DIY

HOME

SOLAR HYBRID HOT TUB

A free and easy thermal assist from the sun. By Eric Muhs

Back when I lived in California, I heated the water for my house and hot tub using some 1970s “energy crunch”-era solar thermal collectors that I found at a junkyard. They were a nice design from Israel, with a large plexiglass collection surface and an insulated horizontal tank with its outlet up top, where the hottest water rises.

Those restored units were all the water heating we needed for about 9 months of the year, and

Photography by Eric Muhs

I installed them on a hillside rather than the roof for easier maintenance. (Alternative energy means lots of repairs; if you’re harvesting energy from the environment, your equipment will be, well, out in the environment.)

Then I moved to a newer house in Seattle with a fancy automatic hot tub on the rooftop deck. But I was appalled when I saw that it cost up to $40 in electricity each month to heat this mass of water

and leave it covered outside overnight.

So I came up with this simple and inexpensive solar system that adds heat to the hot tub during the day so that the main electric heater doesn’t have to work as hard after the sun goes down.

Solar Collectors

The system heats the water by running it through coils of black vinyl hose inside 2 solar collector boxes. A small solar-powered pump draws water out of the tub, runs it through the coils, then dumps it back in. The tub’s original heater and thermostat are not altered, but the heater switches on less.

For the solar collectors, I built 2 open 3'× 3' boxes out of plywood reinforced with blocks of scrap wood in the corners (I had just 1 box originally, but expanded the system later). I put the boxes together using 1¼" wood screws and glue, then caulked the

Make: 135