Part IV: Solar Water HeatingChapter 6. Simple and Batch Water Heaters

Solar Pool Heating on a Budget

How much collector or cover area a pool wants, and why a cover often beats another panel.

Difficulty: Intermediate
Cost: Varies

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A swimming pool loses heat mostly from the surface, mostly by evaporation, mostly at night. The cheapest solar device for a pool is a cover you actually pull on when you get out. Collectors replace heat. A cover keeps the heat you already paid for, from the sun or from a heater.

Unglazed polymer mats are the usual collector. Pool water wants to be around 80 degrees F, and at that temperature a sheet of glazing does not buy you much. A copper domestic collector is the wrong metal in chlorinated water, and it is the wrong temperature machine. Keep pool water out of the house hot-water system.

Difficulty

Level: Intermediate
Time: A day to learn the cover habit, longer if collectors go on a roof
Skills: Honest sizing, and the judgment to stop at the roof and call for help

Cost estimate

Varies

A cover for a small above-ground pool can be under a few hundred dollars. Unglazed collectors sized to the sketch below are often the larger bill, and a roof, a pump, and an electrician move it again. Buy the cover first and see what is left to do.

Piece	Typical cost
Cover you will use nightly	Tens to a few hundred dollars for a small pool
Unglazed collector area	Often the main expense once the cover is in place
Plumbing to the existing filter loop	Hardware plus, on a roof, mounting and flashings

Materials

Item	Qty	Notes
A cover matched to the pool	1	Solar rings, a blanket, or a safety cover are different objects. Read the maker's load limit. A cover is not a walking surface.
Unglazed pool collectors	From the sketch	Polymer panels made for pool water. Not a copper domestic hot-water collector.
Tie-in after the filter	1	So debris is filtered before it reaches the panels. Do not add a new suction opening in the pool.
A pump that can spare the head	The existing one, or a check that it cannot	Roof arrays add lift. If the filter pump is already at its limit, flow collapses or the motor suffers.
Drain-down plan	1	Panels above the water must empty when the pump stops if a freeze is possible.

Cover first, then area

Measure the water surface, not the deck. A 15 by 30 foot pool is 450 square feet of water. Every night that surface is uncovered, you give back a large share of the day's heat. If the pool spends its nights uncovered, much of the collector area you buy only makes up for that loss.

With a cover used every night, a first guess for unglazed collector area is about half the surface, in a sunny and fairly mild site. Without a cover, start near the full surface. A cool, windy, or partly shaded pool needs more area, or it needs a smaller ambition for water temperature. The sketch below does that arithmetic and also shows how many BTU it takes to raise the pool a single degree. That number is why a pool heats slowly and why a forgotten cover hurts so much.

Flow through pool panels is high and the temperature rise across them is small, often only a few degrees. If the outlet is much hotter than the inlet, flow is too low. Follow the panel maker's flow number rather than a domestic-hot-water rule of thumb.

The cover sits on the water. Collectors are a second purchase, sized to a pool that is not naked all night.

Roof, pump, and chemistry

Panels on the ground or a low rack are easier to plumb and easier to drain. Panels on a roof need a structure that can hold them in wind, flashings that do not become a leak, and a pump that can lift the water. Many pool panels are meant to drain back when the pump stops. A check valve that holds them full will freeze and split them on the first cold night. The drainback and freeze pages are about domestic systems, but the lesson is the same: trapped water is what bursts.

Do not add a new underwater suction fitting to "feed the solar." Body entrapment on pool drains is a known hazard, and the fittings you already have were chosen as a set. Tie into the existing filtered loop the way the panel instructions describe, or hire the pool technician.

Chlorine and bromine stay in the pool loop. They do not go through a tank you cook with or a coil in a water heater. A heat exchanger is the only sane bridge if you ever think about sharing hardware, and most houses should not share it at all.

Cover the pool for a week of normal swimming
If the cover never makes it onto the water, fix that before you buy panels. Note the water temperature in the evening and the next morning.
Run the surface sketch
Use the real water size and a depth that is an average, not the deep-end boast. Treat the collector area as a starting guess and add area for wind, shade, or a higher target temperature.
Choose unglazed panels and a place they can drain
Ground rack if you have sun there. Roof only if the structure, the flashings, and the pump head are settled. Read working on roofs before you go up.
Plumb after the filter, with a drain path
No new suction opening. No copper in a chlorinated loop. No check valve that leaves the panels full in a freezing climate.
Confirm a small rise and a warmer morning
Measure inlet and outlet. A few degrees at high flow is success. A large rise means poor flow. Then pull the cover that night anyway.

Solar Pool Heating on a Budget

Difficulty

Cost estimate

Materials

Cover first, then area

Pool surface sketch

Roof, pump, and chemistry

Cover the pool for a week of normal swimming

Run the surface sketch

Choose unglazed panels and a place they can drain

Plumb after the filter, with a drain path

Confirm a small rise and a warmer morning

Further reading