Boiler Not Producing Hot Water?

Why some boilers heat the radiators but not the taps, the safe checks to make first, and what each pattern usually points to.

Last updated: 2026-10-19

“No heating” and “no hot water” are different problems, and the pattern of what works tells an engineer a lot. If the radiators warm up but the taps stay cold, here is what is worth checking.

First, which boiler type do you have?

Combi boilers heat water on demand — there is no cylinder. System and heat-only boilers store hot water in a cylinder. The causes differ significantly between the two.

Combi boilers

• Is the boiler showing a fault code? Note it — the code narrows things down quickly.
• Does the boiler fire when you open a hot tap? If it fires but the water stays cold, the diverter valve or plate heat exchanger is a common suspect.
• Is the flow very low? A combi needs a minimum flow rate to fire. A partially closed valve or a blocked tap filter can be enough to stop it.
• Is the heating working? If heating is fine but hot water is not, the fault is usually in the domestic hot water side rather than the boiler’s main burner.

System and heat-only boilers

• Check the cylinder thermostat — it may be set too low, or the cylinder may not be calling for heat.
• Check the programmer — hot water may simply be switched off or on a schedule you did not expect.
• Is the motorised valve moving? A stuck valve is a classic cause of “heating works, water does not”.
• Is the cylinder actually heating? Feel the pipework near the cylinder.

Safe checks anyone can do

Thermostat and programmer settings, boiler pressure (see our pressure guide), the boiler display for a fault code, and whether other gas appliances are working. Do not remove covers or touch gas pipework.

When to call an engineer

Book a boiler repair if:

• the boiler will not fire on hot water at all;
• a fault code is showing;
• pressure is low or falling;
• you have a system boiler and the cylinder is not heating despite correct settings;
• the water alternates hot and cold.

Get the boiler make, model and any fault code ready — and a photo of the display helps if you contact us on WhatsApp.