A net BTU test is used to determine exactly how many BTUs are, in fact, entering the body of water, as opposed to what the heating unit is rated for. To do this, a flow meter is installed on the effluent side of the heater and readings are taken. This will determine how much water is moving through the unit.
Next, thermometres are installed on both the influent and effluent sides of the heater. The difference between the readings is the ΔT, or the change in water temperature across the heating appliance.
The scenario below is based on a 378,541-L (100,000-gal) pool with an 800,000 BTUs per hour (BTUh) propane-fired, 83 per cent annual fuel use efficiency (AFUE) copper-fin pool heater.
The flow rate across the meter read 310 litres per minute (lpm) (82 gallons per minute [GPM]). Influent temperature was 18 C (65 F), and effluent temperature was 23 C (75 F), resulting in a 5 C (10 F) ΔT.
The formula to determine the real-world BTU input is:
[Flow (82) x ΔT (10)] x 500.4 = 410,328 BTUh
This indicates the pool heater is providing 410,328 BTUs per hour, instead of 800,000 BTUh, which is its published gross input. To determine the energy efficiency of the unit, the following formula must be used:
[gross input (800,000) – net input (410,328)] / gross input (800,000) = 0.487
0.487 x 100 = 48.7%
The old pool heater is rated at 83 per cent efficiency when, in actuality, it is performing at 48.7 per cent. The truth is, 51.3 cents of every dollar spent on propane heads straight out the chimney and into the atmosphere. From the author’s experience, this is not an anomaly. When tested, old, conventional pool heaters usually operate around 50 per cent efficiency.
At full input, an 800,000 BTUh appliance will consume 33 L (8.7 gal) of propane per hour. Assuming an average propane cost of $2 per gallon, the facility is paying $17.52 per hour to fire the heater—$8.98 of which is wasted. The runtime of an average pool heater is 1,500 hours.
The equation for a seasonal outdoor swimming pool is as follows:
$17.52 (cost per hour) x 1,500 (hours) = $26,280 annual propane expense
$26,280 (annual expense) x 0.513 (inefficiency) = $13,481 of inefficiency per year
Now, consider how the numbers would look if the old heating equipment was replaced with new condensing equipment. The author’s company installs 96 per cent AFUE boilers to heat pool water. It is important to note, however, while the equipment is rated at 96 per cent efficiency, after efficiency loss across the heat exchanger (typically a plate-and-frame or shell-and-tube heat exchanger that isolates the boiler water from the pool water), tests consistently indicate the BTU yield is 90 per cent.
