What is the fastest way to heat a pool?
A gas heater is the fastest way to heat a pool, and nothing else is close. It burns natural gas or propane to heat water as it passes through, so it works in cool weather, at night, and on a cloudy day, none of which a solar system or heat pump handles well. A properly sized gas heater raises the water roughly 1 to 1.5 degrees Fahrenheit per hour, so it can take a cold pool up to swim temperature in a single afternoon rather than the days a slower method needs.
That speed is why gas is the default for a pool you heat occasionally: a spa you want hot tonight, a weekend guest visit, or a shoulder-season swim when the air is cool. You fire it up, hit temperature, and shut it off, so you only pay while it runs. The downside is the running cost, which is the highest of any method because you are buying fuel by the therm or gallon. See the best pool heater guide for sizing and picks, and the gas versus heat pump comparison for the running-cost math side by side.
Gas also wins when you want a fixed temperature you control regardless of the weather. Solar can only add heat when the sun is out, and a heat pump slows down as the air cools, but a gas heater hits your set point in July or October the same way. If dependable, on-demand heat matters more than the fuel bill, gas is the answer.
What is the cheapest way to heat a pool?
The cheapest way to heat a pool is solar, and the cheapest single upgrade is a solar cover, because it costs almost nothing to run and stops the heat you already have from escaping. A solar heating system (roof or ground panels the pump pushes water through) has no fuel cost at all once installed; it heats the water for free whenever the sun is shining. In a warm, sunny climate it can add real weeks to your swim season for the price of the panels and the electricity to run the pump you already run anyway.
If you want a powered heater but the lowest possible running cost, choose a heat pump. Instead of making heat by burning fuel, it moves heat out of the air into the water, so it delivers roughly 5 to 6 units of heat energy for every 1 unit of electricity it draws. That efficiency makes it far cheaper to run than gas over a season, which is why it is the standard choice for anyone who heats a pool most days in a mild climate. The best solar pool heater guide and the solar versus heat pump comparison cover which of the two fits your yard and sun.
The catch with both is patience and weather. Solar and heat pumps heat slowly, often under a degree an hour, and both fade when conditions turn: solar stops at sundown, and a heat pump loses output once the air drops below about 50 degrees Fahrenheit. They are the right tools for holding a comfortable temperature through the season, not for heating a cold pool in a hurry.
How much heat does a solar cover actually save?
A solar cover is the highest-value heating purchase you can make because evaporation is the biggest way a pool loses heat, commonly around 70 percent of total heat loss, and a cover cuts that evaporation by roughly 90 to 95 percent. Every time water evaporates off the surface it carries heat away, and that happens fastest at night and on windy or dry days. Trapping the surface stops the single largest leak in the system.
On the gain side, a solar cover left on a pool in the sun can add about 5 to 10 degrees Fahrenheit over several sunny days, and just as importantly it holds the heat overnight so you are not reheating the same water every morning. It works as a magnifier for every other method too: pair it with a heat pump or solar panels and you keep the degrees they add instead of losing them to the night air. The best solar pool cover guide covers thickness, sizing, and the reels that make a big cover manageable.
Use it whenever you are not swimming, especially overnight, and take it off during the day if you are running solar panels so the sun hits the water. A cover is cheap, it needs no plumbing, and it saves both heat and water, so it is the first thing to buy no matter which heater you end up choosing.
How do the three pool heater types compare?
There are three ways to add heat to a pool, and each is best at a different job. A gas heater is fastest and works in any weather, but costs the most to run. A heat pump is far cheaper to run and holds a set temperature well, but heats slowly and needs the air above about 50 degrees. A solar system costs nothing to run and lasts the longest, but only heats when the sun is out and cannot hit a temperature on demand.
Match the tool to how you swim. If you heat the pool occasionally and want it hot fast, buy gas. If you want the water comfortable most days through a long warm season at the lowest running cost, buy a heat pump. If your goal is to stretch the season for free and you have sun and roof or ground space, buy solar and accept that the weather sets the schedule. Many owners run a combination: solar or a heat pump to carry the season cheaply, with a solar cover on top to hold every degree.
Whatever you install goes into the plumbing after the filter so only clean water passes through it, and it only heats while the pump is circulating water through it. That makes pump run time part of your heating setup, not a separate thing; the how long to run a pool pump guide covers setting the schedule so the heater and the sun both have water to work on.
How long does it take to heat a pool, and what does it cost?
Heating time depends on your pool's size, the temperature rise you want, and the heater's output, and you can estimate it with a simple energy formula: BTUs needed = pool gallons multiplied by 8.34 multiplied by the degrees of temperature rise. For a 20,000 gallon pool you want to raise 10 degrees, that is about 1.67 million BTUs. A 400,000 BTU per hour gas heater delivering around 320,000 usable BTUs would take roughly 5 hours for that 10 degree rise, before accounting for heat lost to the air. Get your exact volume from the pool volume calculator so the math reflects your pool.
Warming a cold pool from 60 to 80 degrees is a 20 degree rise, which is a big ask. A well-sized gas heater might do it in a long day of running; a heat pump often needs 24 to 72 hours of continuous operation; solar can take the better part of a week of good sun. This is exactly why gas exists and why a solar cover matters so much, since holding the heat overnight can cut the reheating time roughly in half.
Running cost tracks the same order. Gas is the most expensive to run, a heat pump is much cheaper, and solar is nearly free to run once installed. The bigger the temperature difference between your water and the air, and the more surface area you leave uncovered, the more any heater has to work, so a cover pays you back on every method. Aim for a swim temperature around 78 to 82 degrees; every degree you push above that noticeably raises the bill.
How can you heat a pool for free or nearly free?
You can add several free degrees without a purchased heater, and it starts with keeping heat in rather than making more. A solar cover is the biggest lever, holding overnight heat and cutting evaporation. Beyond that, block the wind, since a windbreak of fencing or hedges around an exposed pool slows the evaporation that steals heat, and keep the water level up so the skimmer is not pulling in cold air.
For adding free heat, the classic do-it-yourself trick is a black solar coil or a length of black hose laid in the sun with pool water pumped through it: dark surfaces absorb sunlight and warm the water on the way back to the pool. Solar sun rings or floating solar mats work the same way on a smaller scale and double as a partial cover. None of these match a real solar panel array, but they cost little and genuinely help a small or above-ground pool.
Finally, work with the sun's schedule. Run the pump during the sunniest part of the day when you have a solar system or DIY coil so it is circulating water while there is heat to collect, and pull the cover off then so sunlight reaches the water. Put the cover back on in the evening. Any pool can be retrofitted with a heater later, including above-ground pools, so start with the free gains and add a powered heater only if the sun and cover cannot get you where you want to be.
Frequently asked questions
What is the quickest way to heat up a pool?
A gas heater is the quickest way, by a wide margin. It heats water on demand by burning natural gas or propane, works in any weather and at night, and raises the water about 1 to 1.5 degrees Fahrenheit per hour, so it can take a pool to swim temperature in a single afternoon. Heat pumps and solar are cheaper to run but heat far more slowly and depend on warm air or sun. Putting a solar cover on the pool speeds up any method by holding the heat you add instead of losing it overnight.
How long does it take to heat a pool from 60 to 80 degrees?
That is a 20 degree rise, which is a large one. A well-sized gas heater can usually do it in a long day of continuous running, roughly 8 to 14 hours depending on pool size and heater output. A heat pump typically needs 24 to 72 hours of continuous operation, and a solar system can take the better part of a week of good sun. A solar cover left on overnight roughly halves the reheating time because the pool does not shed most of its heat by morning.
Can you turn a regular pool into a heated pool?
Yes. Any pool can be retrofitted with a heater, including above-ground pools, which have their own compatible gas heaters, heat pumps, and solar kits. The heater is plumbed into the circulation line after the filter, so clean water passes through it, and it heats whenever the pump is running. The cheapest way to make an unheated pool warmer without a heater at all is a solar cover plus a black-hose or solar-ring setup, which adds free degrees on sunny days.
How do you heat an unheated pool without a heater?
Start with a solar cover, which cuts evaporation by about 90 to 95 percent and can add roughly 5 to 10 degrees over several sunny days by holding the sun's heat in. Add a do-it-yourself solar coil or black hose in the sun with pool water pumped through it, use floating solar sun rings, block the wind with a fence or hedge, and run the pump during the sunniest hours. These free methods work best on small and above-ground pools and can extend the season by weeks without any fuel or electricity beyond the pump you already run.
What is the cheapest way to keep a pool heated?
For the lowest running cost, combine a solar cover with either a solar heating system or a heat pump. A solar system has no fuel cost and heats for free when the sun is out; a heat pump delivers about 5 to 6 units of heat for every unit of electricity, making it far cheaper to run than gas over a season. The solar cover ties it together by trapping the heat overnight so you are not reheating the same water each morning. Gas is the most expensive to run and is best saved for fast, occasional heating.