California Pool Heating Season | CA Pool Heater Pros

General information, not legal, engineering or insurance advice.

California summers warm most pools on their own, but the swimming season is shorter than many people expect. Cool nights, ocean air and winter cold pull water temperatures down, and that is when owners start thinking about a heater. Here's a general picture of how heating needs differ around the state, how to hold on to heat, and how to protect equipment on cold nights.

Why California pools cool off

A pool loses most of its heat from the surface, and evaporation is by far the largest source of that loss.¹Anything that speeds evaporation, such as cool, dry air, wind and clear nights, cools the water faster. Much of California has dry air and large swings between daytime and nighttime temperatures, so even a warm afternoon can be followed by a night that takes heat back out of the pool.

The Department of Energy's solar heating guide gives northern California as an example of a place where pools are used 6 to 8 months a year.² Heating is how many owners stretch that season into spring and fall, or swim through winter.

Coastal, inland and desert pools

Conditions vary a lot across the state, and even within one county. As a general guide:

Coastal areas: milder winters, but cool ocean air, fog and wind can keep water cooler than you'd like even in summer. Solar output drops on foggy mornings, so ask how a solar system performs at your site.
Inland valleys: hot summers and colder winter nights. Pools warm quickly in summer, but heating is often needed to swim comfortably in spring and fall.
Deserts: very hot summers, but winter nights can get cold. A pool that is too warm in August may need heat in winter.
Foothills and mountains: the shortest natural season and the coldest nights, with freezing weather in winter. Freeze protection matters as much as heating.

Your own pool's sun, shade and wind exposure matter as much as the region. Ask a local contractor what owners near you typically do.

What the weather means for each heater type

Water loses heat fastest on cool, dry, windy nights.
A heat pump delivers less heat when the air is cold, so it may lag behind on cold nights and catch up as the air warms.³
Solar collects less on cloudy or foggy days and short winter days.
A gas heater can hold a set temperature regardless of the weather.⁴

Use a cover to keep the heat

Pool covers can save 50% to 70% of pool heating costs, because they cut evaporation.¹ A pool cover or blanket reduces heat loss and helps hold warm temperatures longer.² Covers can also cut make-up water and keep debris out, which matters in a state where water is precious.

Cold-night precautions for pool equipment

Freezing nights are uncommon along much of the coast but happen in inland valleys, the high desert and the mountains. Water left standing in pipes and equipment can freeze and crack them. For pool equipment:

Check your freeze protection. If your pool automation has a freeze-protection setting that runs the pump when it gets cold, make sure it's turned on and working before winter.
Follow your equipment manuals. Heat pumps, gas heaters, pumps and filters each have their own cold-weather instructions. Your heater manual is the authority for your unit.
Solar systems need their own plan. Ask your installer how your solar collectors drain or are protected in a freeze.
Plan for power outages. Freeze protection that depends on running the pump won't work without power. Ask your pool professional what to do if the power goes out on a freezing night.
Call a licensed pro after a hard freeze if you see leaks, cracks or a heater that won't start, rather than running damaged equipment.

Watch your local National Weather Service forecast at weather.govfor freeze watches and warnings, and get equipment ready before a forecast freeze, not after.

Getting ready for heating season

Before the nights turn cool, run the heater and check that it reaches your set temperature.
Clean or service the filter so water flows freely through the heater.
Make sure the area around a heat pump is clear so it gets good airflow.
Have a licensed contractor inspect a gas heater, and check solar collectors and valves, if it's been a while.
Set the temperature sensibly. Lower set points save energy.
Put the cover on when the pool isn't in use.

Sources

U.S. Department of Energy, Better Buildings, “Pool Efficiency” fact sheet.
U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, “A Consumer’s Guide: Heat Your Water with the Sun” (DOE/GO-102003-1824, December 2003).
U.S. Department of Energy, Energy Saver, “Heat Pump Swimming Pool Heaters”.
U.S. Department of Energy, Energy Saver, “Gas Swimming Pool Heaters”.
National Weather Service, weather.gov local forecasts and freeze warnings.