How to MIG weld a water jacket heat exchanger for a wood stove

[jpsousamacedo]

Hi !

I have a wood burning stove in an agricultural building and would love to be able to heat up a vented indirect water cylinder for hot water for taps. Essentially I plan to fabricate a small mild steel box that will act as a back boiler attached to the rear of the firebox with one inlet and one outlet. Since the system is vented there arent massive pressures on the back boiler as the water should never boil. I have done Mig welding before on small car repairs and building driveway gates so I am not a complete newbie but very far from a pro. My question is could I make a water tight box with a mig welder or is it very likely that there will be leaks and how can I minimize this.

Thank you for your help

 

[Bsmith]

We had a Charnwood multi fuel stove at one time which heated the water and radiators, with the covers off it the boiler looked like 2 - 3 mm thick plate that had been mig welded together. Eventually the boiler started to leak and I mig welded a patch on it which did ok for a few years.

 

[waddycall]

Hi !

I have a wood burning stove in an agricultural building and would love to be able to heat up a vented indirect water cylinder for hot water for taps. Essentially I plan to fabricate a small mild steel box that will act as a back boiler attached to the rear of the firebox with one inlet and one outlet. Since the system is vented there arent massive pressures on the back boiler as the water should never boil. I have done Mig welding before on small car repairs and building driveway gates so I am not a complete newbie but very far from a pro. My question is could I make a water tight box with a mig welder or is it very likely that there will be leaks and how can I minimize this.

Thank you for your help

That’s how most are made. Mig welded mild steel. Be careful you don’t take too much heat out of the flue gasses as you could stall/reverse the flue flow and potentially block it with tar.

 

[zzr1200]

Shouldn't be a problem provided you tack up the water chamber fully and then feather grind your tack weld ends, before fully welding to produce seamless welds.

Seamless welds not hard to do, they just take practice and patience.

PS. Depending on your welding/welders capabilities, I'd suggest 4mm plate with a slight gap for the weld root.

2mm plate can distort/warp easily causing welding problems, also there's a bit more meat in the weld seam to allow for more bite and less possibilities of slight weld faults...

 

[gaz1]

if you want less welds to play with then you create a box and fold up each side that means your doing a precut to bend up each side

this means that your only welding at the corners on the boiler box itself then you cut your plate to match the folded sides as you also need to predrill this plate as well one high one low on opposites sides to each other

you will still need to weld the precuts but only after you have welded that plate onto it

the other way is 2 plates and a bar bent to form a square and weld the plates to the bar dont forget to predrill

a warning with back boilers you will still boil your water a back boiler will easily heat a water cylinder up in one hr any longer after that you boil the water you dont notice this as the overflow tank always refills the lost water

since your in an agricultural building you can also use that system to heat rads up as well while your in there

tank feed only to the return side of the boiler not to the feed side

 

[chrismac]

Shouldn't be a problem provided you tack up the water chamber fully and then feather grind your tack weld ends, before fully welding to produce seamless welds.

Seamless welds not hard to do, they just take practice and patience.

PS. Depending on your welding/welders capabilities, I'd suggest 4mm plate with a slight gap for the weld root.

2mm plate can distort/warp easily causing welding problems, also there's a bit more meat in the weld seam to allow for more bite and less possibilities of slight weld faults...

I am in need of some education, what are seamless welds? I have never come across that term before.

 

[Jelly_Sheffield]

I don't want to put you off, but there's some things I think you may not be fully accounting for, that could catch you out badly.

You may wish to read through Approved Document G (paragraphs G3.10 onwards are the relevant bits) as a starting point.

Refering to the Model Engineering Boiler Code to allow you to test your creation safely once built, would also be wise. If you're intending to run it at atmospheric pressure, good practice would be to hydraulically test it to 1 barg (i.e. 2 bar absolute, double the atmospheric pressure of 1 bar absolute).

If you need help calculating the thermal capacity, pressure relief requirements etc. I can probably point you at suitable resources.

would love to be able to heat up a vented indirect water cylinder for hot water for taps

When you say vented indirect, that means the cylinder containing your tap water is vented, not the heating coils, no?

If you hook up your home-made boiler to the indirect heating coils, that part would be a sealed system unless you put a vent, atmospheric pressure tank, tundish or pressure relief valve in to the hot water circuit that the boiler feeds.

Since the system is vented there arent massive pressures on the back boiler as the water should never boil.

You assume that, but you need to actually ensure that there will be sufficient capacity to dissipate all the energy that the HE can absorb (both by pressure relief pushing water/steam through the vent, and by heat dump radiators, with the former being a fail safe.)

You should have a permanently connected ("heat dump") radiator sized to release enough heat to prevent the water ever boiling, in the circuit, this would usually be after the DHW tank, and you need to size it to suit exactly how big your wood stove is, how hot it burns, and how efficiently your design extracts heat, the whole hot water circuit connected to the boiler needs to be designed to operate on thermosyphon circulation too.

If you do ever start to boil the water, then the size of pipework/vent needed to dissipate the steam and pressure will be orders of magnitude greater than anything sized to allow for the expansion of hot water.

This is why approved document G requires a minimum 19mm vent line on all solid fuel boiler systems.

Welding in a ¾ BSPT socket, and fitting a fusible plug pointed in a safe direction (ideally this would vent into the firebox of the stove, otherwise likely down and backwards away from humans and electrics) is worth considering as a failsafe in case it ever overheats or boils dry.

 

[gaz1]

indirect means a coil system that the boiler feeds has its own water tank to feed the open circuit system

you will also need a tank on the cylinder system as well

to make sure you dont have leaks then weld the joints hot not cold best practice is to follow the arc welding side of it start not at the joint weld into it and weld to a corner and do the same with each corner that will make sure you have a really good water tight seal

 

[zzr1200]

I am in need of some education, what are seamless welds? I have never come across that term before.

Sorry wrong term on my part.

If the tack welds aren't ground to a feather edge/end, there's chance of a small pin hole at the end of a tack weld and between the main weld structure, that could cause porosity (ie:- a water leak), at the stop and start point.

Hence a seamless continuous weld.

 

[Turbo]

Why not start with a piece of heavy wall box section steel and weld end caps into it?

To make your water connections buy good quality black iron fittings to weld on. Avoid cast steel ones as they are hard to weld cleanly. The other option would be stainless steel fittings.

 

[gaz1]

Why not start with a piece of heavy wall box section steel and weld end caps into it?

To make your water connections buy good quality black iron fittings to weld on. Avoid cast steel ones as they are hard to weld cleanly. The other option would be stainless steel fittings.

theres no point with heavy guage steel 4-5mm is ample enough and easier to form a larger box using 1" bar 4-5mm

id use stainless steel connection threaded fittings if the boiler box was stainless steel otherwise not worth doing in the long run as the box will fail before those stainless fittings failed

 

[chrismac]

Sorry wrong term on my part.

If the tack welds aren't ground to a feather edge/end, there's chance of a small pin hole at the end of a tack weld and between the main weld structure, that could cause porosity (ie:- a water leak), at the stop and start point.

Hence a seamless continuous weld.

Got you now, good advice,

 

[Jelly_Sheffield]

Why not start with a piece of heavy wall box section steel and weld end caps into it?

Logically it would make the most sense to use make it out of pipe and fittings, and drops of PN16/Sch40 are easy to come by.

6" is the most a common woodburner flue size, so get a drop of DN100 (4") pipe, a pair of butt-weld end caps, and three thread-o-lets for your desired water pipe diameter, and assemble like so:

There's a 350mm length of DN100 PN16 Seamless going for about £12 on eBay at the moment, 4" end caps are about £11 and thread-o-lets are about £13 last time I checked, then a fusible plug is about £6

So all up material costs (assuming you use the existing flue pipe and water pipes) are just under £80.

For a really solid little heater that would (assuming the flue is lagged) be more thermally efficient than a double pipe HE arrangement with a water-jacket around the flue... And has a level of intrinsic safety as if it's ever overheated, it will dump it's remaining water/steam on the fire extiguishing it.

 

[sardine]

I don't want to put you off, but there's some things I think you may not be fully accounting for, that could catch you out badly.

You may wish to read through Approved Document G (paragraphs G3.10 onwards are the relevant bits) as a starting point.

Refering to the Model Engineering Boiler Code to allow you to test your creation safely once built, would also be wise. If you're intending to run it at atmospheric pressure, good practice would be to hydraulically test it to 1 barg (i.e. 2 bar absolute, double the atmospheric pressure of 1 bar absolute).

If you need help calculating the thermal capacity, pressure relief requirements etc. I can probably point you at suitable resources.



When you say vented indirect, that means the cylinder containing your tap water is vented, not the heating coils, no?

If you hook up your home-made boiler to the indirect heating coils, that part would be a sealed system unless you put a vent, atmospheric pressure tank, tundish or pressure relief valve in to the hot water circuit that the boiler feeds.



You assume that, but you need to actually ensure that there will be sufficient capacity to dissipate all the energy that the HE can absorb (both by pressure relief pushing water/steam through the vent, and by heat dump radiators, with the former being a fail safe.)

You should have a permanently connected ("heat dump") radiator sized to release enough heat to prevent the water ever boiling, in the circuit, this would usually be after the DHW tank, and you need to size it to suit exactly how big your wood stove is, how hot it burns, and how efficiently your design extracts heat, the whole hot water circuit connected to the boiler needs to be designed to operate on thermosyphon circulation too.

If you do ever start to boil the water, then the size of pipework/vent needed to dissipate the steam and pressure will be orders of magnitude greater than anything sized to allow for the expansion of hot water.

This is why approved document G requires a minimum 19mm vent line on all solid fuel boiler systems.

Welding in a ¾ BSPT socket, and fitting a fusible plug pointed in a safe direction (ideally this would vent into the firebox of the stove, otherwise likely down and backwards away from humans and electrics) is worth considering as a failsafe in case it ever overheats or boils dry.

A back boiler is just a basic tank with two pipe fittings it's not a nuclear reactor cooling system.

 

[waddycall]

Logically it would make the most sense to use make it out of pipe and fittings, and drops of PN16/Sch40 are easy to come by.

6" is the most a common woodburner flue size, so get a drop of DN100 (4") pipe, a pair of butt-weld end caps, and three thread-o-lets for your desired water pipe diameter, and assemble like so:

View attachment 490979

There's a 350mm length of DN100 PN16 Seamless going for about £12 on eBay at the moment, 4" end caps are about £11 and thread-o-lets are about £13 last time I checked, then a fusible plug is about £6

So all up material costs (assuming you use the existing flue pipe and water pipes) are just under £80.

For a really solid little heater that would (assuming the flue is lagged) be more thermally efficient than a double pipe HE arrangement with a water-jacket around the flue... And has a level of intrinsic safety as if it's ever overheated, it will dump it's remaining water/steam on the fire extiguishing it.

Please don’t do this.

 

[julianthegypsy]

I've made back boilers out of 5mm plate, simply because I got all the parts for the stove I was building laser cut so did them all together. No problem at all.

 

[Popcorn]

A back boiler is just a basic tank with two pipe fittings it's not a nuclear reactor cooling system.View attachment 491051

Don't know about that, isn't the above just a copy of the one from Chernobyl

 

[Jelly_Sheffield]

A back boiler is just a basic tank with two pipe fittings it's not a nuclear reactor cooling system.View attachment 491051

I am literally repeating the best practice guidance from building control on the implementation of solid fuel boilers in central heating and DHW systems...

Please don’t do this.

What do you feel is the issue with that design?

There's a number of commercially made back boilers for retrofit to solid fuel stoves which use the water-tube in flue approach.

That's just an implementation using readily available materials that minimises the number of pressure tight welds needed.

 

[Dozzer]

last woodburner we added back boiler too, was just simple tank that replaced the rear firebrick, with fittings like @sardine is showing.
Wouldn't do it again as it changed the efficiency of the woodburner.
could never get it to burn quite as hot = when you just dont get any soot on the glass. and it used more fuel.
Also if you go thermosyphon circuit, make sure that there are no downward facing pipes on the hot side. mate boiled his system doing that.

 

[gaz1]

I am literally repeating the best practice guidance from building control on the implementation of solid fuel boilers in central heating and DHW systems...

back boilers are banned after 2005 that includes the wood burner due to not being efficent by the morons

my take on that your enjoying your wood fire in the room whilst your doing that your either warming the water cylinder or your heating the central heating

there efficent enough to do that job whilst you enjoy your stove fire at the same time however i wouldnt want to use it as a main supply unless i had to do as then there not an efficent way to do it