By Tom Soukup
Many heated pool and spa systems rely on a single pump or pump package to provide flow for the filtration system and heating units. While this simple approach has provided the bare necessity for circulation of pool water for decades, it is far from an optimum design. When properly sized and installed, providing a separate pump for the heating equipment extends the lifecycle of all system components, lowers power and fuel consumption, and reduces maintenance.
In commercial pool applications, filter pumps are specified based on the size of the pool and the turnover rate dictated by the local governing body or International Swimming Pool and Spa Code (ISPSC). For example, in Pennsylvania, a commercial pool requires a turnover once every six hours, and spas must turnover once every 20 minutes.
All too often, filter pumps are sized to meet the required flow rate and overcome filter head loss, with no consideration given to the flow requirements and head loss of the heat exchanger(s) in the system. This is an issue because heating equipment has a manufacturer-specified minimum flow rate requirement to protect the heat exchanger and attain maximum efficiency.
Most systems seen in the field use an insufficient means of providing flow to the heating unit(s). This almost always includes two tees with a manual diversion valve in between.
This diversion valve is, in theory but rarely in practice, adjusted to provide the correct flow rate through the heaters. Often, the valve is not set correctly because the system lacks flow meters to indicate what the actual flow rate is through the heat exchanger(s).
Moreover, even if a valve is set to the correct flow rate a manual valve does not modulate to reflect whether the heaters are running or not, meaning water is constantly flowing through the heat exchangers. This not only wastes power, but constant circulation through the heat exchangers increases mineralization inside. This reduces the heat transfer efficiency of the heater and further increases pumping resistance, as the channels within the heat exchanger become narrower. It is similar to plaque buildup inside the arteries of the body.
In the less-than-desirable but all-too-common scenario described above, the issue is, the filtration system and the heating system are not hydraulically separated. Both loops are served by the same pump, and the pump is typically not sized for the additional head loss of the heat exchangers.
Aside from the inadequate flow for both the filters and the heating appliance, this creates a scenario in which the pump draws more amperage than intended, leading to wasted energy and premature pump failure.
Marginal improvement
Some manufacturers have recognized this issue and have tried to make an improvement. Some larger commercial pool heaters ship with a built-in pump that serves the heat exchanger of the appliance. However, when you look at the specifications of the appliance, there is a very specific acceptable “developed pipe length” the provided pump can fit. Developed pipe length is the length of the pipe that can be accommodated by the pump, including the pressure drop created by elbows and other fittings.
