The setpoint says one thing and the water says another. The unit is running, nothing is obviously broken, and the pool sits stubbornly a few degrees below where you want it.
Before calling anyone, work through this in order. A heat pump pool heater that is not reaching temperature usually has a cause you can identify yourself, and a meaningful proportion of the time it is not a fault at all.
Step 1: How long has it been running?
The most common false alarm.
A pool is an enormous mass of water. Heating it from cold takes days on a heat pump, not hours. New owners frequently check three hours after switching on, find little change, and conclude something is wrong.
What is normal: gradual gain, measured in fractions of a degree per hour depending on volume, ambient conditions and the unit's output.
What to do: if you have owned it less than a week, or you have just restarted after a shutdown, give it days rather than hours before diagnosing.
Step 2: Is the pool covered?
The second most common cause, and the most fixable.
Evaporation is the largest heat loss mechanism in an outdoor pool, and it peaks overnight when the air cools and the water does not. An uncovered pool can lose overnight most of what the heater added during the day.
The symptom this produces: the pool warms during the day and is back where it started by morning. The heater is working. It is just being asked to out-produce a very large ongoing loss.
What to do: cover it, every night, for a week, and see if the picture changes. If it does, you have your answer.
Step 3: What is the ambient temperature?
Heat pumps collect heat from air. Cooler air means less available heat and lower output.
What is normal: reduced output as the air cools, and every unit has a minimum operating temperature below which it will not run at all.
What to check: the overnight and early morning temperatures, not the daytime figure. That is when your pool is losing heat fastest and your unit has the least to work with.
What this may mean: if it holds temperature comfortably in warmer weather and cannot in cooler weather, the unit may simply be sized for a shorter season than you are now asking of it. That is a specification issue rather than a fault.
Step 4: Is airflow restricted?
A heat pump starved of air works harder for less.
Check:
Has landscaping grown in around it? The most common cause and it develops slowly.
Has anything been stacked against it? Pool equipment, furniture, bags of salt.
Is the coil clogged with leaves, grass clippings or debris?
Is it in a corner or enclosure where it is drawing back its own cooled exhaust air?
Are the manufacturer's clearances still maintained on all sides and above?
What to do: clear the surrounds, hose the coil gently with fresh water, never a pressure washer, which bends the fins permanently.
Step 5: Is the water flow adequate?
The heater needs a minimum flow rate through the heat exchanger.
Check:
Is the filter clean? A dirty filter reduces flow.
Is the skimmer basket clear?
Is the pump basket clear?
If you have a variable speed pump, what speed is it running at? An eco setting may not deliver the flow the heater requires. This is a very common and easily missed cause.
Is the pump running at the same time as the heater? They need to be aligned.
What to do: clean the filter and baskets, and check the pump speed against the heater's stated flow requirement.
Step 6: How long is it actually running?
Not how long you think, but how long the unit is powered and calling for heat.
Check:
Is there a timer restricting run hours?
Is the pump schedule shorter than the heater schedule? The heater achieves nothing without circulation.
Has anything changed recently, such as a new pump schedule to save electricity?
A common scenario: somebody shortens the pump run time to reduce costs, and the heater loses hours it needs.
Step 7: Is the setpoint realistic?
Worth asking honestly.
Every degree costs more than the last, because heat loss is driven by the gap between water temperature and the air. A setpoint several degrees above comfortable, on an exposed uncovered pool in a cool month, may be beyond what the unit was sized for.
What to try: drop the setpoint by a degree or two and see whether it holds. If it holds comfortably at a slightly lower temperature, the unit is working and the ask was too high.
Step 8: Is it actually sized correctly?
If everything above checks out and it still cannot reach temperature in the conditions you want to swim in, this is the likely answer.
The signature of undersizing: the unit runs almost constantly, gets the pool to a temperature slightly below your setpoint, and stays there. It does not fail loudly. It just never quite arrives.
The usual cause: sized on pool volume alone, or sized for summer when you now want a longer season. Volume tells you the mass of water and nothing about the rate of loss, which is driven by target temperature, climate, cover use and exposure.
What to do: get the sizing checked against your actual conditions rather than assuming the unit is faulty. Request a free personalised pool heat report.
When to call someone
Stop troubleshooting and get a technician if you see:
Error codes on the display, particularly repeating ones
Ice on the coil persisting beyond what the defrost cycle clears
New noises: grinding, rattling, or a change in fan sound
Water leaking that is clearly more than normal condensate
The unit tripping the circuit
No heat output at all despite running
Note the error code and the conditions before you call. It saves diagnostic time.
The quick checklist
Has it had days, not hours?
Is the pool covered overnight?
What are the overnight temperatures?
Is airflow clear on all sides?
Is the filter clean and the flow adequate?
Is the pump running whenever the heater is?
Is the setpoint realistic for the conditions?
Was it sized for the season you are now asking of it?
In most cases the answer is in items two, four, five or eight.
Frequently asked questions
How long should it take to heat from cold?
Days rather than hours, depending on volume, the temperature gap and output. Heat pumps are designed to hold a temperature rather than sprint to one.
Why does it warm during the day and cool overnight?
Almost always an uncovered pool. Overnight evaporation returns much of the day's gain.
The display says the right temperature but the water feels cold.
The display reads at the plant, not in the pool. Check with a thermometer at depth, early in the morning.
Could my new variable speed pump be the problem?
Yes, and it is a common one. Low eco speeds may not deliver the flow the heater requires.
Is it broken or undersized?
Undersizing looks like consistently arriving just below setpoint while running constantly. A fault usually produces error codes, noise or no output.
Does it stop working in winter?
Output falls as air cools and it stops below the minimum operating temperature. Whether that matters depends on how it was specified.
Work through it in order
Most pools that will not reach temperature have a cause on this list, and the first four cost nothing to check.
Browse the pool heater range, or request a free personalised pool heat report.
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