Calculating Propane Consumption

[slo-ryde]

I'm trying to find out if my propane is lasting as long as it should when running the furnace. From my research it would seem that one gallon of propane is equivalent to 91,690 btu's. My furnace is rated at 34,000 btu input, which I'll assume means it will use that many btu's for every hour of operation. If I do the math it looks like the furnace will run 2. 7 hours on one gallon of propane (91,690/34,000=2. 69). If my tank holds 7 gallons that should come out to roughly 18. 8 hours total run time for the furnace. It's hard to say just how many hours I'm getting from a tank but I know it's not even close to the 18. 8 hour figure.



I'd like to know what kind of run time everyone else is getting before I start troubleshooting the system. I've already done the soapy water on all the fittings looking for leaks, everything appears to be leak free. The furnace runs fine and puts out a lot of heat. I've checked the flame and it's running blue with almost no yellow at the tip of the pattern.



Russ

 

[frijole]

Calcs

You're correct on your first assumption, the equation is in BTUH. Next, Fuels, depending on the geographical area, have differing heat content. Then, the variable in the equation is, delta T, meaning the difference between outside and inside temp. Obviously, the colder it gets, the more the thing runs. You have only one thing you can control and that is T-Stat temp setting. So, you would have to keep track, in minutes, (or hours), of actual burner run time, add to that water heater run time, then cooking time. It can be done, but WPINTA. Just keep an eye on the tanks and fill them up when needed. When we hunt Elk here in NM, we each bring a 100 lb cylinder with us and hook that up by itself, and if it gets low, we switch to the 2 - 7 lb cylinders while we get the 100 lb filled. Gone as long as 2 weeks on the 100 lb, never have had to fill it up, and it gets darn cold in NW NM during October. Hope this helps some. Ron

 

[slo-ryde]

How many gallons of propane will a 100 pound tank hold? You mentioned 7 pound cylinders, did you really mean 7 gallons (or what is commonly referred to as a 30 pound cylinder)? From what I can gather, these furnaces really go through the fuel. I'm going to assume the 100 pound cylinder probably holds around 23 gallons?



It sounds like my furnace is probably running in "the ballpark" after all. Thanks for the input!

 

[RMcCulloch]

At ~5 lb/gallon, that 30# tank is a 6 gallon tank - not a 7 gallon.



That would change the time from 18. 8 hrs to 16. 14 hours of continuous running [presuming that everything else is running off externally-supplied electricity].



As has been mentioned previously, the water heater, stove, oven, and refrigerator may all be using some of that gas too.



In addition to gas leaks, you may also want to check for heat losses which may cause your furnace to run excessively.

 

[slo-ryde]

Originally posted by RMcCulloch

At ~5 lb/gallon, that 30# tank is a 6 gallon tank - not a 7 gallon.

Information from http://www.altenergy.com/propaned.htm indicates a gallon of propane weighs 4. 24 pounds. I've watched them fill my tanks enough times to confirm that it's taking 7 gallons of fuel.



It should not matter how cold it is outside. The furnace is either on or off, there's no in between. Of course the furnace will run longer in colder weather but it only has one burner setting. So, if the sticker on the furnace says 34,000 btu's input I translate that to mean if the burner runs for 1 hour it will consume 34,000 btu's of energy. Also from the link above one gallon of propane provides 91,690 btu's which would mean my furnace should run for 2. 7 hours on one gallon, give or take. I'm not sure if altitude plays a part, I'm currently at 4500 feet ASL.



I only turn on the water heater in the morning for a shower, it runs for less than 30 minutes. After the shower the water heater is shut off until the next morning. I need to find out what the btu draw is for it and factor that in.



The main reason I was looking for input is the fact I can only go 2 1/2 to 3 days on one 30 pound tank before switching over to the second tank. At night the furnace is turned all the way down to 45 degrees but does come on a few times in the wee hours. When I wake up I set the thermostat to 65 degrees. It takes 30 to 45 minutes to bring the temperature up from 45 to 65 degrees. Once I get cleaned up and have breakfast the furnace gets turned down before I leave. Then at night it runs for a couple hours. I suppose it's not that big a deal, propane sells for $1. 60 a gallon at the RV park. I could have the local propane dealer set up a larger cylinder and they will deliver propane to me for $1. 40 a gallon.

 

[hammersley]

It may be cheaper to run an electric

heater and run your gas generator - or diesel if you have one.

 

[Casey Balvert]

When we camped up near Tupper Lake in New York in late September a few years ago, I recall going through a 30 pounder roughly every three days. The temperatures were around freezing and the trailer was only a 23 foot tag although it was fairly well insulated. My wife did like to keep it comfortable in the trailer. Sounds like you are doing ok for the size of trailer you have.



Casey

 

[Fireman]

Like Hammersly suggested, heat with electric. If you are in an RV park you have unlimited FREE electricity. We use a small ceramic heater, it has a thermostat which we set at a comfortable temp, then turn the furnace t'stat back 5-10 degrees below that. Ceramic heaters dont get hot to the touch and most have safety features like "tip over protection" that turns them off when knocked over. We use ours in the living area during the day and turn it toward the bed room at night. We recently were out for a week and used less than 10# of propane including cooking and showers.

If you are boondocking I have heard that catalitic heaters are much more efficent than your regular RV furnace. Check out Phred Tinseths "poop sheets" at www.phrannie.org lots of good information and unconventional wisdom there.

Fireman

 

[TowPro]

If the heater really says 35,000 btu INPUT your figures would be correct. if its 35,000 output, I would figure 80% efficiency for an input of 42,000btu.



I just installed a 60,000 btu input propane heater in my garage, did the same calculations and they are correct. (and my 60K btu can suck 15. 00 per day of propane at 2. 19 per gallon!)



In my 30' 5er, camping where its around 32 at night I can suck a 30 lber dry in a weekend.



I ran out to Walmart and bought a Polonis Ceramic Disk heater, now I run that and propane comsumption went way down.



Plus with the fan noise from the disk heater, my Daughter stopped getting up in the middle of the night saying "Daddy, did you hear that noise? what was it?"

 

[DPelletier]

Slo-ryde,



I have experienced useage to the same degree as Casey, a 30 lb tank in a 24' trailer lasting about 3-4 days when it's below freezing at night.

As far as your calculations go, you are correct, But... are you saying your furnace is not thermostatically controlled? i. e. when you turn it on, it runs continuously? Most furnaces are rated at BTUH input, which tells you the gas heat energy required to run the unit when its running. Even when its quite cold a furnace generally spends more time cycled off than on. This would skew your calculations.



Dave

 

[BV]

Maybe this will help http://kadina.yp-connect.net/~tcents/metal/data/burner.html

 

[slo-ryde]

Well, from the input it looks like going through a 30 pound cylinder every 2 to 3 days isn't that unusual. I'll have to look at getting a small electric heater to supplement the propane furnace. These things just aren't as economical as I originally had hoped for. I might go ahead and spring for a 100 pound cylinder, it will save on trips to the propane station.

 

[frijole]

Altitude Penalty

slo-ryde: You are correct that altitude plays a part in efficiency. You loose 4% per 1000 ft elevation (above sea level), due primarily to air volume and density). AND 4% per 1000 ft elevation if the heating appliance is actually a natural gas unit converted (legally, with an aga approved kit furnished by the mfg), to propane. A 100 lb tank cures the up-and-down the road problem, but be sure to have two full 7 gallon standard tanks for backup when th big one bites the dust. As mentioned earlier, you pay one electric charge per day at a campground, I find the use of a small 1500w heater really helps with propane use. I wouldn't necessarily mention the electric heater to campground management, in fact, I wouldn't know anything about it... ... . eh?

 

[Prairie Dog]

Not to get off subject but I've always thought it was strange that campgrounds have a surcharge for electric heaters but not for A/C which usually draws even more power.

 

[Dean Upson]

Originally posted by Prairie Dog

Not to get off subject but I've always thought it was strange that campgrounds have a surcharge for electric heaters but not for A/C which usually draws even more power.

They usually charge a couple of bucks extra for the A/C in New England. If the temp isn't less than 28* or so the heat pumps in the a/c's work great.



Dean