Free hot showers! Part 5: Installation and use of the wonky water heater.

Pictured is my home made wide powered water heater in use
DIY wood powered water heater in use

It works!

Yes, indeed. The wonky water heater, cobbled together from old gas bottles, made the trip from Scotland to the tiny house in France. I used it every day for a period of about 3 weeks and I’m pleased to say that it performs as expected. Perhaps even better than I had hoped for.


The photo above shows the water heater installed a few metres away from the tiny house.

One addition to the heater was the home made ‘spark arrestor’ fitted to the top of the flue. It is actually a steel mesh pen/pencil holder (cost 0.69 Eur!). I don’t know how well it really works as I did see the occasional spark escape. Maybe it would have been worse without it.

You can just about see the copper ‘tails’ from the heat exchanger coil connected to the red plastic pipe. It wasn’t too difficult to find the fittings to join the UK 10mm copper microbore pipe to the 12mm PER plastic pipe (PEX in the UK).

The red plastic pipe came covered in protective conduit, so the pipe was simply laid on the surface of the ground for the duration of my summer stay.

At the other end of the plastic pipe I connected short sections of 14mm diameter copper pipe (I used this size of pipe throughout the house). These sections went up through two holes drilled in the tiny house floor. These were then connected to my hot water mixer valve system, via flexible pipes, as shown below.

Pictured is the tiny house hot water mixer plumbing
Hot water mixer valve plumbing
1Mains water in
2Feed to outside garden tap
3Cold feed to mixer valve
4Thermostatic mixer valve
5Hot feed to mixer valve (from water heater)
6Hot water leaving mixer valve – goes to kitchen and shower room
7Mini isolator valves for water heater
8Cold feed to kitchen and shower room
9Cold feed to water heater
10Hot water in from water heater

Admittedly the plumbing ain’t pretty, but it does work. Surprisingly, none of my solder joints leaked but I did have some trouble with the isolator valves which had compression fittings. I tried using PTFE tape and using replacement olives, without finding a reliable fix. After lots of trial and error, I eventually got the leaks down to one small seep, which actually stopped after a few days.

The mixer valve controls the temperature of the hot water that goes to the kitchen sink, bathroom washbasin and shower. The valve itself cost about 34 Euros from Amazon France. It was simple to install and worked very well.

Although the sink and washbasin have mixer taps, the shower does not. Instead I installed a simple ball valve. This is the red valve next to the soap dish in the photo below. In this way, I was able to run the shower pipe neatly around the edge of the corrugated steel shower walls. This was important as I didn’t want the complexity of a conventional shower mixer which would have required holes to be drilled in the shower walls and pipes installed behind them. Another advantage of my system is that it takes up less space inside the shower, which is only 70cm square.

Pictured is my tiny house shower
Tiny house shower


Operating the water heater was simplicity itself. For firewood, I have lots of small pieces of timber offcuts. These were stacked in a cross-cross fashion in the heater firebox and a gas blowtorch used for a few seconds to light it. I could also have used paper, or firelighters, were I less lazy/impatient.

Once lit, the heater smoked a bit for 5 or 10 mins then burned extremely clean. At dusk it was easy to see the flames leaving the top of the flue. Perhaps there was secondary combustion happening in the flue? In any case, it was something of a shame to see this wasted heat. In retrospect a taller thermal store (from a larger gas bottle) would have enabled me to make use of this energy.

Anyway, by the time I had cleared my tools away at the end of the day, the water was up to temperature (70-90 deg C) and I could have a well earned shower!

I found that an indicated setting of 35 deg C on the mixer valve gave a good temperature for showering and washing up (I’ve no idea what the actual water temperature was). Once set, it was not necessary to change it.

It takes perhaps 20 or 30 seconds for the hot water to arrive at the shower head. The little ball valve allows the flow rate to be easily adjusted. With a ‘medium’ flow rate, the shower temperature is maintained without a problem for the length of the shower. This was a nice surprise given the small size of of the copper coil heat exchanger.

I was often in the tiny house alone, so I could afford to be quite indulgent with the length of time spent in the shower and the amount of hot water used. With more care, it should be possible to have 2 or 3 showers without needing to add more firewood. Not that adding a few more sticks and waiting 15mins would be any great hardship.

One final point: in order to hide the messy plumbing, I boxed in the mixer valve system and added a door. This was painted a similar colour to the kitchen units and a small black knob added. Now it looks quite neat and unobtrusive.

Pictured is the hot water mixer cupboard
Hot water mixer ‘cupboard’

Final thoughts

I am really pleased with the water heater – it was such a thrill to get hot water from the tap and see that my crazy idea actually worked!

Although I have no complaints about the performance of the system, there are some improvements that could be made. Fitting some insulation to the thermal store is the most pressing. At the moment it is an effective radiator and loses heat quite quickly.

Another thing I may do in future is fit a flue damper. This might reduce some of the heat being lost up the flue. I suspect that when the fire gets to the glowing ember stage, it would be good to be able to close off the flue so that more of the residual heat is used for water heating.

I’m not sure if it is worth insulating the firebox. Yes, it does lose some heat but it is also quite nice to sit outside next to this fire with a cup of coffee. I imagine in the colder seasons this could become a daily ritual :-). When the firebox door is opened the radiant heat can be felt some metres away.

Unsurprisingly, the water in the thermal store became quite orange and rusty. The gas cylinder and flue are, after all, just made from mild steel. Sooner or later a rust hole will appear. I guess, at that point, I’ll just have to patch it up. I’ve no idea if adding corrosion inhibitor to the water would really help. As it’s not a sealed system it does need to be regularly topped up to replace the water that has evaporated, so I’d be continually adding corrosion inhibitor too. At about £14 per litre, that’s not ideal. What would be ideal is a stainless steel thermal store and flue. If I were to go down that (expensive) route, I would be as well buying a commercially available wood burning water heater and adding my own heat exchanger, if needed.

A final, final point is that I still intend to build a solar water heater to be used in conjunction with the wood burning heater. Time defeated me during this summer’s build (2021) but it’s something I hope to return to in future.

Update – 2 Years later (2023)

So – does the wonky water heater still work?

The answer is … Yes, it does! It has suffered some damage though. The temperature sensor got smashed in storage and my poor welding meant that the lid became detached from the hinge (now it just sits in place). There is also a very slight water leak where one of the plastic pipes connects to the heater. It’s also quite rusty inside (to be expected).

But despite the above I can still connect it up in about 10 minutes, fire it up, and have a hot shower in less than an hour.

It does seem a bit crazy though during the baking hot French summer to be burning wood in order to have a shower. A solar heated system would make more sense. Also, if I am on my own, I will sometimes just use my Hozelock pump up portable shower with hot water heated from the gas hob or from a kettle placed on the wood burner. That way I can keep the firewood for room heating.

In conclusion, despite it’s crudeness, the water heater is still a useful piece of kit. It was also a fun project and it shows what can be done with some recycled stuff and imagination.

Free Hot Showers! Part 4: Wonky Water Heater – Tweaks and Test

Pictured is the wood burning water heater firebox
Water heater firebox

In this blog post, I will describe what changes I made to my DIY wood powered water heater and show how it performed in testing.

In a previous blog post I described the build and test of the water heater. I have since christened it the “wonky water heater” as it’s a bit squinty and done ‘by eye’. Near enough is good enough, as I often say to myself. Anyway, the heater was made from 2 old butane gas cylinders. The bottom cylinder became a firebox and the top cylinder became a 20 litre ‘thermal store’ (i.e. hot water tank) into which was placed a copper coil. Cold water going into the coil absorbs heat from the thermal store and – hey presto – hot water comes out of the coil. The aim was to give enough water for a few hot showers in my tiny house by simply burning scraps of wood or scavenged sticks.


Pictured is the water hear door in closed position
Firebox door in closed position

The first thing I did was to fit a door. The aim of this was to reduce heat loss and – hopefully – transfer more to the water.

The door itself was made from the previously cut out rectangle and the hinges were very inexpensive mild steel items. I fabricated a latch to hold it shut, the handle of which was cut from a wooden broom. At the bottom of the door I cut a slot which I hoped would allow enough air in for good combustion. This actually worked very well. With the door closed the air rushes in and up through the burning wood. You can really hear it roar!

Pictured is the firebox showing internal insulation
Firebox open showing insulation

The second thing I did was to coat the inside of the firebox with a layer of fire cement mixed with perlite. I used just under 3 x 2kg tubs of cement (from Screwfix) and most of a small bag of perlite (from a garden centre). The ratio was about 1 part cement to 2.5 parts perlite, with a splash of water added. The resulting mix was sticky enough to adhere to the walls of the firebox. I used a plasterer’s mini finishing trowel to apply it.

I hoped that this layer of insulation would reduce heat loss and wear on the steel body itself (in the same way firebricks do in a wood burning stove).

In practice, the outside of the firebox still gets very hot so I don’t know how much energy is saved. The cement/perlite layer became very tough when fired, so I’m sure it at least offers good protection for the steel.

I also welded on some rectangular feet to the bottom of the legs (see image at the top of the page). These make the heater more stable and allow it to be bolted down, if required.

Thermal Store

The thermal store got some attention too. The first improvement was the addition of a hinged lid with a slot cut out for the flue and copper pipe. I later added a wooden handle to make it easier to open and close the lid when hot.

Pictured is the water heater hinged lid
Water heater hinged lid
Pictured is the finished wood powered water heater
The finished water heater

The second job was to make the flue demountable. To do this I cut a small section (about 40mm) off the top. I then cut a slot in the small piece so it could be expanded slightly to fit over the stub of the flue. Next it was welded onto the stub to form a collar into which the top part fitted.

The third task was to install the thermometer. The bracket was simple enough – just a small piece of steel flat bar attached to the top of the thermal store with two bolts. The bolts space the thermometer away from the worst of the heat (it’s made from plastic). Cable ties were used to attach the thermometer to the flat bar. A hole drilled in the thermal store allowed the temperature sensor and wire to pass through. The sensor itself dangles in the water about 1/2 way down.

Lastly, the heater was given 2 coats of high temperature black paint from a spray can. This didn’t cover the blue paint of the lid very well (which I had already given a coat of engine enamel) but worked fine on the bare metal.

Performance test

I wanted to find out if the modifications would get the heater up to temperature more quickly and give greater hot water output.

Pictured are some split birch logs
Birch logs used for test

I also tried some ‘proper’ hardwood logs (Birch) rather than pallet wood.

The photo gives an idea of the firewood size. In practice I could only fit one of these at a time (at least until the previous wood had burned down).

Over the course of the test I used 2 full ‘blocks’ and some kindling.

The other change to the test setup was to draw off hot water in 5 litre amounts. I did this by filling up my Hozelock porta shower. In this way I hoped to learn how many showers the heater could produce for a certain quantity of firewood. A ‘shower’ was defined as 5 litres of water at 40 deg C or above.

Pictured is a Hozelock porta shower being filled
Heater used to fill Hozelock portashower

The video below shows the water heater being put through it’s paces.

Test Results

The firebox was loaded with some kindling and 1 block of Birchwood and it lit very easily. The temperature rise in the thermal store is shown below.

Burn timeWater tempNotes
0 minutes18 deg C
10 minutes45 deg C
20 minutes69 deg C2nd log added after 25 minutes
30 minutes90 deg C
33 minutes95 deg C

The temperature of the water in each 5 litre fill of the Portashower is shown below.

Shower NumberShower Water TemperatureThermal Store Temperature
174 deg C95 deg C
261 deg CNot measured
355 deg CNot measured
448 deg CNot measured
544 deg C61 deg C
641 deg C56 deg C
Pictured is the Hozelock Portashower being tested
Hozelock Portashower test

Around 10 mins after shower 6, the temperature in the thermal store had risen again to 65 deg C. By this time the firewood was glowing but there were no flames. It was enough to produce a 7th shower at 42 deg C.

During the course of the test, the outside of the firebox got very hot, so this is still an obvious source of heat loss. In future, I may try to cast an external layer of perlite/fire cement insulation around both the firebox and the thermal store.

I also pressurised the Hozelock Portashower and measured the shower time. With a full 5 litres, the shower lasted 3 minutes and 30 seconds (including occasional stops to pump It up a little).


The water heater got up to temperature more quickly and produced more hot water than during the previous test.

Clearly the temperature of first few showers was far too high. In practice, cold water would have be added to get a normal shower temperature of 40 – 45 deg C.

It’s reasonable to say that the heater would give 8-10 showers at 40-45 deg C from just over 2 blocks of wood.

In practice, one filling of the firebox should be enough to see the thermal store get to 70-80 deg C and provide 3 showers. This is fine for the tiny house, which will probably only be occupied by 1 or 2 people at a time.

Obviously in the winter the initial water temperature will be lower and more firewood would be necessary to get the same hot water output.

In spring and autumn I hope to use a solar heater to pre-heat the water. In which case the heater may use less firewood for the same result.

In summer it’s likely that the solar heater will provide almost all the hot water and the wood powered water heater will be hardly required.

The total cost of the project was around £150. This went on steel tube, steel flat bar, paint, welding wire, screws, nuts and washers, fire cement and perlite, hinges, a copper coil, 2 thermometers, grinder discs and a drill bit.

It should be possible to run the water heater on scavenged ‘windfall’ sticks and branches and waste wood from the tiny house build. So it does live up the promise of “free hot showers” (after the initial build cost). However, it’s likely that I will be buying some hardwood for the tiny house wood stove anyway and some of this will find it’s way into the water heater in winter.

Free Hot Showers! Part 2: Water heater build and test

Pictured is a DIY wood powered water heater
Wood powered water heater nearing completion

It’s done! Today I tested my wood powered water heater and I’m glad to say that it works. In this post I’ll outline the build, the results of the performance test and what improvements could be made.

If you haven’t read Part 1, the aim of this project is to make a water heater that is connected to the tiny house plumbing system but is actually located outside the house.

This has the benefits of not overheating the house in summer and not requiring an (expensive) double wall flue installation. Furthermore, having it outside will safe space and the heater can be ‘quick and dirty’ (e.g. no need for a fancy sealing door).

It has recently occurred to me that this heater could also be useful for ‘topping up’ hot water produced by solar energy. This would be handy in the winter or on cloudy days.

Pictured is a DIY water heater in the early stages of construction
2 gas cylinders and a flue pipe

The Build

The water heater was made from 2 gas bottles which originally contained 13kg of butane.

After emptying the bottles of gas and purging them with water, the top was cut from both cylinders and the metal support ring was also removed from one of the cylinders.

One of the cylinders became the ‘firebox’ (bottom), the other became the ‘thermal store’ (top).

Pictured is some terrible welding on my DIY water heater
Terrible welding – but it did work eventually

The thermal store had a hole cut in the bottom and a piece of 75mm tube welded in place. I had barely used my welder before this, so my welding was terrible. It took a lot of grinding, testing and re-welding before the join was watertight.

Pictured is a Sealey Mighty MIG 100 Welder
Sealey Mighty MIG

By the way, the welder I used was a Sealey Mighty MIG 100. This uses flux cored welding wire (no gas required). It’s quite basic but seems to be fairly decent quality for a budget machine.

The rectangular hole in the firebox was cut big enough so that it could be filled with fairly large pieces of wood.

Pictured is an abrasive wheel
Bosch abrasive wheel for grinder

Removing the original blue paint was achieved using a Bosch abrasive wheel fitted to an angle grinder. This took a little time but the wheel was very effective and lasted for the whole job (still going strong).

Pictured are the water heater’s bolt on legs
Bolt-on legs

Three bolt-on legs were fitted to the firebox and some brackets welded onto each half of the heater so they could be bolted together. This was a safety issue as I didn’t want the thermal store (scalding water) to be able to be easily detached from the base. Prior to this, the thermal store was just sitting in place.

The final step was to put a heat exchanger coil inside the thermal store. This was made from a 10m length of soft copper ‘micro bore’ pipe with a diameter of 10mm. I thought this might be too small but I gave it a try anyway.

Test Procedure

I added 20L of cold water to the the thermal store and connected one end of the heat exchanger to the mains via a garden hose. The other end of the heat exchanger discharged into a bucket via a short length of hose.

Digital thermometers were put inside the thermal store and the discharge bucket.

The firebox was filled with scrap wood from a pallet and the fire lit.

Performance Results

The video below shows the water heater being tested.

The initial temperature of the water in the thermal store was 17 deg C.

After 15 minutes of heating the temperature was raised to 47.5 deg C.

After 35 minutes, the temperature in the thermal store was 70.9 deg C.

Test 1

At this stage I drew off 2 buckets of hot water (10L per bucket). The first bucket had a water temperature of 37 deg C and the second was 34.8 deg C, giving an average of just under 36 deg C. The temperature in the thermal store had dropped to 51.6 deg C.

Next, I added more wood to try to get the water in the thermal store as hot as possible.

Test 2

After a further 20 minutes of heating, the water in the thermal store was 87.8 deg C.

I drew off 3 buckets of hot water. Their temperatures were 51.4, 42 and 36 deg C. So the heater had given 30L of water at an average temperature of 43.1 deg C.

The water in the thermal store had decreased to 60.4 deg C.

By this time the flames had died down and firebox consisted mainly of embers.

Test 3

As a final test I waited for 10 mins, without adding further firewood and drew another 3 buckets of hot water from the heater.

The average temperature of this final 30L of water was 31 deg C.

Regarding wood consumption, after the initial fill, I topped the firebox up once. That equates to, perhaps, 3 planks from a pallet and one or two chunkier blocks.

The heater burned quite cleanly i.e. there was not a great amount of smoke visible from the top of the flue.

During the course of the tests, the water level in the thermal store dropped by 1-2 cm, presumably from evaporation.


I was more than happy with the results of this test.

Firstly, the water heated up remarkably quickly. With a full fire the temperature increased by about 10 deg C every 5 minutes.

The heater performed best when the water in the thermal store was very hot (almost 90 deg C). This temperature could be easily reached in 45 minutes from cold.

With a thermal store temperature of around 90 deg C, the heater could provide 30L of water at an ideal bathing temperature (40 – 45 deg C). If used to fill my Hozelock portashower, this would be enough for 6 showers (plenty for my 3 person tiny house).

I see no reason why it would not be an effective way of topping up the temperature of solar heated water on cloudy days.

The small capacity heat exchanger coil worked surprisingly well at removing heat from the thermal store. I had considered doubling it’s length or replacing it with a larger diameter coil but I’ve decided to leave it as it is.


Heat is almost certainly being lost through the open hole in the firebox. Adding a door may help reduce these losses. Of even greater benefit would be insulating the firebox. It should be possible to do this by adding an insulation blanket (such as ceramic fibre) to the inside walls and base. It these two improvements are made, then more heat would be transferred to the water above, increasing the performance and/or reducing the wood consumption.

External insulation of the thermal store might also improve performance. At the very least the water would stay hotter for longer after it had been heated. I will try to cast a thin layer of hempcrete around the thermal store for this purpose. It will cost me nothing to try it (I have plenty left over from the tiny house build).

Another possible improvement is the fabrication of a lid to cover most of the top of the thermal store. This might reduce heat loss as well as water loss from evaporation. My idea is to make it hinged so that it could be swung back out the way for easy filling of the thermal store by bucket.

Besides the above, I will give the heater a coat of black high temperature paint and cut the flue into 2 sections that can be re-assembled. This will enable easier transport of the heater by car.

Free hot showers! Part 3: A solar water heater after all?

In a previous post I dismissed the idea of a solar water heater as being too complicated and expensive.

However, it is still an appealing idea. Simple, clean, free, effective and ideally suited to make use of the sunny French climate where the the tiny house is located.

Unlike a ‘normal’ house with a ‘normal’ family, my hot water requirements are minimal. Using a Portashower, I know I only need 5 litres of water per shower. The tiny house can only sleep three people (and mostly it would be just me). So I could probably get by with 20 litres of hot water per day, or even less.

Commercial Solar Water Heater Systems

The typical solar arrangement (shown below) with a roof mounted solar panel feeding a large hot water cylinder would be way over the top for my tiny house.

Pictured is a diagram of a typical solar water heating system
Typical solar water heating system

Even a simpler ground mounted solar panel with a thermosiphon header tank is too big and too expensive. For example, this 200 litre system, from the French company Solaris, costs nearly 1300 Euros.

Pictured is a thermosiphon solar water heater
Solaris solar water heater system. Ground mounted and works on thermosiphon principle.

DIY Systems

I know that people have been making DIY solar water collectors for years. Typically these consist of copper or plastic pipes in a grid or coil arrangement, painted black and placed in a glazed insulated box. Hot water from these collectors is normally transferred to a tank – either by pump or thermosiphon.

There is plenty of information out there on DIY solar water builds, such as the one shown below from

Pictured is a DIY solar water heating system
DIY solar water heating system

Going down the DIY panel route has advantages. I could build something small enough to meet my needs and I wouldn’t have to pay the shipping costs of a bulky commercially available panel. But what about the header tank? I don’t need 200 litres. What are the options?

I considered lots of possibilities but the one that seemed most promising was based on an insulated plastic cool box. I could easily get one with about 20 litres capacity (perfect!) and I could probably fit a tap to drain off the hot water as needed. The problem then is that I’d then have to manually top up the panel with cold water, unless I fitted a float valve plumbed to a cold water feed. So even now things are starting to get a little bit more complicated.

Is there an even simpler way?

Pictured is a solar batch water heater
Solar batch water heater

My thoughts then turned to the old idea of the solar batch water heater. At their simplest, these are basically a “tank in the sun”. Typically they consist of a recycled water cylinder, painted black, put in a glazed and insulated box and plumbed in-line with the domestic water heater. Thus the cylinder is both the solar collector and the storage vessel. The hot water coming from the cylinder, into the domestic water system, may require some ‘topping up’ with heat to get it up to temperature, but useful energy savings can still be made.

OK, so the batch water heaters are cheap and simple, but I dont want a giant cylinder in my garden with much more water than I need.

But what if I could make a normal solar collector with a large enough water capacity that it effectively became a batch heater? I.e. no need for a separate tank.

Remember, I only need 5 litres of hot water at any one time.

With this in mind, I came up with the following design:

Mini Batch Water Heater

Pictured is a drawing of a mini batch water heater
DIY Mini Batch Water Heater Design

There is nothing revolutionary in my design. The important point is that the 22mm copper pipe is large enough to give a total water volume of 5 litres for a panel that is around 1 square metre. It might not heat up as quickly as a normal collector (which has a lower water volume) but that’s not too important. If it can get the water to 45 deg C in 30 mins or so, that will be absolutely fine.

I wont go into too much detail regarding the construction. The 22mm copper pipe would be soldered together using standard elbows and tees. Ideally it would be clipped onto a thin metal sheet of some kind in order to increase the amount of heat absorbed, but I think this is optional. The whole lot is then sprayed matt black.

The retaining frame is made from wood and insulated at the rear to reduce heat loss. I think that anything can be used for the insulator but I suspect a sheet of foam would be cheap, effective and easy to get hold of.

As for the glazing, I wouldn’t be too keen on using real glass. Besides the weight and fragility, it would also have to be custom cut. I would go for clear, corrugated polycarbonate. To avoid it fogging on the inside, I would seal the ends with tape. A bead of clear silicone would fix it into the frame.

A pressure reducing valve (PRV) might be an idea if there was any chance that high temperatures would cause water expansion and pressure on the solder joints. I guess this wouldn’t be needed if the heater was attached to a garden hose as this could expand slightly. Of course, if the garden hose was disconnected all the water would drain out! So maybe a non return valve would be a good idea, in which case the PRV might be needed after all.

Some wooden legs either side of the frame would allow the panel to face the sun at, say, 45 degrees. If I was being really clever I could make a cradle so that the panel could be rotated to any angle.

If I wanted to store a greater quantity of hot water then I could fill up an insulated container 3 or 4 times during the afternoon, giving me some for the evening.

Should the panel prove to be effective, it might actually be worth connecting it to the water system of the tiny house. It could conceivably be fixed onto one of the exterior walls of the house and hinged so it would swing outwards to collect the sun.

To conclude – solar is back! If I make it to France this summer (2021) and I finish all the other jobs I have to do, then I will have a go at making the mini batch water heater. Stay tuned to see if it works…

Free hot showers! Part 1: design of a simple wood burning DIY hot water system

Something which had taxed my brain for a while was the tiny house hot water system.

I considered lots of options. As ever, I wanted something simple, cheap and environmentally sound.

A back boiler on a wood burner could have been used to heat a hot water tank by a simple thermosiphon arrangement. Unfortunately, there’s not much room inside the tiny house for a hot water tank. Neither would it be practical in the summer, when it’s already 25-30 deg C inside, to have to light a stove in order to take a shower!

Solar water heating systems seemed to be complicated and expensive and, again, I don’t have much room for hot water storage tanks.

I considered a cheap gas powered water heater, the sort that gives hot water on demand. Apart from the gas consumption and it’s lack of eco-credentials, I was wary of it breaking down in a few years and the likely hassle of obtaining spare parts. Not to mention having to install the thing on the wall inside the cabin (no space!) and have a flue vented to outside.

In the end I decided to build a wood fired water heater that could be located outside the cabin but connected to the cabin’s plumbing system.

This would solve the problem of overheating inside the cabin, it wouldn’t require bulky hot water tanks indoors and it should be relatively cheap, simple and efficient. Nor would I need to install a twin wall flue inside the cabin, with all the expense and hassle that would entail.

Regarding the design, I got some inspiration from my storm kettle (a.k.a Kelly or Ghillie Kettle). The principle is that water is contained in a jacket surrounding a central chimney heated by a firebox beneath. Presumably it is this large heat transfer surface area which accounts for its effectiveness. In use it can bring 1.5 litres of water to the boil in just a few minutes using a handful of dry sticks.

Pictured is Ghillie kettle in use.
Storm kettle

The sketch below shows my design idea of the tiny house hot water heater. It’s based on two salvaged butane gas bottles, one on top of the other. The upper bottle contains the hot water, through which the flue passes. Sitting in the water is a coil of copper pipe that will act as a heat exchanger. The lower bottle is used as the firebox.

Pictured is a sketch for a DIY wood fired water heater
Sketch of wood fired water heater

The insulation would serve three purposes. Firstly, it would reduce heat loss from the hot water, keeping it hotter for longer. Secondly, it would reduce heat loss from the sides of the firebox, thereby increasing the amount of heat transferred to the water via the top of the firebox and flue. Thirdly, it would improve safety by reducing the temperature of the external surfaces.

The design could be improved further by adding other elements, such as :

  • Legs to raise the heater to a more comfortable height.
  • A thermometer to measure the water temperature.
  • A pressure relief valve and/or expansion vessel on the copper pipe leaving the heater.
  • A grate to allow for good airflow under the fuel.

My main concern with this design concerns the heat exchanger. Would it be effective? In other words, would the cold water pick up enough heat on it’s passage through the coil to be of any use?

I did some research and, based on comments in various forums, it didn’t look too promising.

It’s likely that the flow rate of the hot water from the heater would be low. This might be fine for filling up a washbasin or kitchen sink but it’s unlikely to be good enough for a normal shower running from a mixer valve.

So was there any point in continuing with this design? How else could I make a working shower system? ‘

On previous building trips I had used a 5 litre garden sprayer such as the one shown below as an improvised shower. I removed the spray head and cut the tube so that just a little stub was left. This was necessary to improve the flow rate but gave a stream of water rather than a spray.

The image shows a 5 litre garden sprayer
Typical 5 litre garden sprayer!

The last modification to the sprayer was a coat of matt black paint. After this, it functioned pretty well as a ‘solar shower’. By leaving it in the sun in the afternoon, I could have a hot shower at the end of the dirty and sweaty day of building! If I needed a shower at other times (or if I had guests) I simply put a kettle of hot water in the sprayer, topped it up with cold water and it was perfect.

So, getting back to the point, could I use a similar system for my indoor shower room? If so, I wouldn’t need a great flow rate from the wood burning water heater. I’d just need to be able to fill the sprayer with 5 litres of moderately warm water. It should also be possible to fill the sprayer repeatedly so that guests could have a shower too.

With that decided, I wondered if there was a better sprayer system on the market? One with a proper spray head?

After a bit of searching I found that Hozelock make a “Portashower” which has a 7 litre capacity, a good spray head and is pretty cheap (about £30). The reviews are very positive so I have ordered one and plan to use it this summer (2021).

Pictured is a Hozelock Portashower
Hozelock 7 litre Portashower

While searching for the Portashower I also came across lots of electrically powered camping showers. Some of these have rechargeable batteries and others run from a 12v cigarette lighter socket.

As I have a 12v leisure battery in the house, I wondered if one of these systems could also work as a permanent installation in the shower room? In this case I could have hot and cold taps in or near the shower cubicle. These would be used to fill a bucket with warm water which would then be pumped electrically to give a shower. This might be a bit more convenient than the portashower (no pumping and screwing/unscrewing of lid) and the shower could be longer (10 litre bucket).

So, as a plan B, I bought a Ring RS1 portable 12v shower. At around £20, this is even cheaper than the Portashower and also has good reviews. I chose the Ring system as they are a well known brand but if you look on eBay or Amazon there are plenty of similar, inexpensive systems.

Picture is a Ring RS1 portable shower
Ring RS1 12v portable shower

So now I have the shower problem solved … let’s get back to the wood burning water heater design.

I’m fairly confident that the heater will be able to produce 20 litres of hot water quite quickly. If this turns out to be true then the next question will be the design of the heat exchanger coil.

I calculated that a copper coil of 14mm internal diameter and a length of 20m long would have a volume of 3 litres. Mixing these 3 litres of hot water with another 3 litres of cold water will be enough for a shower.

Obviously, if there is still burning fuel in the firebox, the three litres of fresh cold water in the coil would also start to be heated. So while the coil would probably be poor in terms of providing constant hot water (probably wont pick up enough heat relative to the water flow) it should be able to provide hot water in batches.

Quite how easy it would be to neatly make a 20m coil of relatively large diameter copper pipe and then stuff it into a small cylinder is another matter! I suspect it would be a real challenge (and quite expensive if it goes wrong). So I will probably make a smaller, cheaper and easier coil in order to carry out a test. Here in the UK I can get a 10 metre coil of 10mm soft copper microbore tube for under £30, so that will be my starting point.

What if, after testing, the heat exchanger idea turns out to be completely impractical? Time for another plan B! In this case I could directly remove some of the heated water to fill up the shower sprayer, either via a drain tap or a simple hand pump.

The obvious problem with the steel cylinder is that of rusty bathing water. To mitigate this I could try coating the inside of the cylinder with some high temperature paint, such as engine enamel. Would it work? I don’t know, but it seems reasonable. The water in contact with the paint can’t get hotter than 100 deg C, so hopefully the paint itself wouldn’t get much hotter than this, despite the high temperatures on the other side of the metal. Another plus point is that I wouldn’t have to heat the water to near boiling point. Instead, a temperature of 40-45 deg C would be enough and this could hopefully be achieved with a less intense fire in the lower half of the heater.

So, to conclude this blog post, I have (a) a wood burning water heater design, (b) two methods of using the hot water for showering and (c) some contingency plans if the heat exchanger doesn’t work.

At the time or writing I have scavenged two old gas cylinders and have removed the valves and purged them with water. I’ve also bought some 75mm steel tube for the flue, some 30mm steel tube for the legs and some steel flat bar, nuts, bolts and washers. The next step is to build the heater and see how it performs.

I’ll report back in the blogpost “Free Hot Showers! Part 2: Water heater build and test”.