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Old 06-23-2015, 06:38 PM   #1
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Calculating propane usage

This is probably old hat to many, but first time I realized how "one" could estimate how long your propane could / should last.

Did you know that a gallon of propane (Liquid Propane, if you prefer) has about 91,500 BTU (British Termal Unit) of energy per gallon?

A full (80%) 20 # cylinder holds 4.7 gallons of propane; a full 30 # cylinder holds 7.1 gallon or propane, again with a good fill.

So about the calculations.

So, with 4.7 gallons in a 20# cylinder contains 430,050 BTU of energy... a 30# 649,650 BTUs.

All appliances should show their BTUs (per hour).

My furnace 19,000 BTU/hr... water heater 12,000... etc.

Dividing the BTU / hour of the appliance tell you how long this appliance could run continuously on the tank. The trick is estimating how long they run per hour. You know how long a furnace might run or how long you used the stove, but a water heater would be harder to tell.

So, if you divide 430,500 by 19,000 (some know where this example comes from ) it tells us we should get a little over 22 hours out of that 20# tank of continuous use. If it ran 30 minutes per hour, we could get 44 hours of heat from one tank, in theory.

A 45,000 furnace would run a little over 2 (TWO) hours from a tank. It is obvious and eveyone knows a 20# cylinder has no place in big RV. But, seeing the number is interesting.

This water heater alone (12,000 for the Dometic 6 gallon) could run 35.8 hours continously.

My little 6,000 catalytic could run over 71 hours.

Propane weighs 4.2# per gallon, by the way, so 4.7 gallons weighs about 19.7#. The empty tank weight is about 17#, which is also called tare weight. Add together, a full tank should weigh around 37# or slightly over. The 30# weighs about 55# total.

Did you know that companies that exchange cylinders (20#ers) underfill them, "full" might only be 3.5 gallons? If the price is low enough it still could be less exensive than having a gas company fill yours. Personally, I would rather keep my tanks and only exchange if is an emergency.

I need bigger tanks! I need a raise.

Sorry.... my OCD (CDO) is glaring, huh?
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Old 06-23-2015, 06:43 PM   #2
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Calculating propane usage

I got into camping so I wouldn't have to do math! Actually I just use one tank until it changes to the other and fill the empty one. If I'm on the road and away from a propane facility I'll use the rhino exchange.

Since I'm mostly a weekender it has worked. If I'm out more it may be worth tracking closer.
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Old 06-23-2015, 06:59 PM   #3
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Wolfwhistle,
Geez, great writeup. For me personally, I bought 2 tanks from Costco that have the gauge included and just go by that. Sigh.
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Old 06-23-2015, 07:15 PM   #4
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Originally Posted by breezyhillfarm View Post
I'll use the rhino exchange.
One caution with the Rhino and similar exchanges... they use proprietary valves that prevents you from being able to refill those cylinders at any ol refill station.

Once you're in the exchange system, you're stuck with having to use it exclusively.
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Old 06-23-2015, 07:34 PM   #5
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Wow you are over thinking this thing........

All I can tell you is we use two 30 lb tanks a year camping a total of about 30-40 days. On our 36 ft. Puma TH. Closest to winter we now get is Thanksgiving week. Filling them last year at the local Exxon Station was at $22 ea. Getting ready to fill one from last year so will see what the price is this year.

Our 1988 22' camper running the furnace & everything else would last a week cold weather on a 20lb tank.

Really too many variables to do too acurate of calcuations.......
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Old 06-23-2015, 07:37 PM   #6
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Wolfwhistle,
Geez, great writeup. For me personally, I bought 2 tanks from Costco that have the gauge included and just go by that. Sigh.
Those pressure type LP gauges though are about as accurate as an RV's black tank sensor.

Tank pressure is affected so wildly by the vaporization characteristics and boiling point of LP within the tank, convective heat transfer from the area of the tank up to the level of the liquid LP, temp differences between the ambient air and liquid LP itself in order for the LP to extract enough available heat to allow vaporization... not to mention regulator demand and whether or not all of the above will allow the tank to maintain equilibrium or not.

As far as I know there are only four ways to accurately measure LP levels in a tank. Thermal strips, weighing, IR, or ultrasonic meter.

I love my Truma!
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Old 06-23-2015, 07:51 PM   #7
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I have heard if you pour warm water on the outside of the tank you can tell how full it is by where the condensations starts not sure if its true and have not tried it yet.
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Old 06-23-2015, 07:53 PM   #8
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Really too many variables to do too acurate of calcuations.......
Exactly!! And those variables are constantly changing.

Math might get you in the ballpark, but LP consumption is fairly user/rig specific. The best bet is to get a tank weight, go about your business, record daily weights over a period of a couple of weeks, then calculate your average consumption. Once you have daily averages for the various seasons of the year it's not too difficult to guesstimate your LP needs for any given get-a-way.

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Old 06-23-2015, 07:59 PM   #9
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I have heard if you pour warm water on the outside of the tank you can tell how full it is by where the condensations starts not sure if its true and have not tried it yet.
Kind of. If you pour HOT water over the side of your tank then run your hand down the side of it, you can often feel a temperature change between the vapor and liquid levels. The warmer it is outside though the more difficult it can be to feel a change.
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Old 06-23-2015, 08:19 PM   #10
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The math looks good but the appliances do not run 100 percent efficient . The best that they are going to run will be approx . 75percent. So you will have to add that to the usage .
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