Considering new furnace..

Gdetrailer wrote:

dougrainer wrote:

I would not worry about the feed supply wiring. They all have enough capacity to run up to 15 amps dc current and no RV furnace will pull over 15 amps---most are at about 10 amps. The largest 40k BTU draws 9 amps. The smallest(20k btu) draws 7 amps. Doug

Doug,

Of all folks I am surprised that you would say this.

RV manufacturers ARE CHEAP, they DO NOT OVERSIZE ANYTHING.

My RV was pretty much ALL wired with 16 ga with with a couple of items with 14 ga.

My original furnace with 7A draw was supplied with 16 ga wire with a 7.5A fuse..

My one size up replacement furnace NEEDED 10A, wasn't gonna happen on 16 ga wire.. Not gonna put a 15A fuse on 16 ga wire, no way, not on my watch.

So, since I was commited to fully rewiring my TT during my rebuild I repurposed the old 8 ga wire which went from the converter to the battery and ran that to my furnace.. I did under fuse that wire at 15A but I gained voltage stability due to less resistance in the wire run and that means the furnace won't cut off early from a low battery.

Not saying that anyone NEEDS to oversize like I did, but for BETTER furnace operation having a couple of wire ga sizes heavier may do wonders when boondocking..

Furnaces depending on BTU size vary, A LOT on how much current draw is required..

For the OP, found a installation manual for the "SF" series (installation requirement should be similar enough) that may be helpful.. HERE

Going from SF 30 to SF 35 the ducting requirement goes from 36 sq inches (Three 4" ducts) to 48 sq inches (FOUR 4" ducts, top or bottom duct is the same between the two at 56 sq inches.

It IS extremely important to follow the installation instructions, failure to add an extra duct will mean that the furnace may overtemp and not operate correctly at the least, at the worst, overheat and have a potential for damaging the furnace and or your RV.

I stand by my post. IF you have the 30 to 35 year old Trailers you have in your sig, then to compare a 35 year old RV to current is just ridiculous. As to your statement about what RV makers do. You have NO CLUE. I work on all makes and types(40 years worth) and I have never seen any wiring to a appliance that would make me suspect that it was under sized. Doug

dougrainer wrote:

I would not worry about the feed supply wiring. They all have enough capacity to run up to 15 amps dc current and no RV furnace will pull over 15 amps---most are at about 10 amps. The largest 40k BTU draws 9 amps. The smallest(20k btu) draws 7 amps. Doug

Doug,

Of all folks I am surprised that you would say this.

RV manufacturers ARE CHEAP, they DO NOT OVERSIZE ANYTHING.

My RV was pretty much ALL wired with 16 ga with with a couple of items with 14 ga.

My original furnace with 7A draw was supplied with 16 ga wire with a 7.5A fuse..

My one size up replacement furnace NEEDED 10A, wasn't gonna happen on 16 ga wire.. Not gonna put a 15A fuse on 16 ga wire, no way, not on my watch.

So, since I was commited to fully rewiring my TT during my rebuild I repurposed the old 8 ga wire which went from the converter to the battery and ran that to my furnace.. I did under fuse that wire at 15A but I gained voltage stability due to less resistance in the wire run and that means the furnace won't cut off early from a low battery.

Not saying that anyone NEEDS to oversize like I did, but for BETTER furnace operation having a couple of wire ga sizes heavier may do wonders when boondocking..

Furnaces depending on BTU size vary, A LOT on how much current draw is required..

For the OP, found a installation manual for the "SF" series (installation requirement should be similar enough) that may be helpful.. HERE

Going from SF 30 to SF 35 the ducting requirement goes from 36 sq inches (Three 4" ducts) to 48 sq inches (FOUR 4" ducts, top or bottom duct is the same between the two at 56 sq inches.

It IS extremely important to follow the installation instructions, failure to add an extra duct will mean that the furnace may overtemp and not operate correctly at the least, at the worst, overheat and have a potential for damaging the furnace and or your RV.

Gdetrailer wrote:
As mentioned, you can upsize some, don't oversize to much.

I went from a 25K to a 30K with no regrets, warms up the trailer a bit faster.

Couple of things to consider, read the manufacturers installation manual, upsizing most likely WILL require adding at least ONE additional hot air duct. Manufacturers installation manual should tell you exactly how many hot air ducts will be required (and/or the total square inches of outlets) for the size of furnace you selected.

Upsizing also may require upgrading the 12V wire and fuse size from the fuse panel. Sometimes the bigger furnace will have a blower that draws more amperage than the original which may require you to upgrade the wire and fuse.

I would not worry about the feed supply wiring. They all have enough capacity to run up to 15 amps dc current and no RV furnace will pull over 15 amps---most are at about 10 amps. The largest 40k BTU draws 9 amps. The smallest(20k btu) draws 7 amps. Doug

As mentioned, you can upsize some, don't oversize to much.

I went from a 25K to a 30K with no regrets, warms up the trailer a bit faster.

Couple of things to consider, read the manufacturers installation manual, upsizing most likely WILL require adding at least ONE additional hot air duct. Manufacturers installation manual should tell you exactly how many hot air ducts will be required (and/or the total square inches of outlets) for the size of furnace you selected.

Upsizing also may require upgrading the 12V wire and fuse size from the fuse panel. Sometimes the bigger furnace will have a blower that draws more amperage than the original which may require you to upgrade the wire and fuse.

dougrainer,

Thanks for the "rule of thumb". It will be useful when recommending watts of heat.

I do think you meant 1000 btu's per foot, not one.

You can go to a 35BTU. The rule is 1000 BTU for each foot length of the RV, except, now, you have to add a little for slide rooms. Doug

Its a Suburban NT-30k

pianotuna wrote:
Too large may lead to short cycling. Too bad they no longer make two speed furnaces for RV's.

I thought they still did :h: http://www.rvpartscountry.com/Rv-Furnace-Atwood-Excalibur-XT-15000-22000-BTUs.html

groundhogy wrote:
More Mechanical Engineer questions...

Does higher cause fiwer? lol

My original furnace is 30k BTU.

Can I go higher? If so how much and what is the effect?

What is the model of the existing furnace? Some newer same brand with higher or even the same BTUs might not quite fit. Eg, new Atwood ones for older 85xx models are longer and require some sort of fiddling at the back where the ducts go to. Could be a huge PITA.

Too large may lead to short cycling. Too bad they no longer make two speed furnaces for RV's.