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

[Jelly_Sheffield]

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.

That's no big surprise.

In modern boiler design, water-wall is usually implemented as a way to reduce/control the combustion chamber temperatures whilst acting as an economiser (water preheat) for a water-tube system.

As a design water-wall is far less efficient at heat transfer than other designs due to the geometry, but is also robbing the combustion chamber of radiative heat that would be re-radiated from the refractory helping to maintain chamber temperature.

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

That's a different part of the building regs (Part L), and I would agree it does seem like a ham-fisted approach that neglects the benefit of energy recovery for suitably designed appliances.

I was referring to Part G which is about the safety of such systems, and specifically pointing OP to the requirements for venting and heat dissipation to prevent potential boiler failure, system pressurisation, or excessive DHW temperatures... All good stuff!

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

I grew up with a coal-fired back boiler, and whilst I occasionally get nostalgic for it, would absolutely not have one as a primary source of DHW or central heating (there's pretty much always a better alternative).

Fine as supplementary heat, but utterly impractical to depend on once you've had a combi-boiler.

 

[Turbo]

In modern times a back boiler is a way of using waste heat from a fire to supplement a hot water system.

It is old fashioned and inefficient but a lot of folk were glad of them after that last storm left us without power for over a week.

The open fire was their only source of heat and hot water. I know some folk that regretted ripping out the back boiler from the fire.

There were other that had fitted wood burners that needed a pump to circulate the water. But with no electricity to run the pump they couldn't light the fire.

 

[waddycall]

What do you feel is the issue with that design?

Risk of poor flue function through -
Increased resistance due to heat exchanger
poor flue flow due to excessive cooling of flue gas
Blocking of flue from fly ash sticking to tar coating on boiler surface (everyone with a back boiler has seen that)

Risk of explosion if that fusible plug let’s go and dumps water into a red hot firebox.

Bear in mind that a workshop wood burner is probably going to spend a lot of time slumbering, burning unconventional fuels and will probably not see a lot of maintenance. It’s not well maintained and monitored commercial plant.

The flue and ventilation are the only safety features of a wood burner so best not to mess around with them.

 

[Jelly_Sheffield]

Risk of poor flue function through -
Increased resistance due to heat exchanger

That's a fair point, it would be more sensible to suggest using 3" pipe in a section of 8" flue, at which point the resistance is actually about equal with a 6" flue (assuming turbulent flow).

poor flue flow due to excessive cooling of flue gas

Blocking of flue from fly ash sticking to tar coating on boiler surface (everyone with a back boiler has seen that)

Those are issues of operation/maintenance common to any waste heat boiler (as you say so yourself), but I can see how decreased visibility of the boiler would potentially make it worse.

Risk of explosion if that fusible plug let’s go and dumps water into a red hot firebox.

The whole point of a fusible plug is to prevent explosion...

Using a standard 325°C plug, positioned so that it's always in intimate contact with water, it can only fail if either:
- the boiler has been run dry, or
- the vent has blocked and the system is now full of steam pressurised to in excess of 140 bar.
In either case, the situation has already become extraordinarily dangerous.

Yes if it lets go it will result in a pretty violent, partially controlled discharge, but far far less violent than if the actual vessel is allowed to overheat to the point of rupture (an actual explosion)
Discharge into the flue is the safest direction

 

[pedrobedro]

How much heat does your stove generate ? Whatever goes towards heating your water will not be available to heat the rest of the stove and surrounding area. You can't have your cake and (h)eat it as well.

 

[gaz1]

it dosnt need the flue release valve

all it needs is a rad plumbed inline of the feed pipe as with the cylinder tank feed

do not use thermostatic valves you can use normal rad valves but remember theres always some idiot that comes along and thinks they know better

or you can use direct flow to the rad with compression fittings

this will create dual flow and extend the heating time to the cylinder whilst also heating one rad up at the same time ( warning this rad will be red hot to touch )

warning

these idiots come in all forms and sizes they think they know what to do

diy
homeowner

they change rad fittings to thermostatic valves
they close the rad valves down

this where the danger lies as they do not understand the gravity feed system direct rad compression fittings stop this though

 

[Jelly_Sheffield]

it dosnt need the flue release valve

all it needs is a rad plumbed inline of the feed pipe as with the cylinder tank feed

Shouldn't need it.

Assuming the rest of the system is correctly installed, that plug would do absolutely nothing.

But what it does do is offer an allowance to reduce the consequences of an unsafe modification in future by a person who doesn't understand the system, a la:

warning

these idiots come in all forms and sizes they think they know what to do

diy
homeowner

they change rad fittings to thermostatic valves
they close the rad valves down

Exactly why I think a last ditch Pressure Relief system is warranted.

This where the danger lies as they do not understand the gravity feed system direct rad compression fittings stop this though

If they're willing to drain the system and change valves, they're probably willing to drain the system and swap compression elbows out for valves.


You can argue that it's just a home project, and we're not building another ICI Wilton, so this level of foresight and mitigation is not needed...

But if we all agree that the potential for a future accident is foreseeable, is that not worth acting on?

 

[gaz1]

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 part is wrong and also dangerous

fitting a rad after the cylinder all your doing is putting that cylinder into a boiling state its far safer to use a rad also on the feed side so it becomes a dual feed system both cylinder and one rad gets hot ( do not use zone valves )

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

i wouldnt use the plug method due to 2 reasons 3 in total due to expense in buying a larger flue

one its dangerous as you are dealing with boiling water/steam and that is dangerous with a hot fire and they do explode rather well with red hot steam blasting out of the fire through the door and up the chimney at the same time

it also causes a creosote issue and that flue will block due to it being there as you cannot brush the chimney properly

 

[Jelly_Sheffield]

i wouldnt use the plug method due to 2 reasons 3 in total due to expense in buying a larger flue

one its dangerous as you are dealing with boiling water/steam and that is dangerous with a hot fire and they do explode rather well with red hot steam blasting out of the fire through the door and up the chimney at the same time

I feel like I must be explaining this incorrectly, because there is absolutely zero chance that the outcome of fusible plug melting would be worse than the alternative that would result from it not being there.

I can say that with a level of personal certainty from seeing one go from about 15 feet away in the firebox of a Loco (fireman's error).



For the plug to let go, the cylinder itself has to be approaching the point where it will fail, causing a actual explosion.

A melted plug suddenly providing a vent path releases that pressure over several seconds, rather than instantaneously, and a lot of energy is dissipated via expansion through the nozzle, further reducing the temperature and kinetic energy of the contents (relative to a sudden failure).

Yes, the discharge is very violent, and yes that does send very hot materials flying out of any openings...

But with nowhere near the force of an actual explosion, let alone the concussive effects or shrapnel that a failed cylinder entails.

 

[Jelly_Sheffield]

fitting a rad after the cylinder all your doing is putting that cylinder into a boiling state its far safer to use a rad also on the feed side so it becomes a dual feed system both cylinder and one rad gets hot ( do not use zone valves )

If it's a closed loop where you add the radiator won't make a difference to whether the cylinder can reach a boiling state or not.

The key thing to prevent that is to design the overall system to maintain the temperature of the thermal fluid at the cylinder no higher than is required to achieve the maximum desired DHW temperature during steady state operation.

That's in turn is a function of the thermal capacity of the boiler, and the heat rejection capacity of the radiator.

By ensuring that the heat dump radiator has enough heat rejection capacity, that as the thermal fluid goes above the DHW never exceed temperature it can transfer energy out faster than the boiler can put it in, the temperature of the thermal fluid stabilises, and heat transfer in the DHW cylinder drops to zero.

Intuitively the radiator being before or in parallel to the cylinder feels logical, but when you analyse the heat transfer, the only variable that matters is achieving sufficient heat transfer out of the system to prevent energy accumulation.

 

[waddycall]

The whole point of a fusible plug is to prevent explosion..

I was referring to the steam created from dumping a load of water into a red hot fire.
If the system is designed and installed in a conventional and proven way these risks would be reduced. No need to reinvent the wheel.

 

[OJCar]

@ OP

Fill with water.
Place on top of wood burning stove.
Remove from stove when water is hot enough.
(If unable to find a handy pouring watering can metal bucket can be substituted.
DO NOT use plastic bucket).

 

[gaz1]

i will show you what happens when water hits a fire

 

[Morris]

i will show you what happens when water hits a fire

That's not an explosion despite the title.

 

[Jelly_Sheffield]

That's not an explosion despite the title.

My point exactly!

It's undoubtedly violent and messy, but it's not blowing holes in walls or demolishing structures...

Unlike actual back boiler explosions:

 

[gaz1]

for start off thats a gas back boiler not a wood stove boiler

think yourself lucky i dont show an actual vid but this forum is teen to adult friendly so im not posting the vid link as its really graphic

now back to your reference of a plug

whilst this vid shows a possible plug melting onto either coal or wood steam boiler the stoker ( rear ) was lucky not to get scalded and so was the organ player lady front as well

possible plug melting could be boiler gained a leak at the base of the boiler

 

[Turbo]

Do you guys realise that the guy that started this thread was a newbie and this was his first and only post!

He's probably been scared off by now by your arguments over the design of back boilers. Please knock it off!

 

[waddycall]

My point exactly!

It's undoubtedly violent and messy, but it's not blowing holes in walls or demolishing structures...

Unlike actual back boiler explosions:

Not sure I’d want to be warming my toes in front of the fire whether or not it’s technically an explosion. I knew it would end up like this

 

[chrismac]

Do you guys realise that the guy that started this thread was a newbie and this was his first and only post!

He's probably been scared off by now by your arguments over the design of back boilers. Please knock it off!

Either that or he is sitting back with a big grin after putting the wooden spoon back in the drawer,

The broad knowledge on this site is tremendous, its a great help to many, but this has gone a bit too far now in regards to explanations of what to do or not to do, I hope this thread can get steered back on track.

 

[addjunkie]

There were other that had fitted wood burners that needed a pump to circulate the water. But with no electricity to run the pump they couldn't light the fire.

This was me, with my back boiler multi fuel stove, open system interconnected through a heat exchanger to our heat store tank, closed system. Generator or no heating. Fixed it now by ripping it out and fitting a log burner not connected to anything but the chimney.

Back boilers, waste heat recovery all work in a domestic setting but must be a vented system, safety valves and fuseable plugs are all prone to failure, because people mess with them, or swap then out for a plug as they are weeping. Its simple process safety. Steam is scary stuff especially if you keep heating it to a point where it starts to superheat before it goes bang. Its all about enthalpy.