks used feel at least 2/3 full after use.  The only thing they were used for were to run the refrigerator and very little cooking.  I have discounted low battery as a cause as I used a generator to run lights and recharge every evening.  In the morning there was always sufficient battery power to run pump, lights and the furnace blower?

It is a Suburban Furnace.  Any ideas?

I have heard of moisture collecting in the regulator and freezing up - try insulating your regulator to see if that makes a difference.

I started a reply to this last night and then deleted it but your problems really are identical to what happens to us with low batteries. By morning the batts are getting low and the cold temps drain them further. The fan will still run but the combination of lower voltage (lower fan speed) and thin air just doesn't provide enough pressure for the sail switch to remain closed.

Letting the batteries to rest or warm up usually solves the problem temporarily. If it happens again I would start the gen and see if that solves the problem.

One other thing: if your "propane" has a high percentage of butane, 35 degrees is enough to substantially reduce the vapor pressure. The stove and fridge do not need as much gas volume as the furnace, so they usually work no mater what.  If you fill the propane tanks in the south where summer temps are high and freezing rare, they blend in more butane to make the LPG work better in high heat. Then if you take the tank to the mountains r a cold climate, you may have problems with vaporization.

LPG is rarely 100% propane - its a blend of 2 or more gasses, mostly propane and butane but sometimes other light petroleum gasses too.

Jeff said:

I started a reply to this last night and then deleted it but your problems really are identical to what happens to us with low batteries. By morning the batts are getting low and the cold temps drain them further. The fan will still run but the combination of lower voltage (lower fan speed) and thin air just doesn't provide enough pressure for the sail switch to remain closed.

Letting the batteries to rest or warm up usually solves the problem temporarily. If it happens again I would start the gen and see if that solves the problem.

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I suppose a low battery could be the problem on this last trip although it is a brand new battery and I had run the generator for two hours the night before to charge it up.  Maybe two hours isn't enough to charge the battery fully.  I did have this occur  two years ago when we were in a campground with electric hook-up but then the outside temperature got down to 15 below zero.  I always try to start a trip with full propane tanks and a fully charged battery.  If (or when) this happens again, I will jump the battery from the truck and see if that makes a difference. 

Lowell:

We have 4 220ah 6v Trojans that will be down to about 50% if they have to run a furnace all night below 30 degrees.

Two hours of generator time will definitely not fully charge a battery that is down to the 50-60% level. It will bring it to around 85% though.

One battery is not a lot for a night on the furnace. The fan draws around 5 amps, so if it runs 50% of the time for 10-12 hours, you are consuming 25-30 amp hours. That's nearly half of what a fully charged typical Group 24 battery can produce.

ISince this was a new battery, right out of the store, it may not have had a full charge to start with although I did install it on the trailer and connect the trailer to 30 amp shore power for about 16 hours before we left on our trip. Next time we boondock, I will run the generator longer.  In my original post, I mentioned that we also had this occur once when we were connected to shore power in a campground.  In that case it was very cold, 15 below.  Is it possible that at that extreme cold, even with shore power, that the power was insufficient to get ignition?

At 15 below most any LPG may be hard to vaporize.  Your tanks are external and will be at the outside temperature and vaporization cools it even more. Pure propane vaporizes at -44 F., but LPG blends are usually somewhat higher.  Below -20, LPG does not vaporize rapidly, so gas pressure is low and so is the flow volume.