Wood burner, flue boiler idea/questions

[Rossey]

I'm throwing an idea about.

I've always wanted a woodburner (free heat) but we have a baby so I'm not allowed (safety aspect)

Well I'm now moving to a house with a garage and thought would be nice to heat it.

I can get wood from work.

Then it dawned on could I wrap a coil around the flue and heat the water tank in the house?

I would do it using a heat exchanger.

What happens to the water in the coil when it gets too hot and boils?

Could I use a motorised valve to shut the flow from the woodburner when the water is hot enough?

What do I do with the flow then?

Do I use an expansion vessel?

What say I don't want to heat the house but have the burner on, what do I do about the water in the coil?

Would a tank/reservoir be needed?

[greasemonkey]

Search round for a wood burner with a water jacket, maybe.
If you were really lucky, you might find a wood burner with a water jacket, and an oil burner in it. I do know of one. I expect £500 would buy it. It's virtually unused. Only problem for you is, it's in Welsh shire.
You will need and expansion vessel, and a a pump, but you can probably incorporate the existing pump.
It takes an almighty amount of wood to boil the system in a house, so long as the pump is going. There is wood burner in the bungalow at home, a big one, and that has never boiled.
The expansion vessel I have on my tiny oil system in my caravan, is a 25 litre drum. Works fine.

[julianf]

I wrote all the below text thinking you wanted to heat the reactor.  If you want to heat the house its different, but i cant be bothered to write more just now!  Ive left the stuff below anyhow.

We have a 300ltr tank attached to the rayburn here.

If we light it in the morning, without the central heating on, we end up having to run hot water down the drain to keep it under 95c ish.

We have no heat leak rad at the moment...

Rayburn is, i think, 4.5kw on coal.  This is burning wood.  Glogs are way hotter....


Buildings regs dont apply to workshop heaters, so you can do pretty much as you choose, but, still, you probably dont want it getting unusably out of control : )

Personally?

See what stove you can get.  Try and get one with a back boiler.
If you manage that, then see what hot water tank you can get.  Get a reasonable sized "indirect" one...  Ideally mount it upside down (you will see why in a bit)

Plump the back boiler direct to the tank, so that it thermosyphons.  You will need a small headder also.  A tank is £35 on ebay if you wait - less on a good day (my last one was a tenner, the one before that 16 quid)

So, you have the tank, hot, from thermosyphoning.  You have a fair bit of water, so boiling is less of an issue...

Your tank is upside down... So the indirect coil, which would have been in the bottom, is now in the top.

Its 28mm, but not finned, like a thermal store's DHW coil would be, but, still, if you pump your oil through it, its going to get hot.

There will be no restrictions, as there would be with a FPHE, and the only thing heating the oil is water - its kept away from the fire.


Youre looking at the cost of a log burner, with a back boiler, plus hot water tank (£35) plus a headdder tank (free if youre lucky) and you have a controllable heat source (ie only pump the oil through the coil when you want the reactor hotter)

[Rossey]

Found this today

http://love2diy.com/488/

The water side of heating the house.

The only safe way I can see it being done is to close the coil both ends and drop the water out.
Starts getting complicated.

I'm gonna make a waste oil/wood burner out a couple of
Gas bottles.

Am thinking using a 4" flue or would a 6" be better?

[julianf]

Im not a fan of the coil idea at all.

If you are fabricating a unit, better to use something like a bermuda baxi exchanger.  If you get really keen, i have one you can have - ive been keeping it for somthing, but would let it go to someone who had plans in place (ie if its to lurk in someones garden, ill keep it in mine, but if youre actually going to use it, you are welcome to it : )

Or use both your 4" and 6" flues and make them into a water jacket.  Anything but a low contact area coil...


I saw a setup once where somone had got an old oil boiler, and run the flue from their log burner through that.  That looked great - the heat exchanger would be cheap - you can get an old boiler from ebay for £20 on a good day.

Personally, i would still go for the thermal store near to the heat source, and then pump to the house.  If you have a spare coil in your house DHW cylinder, you could use that, otherwise its a case of fitting a FPHE and the extra pump (2 pumps - one for the transfer from the garage and one for the loop on the fphe)

[Rossey]

I like the water jacket Idea and the like the back boiler better.

I'm not quite following (lack of sleep due to baby)

I was thinking of using a fphe to transfer the heat with a pump to circulate.

But I'm still back to my concern if I don't want to or the water in the house is up temperature how can I stop the water being heated?

Hang on just had a brain wave.

I bypass the burner using a 3-2 valve and put a pressure relief valve on the burner side back into the water tank in the garage.

I'll draw some pictures in a bit as that might help.

[Rossey]

Right this is a sketch of my idea using a back boiler heat changer



The 3-2 valve is controlled by a second thermostat on the immersion tank.
I will have a switch in the garage which will turn both pumps  on.

Ive fitted a pressure relief valve not sure i need to now as its an open system.
When the water boilers in the burner it will go  back to the tank.

And is this the bit i need from a back boiler?



I'm going to mount that inside the burner.

[julianf]

The heat exchanger you show is pretty much the one i have (looks like a more modern version or somthing)


Your diagram is both more complicated & dangerous than it needs to be.

You have the stove.  However the heat transfers to the water is irrelvant, but you have it connected to a large, vented tank, by pipework, with no valves.  So it can thermosyphon.  No pumps.

So, the fire is lit, the water heats, and cycles into the tank, passivly.  No valves to fail, no pumps to fail in power outage.

This tank has a headder tank, with the vent etc.  So the first tank is at atmospheric at all times - if it boils, it just vents to the headder. 


So this is all passive - does not need power.


Then you pump from this tank to the fphe in your house.
At the same time you pump from your house tank, through the other side of the fphe.


Your kit list is -

Burner with some way of getting heat into water.
Tank
Headder tank
2 pumps
FPHE
a load of pipe, and some control circuits for the pumps.


If i had a scanner, id do you a diagram...

[Rossey]

Do what I did

Take a picture and load to photobucket.

I have a 2-3 valve in there so the water doesn't get too hot.

I know the vent doesn't need to be there and it won't work as I'm pumping higher than the garage roof.

What size tank are we talking about?

[julianf]

Height makes no difference, as youre pumping from an atmosperic, vented, system (in the garage) to a sealed loop (the fphe)

Providing your pump has enough power to intially fill the sealed loop to the fphe, then it makes no odds where it is.  Its not like a rad that needs to be bled using the head pressure, as youre pumping that section.

Tank size will depend on how long you intend to run the system for, what you are fueling it on, etc.etc.

Without loading, our rayburn (4.5kw on coal, i think) will boil a 300ltr system in about 10 hrs on slow wood burning.  Burn it fierce and youre looking at a fraction of that time.  Load the tank and longer... etc.etc.

Get a cheap tank off ebay, try it, if its not large enough, weigh it in (about £30 for a small ish tank at the moment), and get another.


If you get a tank with an integrated headder, it will make your life easier.

[Rossey]

By tank do you mean a immersion tank?

And with a header you mean this?
http://www.ebay.co.uk/itm/Manco-immersion-tank-plumbing-heating-water-/291229948363?pt=UK_Home_Garden_Hearing_Cooling_Air&hash=item43cea859cb

And will this do for a header?

http://www.ebay.co.uk/itm/Water-tank-/181509772344?pt=UK_DIY_Materials_Plumbing_MJ&hash=item2a42d35438

[julianf]

The first tank is fine.

It will two ports on the tank, probably 1" bsp.  One in the base, and one at the top.  It may have a small 1/2" bsp drain as well, but ignore that for now.

Fit a tee to both ports.
Plumb the top of your burner water jacket to the top tee, and the bottom of the water jacket to the bottom tee.

Site the tank as high above the burner as is possible.  Make the pipework flow smoothly, without any where where an airlock could hold (ie all uphill, no dips)

On the other branch of the tees, fit your pipework / pump to the fphe in the house.

When the pump is running, it will pull the water through the water jacket too, as you will have both the thermosyphon and the pumped loop to the same tees, but this wont matter - the important thing is that, when the pump is not running, it will still turn over through the jacket, without power.


The tank is, possibly, a little small, but its the type of thing you want.

Id not bother with the galvanised tank - its fine and its cheap enough, but simpler to get an integrated, like the first item. 

[julianf]

i missed it off the diagram, but you would need a small expansion tank like this -



on the fphe loop on the house side too, to take up the expansion / contraction in the tank coil, if there is nothing else on the coil (ie if its otherwise sealed) 

[attachment deleted by admin]

[Rossey]

Thanks for all your help Juilan,

I'll start sourcing some bits now.

[julianf]

How are you doing it at the house hot water tank end?

I assume your tank has at least one coil?  And one immersion boss, or two?

Is the coil in use, and, if so, by what?


For example, if you just have the one coil, and its connected to the boiler, then you would have to tee into this, but then the flow will go through the boiler also, unless you fit an additional valve.  And is the boiler on a sealed system, or vented?

If youre tapping into the boiler system, then you wont need an additional expansion on the house side of the FPHE loop - just rely on the existing provisions for expansion.

You can also get these things to replace immersion heaters with a coil.  Theyre about 40 notes off ebay.  Probably dont work that well, judging by the surface area availible, but may also be a consideration.  You would need the additional expansion tank with one of those.


I do think that the garage side of things in the diagram will be fine, but i dont know what your house side is like, so that may require some more thought.