Boiler Not Producing Hot Water – Common Causes

Diverter valve fault

Combi boilers use a diverter valve to switch between heating water for the taps and heating water for the radiators. If this valve fails or gets stuck, you can end up with heating but no hot water, or hot water but no heating, depending on which way it's stuck. Replacing a diverter valve is gas appliance work and needs a Gas Safe registered engineer.

Limescale on the heat exchanger

In hard water areas like ours, limescale can build up on the plate heat exchanger that heats water on demand for the taps. This restricts water flow and reduces or eliminates hot water production, even though the boiler otherwise seems to be running normally. Regular servicing helps catch this before it becomes a complete blockage.

Faulty thermistor

The thermistor measures water temperature and tells the boiler how hard to work. A faulty thermistor can cause the boiler to misjudge temperature, cutting out before water is properly heated or failing to fire the burner at all. It's a relatively inexpensive part to replace once correctly diagnosed.

Low pressure or a lockout

If system pressure has dropped too low, many boilers will stop producing hot water and heating altogether as a safety measure. Check the pressure gauge — if it's low, this may be the simplest explanation, though the reason for the pressure drop still needs investigating.

Airlock in the system

An airlock in the hot water pipework can prevent water flowing properly to the taps even though the boiler itself is functioning. This is more common after work has been done on the plumbing system or after the water supply has been off, and can sometimes be cleared by running taps in sequence, though a persistent airlock is worth having checked properly.

What an engineer checks and in what order

• Confirms system pressure is correct and the boiler isn't in lockout
• Checks for an error code and cross-references it against the fault symptoms described
• Tests the diverter valve's operation between heating and hot water modes
• Inspects the heat exchanger for scale or restriction if flow seems reduced
• Tests the thermistor's readings against expected values

What to check yourself before calling an engineer

• Confirm the boiler pressure gauge is reading within the normal range, usually 1 to 1.5 bar
• Check the hot water temperature dial on the boiler hasn't been turned down or reset accidentally
• Run a hot tap for a minute or two in case an airlock is the cause rather than a genuine fault
• Note whether the fault appeared suddenly or has been gradually getting worse, which helps distinguish a mechanical failure from limescale build-up
• Check whether the issue affects every tap in the house or just one, which can point towards a local blockage rather than a boiler fault

Why this is gas appliance work and shouldn't be attempted yourself

Every one of the faults above, from a stuck diverter valve to a scaled heat exchanger, sits inside the boiler casing. Opening the casing to inspect or replace these parts is gas appliance work under law, and must only be carried out by a Gas Safe registered engineer. This isn't just a legal formality — a boiler that's been incorrectly reassembled or has a poorly fitted seal can develop a dangerous fault that isn't obvious just from looking at it.

What this typically costs to diagnose and repair

Diagnosis and repair of hot water faults is charged at our standard £99 per hour daytime rate with a one-hour minimum, then billed in 30-minute increments after that. A straightforward fix, such as a faulty thermistor, is often a relatively low-cost part once identified. A diverter valve replacement or a heat exchanger clean typically takes longer, and any parts required are always quoted and agreed with you before they're fitted.