Have you seen claims of wood having a burn time 72 - 96 hours?
That you can put a furnace 500 feet from a building? That you can
heat a 5,000 sq ft house with a furnace having a 100 gallon tank?
How about 300,000 BTU from the same furnace? Smokeless furnaces?
Stainless steel doesn't rust and is the best metal to use? They
are all false!
Wood furnace manufacturers are full of hype and false claims
because they want to sell you a furnace. What happens when you
buy their furnace only to find out you have to fill it 2-4 times
a day because it's undersized for the job? Or because they
exaggerated it's heating capacity and BTU rating? You're stuck!
They have your money and you have a lifetime chore.
Filling the Furnace
Sure wood will last 72 - 96 hours in the SUMMER when you're only
heating hot water or your pool. But the fire will sit there and
smolder and smoke and smoke and smoke! Be sure you don't have any
neighbors nearby. Smokeless furnaces? Preposterous.
A furnace will normally have to be filled twice a day. Not too
bad. Once in the morning and once at night. You have to do MUCH
more than that with a regular wood stove or fireplace insert, in
the house, because it's so much smaller.
Having the wood outside saves time and energy and keeps the wood
chips, dirt and bugs outside - along with the smoke - increasing
indoor air quality.
Therein lies another good aspect of owning an outdoor wood
furnace. It's outside next to the wood pile and the wood you can
use is much bigger and cheaper. Instead of split wood 16-18"
long, you can use whole rounds - typically up to 6" to insure a
complete burn. You can't load it with 12" logs as many say!! They
will never burn completely. One or two along with smaller ones is
OK. A cord of split wood costs between 120 and $140 in this area.
30" logs, unsplit, are only $90 a cord because there's a lot less
You do need some split wood especially to get a fire going, so be
sure to include some in the mix.
How Far Away?
Now many makers claim you can put a furnace 500 feet from the
building to be heated. Sure you CAN but how efficient is that?
500 feet of PEX pipe underground times two is 1000 feet. Remember
there is a return line. 180 degree water traveling 500 feet in 60
degree dirt (assuming that it is below the frost line) results in
tremendous heat loss. That cannot be denied. Above the frost line
as many say you can do? Forget it. The ground can be frozen solid
and below 32 F.
Then the water has to be piped back to the furnace resulting in
more heat loss, requiring more wood to reheat it. You will also
need a much bigger pump - consuming more power - to pump the
water that far as well. A typical pump is 110 watts, not much
more than a standard light bulb. Not possible with those long,
The bottom line is that if you started out with 160,000 BTU
capability, you have lost 20-30% of that underground, or 32 -
50,000 BTU, gone. Now you need a bigger furnace too.
Firebox Size and BTUs
Those 100 gallon tanks are too small except for the smallest of
houses. A 100 gallon tank would have to be heated up to 435
degrees F (and we all know that water boils at 212 F) to extract
300,000 BTU - simply not possible.
A BTU is The amount of heat needed to raise the temperature of 1
pound of water by 1 degree Fahrenheit.
That means that if the normal water temperature is 60 degrees F
and it needs to be raised to 180 degrees, that it will take
163,200 BTU to do this. Remember that a pound of water is about
16 ounces. A gallon of water is approx 8 lb.
(170 gal. x 8) = 1360 lb. (A standard Shaver Furnace holds
gallons, so we're using that as an example.)
Temperature change = 120 degrees
1360 x 120 = approx. 163,200 BTU. This is a reasonable figure
using dry hardwood, as has been proven.
Have you ever seen a fire that didn't smoke? People actually
believe these claims!
Use common sense and do your homework.
Using stainless steel in a furnace isn't quite as clear. While
typically stainless steel is not known to rust, such as with
flatware or bright metal trim on a car, it is not being welded.
(I have a 68 Impala with flawless window trim that is stainless
Also, not all stainless is a forever product. There are many
grades and some of them are subject to rusting and corrosion.
Automobile exhaust systems are made from one of the lower grades;
they resist high temperatures but totally corrode. Most outdoor
furnace manufacturers went to stainless steel to get in on the
stainless quality image, but since it s expensive many of them
went to a low-cost, cheap-grade stainless - which is still
subject to rust and corrosion!
Then when the stainless steel is subjected to the high heat of a
welds - especially welding plates together that are .200' to
.250", it loses it's qualities. If it is not carefully retreated.
Post-weld annealing is needed to restore ductility, formability,
toughness and corrosion resistance. Most manufacturers simply
don't do that if they even started with a good grade of stainless
During the manufacturing and welding process for stainless steel,
if the proper quantity and blend of corrosion-resistant and
stabilizing elements are used, then it does indeed become a
forever product. These elements optimize weldability without
the need for post-weld annealing to restore ductility,
formability, toughness and corrosion resistance.
Another negative aspect of stainless is that is expands and
contracts more than stainless. This can lead to stress cracks and
broken welds. You're better off with mild steel.
If the firebox is made of a mild steel 1/4" thick (like a
Furnace), it isn't going to rust through anyway, We have furnaces
34 years old, still in use with no signs of rust. Since the
firebox is constantly being heated, rust doesn't have a chance to
form other than minor (superficial) surface rust.
Make it Simple so it won't break.
So don't believe the claims and buy a simple furnace without
lot of gadgets, solenoids, switches and electrical devise to
break down and leave you without heat. OH, and also avoid
furnaces with all these devices, fans, etc. hanging off the
outside of the furnace - for safety sake.
Check out the SHAVER
OUTDOOR WOOD FURNACE. They've been building
quality furnaces for 34 years! Many, with the original wood
siding, are still in use today. The owner heats his 5000 sq ft
house AND his Hot Water and his Spa with an original 34 year-old
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