7.11 Heating Hot Tubs and Pools

Heating pools and hot tubs require a lot of energy use because we are heating A LOT of water. As we learned earlier in the lesson the amount of energy required to heat water depends on the amount of water and pools an hot tubs use a significantly larger volume of water compared to residential hot water use.

Pool water temperatures typically range from 78-82°F. The volume of water in a pool can vary significantly compared to the size, style and depth of the pool. The manufacturer will likely tell you the amount of water your pool can hold, but you can estimate it based upon online calculators or simple geometry.

Example - Heating a Pool

A round above ground pool that is 4 feet in depth. We can estimate the volume of water required by getting the radius (or diameter) of the pool. If the radius of the pool is 6 feet (diameter is 12 feet) the volume of the water is ${Volume = Pi×Radius}^{2} ×depth$ .

${Volume = Pi×(6 ft)}^{2} ×(4 ft)$

${Volume = 3.14×(6 ft)}^{2} ×(4 ft)$

${Volume = 452.16 ft}^{3}$ so we need to convert that to gallons, knowing that ${1 ft}^{3} = 7.48 gallons$ .

${Volume = 452.16 ft}^{3} \times (7.48 gallons / {1 ft}^{3}) = 3,382 gallons$

So this pool would require about 3,382 gallons of water!

As you can see, this is a relatively small pool, but uses a significant amount of water.

The calculations get more complicated if you have a different shaped pool or have a deep end. However, there are lots of online calculators that will help you estimate the gallons of water needed.

Hot Tubs are another high hot water demand device that may be found in some homes. Hot tubs are typically operated at higher temperatures between 100-104°F. Most of the energy use is to heat the water, over 75%. The remaining energy use can be from the pumps, jets and lighting. In colder climate conditions the energy use may be higher to get that temperature up.

Example - Heating a Hot Tub in Maine vs. Florida

Calculate the energy used to heat water for a 500 gallon hot tub in Maine versus Florida. In Maine the outdoor temperature is 25°F while in Florida is 65°F. Assume the operation temperature of the hot tub is 102°F.

Calculate the amount of energy required in each location.

Recall our Water Heating formula is:

$Q = m \times C_{p} \times ∆ T$

We also know:

$C_{p} = 1 B T U / (l b \times ° F)$

and

$1 gallon of water = 8.34 pounds$

Energy Calculations
Maine	Florida
$m =500 gal× (8.34 lbs / gal) = 4,150 lbs$	$m =500 gal× (8.34 lbs / gal) = 4,150 lbs$
$C_{p} = 1 BTU / lb \times ° F$	$C_{p} = 1 BTU / lb \times ° F$
$Δ T = 102 ° F - 25 ° F = 77° F$	$Δ T = 102 ° F - 65 ° F = 37° F$
$Q_{Maine} = 4,150 lbs× (1 BTU / (lb \times °F)) \times 77° F$	$Q_{Florida} = 4,150 lbs× (1 BTU / (lb \times °F)) \times 37° F$
$Q_{Maine} = 319,550 BTUs$	$Q_{Florida} = 153,550 BTUs$

A hot tub in Maine uses more than double the amount of energy than one in Florida in order to heat the water!

Type of Water heaters for pools and hot tubs

Just like we saw with residential hot water heaters, pools and hot tubs can be heated in a variety of ways including electric, natural gas, solar and heat pump.

Heat Pump Hot Water Heaters

Just like we saw with residential hot water heaters, heat pumps have higher efficiency because they use some of the energy available in surrounding air to help heat the water. Heat pump will work more efficiently in temperatures above 50°F.

Diagram of a heat pump system for pool heating with components labeled 1 to 10. — Heat Pump Hot Water Heater for Pools

Just like we saw with other types of water heating, the heat pump hot water heater is more expensive, but has less operating costs. Heat pump efficiencies range from 3.0-7.0, which means they get more energy out per unit of electricity put in. This isn’t magic, heat pumps just pull some energy out of the ambient air.

Solar Swimming Pool Heaters

Solar Swimming Pool Heaters use a solar collector to convert energy from the sun to heat the pool water. They tend to have the lowest operating cost and can be the most cost-effective solution in some climates.

Most solar pool heating systems include the following:

A solar collector — the device through which pool water is circulated to be heated by the sun
A filter — removes debris before water is pumped through the collector
A pump — circulates water through the filter and collector and back to the pool
A flow control valve — automatic or manual device that diverts pool water through the solar collector.

Pool water is pumped through the filter and then through the solar collector(s), where it is heated before it is returned to the pool. In hot climates, the collector(s) can also be used to cool the pool during peak summer months by circulating the water through the collector(s) at night.

Diagram of a solar pool heating system with pool equipment, solar collectors on a roof, and a conventional pool heater. — Diagram of a solar pool heating system

How to reduce your energy use in your pool or hot tub:

Use a high quality cover to keep as much heat inside. A lot of heat is lost from the top of the pool or hot tub, so keep it covered and insulated as much as possible when not in use. A cover will also reduce evaporation.
Lowering the temperature just a few degrees can reduce your energy costs
Maintaining a constant temperature is more efficient than increasing and decreasing the temperature constantly.
Off peak heating, using a timer to heat during off peak (cheaper) times
Keep filters clean and free of debris

Example - Heating a Pool

Example - Heating a Hot Tub in Maine vs. Florida

Type of Water heaters for pools and hot tubs

Heat Pump Hot Water Heaters

Solar Swimming Pool Heaters

Example - Heating a Freshly Filled Hot Tub

Test Your Knowledge