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Wood burning stoves and backboilers

M

Malc

Jan 1, 1970
0
Ok I'm in the UK but presumably similar things apply world wide?

I'm looking to get a woodburning stove as gas prices here are rocketing. It
would be nice to get one with a backboiler attached so that I could plumb it
into the central heating system and get it to heat the upstairs rooms if
possible and/or the hot water. The snag is that a standard non boiler stove
can be got for less than 200 UK pounds whereas one with a backboiler retails
at over 400UKP. Why the sudden jump,it's not rocket technology after all?
How easy would it be to make a backboiler and retrofit it? Any
recommendations?

--
Malc

"It is inconceivable that anything should be existing. It is not
inconceivable that a lot of people should also be existing who
are not interested in the fact that they exist. But it is
certainly very odd."
 
G

George Ghio

Jan 1, 1970
0
What it comes down to is the fact that if you have to ask then you
probably don't have the required skills to do the job.

This is not to say you could not learn them.

Tools required:

Oxy set

Pipe bender

Metal saw

You will need to know how to braze weld copper

And you will need a plan or an item to copy that will fit the stove

Other than that it's too easy to make a boiler for a stove.
 
G

George Ghio

Jan 1, 1970
0
malc said:
That's an unwarranted assumption. I have never seen the inside of a
stove hence the need to ask.
Not true. The comment is based on metal working skills not whether you
have ever seen a HW Jacket.

Take for example the fact that if you use the common phosphor bronze for
welding any of the joints inside the fire box then you do not have the
required metal working skills needed to carry out the work.

The last boiler I made was to replace the failed unit in my stove. It
was a single coil of 25mm copper pipe. That is to say from the inlet the
pipe came in formed one complete loop and then exited. The inlet was
50mm lower than the outlet.

My stove will raise the temp from around 10c to more than 60c in twenty
minutes in a 60 gal tank.

Do you own or have access to an oxy set? Y/N

Do you know how to use an oxy set? Y/N

Do you own or have access to a pipe bender with a mandrel. Y/N

Do you know how to bend 1in copper pipe without a pipe bender, with out
flattening it? Y/N

If you have the skills then it is too easy to do and you would know this.

There is nothing stopping you from learning what you need to know.

Ask to see the HW Jacket for the stove you are looking at, measure it
and draw it up. Buy the materials. Cut and bend as required. Weld it all
together. Install it in the stove. Maybe a days work start to finish.
Off to the pub for a well earned pint.

And if you build a fire in the stove before you have connected to the
storage tank, well, you get to do it all over again.

It is not hard to do if you have the tools and skills. If you need to
get someone else to do the work... Just buy the bloody thing with the stove.

So tell us, do you have access to the tools and do you have the skills?
 
G

George Ghio

Jan 1, 1970
0
malc said:
George Ghio wrote:




Yes and yes. However I don't have the stove or the fireplace or the
whatever yet. What I was trying to ascertain was whether it was easy
enought to build a backboiler.

Then you will have no trouble in building one. Just bronze weld it.
Remember that the usual phosphor bronze used by plumbers for copper pipe
will burn away if exposed to flame and should not be used within the
fire box.
 
A

AJH

Jan 1, 1970
0
Yes and yes. However I don't have the stove or the fireplace or the
whatever yet. What I was trying to ascertain was whether it was easy
enought to build a backboiler.

Let's look at this another way, many multifuel back boilers seem to
compromise good combustion of wood, as they act to quench the fire
chamber. Non back boiler designs use the combustion chamber exterior
surfaces as their heat exchange with the outside room.

We know we don't want to allow the flue gas to be cooled below its
dewpoint but we may be able to change the design a little and move the
water heating parts outside the stove.

First thing is to raise the flue temperature to about 500C ( don't
expect to keep the fire in, run it hot and hard and store the heat in
the water). I'd try lining the firebox with either refractory or more
easily cast iron cut from a broken drain cover. Now with more of the
heat going up the chimney you can use this to heat the water. Simplest
would seem to be to solder a copper tube wrapped around the flue. You
need to ensure that your heat exchanger isn't so efficient that the
bulk of the flue gas falls below about 150C.

Note the requirement in UK to have a cold feed and expansion pipe
either side of the heat exchanger with uninterrupted direct vent to
safety also consider Part J regulations and certification
requirements. These things tend to make D-I-Y non practical.

AJH
 
A

AJH

Jan 1, 1970
0
We do, if we want 15% more heat from the wood :)

It could be a variable amount of extra heat, depending on dryness of
the wood, I haven't done the calcs recently but IIRC every kg of oven
dry wood completely burned gives a kg of water. If 1kg of wood being
burned has a 50% mc (wwb) then the wood energy released is about
2.7Kw(t) and the water vapour up the chimney contains about 1.1kW(t).


The trouble is many UK chimneys will be cement lined brick and the
products of incomplete combustion will be tarry and acidic, one will
stain through the brickwork the other will eat away the mortar. May be
doable with a fully sealed 316 stainless flue. Again our building
regulations will prove a problem.

I've yet to see a domestic condensing woodburner, I doubt the
combustion can be made clean enough without forced air and feedback
controls, apart from maybe running in vast excess air.

Domestic wood burning has not been very common in UK, largely because
of our concentrated population and lowish tree cover.

AJH
 
One woodstove experts told me "15%."
It could be a variable amount of extra heat, depending on dryness of
the wood, I haven't done the calcs recently but IIRC every kg of oven
dry wood completely burned gives a kg of water.

Wow. IIRC, "kiln-dried" has 6% moisture by weight.
If 1kg of wood being burned has a 50% mc (wwb) then the wood energy released
is about 2.7Kw(t) and the water vapour up the chimney contains about 1.1kW(t).

What are "wwb" and kW(t)? Kw is power, not energy.
The trouble is many UK chimneys will be cement lined brick and the
products of incomplete combustion will be tarry and acidic, one will
stain through the brickwork the other will eat away the mortar.

Outdoor woodstoves and liners and chimneys in the living space...?
I've yet to see a domestic condensing woodburner, I doubt the
combustion can be made clean enough without forced air and feedback
controls, apart from maybe running in vast excess air.

I've been thinking about an outdoor woodstove design with a little forced
air and an insulated combustion chamber and a chimney that heats air which
circulates through a nearby house.

Nick
 
A

Arnold Walker

Jan 1, 1970
0
We do, if we want 15% more heat from the wood :)

Nick
You would need force fired.(blower)At the lower temperature.


Nick

The other guy with the flue mounted coil .....that is an
economizer/normalizer coil.
A good boiler is reverve flow ...in other words, cool feed water/steam is in
the cooler regions
and hot coils like superheat are in hot parts.Granted I am talking relative
to each other in terms
of temp......not actual temp. since a 190F. in the cool area will have you
removing your hand in short time
,if you touched it.....

Any hotwater appliance has a popoff valve.Hotwater heater ,boiler,or
whatever ,as a common sense precaution.
In some cases several.....

Reminds me of another item....most of what I said is for a monotube boiler
,using your tubing.
Unless you use a feed pump, you need a check valve and a flow restrictor
(unless you live in Buckingham not more than two liters
feedwater a minute)on a water line to match the BTU's of steam you are
pushing upstairs. With your existing waterheater being the
" hot tank return".

In otherwords the water enters the stove from the city/well water line
.....is preheated,then turned to steam.
Goes to your radiators upstairs ,then to co-heat your waterheater for
washingclothes /bathes.
Otherwords ,your stove is a tankless boiler/waterheat(depends on waterfeed
rate and fire in stove) ,for folks that don't know what
a monotube is.

Unless you get into some outrageous firing rates....coppertubing should
work.If not, you need to look at braketubing suppliers
(carbon steel) in the superheat regions.The higher the heat the higher grade
of materials you need to run.
I already go part way with that idea ,since I run a woodfired 10Kw homepower
steamplant with the waterheater being plumbed as a aux. condenser.
Granted the main condenser is a jet condenser.I have a problem of of too
much hotwater at full generator load to heat a house and hot waterheater.
(about 50,000 btu's continuous 24/7) Presently looking at biodiesel
processing to address some of that excess heat.

Home heating oil and autofuel prices seems to be a bigger ticket item for
the general public than PVs and windmills.Here at least.....I don't know
about the rest of the country/world.

You have greater control on the heat distribution with water radiators than
a stove on central radiation from the living room/den.
Be it, the old tyme radiators or passive heat floors.
 
A

AJH

Jan 1, 1970
0
One woodstove experts told me "15%."

OK I don't claim to be an expert.
Wow. IIRC, "kiln-dried" has 6% moisture by weight.

Yes but wood is essentially a carbohydrate (OK not quite as it has a
bit too much carbon in the lignin) so for overall energy purposes you
can think of it as carbon with water attached, then you can model all
the energy as coming from the carbon and the water as carrying off
some of the energy as latent heat. With the basic building block being
C6H12O6 you can see the molecular weight is 180, the water content
after combustion is 108, so just over 2/3 the weight of the dry wood
is in fact water, add the free moisture content to this, multiply this
by the heat of the flue gas over ambient and the total latent heat of
water going up the flue and you see the potential.
What are "wwb" and kW(t)? Kw is power, not energy.

Moisture content referred to the wet weight i.e. wet weight basis.

You're completely right about my missing out the hours, I did a quick
mental calculation from megaJoules to kWhr and then mistyped it. The
(t) was for thermal so that it would not be confused with electrical
power. I'd have been better off sticking with MJ.
Outdoor woodstoves and liners and chimneys in the living space...?

I was guessing that the original poster would have an internal
fireplace and brick chimney in one wall, typically originally built to
burn coal on an open grate.
I've been thinking about an outdoor woodstove design with a little forced
air and an insulated combustion chamber and a chimney that heats air which
circulates through a nearby house.

Much the same as my proposition but wet systems (hydronic?) are
prevalent here and there is little forced air circulation. Forced air
has advantages if you can get it to cling to the floor.

The other difference in language is that we call our water heaters
boilers, they are not as they typically put water out in the 90C
region and the return will be ~80C in older ones and around 50C with
condensing boilers and especially with under floor heating.

AJH
 
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