Hot Water Not Working: Diagnosis Guide for UK Homes

No hot water is one of those faults that feels catastrophic on a cold morning but is often straightforward to diagnose. The starting point is knowing what type of hot water system you have, because the common faults differ significantly between system types.

Step 1: Identify your system type

Combi boiler: No hot water cylinder or tank. The boiler heats cold mains water on demand when a hot tap is opened. Hot water is available instantly but flow rate is limited by the boiler’s output.

System boiler with unvented cylinder (Megaflow or similar): A cylinder stores hot water at mains pressure. The boiler heats the cylinder via a coil. Hot water flows at mains pressure when a tap is opened. The cylinder is a sealed pressurised vessel — no loft tank.

Conventional/regular boiler with vented cylinder: A gravity-fed system with a cold water tank in the loft feeding a vented hot water cylinder. Hot water is available under gravity pressure (lower than mains pressure). Typically found in older properties.

Electric immersion heater (no boiler): A directly-heated electric element inside a hot water cylinder. No gas involved.

Combi boiler: no hot water

Check 1: Is the heating working? On a combi boiler, heating and hot water share the boiler but the DHW (domestic hot water) takes priority. If the heating works but hot water doesn’t — the diverter valve is the most likely fault.

Check 2: Boiler pressure. Below 1 bar and many combis will produce reduced or no hot water. Repressurise via the filling loop to 1.2–1.5 bar.

Check 3: Boiler error code. Note the error code on the display and look it up in the boiler manual. Common hot water-related codes relate to: heat exchanger temperature sensors, overheat thermostats, or flow rate sensors.

Check 4: Diverter valve fault. If the diverter valve is stuck in the heating position (rather than DHW), hot water will be cold or lukewarm while the radiators are hot. This is the most common combi boiler-specific hot water fault and requires a plumber.

Check 5: Hot water temperature setting. Most combi boilers have a separate DHW temperature dial or setting (often a tap symbol). If this was accidentally turned down (a common result of children fiddling with the boiler), the water may be warm but not hot.

System boiler with cylinder: no hot water

Check 1: Is the cylinder actually hot? Touch the cylinder — is it warm? If it’s completely cold, no heat has been delivered to it recently.

Check 2: Boiler pressure and programmer. Confirm the boiler is operating normally and the programmer is set to heat hot water (a separate programme to the heating on most systems).

Check 3: Cylinder thermostat. A dial-type thermostat is mounted on the side of the cylinder. Check it’s set to the correct temperature (typically 60–65°C for hot water safety — Legionella prevention requires a minimum of 60°C for hot water storage). If the thermostat has failed, the cylinder won’t call for heat.

Check 4: Immersion heater backup. Most hot water cylinders have an immersion heater as backup. If the boiler circuit isn’t working, switch the immersion heater on. If hot water returns within 1–2 hours, the boiler side of the cylinder circuit has a fault that needs investigation.

Check 5: Cylinder programmer or hot water zone valve. The hot water zone valve (a motorised valve on the pipe feeding the cylinder) opens when the cylinder calls for heat. If this valve has failed in the closed position, no hot water is delivered even though the boiler runs. A plumber can diagnose and replace it.

Conventional boiler with vented cylinder: no hot water

The same checks as system boiler above, plus:

Check the cold water header tank (loft tank). If the ball valve in the loft tank has jammed closed, no cold feed water enters the cylinder and no hot water comes from the taps (the flow stops completely, not just goes cold). Look into the tank — is it nearly empty when it should be full?

Check for airlock. Vented hot water systems can develop airlocks in the pipe runs that stop flow. An airlock typically causes a trickle or intermittent flow rather than complete stoppage. Clearing a hot water airlock involves connecting the cold main to the hot pipe briefly to use mains pressure to push the air out — a job for a plumber.

Electric immersion: no hot water

Check 1: Immersion heater switch. Is the dedicated circuit switched on? There should be a dedicated switch, often a double-pole isolator with a neon indicator.

Check 2: Timer or programmer. If there’s a timer on the immersion circuit, is it set correctly? Timer clock correct after any power cut?

Check 3: Thermostat. The immersion heater has an internal thermostat. If it trips (safety cut-out), hot water stops. Some thermostats have a reset button (a small button under a cover on the head of the element). Press it. If it trips again quickly, the thermostat or element has failed.

Check 4: Element failure. The heating element inside the cylinder can fail, particularly in hard water areas where scale builds up on the element and it eventually burns out. An electrician can test and replace the element — this is electrical work, not plumbing.

Tepid water but not hot: common across all systems

If the water is warm but not hot enough (below about 50°C), common causes:

• Thermostat or temperature control set too low
• Scale on the heat exchanger or immersion element (insulating it from the water)
• Mixing valve fault (a thermostatic mixing valve delivering cooler water than set — check the TMV setting)
• Boiler delivering lower temperature than it should (scale or a fault with the gas valve)