Propane question

[RandandDoug]

Was asked this in a campground, and while I think I have all the answers, I am not at all sure i am ever right.
Fifth wheel with two propane cylinders, if running low, can someone hook up a small, like blue rhino bbq tank to one of the hoses to get through a night, and should it be hooked up like regular. Meaning leaving the switch to the large tank as usual, and letting it switch over if it runs out, using the small tank as a backup?
Thanks

[afidel]

smthbros wrote:
Several people have lamented the efficiency of rv furnaces, but have provided no documentation to support their claim. This is understandable since rv furnace manufacturers seem to not offer efficiency documentation. I found an atwood spec sheet from '08 that listed input btu at 12000 and output at 9160 which works out to 76%.

http://techsupport.pdxrvwholesale.com/wp-content/uploads/2017/08/Atwood-7920-II-Installation-Manual....

Nice, no current specs list the output BTUs, I used 80% figuring it was on the optimistic side, but looks close enough for these kinds of analysis.

[afidel]

Skibane wrote:

afidel wrote:
Propane 91,500 BTUs/gal, RV furnace is maybe 80% efficient, so ~18,000 BTU/$.
Electric is 3,412 BTU/kWh and 100% efficient so 28,320 BTU/$.

Resistance-type electric is 3,412 BTU/kwh.

Heat pump-type electric is several times more efficient than that - so maybe approaching 10K BTU/kwh.

No RV heat pumps currently available work below freezing, there was one model that worked down to 24F but it's been discontinued for years since LCI gobbled up the manufacturer. Even then at least for me it wouldn't do much good, we just got to 25 degrees at 1pm today, temp this morning when the trailer was heating up was 3 😃

[Skibane]

afidel wrote:
Propane 91,500 BTUs/gal, RV furnace is maybe 80% efficient, so ~18,000 BTU/$.
Electric is 3,412 BTU/kWh and 100% efficient so 28,320 BTU/$.

Resistance-type electric is 3,412 BTU/kwh.

Heat pump-type electric is several times more efficient than that - so maybe approaching 10K BTU/kwh.

[smthbros]

Several people have lamented the efficiency of rv furnaces, but have provided no documentation to support their claim. This is understandable since rv furnace manufacturers seem to not offer efficiency documentation. I found an atwood spec sheet from '08 that listed input btu at 12000 and output at 9160 which works out to 76%.

http://techsupport.pdxrvwholesale.com/wp-content/uploads/2017/08/Atwood-7920-II-Installation-Manual....

[JaxDad]

afidel wrote:
Propane 91,500 BTUs/gal, RV furnace is maybe 80% efficient, so ~18,000 BTU/$.
Electric is 3,412 BTU/kWh and 100% efficient so 28,320 BTU/$.

I think you’re overly optimistic, most RV furnaces are more in the order of 60% efficient.

[afidel]

Propane 91,500 BTUs/gal, RV furnace is maybe 80% efficient, so ~18,000 BTU/$.
Electric is 3,412 BTU/kWh and 100% efficient so 28,320 BTU/$.

[CavemanCharlie]

afidel wrote:

jdc1 wrote:
I have an older couple (California fire victims that lost their home) living in my 5th wheel right now that do exactly what you are asking about. Our temps are 30-50* now, so the furnace stays on most of the night. They have 3 different size tanks sitting out there they use.

What are your propane and electric rates? For me here in Ohio with electric at $.12/kWh and propane at $4/gallon it's 50% cheaper to heat with electric than propane.

I think that is kinda rare. Here in southern MN electricity heating is never cheaper then propane. (Unless you are getting the electric for free at a campground.)

I'm really bad at math but, I wonder if you are doing your math right ? You need to know how many BTU's you are getting for those prices to do a accurate comparison.

[afidel]

jdc1 wrote:
I have an older couple (California fire victims that lost their home) living in my 5th wheel right now that do exactly what you are asking about. Our temps are 30-50* now, so the furnace stays on most of the night. They have 3 different size tanks sitting out there they use.

What are your propane and electric rates? For me here in Ohio with electric at $.12/kWh and propane at $4/gallon it's 50% cheaper to heat with electric than propane.

[jdc1]

I have an older couple (California fire victims that lost their home) living in my 5th wheel right now that do exactly what you are asking about. Our temps are 30-50* now, so the furnace stays on most of the night. They have 3 different size tanks sitting out there they use.

[time2roll]

When you are under 50% (one cylinder empty) the propane regulator should have a red indicator. The lever is pointing to the empty. Fill the cylinder and move the pointer. Repeat as needed and you will never run out. Check daily during high usage of the furnace.

Otherwise yes any cylinder that has the correct fitting can be attached. 10lb, 100lb, 250lb, 500+ gallon all work the same.

[rhagfo]

We have 3 30# tanks and a 20# for BBQ and as a backup. We the third 30# after dealing with a nine day power outage. During the outage we placed the 20# on a couple Lego levers and took both 30# so they were both FULL.

[wa8yxm]

RandandDoug wrote:
Was asked this in a campground, and while I think I have all the answers, I am not at all sure i am ever right.
Fifth wheel with two propane cylinders, if running low, can someone hook up a small, like blue rhino bbq tank to one of the hoses to get through a night, and should it be hooked up like regular. Meaning leaving the switch to the large tank as usual, and letting it switch over if it runs out, using the small tank as a backup?
Thanks

Yes you can. Also if you have the proper adapter you can do the same with a motor home..... Done it a few times. (Extend-a-stay or Stay-a-while are the names of the adapters)

NOTE: You don't need an adapter with a Trailer though you may need an overturned bucket to raise the smaller tank to the proper height. (or a few slabs of wood or .. well most anything)

Note 2: On a motor home I've seen some home made "Adapters" to allow external Propane feed.... SCARY.

[2oldman]

Sure, in fact I use only the BBQ tanks because the 7 gallon ones are too heavy.

[philh]

I have two 40lb onboard tanks and have a 20lb spare that is used for a firepit too.