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.

1 thought on “Free hot showers! Part 5: Installation and use of the wonky water heater.”

  1. I appreciate your clear explanations and the emphasis on safety throughout the process.
    It’s evident that you care about your readers’ well-being and want us to tackle these issues safely and effectively.
    Your blog has become my go-to resource for appliance-related issues,
    and I’ll be sure to share it with friends and family who might face similar problems with their appliances.
    Keep up the fantastic work!


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