Worcester Boiler No Hot Water? Common Causes and Fixes

If your Worcester Bosch boiler is giving no hot water, the first question is whether the heating still works. If the heating is dead too, the boiler has probably locked out, so check the display for a fault code, check the pressure gauge, and in cold weather check the condensate pipe. If the heating works but the taps run cold, the boiler is lighting fine and the fault is on the hot water side. On a Worcester combi that usually means the diverter valve, the flow sensor that detects a tap being opened, or a plate heat exchanger clogged with limescale.

First, Is the Heating Still Working?

This single check splits the problem in two:

• No heating and no hot water: the boiler is not running at all. Look for a flashing light or code on the display. Our guide to the Worcester flashing blue light covers how to read it and when a reset is safe.
• Heating works, no hot water: the burner, gas supply and pump are all doing their job. The fault is in how the boiler switches to hot water.

The rest of this guide focuses on Worcester Bosch combi boilers, which heat water on demand. If you have a Worcester system or regular boiler with a hot water cylinder, skip to the cylinder section below.

Checks You Can Do Yourself

• Hot water dial or setting. Make sure the hot water temperature has not been turned right down. On models with a separate hot water on/off control, check it is on.
• Pressure. The gauge should read about 1 to 1.5 bar with the system cold. Low pressure normally shows as a CE1 or similar code and stops the boiler entirely. Our guide to repressurising a boiler walks through topping up.
• Every tap, or one tap? If only one tap or shower runs cold, the problem is probably that tap, mixer valve or shower cartridge, not the boiler.
• Flow rate. Combis need a minimum flow before they fire for hot water. If a tap is only slightly open, or the mains pressure to the house has dropped, the boiler may not detect demand. Open a hot tap fully and wait 30 seconds.
• Eco or preheat setting. Some Worcester models have an eco mode that switches off preheating. The water still gets hot, but it takes noticeably longer to arrive. That is a delay, not a fault.

The Diverter Valve

A Worcester combi uses a diverter valve to switch the boiler's heat from the radiators to the hot water side when you open a tap. If the valve sticks in the heating position, or its actuator fails, the radiators carry on working while the taps run cold. It is the most common reason for heating without hot water on any combi, Worcester included, and we explain the part in what a diverter valve does.

A related sign is hot water that comes through but the radiators also heat up whenever a tap is running. That suggests the valve is not moving fully across.

The Flow Sensor

For the boiler to switch to hot water, it has to know you have opened a tap. Worcester combis use a flow sensor on the incoming cold water for this. If the sensor fails or clogs with debris, the boiler never registers the demand and never fires for hot water. The tap runs, but cold. Sometimes the fault is intermittent, and the hot water works on a full flow but not on a gentle one.

A Scaled Plate Heat Exchanger

London's water is hard, and limescale builds up in the plate heat exchanger, the part where heat passes from the boiler into your tap water. As it scales up, the hot water becomes lukewarm, fluctuates between hot and cold, or takes longer to heat. If your water runs cold, then hot, then cold again, our article on hot water that runs cold then hot covers the pattern in detail. Depending on the condition, the exchanger can be descaled or replaced.

If You Have a Worcester System or Regular Boiler

With a hot water cylinder, the boiler heats water in the cylinder rather than on demand, so the causes are different. Check the programmer is set to give hot water at this time of day, and check the cylinder thermostat on the side of the tank has not been turned down. If both are fine, a failed motorised valve, which is often in the airing cupboard, is the most likely fault. If you have an immersion heater as backup, switching it on will give you hot water while you wait for an engineer.

When It Needs an Engineer

The diverter valve, flow sensor and plate heat exchanger are all inside the boiler casing, so diagnosis and repair are work for a Gas Safe registered engineer. A good diagnosis tests them in order rather than swapping parts until something works. If you tell us the model (it is printed on the front or under the front cover) and whether the heating still works, we can usually narrow it down before we arrive.

You can book a diagnostic visit, or see our no hot water repair service for London. For the same fault on other makes, see Ideal boiler not giving hot water and our general guide to no hot water but the heating still works.