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. Unless it has a
catalytic converter, and then it’s smoke LESS..
A furnace will normally have to be filled twice a day in the dead of winter.
Maybe once a day in the early and late winter. Not bad! Once in the morning
and once at night. You have to do MUCH more than that (5-6 times a day)
with a regular wood stove or fireplace insert, in the house, because the firebox
is 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 and helping your lungs.
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, (like with an indoor fireplace or woodstove) you can use whole rounds –
typically up to 6-10 to insure a complete burn. Despite what you might think
at first, an 18″ x 18″ door is plenty big enough. A 32″ log only 15″ round will
weigh 112 lb and most people don’t or can’t left a log even that big!
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 50-60
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 is usually well 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 80 watts, less than a standard
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.
The bottom line? You must ALWAYS use insulated pipe and bury it
below the frost line. And do you really want to walk that far anyway?
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 170
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. Unless it has a
catalytic converter, ALL fires smoke.
Out furnace generally only smokes when the fire is idling. When the
fire is hot and working hard, there is virtually no smoke.
Furnaces with forced air drafts smoke less than natural drafts.
A fan that feeds air into the bottom of the fire, like a blacksmith
forge is best.
Using stainless steel in a furnace isn’t as clear as it might seem.
While typically stainless steel is not known to rust, such as with
flatware or bright metal trim on a car, remember, it is not being
welded. This weakens the stainless steel.
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 is expensive many of them
went to a low-cost, cheap-grade stainless – which is still
subject to rust and corrosion!
Most stainless steel furnaces are as little as 1/16th of an inch thick,
because it’s so expensive! Do you really want something that thin?
Then when the stainless steel is subjected to the high heat of a
welds – it has to be 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 steel.
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 – except for cracking. 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 mild steel. This leads to stress cracks and
You’re better off with mild steel. Boilers have been
made from mild steel for decades!
We’ve replaced stainless steel furnaces from other manufacturers
that were only 5 and 6 years old! WHY? Because they were split
wide open (due to stress cracks) and couldn’t be repaired.
If the firebox is made of a mild steel 3/8″ thick (like a Shaver
Furnace), it isn’t going to rust through anyway. We have furnaces
25 years old, still in use today – with no signs of rust. Since the
firebox is constantly being heated, rust doesn’t have a chance to
form because the water is virtually void of oxygen..
Make it Simple so it won’t break.
So don’t believe the claims and buy a simple furnace without a
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, like on the front door – for safety sake.
Circuit boards are fine for TVs but no the harsh elements a furnace
is subjected too. One power surge and it’s toast!
Check out the SHAVER OUTDOOR WOOD FURNACE. They’ve
been building quality furnaces for 36 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 35 year-old
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