Heating a pool in winter

The short answer

Yes, you can heat a pool in winter. Commercial and municipal pools do it continuously. The question worth asking is not whether it is possible but what it costs and what it asks of your equipment, and on both counts winter is not summer with a bigger number attached — several things change at once and they all push the same way.

What actually changes

The gap widens, and it widens twice. You are heating from a colder starting point, so there is more water temperature to make up. And the loss rate rises at the same time, because heat flows out in proportion to the difference between the water and the air. A pool held at 82°F in 75°F air is losing slowly. The same pool in 40°F air is losing at several times the rate, permanently.

Evaporation runs harder, not softer. This surprises people, because winter feels damp. But evaporation is driven by the difference between the water’s vapor pressure and the air’s, and warm water under cold dry air is the strongest driver there is. That steam rising off a pool on a cold morning is heat leaving at 1,048 BTU for every pound of water that goes with it.

Wind matters more. Winter is windier in most of the US, and DOE puts a 7 mph surface wind at a 300% increase in energy consumption. That multiplier applies to a loss rate that is already higher.

A heat pump gets weaker exactly now. It extracts heat from outdoor air, so both its output and its coefficient of performance fall as the air cools. Below roughly 50°F of air temperature most units deliver very little, and the cheap running cost that made a heat pump the obvious summer choice is not available to you in January.

Which equipment survives winter

Gas. The one technology genuinely unaffected by air temperature. It burns fuel and transfers the heat, and a 40°F morning makes no difference to that. In summer gas is the expensive option you buy for speed; in winter it is frequently the only option that works, which changes the calculation rather than just the price.

Heat pumps. Progressively weaker as the air cools. Some units carry low-ambient ratings and will keep working further down than others, but the direction is fixed by the physics. If winter heating is your actual goal, do not size a heat pump from its summer rating.

Solar. Nearly useless in winter, and the reason is not cold, it is daylight. Shorter days and a lower sun angle mean far less energy arriving, and in a freezing climate the panels have to be drained down anyway or they split. Solar is a season-extension technology, not a winter one.

Electric resistance. It works fine in any weather and costs about $53.75 per million BTU delivered, which on a full-size pool in winter is a number that ends the conversation. On a spa it remains reasonable.

The constraint nobody mentions until it happens

Heating a pool through winter means you have not winterized it, and that means the plumbing is full of water in freezing weather. That is the real risk, and it is not a heating problem — it is a burst-pipe problem.

• Water has to keep moving in a hard freeze. Still water in an exposed pipe is what splits it, which is why freeze protection on pool controllers runs the pump rather than the heater.
• A power cut becomes a repair bill. An unheated, un-circulated pool in a deep freeze can damage plumbing and equipment within hours.
• Solar panels must be drained regardless of what you do with the rest of the system.

In a climate with genuine hard freezes, this is the argument that usually decides it, ahead of the running cost.

When heating through winter genuinely makes sense

• A mild climate. Where winter means 50°F rather than 20°F, the gap is smaller, the freeze risk is low and a heat pump may still be working. This is a different question from the one someone in Michigan is asking.
• A small volume. Energy scales directly with gallons. A spa or a small pool is a fraction of the problem a 20,000-gallon pool is.
• An enclosure. An indoor pool or a sound enclosure removes the wind and cuts the evaporation driver, which is a bigger effect than any equipment choice on this page. The domestic version of that is a pool dome, though every one we could find is a summer and shoulder-season structure with no published snow load.
• You will actually swim. Heating a pool nobody enters because the deck is freezing is the commonest version of this mistake.

If you are going to do it

1. Cover it, without exception. Every reason a cover matters in summer is larger in winter, because the driver behind evaporation is stronger. An uncovered heated pool in cold air is close to the definition of wasted fuel.
2. Lower the target. DOE puts each extra degree at 10 to 30% more, and that penalty applies to a bill that is already up.
3. Heat in bursts rather than holding a temperature, unless you swim most days. Holding 82°F through February means paying the elevated loss rate around the clock.
4. Get the freeze protection right before the first hard night, not after it.
5. Cost it first. The running-cost calculator takes your volume, your rates and your equipment, and it is a great deal cheaper to be surprised by a number on a screen than by one on a bill.

If the answer is that you are not heating through winter, the equipment still needs putting to bed properly. Winterizing a pool heater covers what freezes, what to drain and why a sealed cover is the wrong kind.

The alternative worth taking seriously

For a lot of pools the right winter answer is to close it properly and put the money into a longer shoulder season at both ends instead. Two extra weeks in April and two in October is a cheaper and more usable outcome than a heated pool in January that gets used twice.

Stretching the swim season is the page for that strategy, and solar cover against winter cover covers what to put on the pool if you decide to shut it down.