Taking a multi-unit approach
The desired approach when using direct-fire, conventional pool heaters is similar to when using boilers—to install an array of smaller units and stage them with modern controls. Instead of using a single pool heater, three or four can be installed together, with a total heating capacity just exceeding the maximum load.
When implemented correctly, this method maintains the pool within one degree of set point temperature while providing redundancy and reducing fuel consumption. Through the use of staging controls, the heaters can be operated with only as much capacity online as needed at any given time, and the ability to rotate the lead unit to ensure even run times is provided. This approach also simplifies equipment maintenance.
It is beneficial to install a booster pump to maintain correct flow rate through the system, according to the pool heater manufacturer specifications. A booster pump not only ensures the heat exchangers see a proper flow rate, but it also reduces the load on the filtration pump.
Curating a design
Last year, the author’s company was called out to a homeowners association (HOA) where the pool complex had an existing 1.7 million BTU conventional heater. It required major repairs every season, and management had finally decided fixing the unit was not their best option.
The non-condensing unit was seeing such low-return water temperatures, the heat exchanger was condensing, especially during start-up and shoulder seasons. This had caused major damage to the heat exchanger. The bypass valve intended to raise incoming water to a safe temperature was manually operated, as is usually the case. However, without constant manipulation, these valves do not work as intended.
As with every application, the author’s company conducted an evaluation of the actual heat going into the pool. On this particular project, only 42 per cent of the BTUs purchased from the gas utility were actually conducted to the body of water. Essentially, only 42 cents of each dollar reached the pool, which was a highly undesirable situation.
The HOA asked the author’s company to present a new heater design. Their team began the process by calculating how many BTUs the body of water needed under design conditions. Factors for this calculation included the water temperature upon opening the pool for the season, the desired water temperature, the heat loss for the pool, and the customer’s expected waiting period between start-up and set point temperature.
This specific job required one million BTUs per hour. The author’s company designed and installed a heating system with three high-efficiency units, at 400,000 BTUs each, for a total of 1.2 million BTUs. Each unit had two stages of input: 200,000 and 400,000 BTUs per hour. With staging controls, the system had six stages of input, ranging from 200,000 to 1.2 million BTUs per hour. Firing capacity could be brought online as needed through the controls.
The units used were also 96 per cent efficient, meaning 96 cents of every dollar spent on fuel went into the pool water—a 54 per cent improvement over the original system.
To ensure the heaters were seeing the correct flow rate, a variable-speed booster pump was installed. The pump had a built-in variable-frequency drive (VFD) which was factory programmed for a certain flow rate. The flow rate was overridden to provide a more suitable speed of 189.3 litres per minute (lpm) (50 gallons per minute [gpm]) per heater, for a total of 567.8 lpm (150 gpm).
