No Hot Water? Common Causes & Fixes UK 2026
No hot water is one of the most frustrating boiler problems—especially in winter. The cause depends on your system type: combi boilers heat water on demand, while system and regular boilers usually use a hot water cylinder and controls. This guide covers the most common UK causes, safe checks you can do yourself, and when you need a Gas Safe engineer.
Step 1: Identify Your System Type
If you have a combi boiler, hot water issues often relate to boiler settings, pressure, flow rate, or internal components. If you have a cylinder (system/regular), the issue could be the programmer, cylinder thermostat, motorised valve, or immersion heater.
Common Cause 1: Programmer or Hot Water Schedule (Cylinder Systems)
Check the programmer is set to provide hot water and the time is correct (power cuts can reset clocks). If the schedule is off, the cylinder may simply be cold.
Common Cause 2: Boiler Pressure Too Low (Sealed Systems)
Low pressure can prevent a boiler from operating correctly. Check the pressure gauge when the system is cold; many UK sealed systems target around 1–1.5 bar. If it’s low, topping up via the filling loop may restore hot water.
Common Cause 3: Flow Rate Too High (Combi)
If hot water goes lukewarm, your tap flow may be too high for the boiler to heat it properly. Try reducing the flow at the tap. If it improves, it may be a flow/temperature rise issue.
Common Cause 4: Lockout or Error Code
If the boiler is in lockout, it may stop producing hot water. You can try one reset if it’s safe and there are no signs of gas smell or leaks. If it returns to lockout, book an engineer.
When to Call a Gas Safe Engineer
Call an engineer if the boiler repeatedly locks out, you can’t restore pressure (or pressure keeps dropping), you suspect a leak, or hot water is inconsistent despite basic checks. Never remove the boiler case or attempt internal repairs yourself.
Upgrading Your Boiler? Get the kW Right
Hot water performance (especially for combis) is closely linked to boiler output and your household demand. Use our calculator to estimate a suitable kW range.
Use the Boiler Size CalculatorFrequently Asked Questions
Why do I have heating but no hot water?
On cylinder systems, it’s often a programmer/hot water setting, cylinder thermostat, or a motorised valve issue. On combis, it can be a diverter valve or hot water flow/temperature problem.
Why do I have hot water but no heating?
That’s commonly a thermostat/programmer issue or a heating-side component issue (like a valve or pump).
Could low pressure cause no hot water?
Yes on many sealed systems. If pressure is below the minimum threshold, the boiler may refuse to operate.
Is it safe to reset the boiler?
It’s usually safe to try one reset if there’s no gas smell or visible leak. Repeated lockouts indicate a fault that needs professional diagnosis.
When should I call an engineer?
If basic checks don’t restore hot water, pressure keeps dropping, or you see repeated lockouts, call a Gas Safe engineer.