Heat Exchanger in a Wood Fired Hot Tub for a Shower




In the middle of nowhere, far from cell phones, automobiles, and power lines, there is a little bit paradise that I frequent. On a point of land overlooking a sheltered bay on the ocean, there was a wood fired hot tub with no shower. The only running water available was cold and that was not good enough for me. So, this is how I fixed the problem:

I used all 1/2" copper pipe for this project. (There are always exceptions.)

1 x 20' copper pipe

8 x 90 deg. elbows

2 x 45 deg. elbows

50' roll of soft copper pipe (it fits 1/2" fittings, but is sold as 5/8" )

male and female garden hose connectors to solder to the 1/2" copper ( or what ever you want to use to connect for your system.)

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Step 1: Find an Unused Corner

There is not a lot of extra space in a wood fired hot tub so I wanted to use an "out of the way corner" that would not interfere with relaxing in the tub..

Have you ever tried to find a corner in a round room? (You don't need to answer that.)

I found the perfect space between the fire box and the wall of the tub.

Step 2: Round and Round We Go

The copper coil comes almost how we want it. First I slipped two lengths of straight pipe between the layers of the coil to help see what was what. I then bent the center a little tighter and worked my way out expanding the coil on one side to make the coil lay with about 1/2" spacing, flipped it over, then expanded the other side.

The two spacing pipes are just long enough so they don't touch the bottom of the tub.

A 90 degree elbow is put on the bottom of each spacing pipe and one is attached to each end of the coil.

After making sure all pipe is clean and flux is on at all the contact points, I think it would have been easier to solder, if I had used two boards and clamps to sandwich the coils tight against the straight pipes. I didn't and it worked but one or two wanted to jump away when I turned it over and soldered the other side.

A 90 degree elbow is then placed on the top end of the spacer pipes and with the coil held in place in the tub, find the angle that they need to be at, to hold the coil on the side of the tub.

I chose to have the pipes go out to the sides so they were not right in front of where you stand when stoking the fire.

So the ends go out of the tub, down the side to a 45 degree elbow (so there is room for your fingers, between tub and pipe, when attaching the hose) and the garden hose adaptors.

Cold water in, warm water out to shower head. There is no mixing of hot and cold for the shower, there is just a ball valve in line for on or off.

Step 3: Bonus Deck

So now when the hot tub is in use, we have a hot shower that is the perfect temperature and you will run out of water before you get a cold shower. If the shower is too hot then so is the tub.

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    5 Discussions


    5 years ago

    Very nice project and a very nice hot tub, I would like one in my yard.
    I was wondering if you know the temperature difference of the hot tub and the shower water, does it feel like te same temperature when you use 20' of copper pipe? Could the coil be smaller (shorter) to still achieve this effect.

    3 replies

    Reply 5 years ago on Introduction

    Thanks, I think it could be shorter but I don't know by how much. I had to buy a new roll for the project and didn't have any other project out there that needed copper too so I just used the whole roll. It kept everything nicely balanced and I didn't want to have to add more later because it was too short. Half the length may have been enough.

    One time I went to have a soak and jumped under the shower and I would not have wanted it any hotter. I didn't get past my knee on one leg for more than 2 seconds when I tried to get in the tub, so I just had a longer shower. I later checked the tub temperature and it was just over 120 F. I am sure the water has come to full tub temperature by the time it leaves the tub.


    Reply 3 years ago

    how long does it take to heat about 200 gallons? I am thinking about doing this but interested in a premade heat ex-changer because I dont wanrt to wait all day to heat the tub


    Reply 3 years ago

    The tub is just over 3' deep and between 5-6' across. If I light a fire and burn small wood the tub heats up in half a day and is faster if the water is moving regularly. I have paddle I use to stir the tub every hour or so when starting it fresh. If I use large wood it takes a full day but you don't have to tend it as often. Once it is going I add wood maybe twice a day and let 3 large pieces smolder to maintain the heat. It is easy to get it too hot. The heat exchanger is just for the shower when the tub is hot. You would have to check the specs. on the pre made exchanger to know if it suited your needs.


    4 years ago on Introduction

    Pretty cool man - this is how I set up my copper coil in my outdoor wood stove: