Propane not empty when furnace shuts off

[kcaravelli]

We have a 33v Suncruiser.
We are full-timers and use external propane tanks when not on the road.
We have a 10 gal propane tanks we hook up to a 3 way valve on the coach. We shut the coach propane valve and run solely on the external tank, leaving the coach tank for backup.
The problem we are having is that the gas furnace is shutting off from lack of propane when the external tank still has about 2.5 gallons of propane in them.
I am also going to be installing a two stage autoregulator on the external tanks and wonder how this will affect things since there is already a regulator on the coach. I suspect the external two stage will be fine since the purpose is to reduce pressure before going to the appliances.

[movin-on]

First...How cold is it outside when this happens...Propane doesn't like cold weather...Just a guess but maybe the liquid isn't turning to gas fast enough when the tank is low...Try it with a full tank..I'm assuming you have 40# tanks...Also check your battery voltage...The furnace needs good voltage to run...

[Clay L]

You might want to take a look at this site LP Gas and LP Gas Generators .

It illustrates what movin-on said.
Scroll down to the last chart. It shows how many btu can be supplied by a given tank at a given temperature. The site concerns generators but the chart is valid for any appliance burning propane.

[Duner]

Well it's probably colder right now than you usually get. I just saw on the news that today all 50 states have snow on the ground with 70.9% of the contiguous US covered by snow.

Maybe you can look to other folks in the rv parks to see how they keep their tanks a little warmer so they don't freeze up......and do it safely!

Maybe let it warm up in the sun (if you get some) and then throw a blanket over it during the night.

[dsbike]

Originally Posted by movin-on

First...How cold is it outside when this happens...Propane doesn't like cold weather...Just a guess but maybe the liquid isn't turning to gas fast enough when the tank is low...Try it with a full tank..I'm assuming you have 40# tanks...Also check your battery voltage...The furnace needs good voltage to run...

I have to disagree with your 1st possibility.
Propane Liquid

Propane exists in its liquid form at or below its boiling point (-44°F) as well as when it stored under pressure. To further explain, if the temperature outside is -45°F, propane will be a liquid and you would be able to pour it out of a bucket. But as soon as the temperature rises to -44°F, the propane begins to boil and thus give off vapor. If the temperature outside is colder than -44°F, propane exists as a liquid.

One possibility with this is if you have moisture in your tank & that in turn could be freezing. One way to check this is to disconnect a gas line with the gas turned off & wipe a q-tip just inside the line & see if it comes out dirty or wet.

I agree with your 2nd possibility if you have bad batteries or you are only operating under battery power, no generator or shore power.

You should check to see if your regulator is set at the proper pressure flow. Apprx 11 inches of water. Not to sure about the dual regulator you speak of, it is my understanding that all the appliances should have the 11 inches of pressure.

[Duner]

Originally Posted by dsbike

I have to disagree with your 1st possibility.
Propane Liquid

Propane exists in its liquid form at or below its boiling point (-44°F) as well as when it stored under pressure. To further explain, if the temperature outside is -45°F, propane will be a liquid and you would be able to pour it out of a bucket. But as soon as the temperature rises to -44°F, the propane begins to boil and thus give off vapor. If the temperature outside is colder than -44°F, propane exists as a liquid. .....

Yes, it has a very cold boiling point, but as the vapor is consumed, the liquid temp drops significantly so the effective temp range is not -44 deg. The smaller the volume of liquid the faster it will chill down and reduce the rate of vaporization. 2.5 gal is not much volume comparing it to the chart that Clay L referenced above.

20 lbs of LP will only supply 13,000 btu's at 20 deg. The chart doesn't go as low as 10 lbs, so the ambient temp is even higher.

We need to know from kcaravelli what the temps he's getting down to.

[a k]

Your right about a low tank. A tank has to be sized for the BTU consumption. I've had to torch the outside of propane tanks on big equiptment to get enough pressure. Used a propane pilot to lite off an oil fired boiler.

See if you are getting frost on the bottom of your tank. That's a giveaway

[dsbike]

Ok, I did not see that link when I first read this. sorry. I now have a few questions on what I just read. Would it be suffice to say that the op could check the pressure down stream of the reg when the tank is full & again when it is near empty? Maybe use his installed tank for the full check & his external tank for the low or near empty pressure check? Just to make sure his regulator is not the culprit.

Also to the OP you don't tell us if when your external tank is low & will not fuel your furnace, can you switch tanks & everything work normal from the main tank at the same temp when the propane stops flowing?

Duner, the Op is using 40 lb tanks which are 10 gal. tanks.

This one of things I like about this forum, I am always learning something.

[movin-on]

That would be my point and I agree 100%

Originally Posted by Duner

Yes, it has a very cold boiling point, but as the vapor is consumed, the liquid temp drops significantly so the effective temp range is not -44 deg. The smaller the volume of liquid the faster it will chill down and reduce the rate of vaporization. 2.5 gal is not much volume comparing it to the chart that Clay L referenced above.

20 lbs of LP will only supply 13,000 btu's at 20 deg. The chart doesn't go as low as 10 lbs, so the ambient temp is even higher.

We need to know from kcaravelli what the temps he's getting down to.