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Old 03-12-2014, 07:37 AM   #1
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LPG excessive consumption rate

My Interstate Ext 2014.5 has a way excessive LPG consumption rate. As per manual on full load should not be over 0,58 gal per hour; however just with AC on, it goes well beyond 1,75 gal per hour.
The dealer checked for any leak and none was found. They say it could be normal. I don't buy it!
Any advise? It is frankly very annoying to look for a LPG station every 12 hrs which is what last the tank.
Thanks in advance
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Old 03-12-2014, 08:23 AM   #2
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Are you running an LPG generator to supply power to the AC? If so your consumption rate will be higher than normal because of the current required to run the AC.

The rate in the manual is at best an average posted as a sales item and running the AC off the generator is not considered normal usage.
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Old 03-12-2014, 08:53 AM   #3
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Are you running an LPG generator to supply power to the AC? If so your consumption rate will be higher than normal because of the current required to run the AC.

The rate in the manual is at best an average posted as a sales item and running the AC off the generator is not considered normal usage.
Full load specs should be just that - at full load it uses ___ gallons of fuel per hour. Often they also give a moderate use or low use number too.

My gasoline generator in the B190, as far as I could determine, used something close to what Onan said it would under the A/C load. It definitely wasn't almost 3 times more!

I'd find an Onan shop (assuming that's who made it) and have them check the generator itself. It definitely sounds high to me. I would wonder about a leak inside the generator itself - an RV shop probably only checked the system in general, with everything off.
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Old 03-12-2014, 09:11 AM   #4
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If you had a leak of a gallon + per hour I would assume you could smell it a block away.

Has this happened for more than one refill?
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Old 03-12-2014, 09:16 AM   #5
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How are you measuring the usage? If the tank is not being completely filled, this would affect the perceived amount used. Jim
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Old 03-12-2014, 09:49 AM   #6
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Originally Posted by Biancoblu View Post
My Interstate Ext 2014.5 has a way excessive LPG consumption rate. As per manual on full load should not be over 0,58 gal per hour; however just with AC on, it goes well beyond 1,75 gal per hour.
The dealer checked for any leak and none was found. They say it could be normal. I don't buy it!
Any advise? It is frankly very annoying to look for a LPG station every 12 hrs which is what last the tank.
Thanks in advance
My manuals don't give a 0.58 gallons/hour figure. May I ask where that number came from? Did that 1.78 gallons per hour figure come from comparing the amount of propane in your last fill-up to the engine hours on the generator?

Anyway, you may be suffering from a misconception. When the tank monitor shows "Empty" that just means the tank is less than 1/3 full, not that it's actually empty. Just as when it shows 1/3 full that lust means it's somewhere between 1/3 and 2/3 full. The only time the monitor will ever show the tank as "Full" is if you fill it in cold weather, and leave it until warm weather; expansion can just barely get the liquid propane level up to where the monitor will show full. At least, that's how it works on mine.

The on-board tank holds 14 gallons at 78% full, according to my 2012 Airstream Interstate manual. If you could fill the tank brim full, it would hold 18 gallons. Of course the OPD valve prevents that. But the point is, 1/3 full on the tank monitor is 6 gallons, 2/3 full is 12 gallons.

So, assuming you're refilling as soon as the tank monitor shows "Empty" and not when all the propane-fueled appliances shut off due to a truly empty tank, you're refilling when you've got somewhere between 0 and 5 gallons left (somewhere less than 1/3 full), and you should be getting somewhere between 9 and 14 gallons of propane per fill-up. Last time you filled the tank, how much propane did you pay for?

Since I have consumption figures for the generator, but not for the furnace, water heater, or stove burners, let's use the generator for illustration purposes. A 2.5kW Onan generator uses 1.0 pounds per hour at no load, 1.5 pounds per hour at half load, and 2.5 pounds per hour at full load. Pounds, not gallons.

Propane weighs 4.24 pounds per gallon at 60F and 4.09 pounds per gallon at 80F, which is about the lowest temperature that most people would turn on the AC. At 90F, it weighs right at 4.0 pounds per gallon, and you'd be running your AC a lot. So let's use that figure.

At 90F, 14 gallons weighs 56 pounds. At full load, you'd use up your propane in (562.5=) 22.4 hours. BUT, your tank monitor would show empty when you've got about 5 gallons left, so at that point you'd actually have only used 9 gallons, or 36 pounds. At full load, you'd run the generator (362.5=) 14.4 hours before the tank monitor showed empty. But you'd still have another 8 hours of runtime left.

Now, if you were also running the water heater on propane at the same time to keep from overloading your generator by running the water heater on electicity, it's not at all unreasonable for a tank of propane to show that it needs to be refilled in just 12 hours, though you might have a full 18 hours of tank capacity.

Does that help at all, or did I just confuse you more?
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Old 03-12-2014, 10:28 AM   #7
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Great explanation, pounds, gallons, etc can make it confusing as well as how the gauges work. Only way to check somethings is to let the tank go completely dry and see what it takes to fill it. May give you a base to work with. Jim
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Old 03-13-2014, 11:04 AM   #8
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Thanks for your, as well as for everyone else comments; my figures come from the actual amount of gallons I have to buy to fill it up after each use (gas station receipt), divided by the hours the generator has been operating, regardless if it shows empty or 1/3. Last time it was 9.6 gallons for 12 hrs of usage with only AC on.
In ref with the comment on the AC not to be calculated on the full load, I respectfully don't think it is that way; as commented, full load is full load!!
In addition please note that my consumption rate comes from the Onan manual that came with the car.
Thanks again to everyone for your comments!!!
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Old 03-13-2014, 11:47 AM   #9
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In addition please note that my consumption rate comes from the Onan manual that came with the car.
Onan must have changed its manuals, then. My Onan manual gives 1.0 pounds per hour at no load, 1.5 pounds per hour at half load, and 2.5 pounds per hour at full load. It doesn't list gallons per hour at all. Your consumption, based on 9.6 gallons in 12 hours, is 0.8 gallons per hour, not the 1.78 gallons per hour you cited in your original post. At approximately 4.3 pounds per gallon, that's about 3.4 pounds per hour. A little high, but not as excessive as you were originally reporting.

As previously stated, at 90F, propane is only 4.0 pounds per gallon, not 4.3, so if you were running the AC on a 90F day, your consumption of 9.6 gallons was only 3.2 pounds per hour. Still high compared to Onan's cited 2.5 pounds per hour, but closer to normal.
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