Gas, Heat Pump or Solar: Choosing the Right Pool Heater

Heating is the largest running cost a pool can have, and the three ways of doing it behave so differently that comparing them on price alone is close to meaningless. One is fast and expensive to run. One is cheap to run and slow. One is nearly free and entirely at the mercy of the weather.

How much energy heating a pool actually takes

Before comparing equipment, it helps to know the size of the job, because most people badly underestimate it.

Raising water temperature takes about 8.34 BTU per gallon per degree Fahrenheit. A 20,000 gallon pool raised by 10 °F therefore needs roughly 1.67 million BTU — before accounting for any losses at all. At 100,000 BTU per therm, and allowing for a gas heater running at around 80% efficiency, that is on the order of 21 therms of natural gas.

That is the one-off cost of getting the water warm. The ongoing cost is replacing what the pool loses, and a pool loses heat continuously through its surface, day and night, in a quantity that depends on air temperature, wind and humidity far more than on anything you control.

The crucial fact is that most of that loss is evaporation, not radiation or conduction. Evaporating water carries away an enormous amount of energy, which is why a pool cover — which does nothing except stop evaporation — is more effective than any adjustment you can make to the heater.

Gas heaters

A gas heater burns natural gas or propane to heat water directly. It is the fastest option by a wide margin and the least affected by weather.

What it does well: it heats quickly, often raising a pool several degrees in a few hours, and it works in cold air. That combination makes it the right choice for a pool you want to use occasionally rather than constantly — a spa on a winter evening, a pool warmed for a weekend and left cold the rest of the week.

What it does badly: it is expensive to run continuously, because you are buying fuel for every BTU. Maintaining a warm pool all season with gas is one of the most expensive things a household can do. Gas heaters also have the shortest service life of the three, typically eight to twelve years, and their heat exchangers are vulnerable to chemistry — aggressive water eats copper.

Propane deserves particular caution. It costs substantially more per BTU than natural gas in most of the US, and a pool heated on propane can produce bills that astonish people who budgeted using natural gas figures.

Heat pumps

A heat pump does not make heat. It moves it, extracting warmth from the outside air and transferring it into the water, which is why it can deliver several units of heat for every unit of electricity it consumes.

That ratio — the coefficient of performance — is what makes heat pumps cheap to run. A well-matched unit in suitable weather can be four or five times as efficient as resistance heating, and typically much cheaper per degree than gas.

The trade-off is speed and weather dependence. A heat pump warms a pool slowly, often over a day or more rather than an afternoon, so it suits a pool held at temperature continuously rather than warmed on demand. And its efficiency falls as air temperature drops; below roughly 50 °F most units lose effectiveness sharply, and below 45 many stop being worthwhile at all.

For a long season in a warm climate — Florida, Texas, southern California, the Gulf coast — a heat pump is usually the cheapest way to keep a pool comfortable. For extending a short season into a cold spring in the north, it is the wrong tool.

Solar heating

Solar pool heating circulates pool water through collector panels, usually on a roof, where the sun warms it before it returns. The running cost is essentially the electricity to pump water slightly further, which on an existing pump is close to nothing.

It is the cheapest heating there is to operate and among the longest lived — panels commonly last fifteen to twenty years. Against that, it has the highest installation cost of the three in most cases, it requires a substantial area of suitably oriented roof, and it works only when the sun is out.

The sizing rule of thumb is that collector area should be somewhere between half and the whole of the pool’s surface area, more in cooler or cloudier regions. A system half that size will still do something, but it will disappoint anyone expecting a specific temperature.

Solar suits a pool that wants to be a few degrees warmer for a longer season rather than a pool that needs to be a specific temperature on a specific day. It also pairs unusually well with a heat pump: solar does the bulk of the work when the sun cooperates, and the heat pump covers the rest.

Comparing them honestly

Sizing, and why undersizing is the usual mistake

Heater capacity is quoted in BTU per hour for gas and heat pumps. Sizing depends on your pool’s surface area — because that is where heat is lost — the temperature rise you want, and how exposed the site is to wind.

Surface area matters more than volume, which is counter-intuitive. A wide shallow pool loses heat faster than a narrow deep one of the same volume, because it presents more surface to the air. Two pools of identical gallonage can have quite different heating requirements.

An undersized heater does not fail; it simply never reaches temperature, running continuously and costing money without achieving the result. An oversized one costs more to buy but reaches temperature faster and then cycles off, which is generally the less bad error. The heater sizing calculator works from your surface area and target rise, and heating time shows how long a given unit will take.

Wind exposure is the factor most often ignored. A pool on an open site loses heat far faster than a sheltered one, and a windbreak — a hedge, a fence, a screen — can reduce heating costs more than upgrading the heater.

The cover is not an accessory

If you take one thing from this article: on a heated pool, a cover is not an optional extra. It is the highest-return purchase available.

Because most heat loss is evaporation from the surface, and because a cover stops evaporation, it addresses the dominant loss directly. No heater upgrade, no thermostat adjustment and no efficiency rating comes close to the same effect for the same money.

A cover also reduces water loss and chemical loss, both of which cost money independently. Our guide to pool covers covers the types and what each one is actually for.

What heating does to your water chemistry

Warm water is chemically different water, and owners often add a heater and then spend a season puzzled by their chemistry.

Warm water consumes chlorine faster, because biological activity rises with temperature. A heated pool needs more chlorine than the same pool unheated, and holding the old target will eventually produce algae.

Warm water also scales more readily. The saturation balance shifts toward deposition as temperature rises, so a pool at 300 ppm calcium that was stable when cool may begin scaling once heated — inside the heat exchanger first, where it is hottest and least visible. This is why a heated pool wants calcium toward the lower end of its range, and why calcium hardness deserves attention before you install a heater rather than after.

And aggressive water destroys heat exchangers. Low pH, low alkalinity and low calcium together will eat copper, and a heater is usually the most expensive copper in the system. A heater failure at five years old is very often a water chemistry failure wearing a different name.

Running costs, in practice

Precise figures depend on your fuel prices, your climate and your target temperature, which is why this site calculates rather than quotes. But the shape of the answer is consistent.

Gas is dramatically cheaper for short bursts and dramatically more expensive for continuous warmth. A heat pump inverts that: expensive to warm a cold pool from scratch, cheap to hold one at temperature. Solar is nearly free to run in either case but cannot be relied on for a particular day.

The single biggest variable within your control is target temperature. Each degree costs, and the cost of the top few degrees is disproportionate because heat loss rises with the difference between water and air. Dropping a target from 86 °F to 82 °F often cuts heating cost by considerably more than the 5% the numbers suggest.

Use the heating cost calculator with your own fuel rate, and see what it costs to run a pool for how heating compares with everything else.

Choosing, in three questions

How do you want to use the pool? Occasional and on demand points to gas. Continuous through a long season points to a heat pump. A few extra weeks either side of summer, cheaply, points to solar.

What is your climate? Heat pumps need air above roughly 50 °F to work well. Solar needs sun and roof space. Gas works anywhere, at a price.

What do you pay for fuel? Natural gas at a low rate narrows the gap considerably; propane widens it. Expensive electricity hurts heat pumps. There is no universal answer, which is precisely why the honest advice is to put your own numbers in rather than trust a generic recommendation.

Sizing by surface area, worked through

Heat leaves a pool through its surface, so surface area rather than volume drives the sizing. This is the part that is most often got wrong, and it produces heaters that run permanently without ever reaching temperature.

Consider two pools of identical volume. One is 16 by 32 feet at an average depth of 5 feet — 512 square feet of surface. The other is 12 by 24 at an average depth of nearly 9 feet — 288 square feet. The second pool holds the same water and loses heat far more slowly, because it presents far less of itself to the air.

Exposure compounds this. A pool on an open, windy site loses heat much faster than a sheltered one, because moving air carries away evaporated water continuously. A hedge, a fence or a screen on the prevailing wind side can reduce heating costs more than a larger heater would, and costs less.

The practical consequence: size from surface area, the temperature rise you want and how exposed the site is, then round up rather than down. An oversized heater reaches temperature and cycles off. An undersized one runs continuously, costs money, and never arrives.

Running a heater without wasting it

Several habits make a substantial difference to what heating costs, and none of them involves different equipment.

• Cover the pool whenever it is not in use. Most heat loss is evaporation; a cover stops it. Nothing else comes close.
• Choose a target and leave it. Each degree costs, and the top few cost disproportionately because loss rises with the gap between water and air. 82 °F is comfortable for most swimming; 86 costs considerably more.
• With a heat pump, hold temperature rather than cycling. They are least efficient reheating from cold. With gas, the reverse — heat when needed.
• Keep the water balanced. Scale inside the exchanger raises fuel use every year, and aggressive water destroys it outright.
• Service the unit annually. A gas heater with a dirty burner or a restricted exchanger burns more for less.
• Shelter the site if wind is a factor. It is a one-off cost against a recurring one.

For anyone replacing equipment, the US EPA’s ENERGY STAR programme is a useful reference point for what efficiency claims actually mean, and worth reading before comparing manufacturer figures that use different baselines.