Pump Power Requirements for Solar Pool Heaters
Will solar pool heating cost more in electricity?
In most cases the extra load on the pool pump is negligible. Here is why. The energy that your pool pump delivers is proportional to the pressure created times the flow rate. For discussion purposes let's assume the flow rate with solar on is the same as the flow rate with solar off so that pressure is the only factor. It doesn’t require much pressure to drive the flow through the piping and the solar collectors. The solar collectors are parallel tubes so the flow splits evenly between them. The water moves quite slowly through the tubes. In fact we set the flow rate so that solar does not restrict flow. There is no need to generate more than 2 psi driving flow through solar. It’s the height that we have to raise the water that matters. Consider that height is pressure. One foot of elevation difference is 0.433 psi. There is pressure in the pool piping with the pump on. Solar on requires that pressure or head to increase to get water up to the solar panels. Solar is an open loop meaning the vacuum breaker prevents negative pressure so we can’t create a syphon.
The water flowing back down the return side does not compensate for the water being lifted on the feed side. If the roof in only one story up the pumping head is only about 10 feet. But the water was already at a certain height. Gas heaters create about 5psi or 11 feet of head so you might have 5-8 psi between the filter and the gas heater where solar normally ties in. Its likely that to get the water up to the roof on a one storey home, the extra pressure that must be generated by the pool pump is zero or even negative meaning the solar collectors are under pressure even with solar off. If the roof is 2 storeys up this is usually considered an ideal condition for solar hydraulics because the pump has to create a little pressure to get the water up to solar. 2 storeys is 20 feet or 8 psi. If we have 5psi there before we add solar to the plumbing then the extra load on the pump in this case is about 3 psi. That’s almost negligible in terms of pumping power requirement. Its when we get to 3 storeys up or more that we have to start thinking about the extra pumping head requirement to add solar. In this case we set the solar speed on your variable speed pump higher when solar is on so you are only creating the extra pressure to raise water to solar when solar is actually on. We very seldomly need the help of a booster pump. Its hard to find a booster pump less than ½ HP. The extra flow caused by a booster pump can often be too much for the pool plumbing and we end up generating too much restriction for the increased flow rate. The default plan is to always just use the pool pump and not add extra pumping capacity.
With today’s variable speed pumps there is lots of power available, so much so that even pressure side cleaning systems no longer use booster pumps. You just turn the VS pump speed up. In this interesting case in Kailua Kona Hawaii we had to deal with a 7 storey rise to get water up to solar. It worked perfectly with no booster pump, just a Pentair Intelliflo variable speed pump running full out. What we had to watch in this case was the air on the return side. We set the return piping so that a column of air would not be developed on the return side. As explained in our Flow Rate white paper forcing water down against a column of air trying to rise in the same pipe can be a big restriction that must be taken into consideration so here we angled the return piping so that air could flow up as water flowed down. We added air vents at the major bends. Its all worked flawlessly for 6 years as of this writing in 2021.