5ht wheel heat loss??

[Jim Smith]

Has anyone ever calculated how long will propane last when heating a 5th wheel? Is there a formula or matrix available. Just a curious question ? You never get this info from the manufacturer. Maybe its because of the many variables. Thanks

 

[DIESELMAN]

Heres a short answer..... First Where on Earth are you? We need to know the Degree Days for Heating where you are at, and NOAA has calculated this for us. In Marquette MI, for example, you needed 366 degree days heating, you can get these values for any period of time.



Now for your 5th wheeler, lets assume that it is a 29 footer, 8 feet wide, with 8 foot high walls. This gives us 1056 SQ ft of wall area, now lets assume that the walls are insulated R11, the Ceiling R14, and the floor, R7.



At 68 degrees inside and 28 degrees outside, your walls each lose about 843 BTU/Hr, the end walls lose about 232 BTU/Hr, the floor loses about 1325 BTU/Hr, and the Ceiling loses about 662 BTU/hr.



And those windows, lets assume about 80 SQ. FT of windows, and being an RV, these are single pane, lets be generous, R1, costing us maybe 1920 BTU / HR.



Of course there are no air leaks, it is sealed tight as a drum, and you never open the door.



This results in 4404 BTU per degree day. Last week in MI, you needed 366 degree day to heat the RV to 65 degrees inside, for the week.



This means you needed 1612000 BTU for the week, or about 25 Gallons of propane. Since July 1, 2004, you would have used 287 Gallons, up till Jan 15 that is.



It must have been cold there last week. Huntsville AL would have needed about 5 Gallons of fuel.



There are a lot of varialbles, but when I design a building, my calcs are usually within a few percent of accurate. I can also calculate the greatest temperature difference that the furnace can maintain, although a 30000 BTU typical is up to the task most of the time.



So, there is no simple answer, and there is no simple chart either.

 

[KPerson]

There are too many variables. If it were indoors out of the wind it would be alot easier to figure out. There should be a way to calculate with the furnace BTU, interior sq. footage and average out temp. & R-Value of your unit. I've never found it though. We Lived in our unit for two years in the winter we averaged 1 - 100 pound bottle every 6 days. That's keeping the unit (34') at room temp. with one 35,000 BTU furnace. That bottle also ran the water heater and stove which we used every day.

 

[EricBu12]

my old 28' toyhauler had 2, 7# bottles... also not very good for cold weather since it is called the desert fox..... I estimate on our trips that all total, it might last a month if you don't have the temp turned up too high especially overnight.



I have now a 36' 5th wheel toyhuler with 2, 10# bottles and have only had 1 big (7 days) trip and cooking turkey and heater and snow overnight 1 night... . didnt hardly use 1 bottle so I would estimate about a month in normal use on a big trailer.

 

[jelag]

Dieselman.....



Your either in the HVAC business or a mechanical engineer or both..... Thank you for a great answer to the question... and maybe a PE



Jim

 

[JStraw]

I use electric heaters, one in the main area and one in the bedroom. It keeps toasty down to the low teens. The gas is used for hot water and /or to take the morning chill off. I fill one 30# bottle every three months. Since electric price is included in campground fees, utilize what your paying for



I would do the same if using a generator while boondocking.

 

[JimB1]

Dieselman, Nice field data reply. Was wondering much the same as your example but heating a Bigfoot 3000 series slide-in. Interpolating your data, maybe 5 gal/day give or take a bit? Jim.

 

[Jim Smith]

Thanks Dieselman, Here is more info on my RV. 36ft Kountry Star, 3 slide outs, 35000 btu furnace. 4 windows in the bedroom slide around the bed, 1 across from the bed. In the living/kitchen area, 4 windows on the rear wall, 2 windows on the left side and 5 windows on the big slide out. We are in Florida for the winter and thats why I was asking the question. I don't use any electric heaters, my wife is not comfortable with them. I filled one 30 lb. tank for $13. 50 just the other day and price seemed. Jim reasonable.

 

[DIESELMAN]

I like these kinds of discussions, I am a Systems Engineer, sort of a PE with absoloutely no specialty because I need to cover so much different ground.



To be any sort of accurate, I would need the size of each and every wall, the size of each and every window, and the size of each door, including those little doors under the dinette. For each of these I would need to know the R value.



Then I would also need to know where you intend to be camping at. heating water is also important, but harder to calculate, do you know how many gallons of water you use in the shower? Didn't think so, its more than you think unless youv'e RV'd alot andknow how to conserve water.



Some Background, R Value is the Fractional value of heat, measured in BTU's, that passes through a wall in one hour, for each degree of difference in temperature between the two sides. Example, R11 means that 1/11 of the heat escapes, or conducts, R28, means 1/28th.



A Window, unless it is low E, is not any better than an opening, except that the wind doesn't blow through it exchanging cold air for warm, so it gets an R1 grade, 1/1!



This doesn't take into account infiltration and convection, just conducted heat through surfaces.



While you can calculate Heat loss in BTU through the walls at a given temperature, how often is it exaclty 68 inside and 20 outside?



Calculations use Degree Heating Days, and Degree Coolong days. These are based on 65 degrees as a standard by NOAA, and you can get the Degree days for most areas of the country off any good weather site.



A degree day is a one degree difference in temperature from the baseline 65 degrees, for one day. If it is 65 inside, and 50 Outside, this is a 15 degree day difference, if it was that temperature all day, which it never is, so the calculation is made all day long for the sum of the entire day.



This makes it easy to calculate a day, for instance, at 5 in the morning, it is 15 degrees outside, at noon, it is 50, and at 6 in the evening, it is 70. Degree days takes the entire day into account, giving the total difference over the day, our sample day might actually be 45 for instance.



Add the degree days up over an entire season, and use the calculation for how much heat is lost through the walls and windows per degree difference, and thus degree days, and you know how much heat you need over that period of time.



Since we know that a Gallon of propane contains about 91500 BTU, give or take, and the average furnace maxes out at 70% efficiency converting energy into heat, you can figure how much propane will be needed, or electricity, or waste heat, or steam, whatever.



You can also figure how much some insulation might save you, if your home is R11, and you increase it to R28, and it cost you $500 for heating, you will save about $28 over the same period, of course this is very simplified.



I build radio sites that may see temps of -40 to +100, need to be kept at 60 inside, have equipment that adds to the heat, and uses heat from the generator, burning fuels, and electricity stored in batteries, wind and solar sources to heat the building, and need to constantly calculate the best source to draw power from to heat and operate that site. Get this one wrong and you are travelling to some remote place in -40 temps to deliver some fuel, and expalining it to your customer.



So, to calculate how much fuel your RV will need, you need to get a lot of details together, little changes make big differences.

 

[LesStallings]

DieselMan,



If you have a bunch of extra time and happen to be in the neighborhood, why don't you figure out my insulation needs on a new 40'x75' metal garage/shop building??



Oh! I know the answer: insulate it the best available for the amount I can afford!!



That was easy.



Thank you very much for your great responses.

 

[JimB1]

Les, Metal buildings have big condensation problems behind insulation, wetting insulation which then loses R value. Not a unsolvable problem but often overlooked and/or not clearly called out by sales agents. Want a large retirement workshop myself in the size range you're calling out and cold northern climate. Haven't worked through all the numbers yet but wood will get a much more serious look than when I first started planning. Jim.

 

[LesStallings]

Jim,



Since my garage/shop (retirement play house) will be in Nashville, I need to be concerned with the high humidity. The building will have a full time heat-pump keeping things reasonable but certainly not low humidity.



I've been considering the vinal backed insulation panels (R-10) that can be provided by the metal building company OR a higher R-value spray foam insulation.



Do I need to figure out some kind of air space between the metal and the insulation? I'm not sure how to accomplish that in an economic way...

 

[jelag]

Isn't this an interesting thread..... my kid the engineer would go nuts on here... .

 

[JimB1]

Les, Sure wish I knew the easy answer. 2 1/2 yrs out from retirement (at 58) for me so I've not done the in-depth research yet, however future fixed-income is the incentive to make sure I've taken "total dollar" analysis on all projects and toys. Journal of Light Construction www.jlconline.com is about the best residential construction resource knowledge base I've ever seen. Lots of free info at their website and annual membership fee is dirt cheap given that their archive goes back 8 years or so and has great well organized search engine. On the quality level of this site, except they are residential construction type site. Good luck, Jim.

 

[Eric_77]

Jim,



Since my garage/shop (retirement play house) will be in Nashville, I need to be concerned with the high humidity. The building will have a full time heat-pump keeping things reasonable but certainly not low humidity.



I've been considering the vinal backed insulation panels (R-10) that can be provided by the metal building company OR a higher R-value spray foam insulation.



Do I need to figure out some kind of air space between the metal and the insulation? I'm not sure how to accomplish that in an economic way...

the spray on insulation would be the best as it protects the metal stucture from the moisture having contact and as you say has the highest R values. Also it seals air leaks. It is considered the best commercially available insulation today.



PS I am assuming you are talking a polyurethane foam not a spray on cellouse or fire retardant



Here is a link to a pretty good fact sheet from the US gov insulation