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Old 10-06-2012, 09:50 AM   #1
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Furnace sizing

I need some guidance on what to do about a furnace for my 20' Argosy motorhome. Right now I believe it has a 16,000 btu furnace installed. I have sitting on the shelf two 30,000 btu furnaces that look to be in great shape.

The question is would installing a 30,000 btu furnace in a space where a 16,000 btu furnace would normally be installed be a good move. My first thought is the 30k unit is going to cycle way to often.

Any thoughts?

Thanks!

Brad
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Old 10-06-2012, 12:22 PM   #2
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Your first thought is correct. You will be treated to brief periods of "blast furnace."
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Old 10-06-2012, 01:32 PM   #3
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You may be able to just put a smaller orifice in it for the gas burner. Someone here compared all the parts of 2 Suburban furnaces with different BTU ratings and the only difference was the size of the orifice and more money for the larger BTU version.
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Old 10-06-2012, 01:47 PM   #4
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The larger ones will take more 12 volt power to run, and although in theory they will run less time I think you will find that in practice the cool down cycle will actually cause a significant increase in overall 12 volt energy consumption.

The larger furnace will need more ductwork to get the heat out of it than the current one you have probably has connected.

The short cycle of on and off, in addition to the temp swings may prove to be annoying.

The 16,000 btuh unit you have now is probably still oversized for anything but the coldest use (below 15F) you may encounter. The 30,000 ones are vastly oversized, as you realize.

I am camping in Montana near Glacier National Park right now. Night before last went down to 20F. The 16,000 buth unit in my Argosy 20' trailer ran about a 50 to 60% duty cycle at those temps.
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Old 10-06-2012, 02:22 PM   #5
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Originally Posted by drboyd View Post
Your first thought is correct. You will be treated to brief periods of "blast furnace."
I've seen what happens when you oversize an A/C unit in a house so I assumed over sizing a furnace should have some similar affect with regards to its operation.

Thanks for the feedback.

brad
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Old 10-06-2012, 02:23 PM   #6
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Originally Posted by ventport View Post
You may be able to just put a smaller orifice in it for the gas burner. Someone here compared all the parts of 2 Suburban furnaces with different BTU ratings and the only difference was the size of the orifice and more money for the larger BTU version.
Now that's interesting!

I'm going to have to do some research along this line to see if I can figure out what size orfice might work to drop the btu's down to around the 16k range.

Thanks!

Brad
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Old 10-06-2012, 02:27 PM   #7
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Originally Posted by idroba View Post
The larger ones will take more 12 volt power to run, and although in theory they will run less time I think you will find that in practice the cool down cycle will actually cause a significant increase in overall 12 volt energy consumption.

The larger furnace will need more ductwork to get the heat out of it than the current one you have probably has connected.

The short cycle of on and off, in addition to the temp swings may prove to be annoying.

The 16,000 btuh unit you have now is probably still oversized for anything but the coldest use (below 15F) you may encounter. The 30,000 ones are vastly oversized, as you realize.

I am camping in Montana near Glacier National Park right now. Night before last went down to 20F. The 16,000 buth unit in my Argosy 20' trailer ran about a 50 to 60% duty cycle at those temps.
This is great info. I don't know that I'll ever try camping in 20F temps but you never know what the future brings. If a 16k is keeping your 20' Argosy comfortable at these temps then as everyone suggests anything larger just won't work.

Thanks all for the feedback. If I can't re-orifice one of my 30k units (assuming it is possible) then I'll keep my eyes open for a good buy on a 16k.

Thanks!

Brad
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Old 10-06-2012, 05:19 PM   #8
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Originally Posted by bkahler View Post

Thanks all for the feedback. If I can't re-orifice one of my 30k units (assuming it is possible) then I'll keep my eyes open for a good buy on a 16k.

Thanks!

Brad
To re-orifice to say half the heat output, you would still have:
A) too much combustion air from the combustion blower wheel
B) A burner with a size too large and a flame too small.
C) The need to adjust the combustion air down to that needed by the smaller orfice. (different from A, above)
D) A main blower which would still be using the higher amps at a lower output.
E) Possible ignition and/or fan control problems with half heat output in a larger chamber.

Suburban may change the orifice to vary the output for say a 30k, 34k or 40k furnace at the factory level. They may find that engineering to a mid point allows them to move up or down a few thousand btuh by orifice change only. I don't think you would be wise to try halving the output by orifice change on your own.

Some of the above is speculation on my part but things to think about.
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Old 10-06-2012, 07:22 PM   #9
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Quote:
Originally Posted by idroba View Post
To re-orifice to say half the heat output, you would still have:
A) too much combustion air from the combustion blower wheel
B) A burner with a size too large and a flame too small.
C) The need to adjust the combustion air down to that needed by the smaller orfice. (different from A, above)
D) A main blower which would still be using the higher amps at a lower output.
E) Possible ignition and/or fan control problems with half heat output in a larger chamber.

Suburban may change the orifice to vary the output for say a 30k, 34k or 40k furnace at the factory level. They may find that engineering to a mid point allows them to move up or down a few thousand btuh by orifice change only. I don't think you would be wise to try halving the output by orifice change on your own.

Some of the above is speculation on my part but things to think about.
Nothing is ever easy is it

I'm going to spend a little time researching Suburban part numbers to see if I can find what the variations might be in parts for the model that I have. If nothing else it will kill an evening

I appreciate your feedback.

Brad
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