There is a certain way that all solar pool heaters should be integrated into the plumbing of a new or existing pool. Solar panels are always installed after the pool filter and a series of vales controls operation. One of the questions we get is whether we plan to install the solar pool heater before or after an existing gas heater or electric heat pump.

Where to Start the Solar Feed and Return Plumbing

Typical plumbing for solar pool heater with an existing heater.

Every major manufacturer of solar pool heating equipment requires or recommends in its manual that the solar heater is plumbed in before the auxiliary heater. The reasons for this are as follows:

• Heat transfer relies on a concept called Delta T, which says that the difference in temperature between the incoming water and the surface of the solar collector determines how much heat is transferred to the pool water. Heating water before it goes to the solar panels will at times make solar panels ineffective, and can even cool the water, whether the solar valve is automated or not.
• Because of the Delta T concept, we rely on a high flow rate for effective heating, and water returning from the solar panels is by design only a fraction of a degree to a couple of degrees warmer. If that is not the case, it’s a poorly designed system or the flow rate of the solar collector is less than ideal under the conditions. That’s why we use only 2″ plumbing and panels with the lowest resistance to flow in the industry. We mention this because…
• It’s true that electric heat pumps suffer from the same issue with incoming water temperature affecting efficiency, but the very small increase in temperature from solar will be negligible in terms of heat pump performance and energy use.

The overall performance of the combined system requires that pool water flows through the solar panels before any auxiliary heater.

Safety With Multiple Heaters

There is a potential hazard if solar panels are plumbed in after a traditional heater. For example, if you heat spa water to 104 with a traditional heater and then send it to solar panels, the water could be heated beyond safe temperatures and cause a safety issue. While less likely, this can occur even if temperature limiting controls are used for the solar heater because the water sensor must be installed before any other heat source to function properly. Manually operated solar heaters pose a real increased risk in this scenario.

When a system is installed correctly, solar heated water will not be further heated by the traditional heater because the water temperature sensor is internal and would prevent operation. Regardless, we recommend that automated solar temperature controls always be used when there is a small body of water like a spa present.

Downsides of Doing it Right

There are a couple of minor issues that you should keep in mind when installing solar pool heaters with traditional heaters.

• Because the incoming water to the traditional heater may be preheated by solar energy, the heater may not come on as expected because it is tricked into thinking the pool water is warmer than it really is. To correct this, set the desired setpoint in the heater control panel a few degrees higher than actual desired temperature. You may need to do this in your pool automation system depending on how the heater is connected and programmed.
• When solar heaters first start up, air is purged from the system. This air can cause heat pumps and gas heaters to shut off for safety and self-protection because they sense a low-flow or no-flow situation. Most heaters will restart automatically when flow is sensed again, although some heaters may have a delay built in.

Plan View of Solar Pool Heater Plumbing Recommendation