that would be running all the time correct?

 

Correct. Furnaces are propane pigs, but in cold weather dry camping, you will be without 12-volt power long before you run out of propane.

Steve

 

When I was at Elk camp a couple weeks ago the temperatures were averaging 15° F at night and no more than 50° F during the day. My 30 lb tank lasted 7 days and that included running the stovetop and oven. I wouldn't call my trailer well insulated. To go 7 days is, in my opinion, very good.

PV = nRT is the formula (aka the Ideal Gas Law) that explains why there seems to be less energy in propane as the temperature drops. Lower temperature equals lower pressure. This sort of goes back to my thread I started recently asking why my furnace stopped working. If the temperature drop was enough to cause the pressure to drop then the furnace may not have seen as much flame and therefore decided to shutdown. The weak flame at the stovetop is supporting evidence.

 

I have monitored charging voltages using a "dumb" charger - the classic Shumacher 2/6 amp 12/6 volt unit. What happens is when a battery gets colder the internal resistance increases, so the higher charge voltage takes care of itself, sort of. The unit is "sized" through the transformer to top out at a relatively fixed voltage, though this is definitely temperature dependent. In warm weather it will peg to the high side and would definitely boil a battery dry in short order, a couple of days. However in cold weather the voltage is limited due to increased internal battery resistance to a safe level and in truly cold temperatures could be left connected indefinitely as it is no more than a "float" voltage. I use "smart" chargers on my trucks but there is no substitute for the old school chargers too. I suspect an alternator is similar, the set point is designed to be close for the widest possible range of conditions.

 

Mad props for breaking out the ideal gas law!

My family don't like cold weather camping, but we do get some frosty nights in the rockies even through the summer. I estimate our usage at under a pound per day in those conditions. If it was truly cold, it would be at least triple that I think.

However agree with the comment above that without shore power you'd run out of 12v to spin the furnace fan long before the propane tanks were empty.

Anyway, sounds like the OP has a plan now, but this has been a really great discussion.

 

That's about all I remember from chemistry class in college. That and the chemical compounds for TNT and sugar.

 

That would be due to the pressure regulator not doing its job. Again easy enough to verify. The two stage regulator is there to take care of changes in temperature and normally does a good job. If not, you would have little flame at low temperatures and flame lifting off the burner at high temperatures. Of course when the temperature is very low and the tank is not full enough, wetted area is decreased inside the tank and you have less vapor and less pressure so there is a point at which the regulator can not compensate.

Steve

 

Why would you fill a 20-pound tank to only 80%?

When legally "full" (not "literally" full), a 20-pound tank of propane typically has about 19.85 pounds in it, plus tare weight (the weight of the steel tank and the brass valve). Some sloppy local fillers (not "exchangers") only put around 19.7 pounds in them.

Exchange tanks have only about 15 pounds of propane in them, which those suppliers tell us is for our "safety", which is complete B.S. They do that yet today, because back when the first major propane shortage occurred, they decided to only put about 17 pounds in a newly-refilled exchange tank, and not have to up the price of an exchange. It was a way of increasing the "price per pound" without most folks noticing.

Once the first big supplier (like Blue Rino) got away with it, all the propane suppliers followed suite. They initially got in trouble with the Feds for this, as they had not said anything on their labeling about it, So, they started showing the true content, which got them off the hook, legally. Later, they reduced it to 15. Today, it's the norm. Our safety? Yea, sure.......

A 20-pound DOT-certified cylinder can safely hold all 20 pounds of propane. They are at least 20% larger in volume than 20 pounds to allow for the liquid propane to expand safely, without leaking, even in higher summer desert conditions.

Another legal scam is the practice by local re-fillers of charging a "minimum of 5 gallons" for a re-fill. Filled properly, there is NO WAY the 20 pound DOT cylinder can hold 5 gallons of propane (unless it's illegally over-filled). It's a math problem you can work out for yourselves if interested. Hint: Google "propane weight chart".

There is SO much misunderstanding about this subject out there, I thought I would help clear this up for FTEers.

The vast majority of the rest of the world will just have to remain unaware, I guess.

Pop

 

I understand that this is/can be a hot topic. The "80% rule" is to allow for expansion. Some places will only put ~15 lbs. in a "20 lb. tank", and some will do more.

I was being conservative. Either way, a 25K furnace will burn through a lot lot of propane during a long and cold night. And it will take another ~5K BTU's to heat six gallons (~50 lbs.) of water from 40F to 80F.

Regards,
Jim / crewzer

 

With the built-in over fill protection the limit is there with the float. Year ago before the overfill protection devices, one of my tanks was overfilled by the operator continuing to fill after the outage valve give the "stop-now" signal. The next day sunshine bathed the tank the the relief valve dumped a ton of vapor into the air. Luckily no one was walking by smoking.

Steve

 

Hot topic? Not at all. It's not really intelligently debatable.

If they do that to you at a local refiller (notice I didn't say "exchanger"), find somewhere else that knows what they're doing. A "real" propane place.

Going back pre-maturely for a re-fill at a short-sale refiller is just a waste of your time.

Simply exchanging tanks (unless the cylinder is "timed-out") is not only a waste of time, but money, too.

You are completely correct about the amount of propane it will take to get comfortably through a cold night, not to mention the 12-volt battery power if not on AC shore power.

Pop

 

Therein lies the problem: float voltage is lower than absorption voltage.

Alternators are convenient tools for more-or-less charging RV batteries when traveling. However, whenever possible at home or at camp, I've always used a three- or four-stage TC-charger (plug-in or solar) to properly charge and equalize my RV's lead-acid batteries.




Regards,
Jim / crewzer

 

Yup. Before the OPD valve, we had to depend on the "dip tube" to determine when the tank was "full".

It was the stupid or careless ones like yours that caused the OPD to become necessary by law. Gub'ment protecting us.

Today's knowledgeable propane refillers will still open the little vent valve and watch for liquid to come out via the dip tube, because OPDs are NOT fail proof! Sometimes they stick. Additionally, they can still be over-filled if the tank isn't reasonably level.

Pop

 

Solar really is a great way to go, and it's in my future! That stand looks very nice, too. Did you build it yourself, or is it commercially available?

Though you'll unlikely ever need it with your fine set-up, have you ever heard of a "trail charger? It's a "black box that takes in (more-or-less) 12 volts, and acts as a true charger on the output side. It compensates for any I^2*R losses in the vehicle's wiring, to give the "house" battery a more-complete charge while under-way.

I've only used one, ever, and it was a cheap one (I think was a "Tow Charger" made by Tekonsha) to keep the break-away battery charged on a small enclosed utility trailer.

The OTR crowd uses them, but they are much pricier, and much higher quality, at around $300. Not sure if they are temperature-compensated, though.

Pop

 

The stand is a 2' x 4' folding table from Lowes; it's very handy. The solar module is an old 123W Sharp, and the charge controller (not visible) is a Morningstar TrakStar SunSaver MPPT-15L. The charger's max output is 15 A, so it's good for ~200W of solar at 12V nominal.

Solar is indeed the way to go! I used to work at OutBack, Schneider (Xantrex), and Danfoss, and I'm familiar with many multi-stage battery chargers, but not with the specific model you mention. The DC-DC converter type charger makes sense though. And, IIRC, Balmar makes multi-stage alternator regulators with TC. I haven't had a chance to try one -- yet...

Thanks,
Jim / crewzer