Re: Combination Oil/Wood Water Boiler

On Sat, 31 Mar 2007 11:43:01 -0600, "nonsense@xxxxxxxxxxxxx"
<nonsense@xxxxxxxxxxxxx> wrote:

Larry Caldwell wrote:

First thoughts:

A 40 gallon reservoir is too small for domestic hot water, much less
domestic heat. You are going to have to keep a wood fire going ALL the
time to get even heat, one of the great advantages of hot water heating.
I would think you would need at least 500 gallons of water storage, and
even with that the oil burner would come on a lot.

Efficient wood combustion requires a high temperature in the fire box,
at least 500 degrees and 800 degrees is better. A water jacket very
efficiently cools the fire box. In the 19th century, steam technology
took a great leap forward when they invented the fire tube boiler, but
these folks brag about not using tubes?

In industrial applications tube type boilers lose a lot
of their efficiency unless accumulating soot is removed
frequently. I expect the same is true for smaller wood
fired applications. Even when the wood is free, such an
operation is still labor intensive.

Wood furnaces don't soot up like industrial boilers where the crud
that collects on the tubes is often semi-fused. Creosote collects in
the upper parts of the fire box and in the chimney but it isn't as
adherent as fused fly ash. My experience is that in the off-season
the stuff absorbs moisture, curls off the surfaces and either falls
away or can be easily brushed away. Either way, not a big problem.

I'm not surprised to see Larry running off about what he knows
nothing. If I could inject a little actual experience with a water
jacketed wood furnace.

Last summer I helped a friend install a pad-mounted outdoor automatic
water jacketed wood burning furnace. I'm sitting here and for the
life of me I can't recall the brand name. Sorry.

This furnace is about chest high and the combustion chamber reaches to
within a foot of the ground and about 6 inches from the top. It is
water jacketed on all 4 sides and the top. Water is at atmospheric
pressure with automatic fill and an atmospheric condensing vent.

The fire control is a solenoid-operated damper and a squirrel cage
forced draft fan. A thermostat in the water jacket controls the fan
and damper and keeps the water at 180 deg.

He heats an uninsulated country house and a 6,000 sq ft insulated
metal building shop plus hot water through a 40 gallon tank (more than
adequate).

The unit is very well insulated - even at high fire no place on the
outside rises more than 20 deg F over ambient.

The firebox is huge, capable of holding 7 or 8 8-10" diameter unsplit
logs, which is what he burns in it. This loading is good for a day to
a day and a half even in the coldest weather we had this year (low
teens). He built a fire last fall and it burned all winter, only
letting it burn out last week.

The fire either burns at full fire with forced draft or is completely
deprived of air. This works very well. There is a little smoke
during the transition to full fire but the forced draft quickly brings
the smothered fire back to life.

The flue is about 4 ft of 8" stainless steel welded *** metal pipe
that sticks straight up out of the top. I was there recently with my
infrared pyrometer. The weather was 50ish. The hottest spot on the
flue at high fire was about 250 deg F. Just right. So much for the
Caldwell theory on water-jacketed furnaces.

I asked him to keep track of the wood he burned. He did but not very
well. He previously heated his house with a conventional wood stove
and said that this thing used far less wood than the wood stove even
though it heated two buildings.

The 40 gallon tank is more than adequate for two people. In reality
the furnace can heat the water in real time so the tank capacity
doesn't matter. He has an electric heater for summer use.

At the end of the season the walls of the firebox/water wall above the
active burning area are coated with a thin shiny layer of jet black
creosote. Really more of a film, as it retains the texture of the
stainless steel underneath. This film established itself early on so
I don't think it will get any thicker.

I admit that I had my doubts about a water wall furnace but since the
company offered a money-back guarantee and this was otherwise the best
unit we could find, he bought it. I'm impressed. Very impressed. I
have a feeling one of these will appear at my place to replace a wood
stove.

John
---
John De Armond
See my website for my current email address
http://www.neon-john.com
Cleveland, Occupied TN
All great things are simple and many can be expressed in single words:
Freedom, Justice, Honor, Duty, Mercy, Hope. -Churchill
.

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