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Very little on this topic webwide. I know that narrowboat owners are embedding coils in firebricks so that's the way i'm going, also i can do this for free which is important.

It's a 4.7kW stove and i reckon i can get about 1.2m of 13mm 316SS tubing bent up and recast into a new back firebrick. I'll drill the stove casing and use clear drilled Swagelok fittings screwed into the stove and just nipped over the tubing to make it airtight; the tails will connect to copper plumbing tubing outside the stove.

The problem i've got is working out how much heat in watts will be directed at the coil, and how much temperature differential this will work out to on the inlet/outlet. I'm using a 1500w small double radiator. Anyone have a guess in heat Watts per metre of tubing?

The best guess i can arrive at is 500w. You lose 25% up the flue which leaves roughly 3.9kw to be distributed equally in all directions, and not all of it gets out, some of it bounces around inside. Estimations from thinking about it and looking at the construction of boiler stoves is that it goes by surface area of the firebrick minus a bit. I plan to tack tig wire between the loops to capture more, but heat cycling welds is problematic.

A 4.7 kw stove is generally understood to mean one that produces under optimum conditions 4.7 kw of useful heat, and not 4.7 kw in total including flue losses.

As to how much heat could be captured by an improvised water coil, who knows ! I feel that there are too many unknowns to produce any reliable estimate.
Be very careful in experimenting, if the water overheats and boils then a steam explosion is possible. That could be fatal, or start a fire.
A stove with a proper back boiler might be a better option.

It'll be running at atmospheric and pumped with a small pump of some type. My original fag packet design had a cooling loop managed by a TCV which diverted inlet water up to the expansion tank in the loft, this then became a cooling pond with the pump suction drawing out from the bottom. The 'cooling pond/expansion tank' will be an old bog cistern screwed to the chimney breast. I now don't think i'll be able to harness enough heat to need the cooling loop, so the cistern will have a single entry and be vented to atmosphere.

I'd love to buy a boiler stove, but i live on my own so i don't really have the brass, and in a crashing world i want to learn to do as much as i can for free. I want to get my mind comfortable with the idea of taking on new challenges by working through it methodically and taking advantage of what i can get for free.

One of the hidden challenges of a post-crash or crashing in steps world will be becoming comfortable with the idea that you CAN do it yourself. There is an epidemic of people sleepwalking into practical paralysis these days; how many people do you see changing a wheel by the side of the road? None, they think it's too complicated and ring the AA. People with practical talent have learnt to talk themselves out of it and ring an 'expert'.

I am in general in favour of DIY and experiment.
Anything involving improvised heating coils and stoves scares me a bit though. Solid fuel is in some respects more dangerous than gas or oil. A gas or oil burner can be extinguished almost instantly if something might otherwise blow up. Solid fuel can not.

What is the object of the exercise ? Might there be a safer way to achieve the result ? Gravity circulation wont work well or reliably with narrow pipes, and pumps scare me in this sort of application due to the risk of the pump or electricity supply failing with a hot fire.

Interesting project. I guess you'll use "heavy" refractory cement to cast the block - to conduct heat better. Would it be worth wrapping the stainless tube in thin plastic or similar, so that after the initial burn there would be a small gap between the tube and the concrete ? I know it's not ideal for heat transfer but it might stop the expansion of the pipe from cracking the refractory. Better would be encasing the tube in a powder - like the heating wire in an electric heating element - I'm not sure how you'd do that.

For initial testing you could put an additional layers of soft refractory board between the cast block and the firebox, to limit the heat transferred until you are comfortable with it, then remove as necessary.

I look forward to seeing your results.

Whilst I am aware of one SOMEWHAT similar improvisation, I am rather nervous of posting details in case any serious accident results.

Catweazle wrote: ↑03 Mar 2022, 21:14 Interesting project. I guess you'll use "heavy" refractory cement to cast the block - to conduct heat better. Would it be worth wrapping the stainless tube in thin plastic or similar, so that after the initial burn there would be a small gap between the tube and the concrete ? I know it's not ideal for heat transfer but it might stop the expansion of the pipe from cracking the refractory. Better would be encasing the tube in a powder - like the heating wire in an electric heating element - I'm not sure how you'd do that.

For initial testing you could put an additional layers of soft refractory board between the cast block and the firebox, to limit the heat transferred until you are comfortable with it, then remove as necessary.

I look forward to seeing your results.

Heavy refractory cement? The formula i've got is:
1.5 parts cement
2 parts silica sand (how does this differ from building sand???)
1.5 parts perlite
2 parts fireclay

This was researched when i was intending on building a blast furnace for smelting lead ore, it doesn't work very well in a fire surrounded by bricks, i lost quite a bit of product in the bottom of the fire bed.

I'm going to wrap the coil in newspaper and masking tape, i think it definitely will need some room to grow.

No idea what refractory board is, but thanks for the tip!

adam2 wrote: ↑03 Mar 2022, 21:01 What is the object of the exercise ? Might there be a safer way to achieve the result ? Gravity circulation wont work well or reliably with narrow pipes, and pumps scare me in this sort of application due to the risk of the pump or electricity supply failing with a hot fire.

The object is to be less reliant on gas and warmer. My house is old and draughty and the cheap 80s extension is a heatsink. The place is full of rats due to masses of surprise disused drainage running under my house and the whole row of 4, it's big stuff ranging in size from 4" bog legs to an 8" main sewer. I'm going to have the extension gutted, the drains dug out by machine, and the extension rebuilt to modern insulatory standards plus a new roof of the heritage structure. However builders are booked up for years and it's hard work even getting one to answer an email.

I could use a standby pump, but like you say if power fails...? It could be SELV with a lead acid backup. I'm not at all worried, if the worst happens steam will force all the water up into the expansion tank. It will make a right old racket as steam superheats and blasts fluid out, but the pipes should take the pressure and it is vented.

To me, putting something that you want to transfer heat into - in your case water pipes - inside something that is designed not to transfer heat very well, fire brick/cement does not seem to make much sense. Sure it is very safe but is not efficient.

The only ‘back boilers’ I have seen are basically metal boxes that have a pipe inlet and outlet. Whether it is to maximise surface area or heat transfer I am not sure. In my experience of a fire with a back boiler and one with a similar shape and size without a back boiler, the one with puts out a lot less heat to the room.

Fully understand this is a DIY job but could you not weld a metal box out of thick plate and pressure test it for leaks etc instead of doing it with tubes. Also thin pipes and gravity circulation do not get along.

POSTED RATHER RELUCTANTLY AND TO BE USED WITH CARE AND ENTIRELY AT YOUR OWN RISK

I am aware of one case in which a water heating pipe was retrofitted to an existing stove.
Heavy duty steel pipe was used, running across the width of the stove, about halfway up and thereby directly exposed to the heat of the fire. Being directly exposed to the fire, the small surface area of the pipe picks up significant heat.
This pipe protrudes a few inches each side of the stove.
One end is rigidly fixed to the side of the stove by means of locknuts engaged with a thread cut into the pipe.
The other end of the pipe is a sliding fit through a VERY SLIGHTLY over sized hole in the side of the stove. This pipe entry needs sealing with glass fibre rope or other suitable material, to avoid unwanted air ingress to the fire.
On the inside of the stove, make good the fire brick or fire board in some suitable way. A SMALL gap in the fire brick around the pipe entry is acceptable as the pipe full of water at less than boiling point will reduce any overheating of the steel work of the stove.

Use one inch or larger pipe to minimise friction. Connect the two ends of the pipe via right angle elbows to vertical flow and return pipes to the rest of the system. Circulated fine by gravity, no pump needed.
The prototype was used to heat bath water in a large cylinder for a family. Heat into the water was estimated at between one and two kilowatts, from knowledge of water capacity and rate of rise of temperature.

Uncertain how long the steel pipe would last, directly exposed to the fire. Once the idea has been proved, keep a couple of spares, ready threaded and cut to length.

POSTED RATHER RELUCTANTLY DUE TO THE POTENTIAL RISKS, USE IS ENTIRELY AT THE RISK OF THE USER. BE CAREFUL.

In my previous house I had a Dunsley Yorkshire stove with backboiler. It was excellent and I wish I had brought it with me. The system had two pumps, one for the heating coil in a 210 litre stainless tank, the other for the main radiator circuit. It also had a 35mm gravity circuit to upstairs bathroom radiators, as a way to dump heat if the pumps failed. Thermostats on the boiler outlets controlled the pumps, so that they always came on when the stove was warm but shut off when the fire was out, so as not to circulate warm water through the stove when it was unlit and dump heat up the chimney.

The Yorkshire has two combustion chambers and meets DEFRA air quality standards, it also has built in adjustable thermostat that controls the air intake.

Sadly, it's now £2500, so I understand why a DIY option is attractive.

Some stove manufacturers offer an add-in back boiler, like this one https://salamanderstoves.com/product/sm ... ck-boiler/ , perhaps it's worth looking at their designs.

Catweazle wrote: ↑04 Mar 2022, 18:05 Some stove manufacturers offer an add-in back boiler, like this one https://salamanderstoves.com/product/sm ... ck-boiler/ , perhaps it's worth looking at their designs.

Yes!!! That's what I remember a back boiler looking like though that one is tiny compared with the one from our Rayburn that we had to take out as it froze up while the house was under construction and split. We fixed by welding.

One other thing you may consider is in Australia there used to be water heaters called 'chip' heaters (the chips being chips of wood that were fed into them). They were water heaters that were used up until large scale electrification of suburban housing that were to the best of my knowledge a large rocket stove used for heating water for baths. The water was 'once through' and not circulated and the system was always kept open so no pressure could build up.

I decided not to go for a coil in the end, but turned my stove pipe into a heat exchanger and used my existing flexible ducting/blower setup to move hot air about. The only problem is that the air coming off the heat exchanger is hot enough to damage the ducting and tape holding the bits together, and the 12v blower gets really hot. When you turn the blower up high, fumes come out of the air vent and set the smoke alarm off, so something is operating well over its rated temp. The first 3m of ducting up into the loft is ally 300degC rated, but the rest is the plastic/foil 140degC stuff held together with gaffer tape.

To do this properly what ducting and fan setup am i looking at? Where might the parts be purchased? Thanks in advance

I have a fire with a metal flue and I miss the old brick chimneys. The metal gets way too hot and I worry about it setting fire to the house.

Maybe you should try to build a "tile stove" with a huge thermal mass in the middle of you heat losing extension.