False Claims by Builders!

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 outdoor wood 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 furnace? Preposterous. Unless it has a catalytic converter, and then it’s smoke LESS..

smokeless outdoor wood furnaces

Smokeless outdoor wood furnaces? Preposterous!

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 labor involved.

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 light bulb.

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) = 1400 lb.  (A standard Hyprotherm FLRH-185 Furnace holds 185 gallons, so we’re using that as an example.)

Temperature change = 120 degrees

1400 x 120 = approx. 168,000 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.

Stainless Steel

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 broken welds. 

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 quality mild steel 1/4″ thick (like a NC Furnace), it isn’t going to rust through anyway. We have furnaces that are 10 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!


Heating with Wood is Carbon Neutral

3rd article:

Northern Ontario Business
Outdoor wood furnace market on fire

4th article:

Trouble Shooting Guidelines for Outdoor Wood Furnaces and Smoke

5th article:

Mild Steel vs. Stainless Steel in Outdoor Wood Stoves

6th article:

Have Wood?

You can cut your energy bills to almost zero!