Identifying the challenges
One challenge that has made it difficult for some business owners and service providers to offer their clients this newer pump technology is the simple fact most swimming pool components were not designed around this low-flow or variable-flow concept.
In fact, most system components on the market today were created for single-speed pumps, which blast large volumes of water at a constant high-flow rate for short operation cycles, rather than using a low-flow rate for much longer operational periods. Therefore, when a pool owner switches from an inefficient pump to a variable- or two-speed pump, compromises have to be made to the overall pool system.
For example, the pool’s plumbing system will see a noticeable drop in efficiency in its ability to effectively agitate and skim the water surface, in addition to a lack of ideal circulation.
One way to resolve these issues is through the pool’s return units. Pool returns play an important role in water circulation and are a major component in providing proper agitation, circulation, and skimming action. Proper pool return (orifice) size is therefore integral to an efficient circulation system when operating a pool at low-flow rates.
Using the wrong pool return, in terms of its size, can create excessive back pressure on the pool’s plumbing system. This back pressure, which is calculated as part of the pump’s total dynamic head (TDH), can cause unwanted stress on the system, leading to higher energy consumption as well as decreasing the lifespan of other pool components and equipment.
For example, in terms of pump motors, the more stress placed on the motor, the more work it has to do, which in turn increases the chance of it failing early on in its life cycle. One common problem is the increase of wear of the motor’s bearings. This is easily detected by an increase in noise from the motor until complete failure.
Another problem that is often unnoticed is over agitation. This can lead to increased rates of evaporation, which in turn results in greater heat loss. This causes longer heating times, which further translates into increased heating costs for the pool owner.
Over agitation also reduces the effectiveness of providing the proper flow required to push debris into the pool’s skimmer pots. This increases the chance of debris sinking to the pool floor; the more debris that sinks to the bottom of the pool increases the amount of chemicals required to prevent algae growth, as well as the amount of time spent on pool maintenance.
How can these barriers be overcome?
One way of skirting around this issue is to set the pump to operate at a low-flow rate (to save money) and install smaller orifice returns, in correlation to the flow rate, to enhance pump performance (i.e. agitation/circulation). However, as stated earlier, certain components require higher flow rates to operate properly (e.g. heaters, chlorine generators, etc.). Therefore, in this scenario, one could keep the flow low, but when this equipment needs to be operated, the flow needs to be increased, which will then lead to over agitation as well as increased back pressure and energy usage.
On the opposite end of the scale, if larger orifice returns are installed, for the purpose of minimizing back pressure, TDH, and over agitation within the system, these returns become inadequate when it comes to efficient agitation and circulation as the majority of the time the pump will be operating at a lower flow rate. The pool owner could increase the pump’s speed or flow rate; however, it does not allow them to take advantage of the energy efficiency benefits nor the advertised cost savings associated with these pumps.
An unlikely scenario would see the pool owner swap out the returns (e.g. a smaller return for low-filtering cycles and larger returns for heating cycles) whenever the situation demands it. It would not take long, however, for the pool owner to realize this exercise rarely happens due to the mere inconvenience of manually changing the pool returns.
Therefore, the ideal way to resolve most of the issues is to use a variable orifice pool return that is capable of self-adjusting its opening based on changing flow rates. These returns provide a balance between efficiency and the desired performance the end user expects from their pool pump.
A swimming pool equipped with variable orifice pool returns enables the pool owner to maximize return flow velocity (i.e. agitation/circulation) at low-flow settings, while minimizing back pressure and TDH issues when running the pump at high-flow settings.
Flow versus watts
When comparing the three most common pool return sizes (9.5, 19, and 25 mm [3/8, 3/4, and 1 in.]) to a variable orifice return, no matter what size eyeball is used, the lower the pump’s flow rate, the more energy-efficient the pump becomes (see Figure 1). As stated previously, this is also when filtering and heating are most efficient.
From 57 to 76 litres per minute (lpm) (15 to 20 gallons per minute [gpm]), there is little difference in energy usage from any given pool return. Where the difference lies, however, is in the agitation and circulation provided by each pool return. The smaller the orifice, the better the agitation and circulation. However, when using a standard 9.5-mm (3/8-in.) pool return, in conjunction with a variable-speed pump, any change in flow rate above 76 lpm (20 gpm) reduces efficiency.
By using a variable orifice pool return, the orifice becomes larger as the flow increases. This adaptation enables the pool return to provide ideal agitation and circulation at lower flow rates, while also instantly adapting to higher flow rates to remain efficient when demand requires an increase in flow.
