No Hot Water: Causes and Fixes
No hot water almost always traces back to one of a handful of causes. Knowing whether the system is a combi, a stored cylinder or an immersion tells you where to look first — and stops you chasing the wrong fault.
A call-out for no hot water is one of the most common jobs a plumber gets, and the cause depends heavily on the type of system. A combi boiler makes hot water on demand, a vented or unvented cylinder stores it, and an immersion heats it electrically — each fails in different ways, so the first question is always what is actually installed.
This guide runs through the usual causes by system type, from combi diverter valves and low pressure to immersion elements, thermostats and airlocks, and sets out a logical order to work through so you find the fault quickly rather than swapping parts on a hunch.
Key takeaways
- Identify the system first — combi, stored cylinder or immersion — because each fails differently.
- On a combi, a stuck diverter valve is a classic cause of heating working but hot water not (or vice versa).
- On a cylinder, suspect the immersion element, its thermostat, or a boiler/motorised valve not heating the coil.
- Check the simple things first: power, gas supply, boiler pressure, timer settings and reset before condemning parts.
- An airlock or failed pump can stop hot water reaching the tap even when the water is being heated.
Combi boilers: no hot water on demand
On a combi, hot water is made instantly when you open a tap, so the fault usually sits with the boiler itself. Start with the basics: is there power, is the gas on, is the system pressure in the normal band (typically around 1 to 1.5 bar cold), and does the boiler light and fire when you run a hot tap? A boiler locked out on a fault or sitting on low pressure will not make hot water.
If the boiler fires for heating but not for hot water — or heats water but the radiators are cold — suspect the diverter valve. The diverter directs flow between the heating circuit and the hot water heat exchanger; when it sticks, one function works and the other does not. A scaled or blocked plate heat exchanger produces lukewarm water that never gets properly hot, and is common in hard-water areas.
Heating fine but no hot water?
On a combi that classic split — one function working, the other not — points straight at the diverter valve rather than a general boiler fault.
Stored cylinders: the coil isn't heating
With a hot water cylinder, the water is heated by a boiler through a coil, by an immersion, or both. If a boiler-heated cylinder has gone cold, check the programmer and cylinder thermostat are calling for hot water, then check the motorised (zone) valve is opening to let hot water through the coil — a failed valve or actuator is a frequent cause of a cold cylinder while the heating still works.
Bleed and confirm the boiler is actually firing for hot water, and that the pump is circulating. On a gravity-fed hot water system a sludged coil or an airlock in the primary can stop the cylinder heating even though everything else looks right.
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Immersion heaters and thermostats
An immersion heater is a simple electric element with its own thermostat. If the immersion is the only heat source and there is no hot water, isolate it and test the element for continuity and the thermostat for operation — a burnt-out element or a tripped/failed thermostat are the usual culprits. Many immersions have a manual reset on the thermostat that trips on over-temperature.
In hard-water areas immersion elements scale up and eventually fail. If you are replacing one, check whether the cylinder is vented or unvented, because an unvented cylinder's controls and any works on it fall under stricter requirements and the element must be the correct type and rating for the cylinder.
Airlocks, pumps and flow problems
Sometimes the water is being heated but simply is not reaching the tap. An airlock — trapped air in the pipework, common after draining down or on low-pressure gravity systems — can block the hot flow while cold runs fine. Clearing it often means forcing mains-pressure cold back through the hot outlet or bleeding the affected run.
On systems with a shower or circulating pump, a seized or failed pump stops delivery. And a partially closed isolation valve, a blocked filter or a collapsed flexi tail can throttle the hot supply to a trickle. Confirm you have flow before you assume the water isn't hot.
Work simple to complex
Power, gas, pressure, timer, reset — rule out the five-minute checks before you start pulling diverter valves or elements. It saves the customer money and you time.
From guidance to action
Frequently asked questions
Why is my heating working but I have no hot water?
On a combi boiler this usually points to a stuck diverter valve, which directs flow between heating and hot water. On a stored-cylinder system it more often means the cylinder thermostat or the motorised valve feeding the coil has failed, so the heating circuit heats but the cylinder does not.
How do I know if my immersion heater has failed?
Isolate the immersion and test the element for continuity and the thermostat for operation. A burnt-out element, or a thermostat that has tripped on over-temperature, is the usual cause. Many immersion thermostats have a manual reset button to try first.
Can an airlock stop hot water?
Yes. Trapped air in the hot pipework, common after a drain-down or on low-pressure gravity systems, can block the hot flow while the cold still runs normally. The water may be hot in the cylinder but simply cannot reach the tap until the airlock is cleared.
Related guides
How to Fix a Dripping Tap
Cure a dripping tap: tell washer taps from ceramic-cartridge taps, isolate safely, replace the worn part, and fix leaks at the spout or base.
Unvented Cylinder (G3) Requirements
G3 requirements for unvented hot water — safety devices, discharge pipework and the D1/D2 rules.
Water Hammer: Causes and Fixes
Why pipes bang, the difference between water hammer and expansion noise, and how to diagnose and fix it.
