Introduction: How to Build a Wood Fired Hot Tub

In this instructable I will describe how I built a hot tub for less than $100 NZ ($75ish USD) from an old bulk liquids tank, a gas bottle and some stainless tube and junk that I had laying around.

This tub is cheap and relatively easy to build and is also useful as a plunge pool in hot weather too. Oh and you can cook on the top of it too!

Step 1: Stuff You Will Need

- 3 metres of 25mm (1in) stainless tube
- 1 old LPG gas bottle (the kind used for barbecue grills)
- 1 bulk liquids storage tank (the 1000litre kind with a steel palletised cage)
- enough old corrugated iron to cover the outside of the tank
- scrap polystyrene packaging
- 1 closed cell foam camping mattress
- a roll of soft narrow gauge binding wire (or equivalent - you could use cable ties at a pinch)
- scrap metal parts for the legs and decoration
- a canoe paddle or something similar

- arc welder (and the safety gear to go with it)
- angle grinder with metal cutting and grinding discs
- pipe bender large enough to bend 1 inch stainless tube
- tin snips
- heat gun / paint stripper
- box cutter
- strong pliers with a sharp cutting edge
- the trusty knockometer
- various clamps
- magnetic welding clamps are handy
- hacksaw (omit if you're deft with the angle grinder)
- handsaw/drawsaw or equivalent
- narrow nail punch/set or a strong nail
- cordless drill with a 2 - 3mm bit
- a vivid or permanent marker pen

- a gas axe is an optional extra here if you have one and aren't afraid to use it

Step 2: Shorten the Cage

First thing to do is unbolt and remove the crossbars that retain the plastic tank in the cage and slide the tank out (to allow you to cut the cage without inflicting damage to the tank).

With a hacksaw or angle grinder remove the top of the cage by cutting the vertical bars flush with the next horizontal bar down.

Remove any sharp edges with the angle grinder or a file to prevent them from rubbing through the plastic liner.

Cut an inch or two out of one of the short sides of the ring you've just cut off and weld back together. The steel is quite thin so it might help to insert a chunk of steel bar inside the joint
to give you something to weld to. I guess if you didn't want to weld this bit you could drill through lot and bolt or wire it together instead.

Slide the ring into the top of the cage so that the short vertical bars are wedged inside the bars of the cage.
Note: One side of the ring will have the vertical bars on the outside of the cage - don't worry about this - it's covered up later by the roofing iron and it leaves enough room to get the tank back in.

Wire the ring to the cage using soft steel wire (I used the mesh segments from an old single bed frame and folded them out to use as ties)

Step 3: Open the Tank

If you put the tank back in the cage it makes this step easier.

With a vivid marker, mark a line just below where the curve of the top starts. You can use the lip of the cage as a reference for your line.

Cut the top off with a hand saw.

Save the top to use as an insulated lid.

Step 4: Fold the Plastic Liner Over the Top of the Cage

Cut a deep slot in each corner of the plastic liner down almost as far as the cage.
Using a heat gun and brute force, heat and bend the plastic flaps over the cage rim. Use clamps and lengths of wood to hold the flaps in place until they cool.

Note: This step is made easier if you can do this on a warm windless day or inside.

Step 5: Clad the Cage

Apologies for the lack of pics for this step

Cut old corrugated roofing iron to length so that the corrugations run vertically. Cut enough to go around the tub.

Tip the whole thing on it's side to make it easier to work on.

Remove the plastic liner.

Start from the edge of one sheet and use a fat nail as a punch or drill to make 2 holes either side of the upper and lower horizontal bars of the cage. Bend the wire into a long U and poke the ends through the holes and twist together on the inside of the cage. Make sure you twist them so that they don't poke outwards into the interior space (where they'd snag on the plastic liner).

Continue around the tub adding more sheets as required.

At the top edge of the tub you'll need to tidy up the edges of the iron. With tin snips cut down the top of each ridge to a depth of about 2 - 3 inches. Make sure one side is just under the other and smack the ridge sharply with a hammer - this makes it form a nice bevelled edge with no sharp protrudy bits.

Step 6: Insulate the Tub

I used scrap polystyrene packaging and cut it up into shapes that would fit between the bars. I toyed with using expanding foam but I reckon it's less wasteful and cheaper to do it this way.

Cut a large piece of closed cell foam to insulate the base. You could use foam camping mattresses or similar. I just happened to have a huge piece laying around.

Step 7: Bend Up the Heating Coil

Sorry I don't have any pictures of the next step - it all happened in a bit of an excited rush.

I used 3m of 25mm (1 inch) stainless steel tube for the heat exchange coil. This makes about 2 full turns and 2 half turns at approximately the inside diameter of the LPG bottle I used. You may want to increase the number of turns or vary the diameter of the coil depending on the gas bottle you use. The only word of warning I have is that the more coils you have the less well the fire will burn - the cold water in the coil reduces the efficiency of the fire.

I used an ancient manual screw type pipe bender which was a lot of work to use. You might want to take it to a muffler/exhaust shop and get them to bend it for you ( a local high school might let you use their workshop also)

You need to leave about 40 to 50 cm of straight tube at each end of the coil to give you enough distance to the tub (you don't want the flames and smoke to spoil your evening)

Step 8: Open Up the Tank

Warning!: This step involves explosive flammable gas and you should take all necessary steps to protect your safety yadayadayada - in other words exercise common sense and if you blow yourself up don't come crying to me.

You need to make sure there is no more gas in the tank. Open the valve all the way and leave it for a few days. If your gas bottle has a pressure reducing valve or quick release nozzle fitted, remove this also because it will not let all the gas out.

Use a heavy monkey wrench and hammer to remove the gas tap and valve gear on the top. You may need to insert a heavy steel bar through the handles to get more leverage and stop the gas bottle from rotating annoyingly while you're whacking it. It would be prudent to use a wooden mallet to hit the wrench with to avoid sparks and do this operation outside in a well ventilated space.

Fill the tank with water to remove any traces of gas and empty it before cutting it through the equator with an angle grinder.

Step 9: Cut the Tank to Take the Heat Exchange Coil

Now you will need to cut some keyholes into the top and bottom hemispheres to take the coil.

Mark the location on the bottom half of the tank (bearing in mind that you want to place the more of the coil at the top of the tank to get more of the heat) and cut with an angle grinder.

Insert the coil into the keyhole and then hold the top hemisphere over the bottom, balanced on top of the coil. Mark the location of the outlet pipe on the upper hemisphere then measure how deep you need to make the cuts.

Make the slot wide enough to admit the pipe without having to rotate the top half at an odd angle to account for the angle of exit of the tube.

Step 10: Cut the Hole in the Top of the Tank

You need to make a hole in the top of the tank to allow you to add wood and get a bit of a draw going.

The first thing to go is the handle/valve protection collar around the top. With a marker pen draw around the inside of the ring and continue the line in an arc across the gap.

Cut the ring at the tabs flush with the top of the bottle and grind of any remaining weld material. Trim the bottom of the collar to remove the remains of the tabs.

Cut a circular disc of steel out of the top with an angle grinder or a gas axe if you have one.

While you've got the angle grinder out remove the paint on both sides of the collar and around the lip of the hole you've just cut to prepare it for welding.

Save the resulting disc to use for the bottom ventilation ring.

Now you can weld the collar back onto the tank so that it surrounds the hole.

Step 11: Create the Ash Grill and Ventilation Holes

Next task is to create holes in the base to allow air to come through and ash to fall out of. You can go about this any way you like. The simplest thing would be to cut a cross into the base inside the steel ring and cut some slots in the sides where the upward curve starts. (This is what I do for my gas bottle braziers). I went a bit overboard with this one and made it a bit more decorative and added a little bit more volume to boot.

What I did was cut a disc out of the centre then a whole bunch of little ray like cuts around the edge. I then bent these outwards by smacking the tabs with a hammer and on welded the disc that I cut out of the lid earlier. This disc was attached so that the concave side was facing out - I figured that that way the ash would fall out better and not pool in the dish.

Step 12: Decorate the Heater

Here you can go to town with decorating you burner with bits of scrap or by cutting out shapes in the sides of the tank. You should also add some legs to hold the thing the correct height off the ground.

Depending on what you are doing to decorate it you may want to wait before you weld the two halves together because if you're making deep or awkward cuts with the angle grinder you'll run the risk of cutting into the coil.

To get an idea of the ideal height, think about where the top water level will be when nobody is in the tub. You want the top inlet from the heater to come in an inch or two below this level and make your legs the correct height to account for this.

Step 13: Perforate the Tub

With the heater leaned against the spot on the outside of the tub that you want it to be, mark the locations of the outlet and inlet pipes on the corrugated iron cladding.

Think about where the hot tub is going to end up before you cut the holes to make sure you have it in the best place for your situation.

Drill out the holes and smooth off the sharp edges.

Now slide the liner all the way into the tub shell and mark the location of the pipes. Cut the holes for the pipes making sure you only make them as large as the outside diameter of the flanged pipe fittings you are using.

Step 14: Attach the Hardware

Now slide the liner back out to expose the holes you made and firmly attach the flange fittings.

Slide the line back down whilst bending the sides in to get the threaded part of the fitting past the rim.

Pull the threaded ends of the flange fittings through the holes you made in the cladding.

Slide two hoseclamps onto each of the heater tubes and slip the flexible hose over the threaded end and the stainless pipe from the heater. Tighten the hose clamps so that they won't leak.

Now you're done!

Note: You might want to move the tub to it's full time location now before you attach the heater. It's heavy so get a friend. I actually made a trailer to allow me to move it on my own (I do have friends but they were at work at the time).

Step 15: Fill the Tub and Light It Up

Fill the tub with fresh or sea water and set a fire in the heater. It took about 4 hours to heat the whole tank so start early.

You also might want to place the heater on a metal tray to catch any ash and hot coals that fall out.

The heat exchange coil will get covered in creosote because it's so much colder than the fire - this is to be expected.

The other thing you can do is make an insulated lid for the tub from the top of the tank and some more corrugated iron.


Please excuse the camera work - there was a newbie at the helm...

Step 16: Improved Rim

Ok so I've added a nicer rim to the tub.

It's rough sawn pine that's screwed at the corners to form a square frame. This is then screwed to the folded flap of the plastic liner.

I have some really nice hard wood decking that I'm going to attach to the rim of the tub but I'm still scratching my head over the best means of attaching it.

I am actually pretty happy with the rim that's on here now as it's nice and smooth and tidies the whole thing up somewhat.

There is also room for a bottle of beer in the apexes of the corners!

Step 17: Other Options

Solar boost
- I have also added a solar boost loop to this by attaching a T-junction to the hot water inlet for a garden hose attachment.
- I have 30m of black polytube that I attach to the bottom outlet and hot water inlet. Use a one way valve on the cold water outlet pipe at the bottom of the tank to stop the tub from reverse thermo-siphoning at night. String the tubing out on the lawn in full sun. I don't know how effective this is yet - I'll keep you posted - but it should add a couple of degrees at least and reduce the burn time.

Either I haven't had this connected long enough in hot enough conditions or my loop is too short, but this technique's a non-starter :(
On the up side the new house I'm building has a 30 evacuated tube solar hot water heater and a wood fired wetback so hot water should be in plentiful supply in summer!

"Active pumping"
- I am toying with adding a 12v pump to the cold inlet to scoot the water through a bit faster. The thermosiphon works but 4 hrs is a long time to wait.

I recently bought a 2nd hand intex inflatable pool pump and set this up to pump water from the bottom outlet of the tub to the cold inlet of the heating coil.  You need to buy a few doohickeys to connect everything up properly but it's very straightforward. The results are interesting.  The heat up times are not dramatically affected by active pumping (it may give you a 1/2 hour on the thermosiphon method), but it totally eliminates the flash boiling of the water in the heating coil.  This is great because I've been poached a few times by the hot water inlet!

BTW - Thanks everyone for your comments and support!

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