Top of Page

The Ultimate Guide to Sizing Outdoor Swimming Pool Heaters for Maximum Comfort

When it comes to enjoying your outdoor swimming pool year-round, having the right-sized pool heater is essential. A properly sized heater ensures your pool water stays at the perfect temperature, providing maximum comfort for you and your family. In this comprehensive guide, we'll take you through everything you need to know about sizing outdoor swimming pool heaters. From assessing pool dimensions to considering seasonal variations and energy efficiency, we've got you covered.

In this ultimate guide, we'll walk you through the step-by-step process of sizing outdoor swimming pool heaters to ensure maximum comfort and enjoyment in your pool, no matter the season. Let's start exploring.

Sizing Outdoor Pool Heater:

1. Determine pool capacity in gallons. Obtain from Architect if available. Length x Width × Depth × 7.5 Gal/Cu.Ft. (If depth is not known assume an average depth 5.5 Feet)

2. Determine heat pick-up time in hours from Owner.

3. Determine pool water temperature in degrees F. from the Owner. If Owner does not specify assume 80°F.

4. Determine the average air temperature on the coldest month in which the pool will be used.

5. Determine the average wind velocity in miles per hour. For pools less than 900 square feet and where the pool is sheltered by nearby buildings, fences, shrubs, etc., from the prevailing wind an average wind velocity of less than 3.5 mph may be assumed. The surface heat loss factor of 5.5 Btu/Hr/Sq.Ft.°F. in the equation below assumes a wind velocity of 3.5 mph. If a wind velocity of less than 3.5 mph is used, multiply equation by 0.75; for 5.0 mph multiply equation by 1.25; and for 10 mph multiply equation by 2.0.

6. Pool Heater Equations:

htotal = hsurface + hheat-up  


htotal = total heat load (btu/hr, W)

hsurface = heat loss from pool through the surface - mainly evaporation of water (btu/hr, W)

hheat-up = heat load required to increase the pool temperature (btu/hr, W)

Surface Heat Loss due to Temperature Difference

The heat load required to replace the surface heat loss due to the temperature difference between the pool surface and the ambient air can be expressed as

hsurface = ks x ΔTaw x A   


ks = surface heat loss factor - for sheltered positions with average wind velocity 2 to 5 (mph), the surface heat loss factor is in the range 4 to 7 (Btu/hr ft2 oF)

ΔTaw = temperature difference between the air and surface water in the pool (oF)

A = surface area of the pool (ft2)

Heat-Up Load

The heat-up load depends on the volume of the pool and the heat-up load can be calculated as

hheat-up = (8.34 x V x ΔTw )/ Δ                                           


ΔTw = difference between initial temperature and the final temperature of the water in the pool (oF)

Δt = heat pick-up time (hr)

Example - Pool Heating

A pool with dimensions length 30 ft, width 20 ft and dept 6 ft is heated from 50oF to 75oF. The volume in the pool (in gallons) can be calculated as

V = (30 ft) (20 ft) (6 ft) (7.5 gal/ft3)

    = 27000 (gal)

The heat-up heat required to heat the pool up in 24 hours can be calculated as

 hheat-up = (27000 gal) (8.34 lbs/gal) ((75 oF) - (50 oF)) (1.0 Btu/lb oF) / (24 hr)

    = 234562 Btu/hr

  • 1 Btu/hr = 2.931x10-4 kW = 0.252 kcal/hr

Note that this value is without the increased heat loss through the surface due to the increased temperature difference and increased evaporation when the temperature increases.

If the ambient temperature is 65 oF and the wind conditions is moderate, the heat loss through the surface due to temperature difference (when the pool is heated up) can be calculated as

hsurface = (5 Btu/hr ft2 oF) ((75 oF) - (65 oF)) (30 ft) (20 ft)

    = 30000 (Btu/hr)

Finally, have your selected heater professionally installed, and make sure to follow the manufacturer's guidelines for maintenance to ensure it runs efficiently.

Keep in mind that while this guide provides a general approach to sizing a pool heater, consulting with a pool professional is recommended for accurate calculations and to address any specific considerations related to your pool and climate. Proper sizing will not only maximize comfort but also help you save on energy costs in the long run.

Post a Comment