DIY Wood Fired Hot Tub
Hot tubs are great! Who doesn’t love a relaxing soak after a hard day? You can have a nice hot soak even when living off-grid with a wood-fired hot tub.
How hard is it to construct your own wood-fired hot tub?
Besides wood and water, the basics include a tub, a wood-fired method of heating water, and probably a system for circulating your nice warm water. However, there are additional considerations in keeping the water hot and clean, as well as possibly filtering and sanitization. Lastly, you’ll want to look into the laws and limits that may affect your use of this sure-to-be-popular backyard attraction.
Generally wood-fired hot tubs are made a few different ways. Usually they start with a farm stock tank and a 55-gallon drum converted into the furnace. The furnace can be set right in the stock tank or off to the side with the water piped through the drum to heat it up.
Choose the Tank You Want to Use for Your Wood-Fired Hot Tub
Converting a stock tank for a tub
Tractor Supply is a great source for stock tanks, but finding one deep enough is hard. Most stock tanks are two feet deep. They come in round or oval shapes. A six foot diameter stock tank will hold 390 gallons of water. We fit five adults, 2 kids, and the 55 gallon drum heater. See picture to the right.
An oval stock tank 2′ wide and 4′ long holds 103 gallons and will seat two people scrunched up.
Most DIY hot tub makers use galvanized stock tanks, but Rubbermaid also makes stock tanks from molded polyethylene for superior performance and long-lasting durability.
Bottomless stock tanks can be deeper
Bottomless stock tanks are just how they sound. You get just the sides and need to create a concrete base and use a liner to create a pool. These can be deeper and larger than basic stock tanks. The expense defeats the purpose of “DIY inexpensive”. If you’re going to go to this extreme, you might just as well buy a small hot tub already made.
Amazon sells galvanized metal raised garden beds that would also work well. Use a pool liner to prevent leaks and add a bit of insulation.
Cedar Hot Tub
A cedar barrel is almost an iconic wash tub. They were invented before metal was available and did a good job of holding water. However, cedar hot tubs do leak, are a bit tricky to assemble, and need to be done just right. That’s why I recommend a kit rather than trying to cut and shape the staves yourself.
Because cedar is wood, it can grow mold if not properly cared for. Unfortunately, both chlorine and bromine can damage the wood.
Determine the Type of Firebox You Can Make
In the picture shown above, Mark put a 55-gallon drum in the tank. The hose you see running to a separate enclosure houses a good filtration system. Enclosed is also a Shop-Vac for getting more air into the combustion area.
In-the-tub firebox
Putting a 55-gallon metal drum into the tank seems easy. No plumbing needed. But you need to make a way for combustion air to get to the fire area, and a way to get the wood into the drum without burning yourself. A bellows is often used to stoke the fire hotter.
External Firebox Heaters
The TimberLine Wood Water Stove naturally circulates the water using no electricity. Its design is based on Thermosiphon. Click on image to see details.
An external firebox has many advantages over the internal firebox.
- You have more room in your tub for people.
- Less chance of burning yourself on a hot spot.
- Easier to design and control airflow to combustion area.
- Keeps ash and wood debris away from tub.
- May require a pump if thermosiphoning¹ isn’t enough.
¹“Thermosiphoning is a method of passive heat exchange, based on natural convection, which circulates a fluid without the necessity of a mechanical pump”- credit Wikipedia.com.
Thermosiphon design does not need electricity to pump water.
The external water heater shown to the right is a “thermosiphon” design. The firebox is double-walled with about a 1.5″ gap that contains the water, making the entire firebox (except the front) a heat-transferring surface. Additional heat transfer takes place in a water-filled flat baffle tubes that runs horizontally through the firebox. The baffle deflects the path of the fire, so it gives up more heat into the water jacket before going up the chimney.
Heat is easily controlled by adjusting the air draft control for heating the water and maintaining the water temperature.
The dimensions of the TimberLine BonFire Stoves are 18″ wide x 29 ” long x19″ high with at total weight of 66 lbs. The fire box can accommodates 28″ logs. The BonFire Water Stove comes with a 3/4″ drain plug to completely drain the internal water jacket preventing any damage that could be caused by freezing. Also included are a temperature gauge to regulate the hot water and a safety pressure valve.
Building Your Own External Wood Fired Hot Tub Water Heater
You can build your own hot tub water heater using a 55-gallon drum for about $650.00* according to the folks at Instructables.com.
They built a door into the front of the barrel and lined the bottom with fire bricks. The the door the legs kits shown in the widgets section of this page for details.
A copper coil was built into the smoke stack and an electric pump circulated the water thru the coil. The builder used a Shop-Vac to force air into the fire box to make the fire burn hotter.
Here’s a good YouTube® video showing an upgrade to the barrel stove type of water heater.
Things you need to think about
When you buy a hot tub it usually has a nice lid to keep the water hot and clean. The sides of the tub are also insulated. The tub is usually deeper that a stock tank.
A manufactured hot tub using an electric heater may cost about $10.00/mth. in electricity but the water will be nice and hot when you’re ready to use it and the temperature will be regulated at 104 degrees.
If you will be using the same water for long periods of time, you’ll need to add a filtering and sanitizing system of some kind.
Redneck Wood-Fired Hot Tub Video
Here’s a YouTube® video showing the simplest way to make a woo- fired hot tub. It shows just the bare basics of heating water for the tub. It took about 2 hours to raise the temp. of the water from 56° to 102°F.
Laws and limits that may affect your use of a wood fired hot tub
Believe it or not, the EPA has strict emission standards for wood-burning boilers. It’s possible that your local authorities, even your homeowners association, could give you some grief. From Obadiah’s.net:
“On May 15, 2015, Step 1 of the NSPS revisions for wood boilers went into effect. Any EPA-certified wood boiler manufactured on or after that date is required to adhere to the 0.32 lb/mmBtu heat output.
Note that it is still legal to sell or own a wood boiler that is not EPA-certified, and that it is only illegal to operate a boiler that does not comply with state or city air quality laws. Chances are your local state laws already reflect the EPA’s update to the NSPS, but please check with your Department of Environmental Quality to find out what regulations apply to you before purchasing a wood boiler.”
Living Off-Grid with Jake and Nichole
Here is a YouTube® video from Nichole and Jake. They live off-grid in a yurt and made their hot tub from a stock tank and a separate firebox. They catch the rain from the shed roof to fill it then send the used water to the garden.
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