Calculating pool heater efficiency and potential retrofit savings

by arslan_ahmed | May 2, 2023 4:00 pm

[1]

By Tom Soukup

When a facility is considering replacement of its standard efficiency pool heating equipment with new, high efficiency appliances, it is prudent to select a contractor who has a firm understanding of the unique challenges pool heating poses and knows the difference between conventional and condensing gas-fired appliances.

Mid-Atlantic and Northeast regions have variable shoulder seasons; and many pool companies serve major metropolitan areas, where it is common for facilities to keep outdoor pools and spas open for three seasons or even year-round. In such instances, the potential to reduce fuel consumption through heating appliance upgrades is tremendous.

What is a British thermal unit (BTU)?

Before diving into specifics, it is important to define what a BTU is. A BTU is the measure of energy needed to raise 3.8 L (1 gal) of water by 0.55 C (1 F), in one hour; 3.8 L (1 gal) of water weighs 3.8 kg (8.3 lb). To keep things simple, this article uses round numbers and accurate averages.

Relating the conversion to pools, it requires 833,000 BTUs to raise the temperature of a 378,541-L (100,000-gal) pool by 0.55 C (1 F). This does not account for evaporation or heat loss through the surface, shell, or associated piping.

[2]

Trying to determine the efficiency of a pool heating system by accounting for all the above is nearly impossible due to any number of variables, including outdoor ambient temperature, wind, ground temperature, etc. However, there is a much easier way to determine the efficiency of an existing pool heating system, and the potential energy savings that may be realized with a retrofit.

Calculating efficiency

Accurately quantifying potential fuel savings may weigh heavily into the decision of when to upgrade or whether to upgrade at all.

The following is to determine the actual fuel efficiency of existing heating equipment. This method works even if historic gas or liquified petroleum (LP) expenses are not known, which is often the case because the gas meter or LP tanks serving the system may also serve other portions of the facility for space heat, a commercial kitchen, domestic hot water production, etc.

[3]

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.

[4]

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.

[5]

The firing rate of the new system and the old system are identical, meaning both heating appliances have the same input (800,000 BTUh). They both consume $17.52 per hour, but the new system only gives up 10 per cent efficiency, meaning only $1.75 is wasted per hour.

The additional bonus is, on this project and many others, the company has found runtime hours to fall by 33 per cent. This is because the new unit is adding heat to the pool at a higher rate than the old unit. The new equation is as follows:

1,500 (original runtime hours) x 0.33 (percent reduction in runtime hours) = 495

1,500 (original runtime hours) – 495 (reduction in runtime hours) = 1,005

$17.52 (per hour) x 1005 (annual runtime hours) = $17,607

To find the potential fuel savings, subtract the new heating cost from the old heating cost:

$26,280 (pre-retrofit annual fuel cost) – $17,607 = $8,673 saved per year.

The reduced runtime—thanks to more BTUs making it into the water and less heading up the chimney—provides the bulk of the savings. The body of water still needs the same BTUs, it is only getting it quicker post-retrofit. The reduced runtime also means less carbon emissions and less wear on equipment. Depending on the application, other control strategies to reduce input (total installed capacity) can be used by staging multiple, smaller units; however, this is another topic on its own.

Savings add up

While $8,673 is nothing to scoff at, it is to be noted some companies service many homeowner associations (HOAs), which have half a dozen or more outdoor pools of this size, and these clients would stand to save more than $50,000 each year. The energy consumption and savings potential are relative to the size of the pool facility.

One can also imagine how cost savings would rise if the cost of LP, or in many cases natural gas, rose. Finally, this author would not be surprised if high efficiency equipment becomes code-mandated for pool heating applications in the not-so-distant future.

Author

Tom Soukup is the principal of Patriot Water Works Co., with more than 20 years of experience as a hydronic designer and installer. He specializes in high-efficiency and green technology and brings his expertise to custom hydronic work, pool heating, and agricultural projects. He can be reached at twsoukup@patriotpaterworks.com.

Source URL: https://www.poolspamarketing.com/general/calculating-pool-heater-efficiency-and-potential-retrofit-savings/

---