Solar Thermal Systems for Seasonal Pool Heating

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Swimming pool heating is the single most popular use of solar thermal systems: more collectors are sold for this purpose in the USA than for any other installation. In fact, solar pool heating is so popular that, on average, seven pool collectors are sold for every flat plate or evacuated tube model.

Why are they so popular? Because they're cheap, simple and save lots of money – a solar thermal system for a seasonal pool can pay for itself in as little as two or three years!

Pool collectors are typically made of special plastic and consist of a collector, a bunch of PVC pipes and a controller (plus a diverter valve). They heat the pool water directly, usually with a draindown setup, and are quite simple to install because they can use the pool's pump for circulation.

A typical solar thermal setup for seasonal pool heatingMost pool collectors are made of polypropylene plastic because they are direct systems and the water is usually chlorinated, which would corrode copper. The same goes for the PVC pipes. The collector is usually tied to its mounting location (a flat surface) with special manufacturer-supplied straps (so you should read the specific instructions for the model you buy) and last 15-20 years. Most models come with long warranties.

The typical solar pool heater setup uses two sensors attached to the controller. One measures the temperature of the water in the pool, the other tracks the sun's radiation. The desired water temperature is set on the controller, which usually works in conjunction with the filter timer, and a diverter valve controls where the pool water goes. This valve is installed just after the pool filter so that the water going to the solar collector array has been filtered.

When the pool is cool and the sun is shining, the controller opens the motorized diverter valve (also called an actuator). The pool's own pump is usually enough to push the water round the collector circuit, though you can add a booster if needed. The water heads off into the collector array and is heated, but it doesn't stay there: it circulates directly back round to the pool.

With each pass, the water gains about 1°F. It's a slow, persistent process that can heat the pool to as much as 15 degrees above ambient temperature in most locations.

When the pool is warm enough or the sun is not shining, the valve remains closed and the water goes directly from the filter pump exit back to the pool.

As mentioned above, pool solar heating systems are usually draindown installations. When the sun stops shining or the pool has finished heating, the controller closes the valve and most of the water in the collectors runs naturally back to the pool. However, to ensure that all the water runs out, a vacuum breaker is usually installed at the bottom end of the collector array, at the opposite end to where the water enters.

Most solar thermal pool heating systems have a collector area of between 50% and 100% of the pool size. However, the exact surface area is very