Since 1990, HydroTher Hot Tubs have been the #1 choice of architects, consultants, designers and facility operators for commercial aquatic applications.
Many heated pool and spa systems rely on a single pump or pump package to provide flow for the filtration system and the heating units. While this simple approach has provided the bare necessity for circulation of pool water for decades, it’s 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 pool code. 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 we see 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. More often than not, 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 that 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’s similar to plaque buildup inside the arteries of the human body.
In the less-than-desirable but all-too-common scenario previously described, the issue is that 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.
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’s a very specific acceptable “developed pipe length” that the provided pump is able to accommodate. 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.
For example, the installation and operation manual of a pool heater may specify that no more than 15 feet of developed pipe length can be accommodated. Some installers may use no more than 15 lineal feet of pipe, but when elbows and fittings are accounted for, the real head pressure of the loop equals that of, say, 60 feet of developed pipe. In a real-world application, it’s very hard to fall within manufacturer specifications when a pump is supplied with the heating unit. The manual rarely reflects a real-world application.
We customize all of our systems by hydraulically separating the filtration loop from the heating appliance loop and sizing our pumps for each.
Instead of using a diversion valve to supply water to the heating appliance, we provide hydraulic separation via closely spaced tees, so the flow rate through the heater loop does not affect the flow rate through the filtration system, or vice versa. The independent heating loop is circulated with a VFD-powered pump. We calculate the developed pipe length and heat exchanger pressure drop to select a pump. This gives us a number very close to the actual performance needed, but actual pump performance is perfected through one final step.
By installing a flow meter on the effluent side of the heat exchanger, the VFD on the pump can be calibrated to perfectly match the specifications provided by the heating appliance manufacturer. If the heat exchanger is not getting enough flow, we increase the speed of the pump. If it’s too high, we reduce the speed of the pump.
Through our unique control strategy, we only run the booster pump when there’s a call for heat. As a failsafe, we also provide secondary flow-proving, so the heaters will not fire without flow through the heat exchanger.
Through the controls, we program a cool-down period after the call for heat, which protects and extends the life of the heat exchanger. We often find that a procedural problem in the field occurs when maintenance personnel backwash filters (or turn off the pump for any reason) without either shutting off the heaters or allowing them a cooldown period. A safe cool-down period can be determined by the temperature differential between the supply and effluent side of the loop, but five minutes is typically sufficient.
Installing a heating system booster pump, hydraulically separating the filter loop and heating loop, and using a well-designed control system, provides many significant benefits; Appliance heat exchangers are protected from corrosion, sooting and mineralization, service life of the system’s filter pump are increased, electrical consumption is reduced, fuel efficiency is increased, and maintenance is reduced.
The vast majority of existing pool heating systems are not designed or installed as effectively as possible, which leaves a lot of room for improvement.
By: Tom Soukup - Aqua Magazine
5170A Timberlea Blvd.