Spa Heating

Q: I'm considering trying to solar heat my hot tub. The heating requirements are a bit odd. It needs to reach about 101F for perhaps 20 or 30 minutes a day, usually after dusk. The rest of the time it doesn't really matter what the temperature is. It's about 500 gallons of water. With these parameters, it seems like conventional pool and spa heaters don't really do the job. They might keep the tub warm all day, rather than reaching a high temperature for a short period of time. I'd still need to fire up the gas heater to get to operating temperature. I'm thinking perhaps the thing to do is use a themosiphon to heat some water to above the operating temperature, and pump it into the tub just prior to use. Any thoughts?

A: I am considering the same thing. I figure the best way would be to use a fairly efficient type of panel to get the most heat exchange. I'm planning on building a glazed copper panel myself with some tubing I have around. Mine has to fit two roof panels on the gazebo. I also bought a used controller on eBay. If it gets hot enough during the day then it should hold the heat until you get in as long as it is well insulated. Worst case, you'll run your heater a lot less even if it's kept at 90 or so by the sun. PS do search on "solar hot tub" on yahoo. I recall seeing a few projects that worked well. I have had great success using Solar to heating my spa over the last 5 months. After considering a solar water panel and pumping the water, a few experiments confirmed a better method. For years, blue plastic "solar blankets" have been sold to pool and spa owners. The problem is they don't work very well. My tests with "solar blankets" only give less than 2 degree rise in the spa on the best day. On some partly cloudy days, the temperatures would actually drop. A better method is cheap bubble wrap ($4 to $6). It comes in large rolls. Buy the large bubble size. My spa needs 3 strips, which are placed parallel to each other. Preparation: Lay the 3 strips on top of your spa's water, smooth side facing out. Use a pen to trace the contours of the interior of your spa. Remove the strips and lay on a flat surface, in preparation to cutting. Then trim bubble wrap with simple sissors to match the contours of your spa. Finally place the back on the water. My location is coastal San Diego, which is usually cool and sometimes foggy. Also the spa gets full sun all day long. Obviously, your results will vary with your location(s). The spa is above ground, electric heater, with lots of insulation on the sides, and a removable 3" insulating cover. At this point, the heating in the spa is set to 80F (essentially off). In the morning, I remove the insulating cover so the sun can reach the spa. At night, I put the cover back on. Results: on Sunny days, there is get a 5F to 10F degree rise. Sometimes, it gets too hot and cold water must added! On foggy days, do not bother removing the cover. The bubble wrap has lasted so far... not counting the obsessive desire of my wife to pop the bubbles :-) A friend of mine is investigating more durable materials. In the mean time, I am enjoying the dramatic reduction in electric bills.

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