Hot Tub Won't Hold Temperature? Losing Heat Overnight — Here's How to Fix It

Your tub gets up to temperature fine — but by the morning it's dropped several degrees, or it just can't seem to stay at 40°C. This is one of the most common complaints we hear across Bournemouth, Poole and Christchurch, and the good news is that most cases are heat loss, not a broken heater. A tub that heats but won't hold is usually losing warmth faster than the heater can replace it — and that's often free to fix.

This guide covers all brands — Lay-Z-Spa (AirJet and HydroJet), Intex PureSpa, MSPA, CleverSpa and hardshell hot tubs. Work through it top to bottom: the free checks come first, the engineer-only faults last.

💡 First, set your expectations An inflatable hot tub will always lose a few degrees overnight in a British garden — that's normal. The heater on most Lay-Z-Spa and Intex tubs cannot run at the same time as the bubbles/jets, so heat recovers slowly. A drop of 1–3°C overnight with a good lid is expected; a drop of 5°C or more, or never reaching target, points to a fixable problem below.

Quick Checks First (No Tools)

1. Check the Lid, Clips and Seal

The lid is responsible for the majority of heat loss. Go round it carefully:

Is it fully clipped down with no gaps at the edges? Even a small gap vents heat all night.
Is the lid waterlogged or sagging? A cover that has soaked up water loses its insulation and should be dried or replaced.
Is the inflatable lid still firm? On Lay-Z-Spa tubs the lid bladder can deflate over time — top it up so it sits tight against the rim.

A worn or poorly fitting lid is by far the most common reason a tub bleeds heat overnight, and a replacement cover is cheap compared with running the heater harder.

2. Make Sure the Heater Is Actually Set to Hold

Many tubs are smarter — and trickier — than people realise:

Check the set temperature hasn't been knocked down. Press the temperature button to confirm the target (usually up to 40°C).
On Lay-Z-Spa and many tubs, the heater only runs while the heating symbol is lit and pauses when the bubbles are on. Running the massage/jets for long sessions stops heating — the tub will cool, then slowly recover afterwards.
Some models have an economy or timer mode that only heats during set hours or filter cycles. Make sure it's set to heat continuously to target if you want it always hot.

3. Insulate Against the Weather

Bournemouth gets cold, damp and windy nights even in summer, and wind chill strips heat from an exposed tub fast. Cheap wins:

Put a thermal/foil blanket on the water surface under the lid.
Stand the tub on an insulating base mat — a lot of heat escapes downward into a cold patio.
Shelter it from the prevailing wind with a fence panel or in a corner.
Don't run the bubbles for long periods on a cold night — they look cosy but they actively cool the water by aerating it.

If the Lid and Settings Are Fine But It Still Won't Hold

Weak or Limescale-Coated Heater Element

In our hard-water area, the heater element builds up limescale, which acts like a blanket on the element and makes it heat slowly and inefficiently. A scaled or ageing heater can no longer keep pace with normal heat loss, so the tub plateaus below target or drifts down through the day. Descaling (or, for a badly furred element, replacement) restores performance. Using a scale inhibitor in hard water prevents it returning.

The Tub Heats Very Slowly From Cold

If it takes far longer than the usual 1–1.5°C per hour to warm up, that's a sign the heater is underperforming — most often scale, but sometimes a tired element or a relay that isn't reliably switching the heater on. If your tub never reaches the set temperature no matter how long it runs, see our dedicated hot tub not heating up guide.

Faulty Temperature Sensor or Thermostat

If the displayed temperature looks wrong — the water feels hot but the panel reads low, or vice versa — the temperature sensor (thermistor) may be drifting. A faulty sensor can make the controller shut the heater off too early (so it never holds target) or read inaccurately. This often shows alongside an error code; check our error code guide. Sensor diagnosis needs a multimeter and is engineer territory.

Heater Relay or Control Board

If the heater cuts in and out, or the heating light flickers on and off when it should hold steady, the relay on the control board that switches the heater may be failing. Left unchecked this gets worse, so it's worth having looked at.

⚠️ Worth ruling out a slow leak A tub that loses both water level and temperature may have a small leak letting warm water out and cold air in. If you're topping up more than usual, check our hot tub leaking guide too.

Lid's Good But It Still Loses Heat?

If the cover is sound and your settings are right but the tub still won't hold temperature, it's likely a scaled or failing heater, a relay or a sensor. We'll match you with a local engineer who knows your brand.

📞 Get a Free Quote

How Much Does a Heat-Loss Repair Cost?

Typical costs in the Bournemouth area:

New thermal blanket / base mat: £10-30 (DIY)
Replacement inflatable lid / cover: £30-90 part
Descaling the heater element: £0-15 DIY, or part of a service
Temperature sensor (thermistor): £15-40 + labour
Heater element replacement: £40-100 + labour
Control board / relay: £80-200 + labour
Typical engineer call-out: £40-80 (often includes the first 30 mins of labour)

Most "won't hold temperature" jobs turn out to be a lid, insulation or a descale — well worth checking before assuming the heater has failed.