Why does my heat pump work fine in October and not in January?

Because a heat pump moves heat out of the outdoor air rather than creating it, so its output falls as that air gets colder. The federal test procedure rates these units with the surrounding air at 80.6 degrees Fahrenheit dry bulb, which is close to an October afternoon here and nothing like a January night. The capacity printed on the unit was measured under those summer conditions. Nothing has failed when it delivers less in winter; it is being asked to work a long way outside the condition its published number describes.

Is a pool cover really worth the hassle?

On the numbers, yes, and by a wide margin. A controlled study run for the National Plasterers Council measured evaporation reductions of 93.9 percent for a solid track cover, 95.9 percent for foam and 94.9 percent for a bubble cover, with floating solar disks at 50.1 percent (Muleta, 2016). Since evaporation is by far the largest heat loss from a pool, that is the biggest lever available without buying equipment. The study is also honest that its figures are maximum possible efficiencies, because covers were left on. A cover only works while it is on the water.

Would a bigger heater solve it?

Sometimes, and it is the most expensive way to find out. A larger unit adds heat faster, which shortens recovery time and raises the temperature the pool settles at. What it does not do is change why the pool is losing heat, so you end up paying to run a bigger machine against the same losses. Address the loss side first, since it is cheaper and its effects are permanent, then size any replacement against the conditions you actually have rather than against a rating measured in 80.6 degree air.

Why does the pool feel colder on a dry, windy night?

Because both of those conditions accelerate evaporation, which is the dominant heat loss. Dry air accepts more water vapor than humid air, so evaporation runs harder at the same temperature. Wind sweeps away the humid layer sitting directly on the water surface and replaces it with drier air, keeping the process going. The published energy model for pools lists wind speed, air temperature and relative humidity among the variables that make outdoor pool energy hard to predict, and a cold front moves all three at once.

How can I check the heater myself before calling anyone?

Three things, none of which require tools you do not have. Read any error code on the display, because most units will tell you what they think is wrong. Check the filter, the pump basket and the flow, since a heater cannot put heat into water that is barely moving and a restricted system imitates a weak heater exactly. Then feel or measure the difference between the pipe going into the heater and the pipe coming out while it runs. A clear rise means it is transferring heat; no rise while apparently firing points at the equipment.

Does salt water shorten the life of a pool heater?

Salt chlorination raises the total dissolved solids in the water, and more aggressive water is harder on the metal it circulates through, including the heat exchanger. Coastal air does its own damage from the outside. Neither makes a salt system a bad choice, and plenty of Naples pools run one for years without trouble. What matters is that the water is kept in balance rather than run acidic or low in calcium hardness for long periods, because that is what turns an ordinary service life into a short one.

Insights · Pumps, heaters and automation

Why is my pool heater not keeping up?

A heater that ran the pool to temperature in October and cannot manage it in January has usually not developed a fault. Two things changed, and neither of them is inside the machine. The pool started losing heat faster, and the heater started producing less, both for reasons that are written into the federal test procedure and the published energy model.

Clayton Waggoner, Owner and Managing Member · Published August 8, 2026 · 12 min read

A pool heater does not set the water temperature. It adds heat at some rate, the pool loses heat at some other rate, and the water settles where those two meet. Every complaint about a heater not keeping up is a statement about that balance, so the useful question is which side of it moved.

The short version

I.Heat pump pool heaters are tested at 80.6 degrees Fahrenheit dry bulb air. That is the condition behind the rating, and it is nothing like a January night here.

II.In the published pool energy model, the evaporation term is roughly nine times the convection term. Most of the heat leaves as water vapor.

III.A measured study found solid, foam and bubble covers cut evaporation by about 95 percent, and solar disks by about half (Muleta, 2016).

IV.Wind speed, air temperature, humidity and solar exposure are named as the variables that make outdoor pool energy hard to predict. All four move together on a cold front.

V.The heater is genuinely at fault when the symptom is a sudden change, an error code or short cycling, rather than a gradual seasonal loss of ground.

Is the heater broken, or is the pool just losing heat faster?

Usually the second. The water temperature is where heat input and heat loss balance out, so the same heater that held temperature in mild weather can lose ground in cold weather without anything having changed inside it. The equipment did not degrade, the arithmetic did.

Think of the pool as a bucket with a hole in it. The heater is the tap. The water level, meaning the temperature, holds steady when the tap matches the hole.

Widen the hole and the level drops even though nothing happened to the tap. That is what a cold, dry, windy night does to a pool.

The published model names the variables directly. For outdoor pools the parameters that can vary significantly are wind speed, pool water temperature, temperature of outdoor air, relative humidity for outdoor air, solar radiation and the time the pool is in operation (ENERGY STAR Portfolio Manager Technical Reference, 2023)¹.

That same document notes that energy consumption in outdoor pools is more difficult to calculate than indoor pools because of this variability (ENERGY STAR, 2023)¹. Even the people building the model treat outdoor pools as the hard case.

A cold front in Southwest Florida moves several of those at once. Air temperature drops, humidity drops, wind picks up, and the sun is lower and weaker. Every one of those widens the hole.

So a heater that suddenly seems undersized in the first week of January is frequently the same heater that seemed generous in October, being asked a much harder question.

The local record shows how wide that swing can be. The National Weather Service series for Naples runs from 1942 to 2025 and its January column carries a maximum high of 89 degrees against a minimum low of 26 (NOAA National Weather Service, Naples climatological records)⁴.

Both of those are January. A heater sized against the pleasant end of that range will be found out at the other end, and the owner will experience it as the equipment letting them down rather than as a specification that only ever covered half the month.

Where does the heat actually go?

Mostly into the air as water vapor. In the published energy model the evaporation term is far larger than convection, by roughly nine to one. Every gallon that evaporates takes a substantial amount of energy with it, and that is happening across the whole surface all night.

The model gives the terms side by side. In its simplified form the evaporation contribution is 368.56 multiplied by the activity factor and the pool area, while the convection contribution is 40.88 multiplied by the pool area (ENERGY STAR Portfolio Manager Technical Reference, 2023)¹.

Those coefficients are for an indoor pool, where the model treats radiation as zero and only evaporation and convection are considered. The ratio between them is the point: the evaporative term dominates.

That is counterintuitive, because evaporation is invisible. Nobody watches heat leave a pool the way they watch a heater run, so the loss that matters most is the one nobody sees.

It also explains why the water feels colder on a dry night than a humid one at the same air temperature. Dry air accepts more water vapor, so evaporation runs harder.

And it explains wind. Moving air replaces the humid layer sitting on the water surface with drier air, which is why an unscreened pool on an exposed lot behaves differently from a screened one in a sheltered garden.

The activity factor in the model exists to correct evaporation loss depending on how the pool is used (ENERGY STAR, 2023, citing ASHRAE)¹. Swimmers splash, wet bodies leave the water, and the surface is disturbed. Use costs heat.

Still pool water at the surface, light moving across it — The largest loss in the model is the one nobody can see happening.

Why does a heat pump stop keeping up in January?

Because the rating on the box was measured in summer air. The federal test procedure sets the surrounding air at 80.6 degrees Fahrenheit dry bulb. A heat pump moves heat from the air into the water, so when the air is colder there is less to move and output falls.

The test condition is specified in regulation. For electric heat pump pool heaters the air temperature surrounding the unit shall be at the high air temperature, mid humidity level of 80.6 degrees Fahrenheit dry bulb (10 CFR Part 430, Subpart B, Appendix P)².

Read that next to the complaint. The published capacity was measured with the machine sitting in 80.6 degree air. In January here the air around that unit at four in the morning is frequently in the fifties or lower.

A heat pump is not a furnace. It does not create heat, it relocates it, taking energy out of the outdoor air and putting it into the pool water. Colder air holds less available energy, so the same machine delivers less.

Nothing in that is a defect, and no service call fixes it. The unit is behaving exactly as designed, at a condition the design was never rated at.

This is also why the shoulder seasons feel fine and midwinter does not. In October the air is still close to the test condition. In January it is a long way from it, on precisely the nights an owner most wants the pool warm.

It is the single most useful thing to understand before spending money, and it is the difference between the two heater types explained on heat pump pool heaters and gas pool heater installation, where output does not fall with air temperature.

Air temperature around the heat pump

40 Frated at 80.6 F95 F

At or above the federal test condition

tested at 80.6 F dry bulb (10 CFR 430 App. P)²

Well below it, output falls

a heat pump moves heat from the air, so colder air yields less

Does a pool cover actually work, or is that just standard advice?

It works, and the effect is larger than most owners expect. A controlled study (Muleta, 2016) measured solid track, foam and bubble covers cutting evaporation by roughly 95 percent, with solar disks at about half and liquid suppressants at about 15 percent.

The study was run at a dedicated research facility with multiple identical pools, weather data from a station less than half a mile away, and a correction applied for leakage measured separately (Muleta, California Polytechnic State University, for the National Plasterers Council, 2016)³.

The measured evaporation reduction efficiencies were 93.9 percent for a solid track cover, 95.9 percent for a foam cover and 94.9 percent for a bubble cover (Muleta, 2016)³.

Solar disks, which are the floating rings some owners prefer for convenience, came in at 50.1 percent (Muleta, 2016)³. Half the benefit, and considerably less handling.

The two liquid evaporation suppressants tested reduced evaporation by 14.4 and 15.8 percent (Muleta, 2016)³. Real, and an order of magnitude below a physical cover.

The authors are careful about their own numbers, and so should anyone quoting them be. They note that the covers were not removed from the pools during the study other than for cleaning and measurement, and that in reality covers have to come off when the pool is occupied, so the reported figures should be considered as maximum possible efficiencies (Muleta, 2016)³.

That caveat is the honest frame. A cover that lives in the garage achieves nothing. What the study establishes is the size of the prize, not a promise about your habits.

What it changes

How do I tell a heat loss problem from a broken heater?

By the shape of the symptom. Heat loss problems track the weather, come on gradually and affect how fast the pool recovers. Equipment faults tend to be abrupt, are often the same regardless of conditions, and usually announce themselves at the machine.

Start with timing. A pool that has lost ground steadily as the season turned, and recovers on the first mild week, is describing the balance rather than a fault.

A pool that was fine on Tuesday and cold on Wednesday, with no weather change, is describing something that happened to the equipment.

Look at the machine while it runs. A heater that fires and then stops within a few minutes, repeatedly, is short cycling, which points at flow, a pressure switch, a sensor or a limit rather than at the weather.

Check for an error code before anything else, because most units will tell you what they think is wrong, and that costs nothing to read.

Feel the difference across the heater. There should be a measurable rise between the water going in and the water coming out while it is running. Little or no rise with the unit apparently firing is an equipment answer.

Then check flow, because a heater cannot transfer heat into water that is barely moving. A dirty filter, a clogged basket or a failing pump all reduce flow, and all of them show up as a heater that seems weak.

Where the pad has been added to over the years, the plumbing itself is sometimes the constraint, which is the argument made on equipment pad rebuilds rather than replacing one component at a time.

When is the heater genuinely the problem?

When the evidence points at the machine rather than the conditions. A unit that will not fire, throws a code, fails a temperature rise check across itself, or has visibly corroded internals is a repair conversation and not a physics lesson.

Age and salt air matter here more than in most of the country. A heater on a coastal lot is living in an atmosphere that attacks metal continuously, and heat exchangers do fail from the inside.

Chemistry matters too. Water that has been run aggressive attacks the same heat exchanger the manufacturer warranted on the assumption it would not be.

A gas unit that will not light, cycles on a limit, or shows soot or scale on the burners is a service item. So is a heat pump with a failed compressor, fan or reversing valve, which behaves quite differently from one merely working in cold air.

The distinction worth holding is between capacity and function. A working heater in cold air produces less. A faulty heater produces erratically, or not at all, and does it in any weather.

Diagnosis costs a fraction of replacement and answers that question directly, which is why pool heater repair starts with establishing which of the two you have before anything is quoted.

And if the answer turns out to be the balance rather than the machine, the fixes are the ones above: a cover, shelter from wind, realistic expectations on the coldest nights, and sizing that reflects the conditions you actually have.

If the equipment pad is going to be opened during other work, that is the cheap moment to correct any of this, which is one of the arguments in resurface or remodel.

“The rating on the box was measured in 80.6 degree air. In January it is being asked a question it was never rated to answer.”

Interactive · Before you call it broken

Is it the heater, or is it the balance?

Four questions that separate an equipment fault from a heat loss problem. They cost nothing to answer and they change what you should be paying for.

Ask Clayton

Questions this article usually prompts

Tap a question. Clayton replies himself, here and on the phone.

Clayton Waggoner Owner and Managing Member · Splash Pools Online

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If this is the situation you are in

Pool heater repair in Naples, diagnosed not replaced →Pool heat pumps, which genuinely work in this climate →Gas pool heater installation, sized to the whole run →Equipment pad rebuilds, so the next repair is a part swap →

References

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