6 min read · TopTubs Lincolnshire Ltd
A hot tub that won't heat is one of the most common service calls we get — and a good proportion of them turn out to be something straightforward. Before you call an engineer, work through this checklist.
It sounds obvious but it's surprisingly common. Check your control panel shows the correct target temperature (typically 38–40°C). If the set temperature is lower than the current water temperature, the heater simply won't activate.
Also check whether the tub is in economy mode or sleep mode — these reduce the set temperature significantly to save energy. Press the Mode button on your controller to cycle to Standard mode.
Check your control panel for any error codes — particularly FLO, OH, DR or HL. These safety codes prevent the heater running until the condition is resolved. See our error codes guide for what each means.
The heater will not run if flow is insufficient — a blocked filter is the most common cause of heating failure on otherwise healthy tubs. Remove the filter, rinse it thoroughly, and restart the tub. If it begins heating, you've found the problem.
If the water level has dropped below the skimmer inlet, the pump draws in air, flow reduces, and the heater protection kicks in. Check the waterline — it should be 2–3cm above the jets.
Air locks are common after a refill or if the water has run very low. The pump runs but can't circulate properly. Signs: you can hear the pump running but feel little or no flow from the jets.
Fix: Turn off the tub. Briefly loosen (don't remove) a union fitting on the pump and let air escape, then retighten and restart. Alternatively, turning jets on and off rapidly several times can help shift an air lock.
Check the electrical panel — a tripped GFCI (RCD) breaker is common after power fluctuations. Reset it once. If it trips again immediately, do not reset it — call an electrician or us.
Many hot tubs have a small dedicated circulation pump that runs continuously to filter and heat the water. If this pump fails, the tub won't heat even if the jets pump works fine. Listen for a quiet continuous hum from the equipment bay — absence of this in a tub that normally has it suggests the circ pump has failed.
Call us if: the checks above don't resolve the issue, the breaker keeps tripping, you can smell burning from the equipment bay, or the heater element itself has failed (common after 5+ years). Heater element replacement is a straightforward job but requires draining and accessing the equipment. Book a diagnosis.