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What Can You Expect From a Solar Pool Heater in Seattle, Washington?

The curves above represent a full sized solar heater for a typical 16x32 pool with 25% shading in Seattle, Washington. 8 collectors is 75% of pool area in collector area. We've plotted the daily maximum pool temperatures. Note we size systems higher if you won't use a cover, in this case over 100% of pool area. We could go even higher. As you can see a cover helps a lot. In this region, there is some humidity in season so a pool loses significant heat by evaporation. A pool with a full sized solar heater and cover will peak out daily 15 degrees higher than an unheated pool. Solar bumps the daily peak temperature of a covered pool 10-13 degrees. Solar bumps the daily peak temperature of an uncovered pool about 10-13 degrees as well. Note that we wouldn't let the pool get so warm thru the summer. We have control of the high temperature either by turning solar off, reducing pump run time, setting the auto controller to a maximum and we can even run the solar at night to bring the pool temperature down if we want. We're showing what would happen if we didn't limit the pool temp because the data is meaningful.

I have a lot of personal experience with pools in this region so I need to note that these curves are pessimistic in my opinion. I'd appreciate feedback from our customers in this area. I think the spike of temperatures has been downplayed in the simulation's weather profile. What happens is that some years there is a nice stretch of good weather in April and we find our solar heated pools get to 80F consistently. In years when April stays gloomy, we don't. I think the typical weather profilers are unwilling to say a typical year gets 3 days of good weather in a row. The reality is that when the nice April weather surprises us, the solar heated pools are ready for a party and those with a gas heater will not even consider the idea of turning the gas on for another 5-8 weeks.

All these curves are based on a typical meteorological year which is hour by hour values of air temp, solar radiation, wind levels and humidity based on 10 years of real weather data but the "tmy" isn't average weather data. It is typical weather generated using a complex algorithm and perhaps manually to some extent.

It would be very costly to keep a pool above 80 with a gas heater in the months solar can't do the job here. Realistically you just can't heat a pool in the off season in Seattle. Note that the use of a cover will allow a few extra weeks of pool use in the fall and spring and thru the summer you can forget about the cover. Solar can make up for the need to use a cover.

Note that you may not be impressed by what a solar heater does in this case so it is helpful to look at the alternatives. A gas heater is the only realistic alternative. We've done the computer simulations for the same season you get with solar but instead using a gas heater to maintain just 84 degrees throughout. We did it for the season May 15 to September 15 with and without a cover. We found that a gas heater would suck up 58 GJ in the first case (covered pool) and $144 in the second case(uncovered pool). At a rough cost of $10/GJ when you assume the gas heater is 80% efficient and the cost of the actual fuel is $8/GJ you see that in the first case (covered pool) solar does what $580 in gas would have done and in the latter case (uncovered) what $1440 in gas would have done. The cost of the typical manually controlled (using the pool timer) solar heater in the covered case would have been $2,400 including pipe (4 season payback period). In the case of the uncovered pool the larger solar heater would have cost $3,600 (2.5 season payback period) so the economics are attractive in either case and we can afford to add automatic controls and put solar on east or west facing roofs where there is a performance penalty and still get very good bang for the buck. Note solar and gas can operate together solar automatically shutting gas off when it is on and capable but this strategy is not the norm. Normally we'd shut the gas heater off and let solar do the maximum amount of the work possible in the case of a residential pool where we can live with pool temperature fluctuations. Here I've linked the computer simulation summaries for the two cases. Covered Pool and Uncovered Pool

Here is a testimonial of sorts from one customer in the Seattle area who has had his Powermat solar pool heater for 6 years. Contact Bill Rowling, our installer in the Seattle area, through his website at

The above curves represent a typical 16x32 inground pool with a little shade (25%) heated or not with an aged solar cover and an 8) 4x12 solar collector system. We assumed the cover would always be on the pool 20 hours a day and for 4 hours the cover would come off (for the 2 plots with covers) and moderate pool activity would occur. We created these charts using Enerpool solar simulation software. We do not have confidence in any other computer program like this. Click here to learn about this software and its history. You can even download the software yourself and put it to use if you're an engineer type.

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