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
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.
Gulfstream’s HE line of heat pumps recommends installation of the heat pump prior to the solar heater. As you mention the heat pump inlet water temp will be reflective of the pool/spa but also their concerns is the turbulent air purging can be damaging to the heat pump.
I recently had a Gulfstream heat pump installed and I understand their rationale of pre solar heater location but because the solar heaters are further downstream from my pump I’m noticing I need to work the pool pump at higher RPMs to keep the VRV closed. I have an intellifloXF.3HP and need to be at 3250 rpm whereas with the prior heat pump installed after the solar panels I could run the same pool pump around 2500-2800 rpm to keep the VRV closed. Is there a less resistant VRV that will stay closed at a lower pressure? Is this plumbing order not ideal?
I have little use for running both solar and electric pump at the same time. Mainly the electric heat pump is for the hot tub which I would run in the evening when solar heater would not be running.
Thoughts?
Hi Alexander,
If you put the heat pump in front of the solar heater, it will render the solar heating far less effective, because you will be sending water to the solar heater that is already warmed, resulting in a lower “Delta T,” which is the difference in temperature across the heat exchanger. This reduces the amount heat transfer. While it’s true that the same applies to the opposite configuration, the heat gain from the solar panels is minimal by comparison typically, so the heat pump does not become much less efficient.
You could relocate the VRV, although that is not ideal for draining purposes. You may get slower draining and excessive noise.
You could also install additional valves to bypass the heat pump when not in use.