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.

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.

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 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).

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.

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”.