How would you like to take some lawn waste, use it to heat your hot tub for 18 months and be left with a huge pile of rich compost at the end of the project?
It sounds like the last scene from Back to the Future, right?
Well, Tom Bartels of Durango, CO is a very inventive gardener and he has figured this out. The best part is that it’s a project that anyone can do with enough space and the time to assemble it. The materials are all readily available, no flux capacitor necessary.
How It Works
You know how compost gets hot as it decomposes? Most gardeners would be familiar with that effect but few have ever considered harnessing that heat for any real purpose. But it can be done.
Tom uses a large quantity of wood chips (and a few other minor ingredients) to power his off-the-grid hot tub. It ends up being a 6 foot tall compost pile that he insulates with leaves. The hot tub itself is insulate only with straw bales and a foam topper.
He uses an electric pump to circulate the closed-loop system, which costs him a whopping $3.27 per month in electricity. Hook it up to a solar panel and it would be totally off the grid!
Consistently Hot Water in Freezing Weather
The water temperature in the hot tub stays between 104 to 110 degrees Fahrenheit ALL DAY, EVERY DAY. Any time he wants to take a dip, the water is piping hot, with NO fossil fuels used to heat the 250 gallons of water.
All his data collection for the project was done over 2 months from mid-December to mid-February in Colorado with nighttime temperatures in the single digits.
Tom estimates that this heat production will continue for 18 months (!) after which the temperature will decline due to the compost maturing.
Pays for Itself in Year One
The cost to set all this up was about $700, mostly for the pump and pipes. But get this…the market value of the organic compost is $700-$1,200!
So it pays for itself (or more) in the first year and every year thereafter is effectively free because the pump and piping are reusable and the wood chips that provide the fuel are waste material that landscaping companies are happy to get rid of.
Use It for Anything
Tom made a hot tub but what he created is just a system that produces heat from yard waste. You could use that heat for any purpose including:
- Heating a house using a radiant floor
- Heating a green house
- Heating aquaponic fish tanks
- And more, it’s limited only by your imagination...
For a lot more great gardening tips, see our guide on 31 Ways to Make You an Organic Gardening Guru.