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”.

Why I chose a biolitter compost toilet

In this post I’ll explain why I chose to use a compost toilet and, more specifically, a simple biolitter type.

Water Savings

The waste of an expensive commodity like mains water was something I was keen to avoid. After washing, toilets are the biggest consumer of domestic water. A report by the Energy Saving Trust in 2013 found that 22% of water consumed in UK homes was used to flush toilets. Furthermore, it just seems crazy to use high quality drinking water to flush a toilet.

An alternative is to use a compost toilet. Having previously read about simple compost toilets (and seen them in use at an eco-centre) I decided early in the design process that I would use one for my tiny house.

Free Garden Compost

A further bonus is the recycling of human waste and kitchen scraps to make a high quality compost that can be used in the garden. To be fair, it’s not completely free as you normally have to buy the ‘biolitter’ (sawdust, wood shavings, etc) to which the waste is added. However, I had lots of hemp left over from the build which was ideal for this purpose.

The Biolitter Toilet

The compost toilet system I chose was popularized (in French speaking Europe at least) by Professor Joseph Országh of the Université de Mons-Hainaut in Belgium. He called his system the “toilette à litière biomaîtrisée” or TLB . This can be literally translated as “biocontrolled litter toilet” or “biolitter toilet” (BLT). US readers may be familiar with a similar system called Humanure proposed by the American Joseph Jenkins.

For the benefit of this website’s English speaking readers, I’ll stick with the term Biolitter toilet (or just compost toilet) and avoid the acronym BLT, for obvious reasons!

Whatever the name, the principle is the same. In place of a normal toilet is a bucket with a volume of 20-40 litres. Around the bucket is built some kind of containing ‘box’ with removable top. This top has a hole around which is fixed a toilet seat and lid.

After each visit to the toilet, the waste (solid and liquid) is covered by a layer of ‘litter’ which is typically wood chips, wood shavings or sawdust. The cellulose in the litter biologically inhibits the enzymatic reactions in the excreta that are responsible for odours. Nevertheless, the bucket should be emptied at least once per week.

The bucket’s contents are then composted for at least two years (along with kitchen food scraps etc) after which the compost is safe to use in the garden.

I briefly considered other types of compost toilet but they seemed to be more expensive and complicated. I didn’t see the point of buying a commercially made item when I could make a very simple, proven and low cost system myself.

The Biolitter toilet in practice

I used a home made outside biolitter toilet for two summers while the tiny house was being built. It was very simple and convenient. You can read about it here.

In the summer of 2021 I started using a biolitter toilet inside the tiny house. This time I used a plastic bucket type toilet with an integrated seat and lid and it was placed in the shower room. This toilet is very compact which was ideal for the small space available. In practice it has worked well. Even my teenage daughter has given it the thumbs up for it’s practicality and surprising lack of odour! She even says that she will use a compost toilet in her house when she is older.

My tiny house design philosophy

Pictured is a rough sketch of the tiny sleeping area and bathroomping

“Design Philosophy” is a rather grand term but I think it’s useful to try to explain my way of thinking with regards to design.

Phrases such as “near enough is good enough”, “if it looks right, it is right” and “added simplicity” ring true with me.

This is a rustic cabin type building. I wasn’t going to be anguishing over paint shades or skirting board styles.

Although my budget wasn’t huge, I didn’t want to compromise on the important bits of the build. To me that meant using healthy, durable and efficient materials for the fabric of the building. So the likes of glass fibre insulation, plasterboard and PVC were to be avoided where possible.

On the other hand I did try to save money on less important things, so I didn’t buy ‘big brand’ kitchen appliances or fancy furniture.

It would be wrong to say that I made it all up as I went along. For safety’s sake I had to do some calculations. Two examples are in the design of the foundations and the sizing of the floor joists. One becomes expert at finding nuggets of helpful information on the internet. This could be the blogs of fellow builders, local government websites, forums, etc.

However, I think people often go ‘over the top’ in conventional building. One bugbear of mine is in the over use of concrete. I remember once having to knock down a wall of a small house extension. The wall had been cast from reinforced concrete and would have survived a nuclear blast! Sledgehammer blows just bounced off it. This is an extreme example but illustrates the kind of overkill that can happen when people want to make something ‘nice and strong’.

So I wanted my design to be be ‘good enough’, not excessive.

Common sense and some lateral thinking was what I hoped to employ. I was keen to avoid complicated and expensive ‘stuff’. Simple, clever and reliable was what I was aiming for.

Fortunately, I was able to call on the experience of a friend and ex-colleague who is a very skilled and knowledgeable carpenter and general builder. He was able to give some advice on construction methods and appropriate wood sizes which helped me make sensible design choices.

Some of my design was done ‘on the hoof’. I remember sketching my idea for a mezzanine sleeping area one night, then building it the next day. This has a lot to commend it. By this stage of the build I knew exactly which materials I had lying around and how I could best use them. Sometimes it’s better to have a general idea of where the design is going but be open to change as the project proceeds. You learn to trust that solutions will appear when needed.

Pictured are photos of the sleeping area and bathroom
Sleeping area and bathroom

Time and labour constraints meant that I couldn’t indulge in any architectural flights of fancy. It had to be quick and simple. It needn’t be sleek or flashy but I hoped that it would have some kind of honest and simple beauty, if only by virtue of the natural materials used.