When spending a night under the stars, what better way to enjoy it than from a piping hot bath-tub? Of course, when camping you aren't usually blessed with a ready source of pressurized clean water or a boiler. This instructable is about a couple of amateur DIYers making a wood fired convection pump outdoor bath. We have (or I should say had) zero plumbing experience and made some mistakes along the way, but we did get there in the end!

Headlines:

Total expense - £200 (ish)

Time taken - a couple of full days all added together

Experience required - not much!

Time to heat completed bath - 2 1/2 hours

Step 1: Concept

So of course the obvious solution is to just buy something that does the job right in the first place isnt it? I must admit I had been looking enviously at the DutchTub for some time, though of course held back by the £5k price tag! DutchTubs work in a very simple to understand way, and a quick bit of youtube hopping will provide a wealth of videos showing how with some simple tools and kit you can easily mimic their design using a conventional bathtub. If of interest, this video is particularly good. The idea was born, we'd make one ourselves.

The concept of the pump is simple. If you heat water it expands. If you heat water in a pipe, it's expansion can be manipulated into a useful direction. By coiling pipe around and around in the same location, the area exposed to your heat source is increased, therefore giving the water inside the pipe more of an opportunity to pick up energy.

The heat of a fire is the obvious method of heating a pipe full of water, and exactly the method we will employ here. But how to get that water in and out of the bath? We don't have any pressure to work with do we? Well we actually do.... a bit. When full, the water at the bottom of the bath will actually be under the pressure of all the water above it. For example, if i was to poke a hole in the bottom of a carton of juice and another at the top, juice would come out of the bottom hole faster than the top one right? Same principle. That little bit of pressure is going to be useful to us for this bath.

If we combine that pressure effect with the fact that we know the water in the heated section of the pipe is going to rise, we can start to imagine a pumping mechanism, where hot water forces it's way up the pipe and into the top of the bath, which is replaced immediately by the slightly pressurised water at the bottom of the bath. So long as we keep the coil hot, the pump should keep working.

So, lets put that all together. We're going to coil a pipe, make the top and bottom end of that coil long and straight and connect them to the bath near the top of the waterline and then close to the bottom of the bath. Simple stuff hey. Lets find out.

Step 2: Gathering Materials

First things first, you need a bath. Being of the naive and overly ambitious type I went for a 3m long monster. In terms of the material type (iron, plastic, tin...) I went for plastic. A nice heavy cast iron bath is beautiful but try dragging that around a muddy field. Tin would have been nice but I couldn't find one online in my price range. The plastic beast you see here cost me a princely £30 on ebay. As I had already settled on heating the bath externally, I figured that the portability and durability gained from having a plastic bath was a fair compromise over something prettier or more fire retardant.

Then it's just the tools and kit. Divided lists below:

Tools:

Cordless Drill
21mm Hole Saw
15mm Pipe cutter
2 pairs of pliers
A hammer
A thermometer
A Sealant gun

Fittings and materials:

A bath
15mmm copper coil
15mm tank connectors
Spare olives
Silicone sealant
Plumbers jointing compound

Step 3: Coiling the Pipe... and How Not to Do It!

So of course the first thing to do is take some copper pipe and coil it into this mythical pump shape thingy. Shouldn't be too hard eh. Well if you've never worked with pipe before then you might be forgiven for thinking that to bend it you just put some welly into it. This was my thought, chuckling to myself as I walked past the expensive pipe-benders at the DIY store, carrying a few lengths of 22mm straight copper pipe on my shoulder to the check out. Take a look at the photos to see what results that will get you. There was some logic to the decision, I figured thicker stronger pipe would be better equipped to handle the heat from a fire. Maybe so, but bending it without tools is a fast way to waste some serious money and get in a right kerfuffle along the way.

So, a closer inspection at some examples of the work of others showed that buying a coil (not straight lengths) of thinner pipe (15mm to be precise) would be far more manageable and just as able to withstand a few burning logs resting against it. So, coil bought and leftover straight pipe returned to the DIY store I began creating my pump.

This was surprisingly quick and easy. I looked around the garden shed to see if I could find a big cylindar to use as a mold for my basic shape. I found the gas cylindar to a patio heater and found this to be perfect, as it was heavy enough to push and pull against as well as being roughly the right size. You however might try anything of this rough shape and size.

I put the coil around the gas cylindar and started to pull lengths together into the cylindar to make the pipe match the shape. I found that by moving up the coil one length at a time I was very quickly able to pull the entire thing into a tighter spiral without any kinking. I found about an arm's length to be right distance to pull on for this with the 15mm pipe.

Once that was done I found it easy to then straighten out the bottom and top ends of the pipe into roughly one meter long lengths without kinking it. The reason for this is that especially with this project using a plastic bathtub, we don't want those flames too close. I can imagine how having too great a distance might effect the efficiency of the pump - thankfully this eyeball method worked out in the end. So if you're following this step by step then go for about a meter.

A note on bending this type of pipe. I think as a general rule with this bush-bending technique is not to attempt to bend too long a piece of pipe, as this concentrates the movement onto a smaller area (counter intuitive though that may sound).

Ok. Now to connect this contraption to a bathtub.

Step 4: Connecting the Coil to the Bath

I ummed and aahd alot over this. Bear in mind that I've never so much as fixed a leaky radiator before, so I hadn't a clue what kind of fittings were out there. In the end I thought better of trying to solder anything and followed some youtube tutorials on how to use compression 'tank connector' fittings, after seeing Mr HomeMadeModern use them in his video which you can find a link to at the beginning of this instructable. My alteration was to use brass, not plastic fittings, just because I liked the continuity of them with the copper pipe. The procedure was a little fiddly but pretty straightforward.

Using the exact sized hole saw (21mm for these fittings) I made two nice neat holes. One at the lowest flat section of the bathtub (so i could get a nice surface to join to) and one just beneath where I expected the waterline of the full bath to come. Because the bath was plastic this took a matter of seconds.

Then, after taking the fittings apart, I put the threaded section through the hole with the flat end on the inside and the thread exposed outside the bath, ensuring the rubber ring was on the inside where the water would be. I then put the first 'nut' onto the thread but didn't tighten it up just yet. At this stage I put a liberal amount of 'plumbers gold' sealant all over the place, which turned out in the end not to be a smart move. If I were you I wouldn't bother (read on to find out why). Then I gave that first nut a good old tighten and prematurely congratulated myself on my first bit of plumbing.

After giving the sealant some time to set I then coated the inside of the second 'nut' and it's olive (not the green sophisticated thing, this is the name for a little ring of copper that crushes as you tighten the nut over it, forming a bond) with liberal amounts of plumbers jointing compound. No idea what the stuff does to tell the truth but all the youtube vids I watched recommended it and tub is just a couple of pounds. The I loosely placed this nut and olive onto the end of the thread before shoving the straight ends of the pipe from the coil into the fittings. This required a fair bit of force, and I found that even a slight bend in the ends of those pipes prevented them from going all the way into the fittings. Some hasty work with the pipe cutter and a good deal of shoving and they were in. Then I tightened the end nut with olive over the pipe, using a pair of pliers on the inside of the fitting to aid purchase and prevent the whole fitting from spinning round and round. This was fiddly but worked after some grunting and puffing. The downside was that all the movement dislodged most of the sealant I had patiently waited to set. Waste of time. My recommendation is if you are inclined to use some sealant as extra security, use it last thing when all your fittings are in.

Ok, so now the coil is made, fitted and hanging preposterously off the side of a bath in the middle of a field. In theory, 90% of the work has now been done.

Step 5: Finishing Touches: Fire Pit and Coil Support

Now I would like to make a note here that neither of the following pieces are intended as permanent. They are made from scrap and were designed on the spot. However, they work great! Particularly the fire protector. So, if you're inclined to copy this instructable I advise taking note of the principle and devising your own method.

Ultimately this fire will sit in a fire pit I hope, either made of metal or of stone and mortar. For this first run however, we just wanted to prevent the heat from the bath from being too close to the plastic side of the bath. What was to hand that could facilitate robust heat reflection? Well we are fortunate enough to have the pieces of a broken wood burning stove lying around the garden. Along with the bent beyond use (so we all thought) pieces of the big old 22m copper pipe, garden wire and some hasty drill work, we were able to create a sort of hinged pair of feet that propped up a big piece of old cast iron. This formed a nice solid barrier between the intended fire and our fragile plastic tub. Looks alright at least.

Now, the coil is made from 15mm pipe. Its malleable and inclined to twist and bend all over itslef. To be honest I'm not even sure that that is a problem, so long as the pipe's integrity and the seals to the bath are maintained. However it seemed sensible to support the weight of the coil and try to maintain the spirals. A bit more bent 22ml pipe and garden wire and we had made an arch that made a temporary structure for the coil to hang off. So far so good.

Step 6: Testing. Will It Work??

Well the first thing to do of course is to collect a whole load of wood. We didn't know quite how much we would need so we just collected tons. Again, we are fortunate enough to be doing this next to a forest so finding dead dry bits isn't a challenge. Obviously if you don't have access to firewood of that nature you need to consider your fuel source a bit more carefully.

Second thing is water. Oh how we puzzled over this. Yes, we're right next to a river. But what about when it isn't mid July and the river is full of silt and crud? There is a natural spring nearby and a future iteration of this bath might include creating some kind of pipe fed supply from the spring. For now and for this test we were happy to lug buckets up the banks and fill the bath with river water. Of course, we filled the bath to well above the top hole to the coil. Due to it being a relatively dry summer the river was slow, clear and clean and so the bath water was relatively clean also. A filtration system might work as well.... Food for thought.

Ok. Firewood - check. Water - check. Bath all set up - check. Nosey neighbors...well if you aren't counting the sheep we should be alright. Lets do this.

We started the fire in the centre of the coil and let it die down a bit. We then spread the fuel around the coil and built it back up again so that burning wood was in direct contact with all of the pipe. For reference, the river water was 10 degrees C and 10 minutes in the bath water was no different.

Not long after, maybe as soon as another 10 minutes later, I tried placing my hand on the upper pipe entering the bath to feel if there was any temperature change I could feel with my hand. I was treated to a minor burn for this inquisitiveness. So something was happening! I placed my hand over the hole of the top entrance pipe to the bath and was nastily burned again! Great stuff!

So we heaped a bunch more firewood on the coil and sat back to let things carry on a bit. About 5 minutes later we ran like mad from the bath thinking it was about to explode......sudden and boisterous sounds of clanging and clinking and clacking were erupting from the pipes, though to the eye they were entirely stationary. What could this mean?

Calmed down and thoughts recollected, we realised that inevitably, air pockets must still be present in the pipes and they were working their way up and down it as the heated water moved in and out of them. We decided to let things carry on.

We draped a tarpaulin over the bath as a temporary method of keeping in some heat, piled on more wood and began to take comfort from the clanging sounds of industry which was scaring the nearby sheep off to another field. This was in fact a great thing, as when the clanging ceased we knew to return to the fire and pile on more fuel. This meant we could walk away from the bath and get on with other things whilst we waited for convection to do it's base work.

I would estimate that it was between 2 and 2 and a half hours between starting the fire and our decision to stop adding fuel. A thermometer check showed nearly 60 degrees C. Time to take the plunge.

We learned pretty quick that 60 degrees is way too hot for even the most leather skinned of bathing enthusiasts. We bailed out a couple buckets of hot and replaced them with cold from the river. Elbow check complete, we dipped in and joyous bathing commenced! Uncork the sparkling fizz and light the dinky tealights!