Worcester Boiler No Hot Water: Causes and What to Check

First, is it a combi or does it feed a cylinder?

Worcester Bosch make both, and the same symptom points at different parts depending on which one is on your wall. A combi heats water on demand as it passes through the boiler, so there is no hot water tank anywhere in the property. A system or regular boiler heats a stored cylinder, usually in an airing cupboard, and the taps draw from that.

The quickest test is to look for a cylinder. If you have a tall insulated tank in a cupboard, you are not on a combi, whatever the boiler looks like. If there is no tank anywhere, you almost certainly are. Do not go by the letters in the model name alone, because Worcester has reused similar suffixes across combi and system ranges over the years; the manual or the data badge on the appliance states the boiler type plainly.

This matters because a combi with no hot water is a boiler repair, and a system boiler with no hot water is very often a controls or cylinder repair with nothing wrong inside the boiler at all.

Is the boiler showing a fault code or a flashing light?

If the display shows a code or the blue indicator is flashing, the boiler has stopped itself for a reason and hot water is simply one of the things it has stopped doing. Write the code down before you touch anything. It is the most useful thing you can give an engineer, and a reset clears it from the screen.

You can press reset once or twice. If the boiler fires and stays running, watch it over the next day. If it locks out again, stop resetting. A boiler that keeps locking out is protecting itself from something, usually an ignition, gas supply, pressure or overheat problem, and repeatedly overriding it does not cure the cause. Our guide to a Worcester boiler flashing blue light goes through what the common lockouts mean.

The checks worth making before you call anyone

A good share of no hot water calls end with one of these, so they are worth two minutes of your time before you book anybody.

• Look at the pressure gauge. Below about 1 bar with the boiler cold, many Greenstar models will refuse to fire at all. Topping up through the filling loop is covered in the manual for your model.
• Light a gas hob, if you have one. No gas at the hob means a supply or prepayment meter issue, not a boiler fault.
• Check the hot water temperature control on the boiler has not been turned down or off. On a combi, a knob or button set to minimum gives you water that barely feels warm.
• On a system or regular boiler, check the programmer is calling for hot water, not just heating. They are separate channels, and a power cut can reset the clock.
• In freezing weather, look at the white plastic condensate pipe where it leaves the house. If it has frozen, the boiler locks out and will run again once it is thawed with warm, not boiling, water.
• Run more than one hot tap. Hot at the bath but not the kitchen means a tap or valve problem, not the boiler.

Heating works but no hot water on a Worcester combi

This is the classic Greenstar combi fault, and it almost always sits in one of three places. The diverter valve is the part that switches the heat from the radiators to the hot water when you open a tap. When it sticks in the heating position, the radiators stay hot and the taps run cold. When it sticks halfway, you get warm radiators in summer and lukewarm water at the tap.

The second place is the flow sensor, sometimes called the turbine or flow switch, which tells the boiler that a tap is open. If it does not register the flow, the boiler never knows you want hot water. A common tell is that the boiler stays silent when you open a hot tap fully but fires for the heating as normal.

The third is the plate heat exchanger, the small block where the heating water hands its heat to the tap water. London water is hard, and scale builds up on that plate until it can barely pass heat. The symptom is water that starts warm and never gets properly hot, or a boiler that fires and cuts out repeatedly while the tap runs. None of these three is a homeowner repair, but all are ordinary parts, and a combi with one of them failed is usually worth repairing. The wider picture is in our guide to no hot water from a combi boiler.

No hot water from a Worcester system or regular boiler

With a cylinder in the picture, the boiler is often working perfectly and never being asked to heat the water. The request travels from the programmer to the cylinder thermostat, then to a motorised valve on the pipework that opens a route to the cylinder coil, and the valve tells the boiler to fire. Any break in that chain gives you heating without hot water.

The motorised valve is the usual suspect. Its motor or its internal switch fails with age, and it either stops opening or stops telling the boiler it has opened. The cylinder thermostat, a small dial strapped to the side of the cylinder, can fail or be knocked to a low setting. And if the cylinder also has an immersion heater, switching it on is a quick test: hot water from the immersion means the cylinder is fine and the fault is on the boiler side of the system.

If you have a mains pressure unvented cylinder, as many London flats and newer houses do, the no hot water but heating works guide goes into the valve and thermostat checks in more detail.

What an engineer does on the visit

We start with whatever the boiler is reporting about itself, then work through the hot water circuit in order rather than swapping parts on a guess. On a combi that means confirming the flow sensor registers a tap opening, watching whether the diverter moves across, and reading the temperatures either side of the plate heat exchanger. On a system boiler it means testing the call for heat at the wiring centre, checking the motorised valve opens and switches, and proving the cylinder thermostat.

Most of these visits end in a repair rather than a replacement. Replacement becomes the honest recommendation when the boiler is old, parts are hard to get, or the same part keeps failing because scale or sludge has not been dealt with. A boiler diagnosis visit is how that question gets answered with evidence rather than a sales pitch.

Anything that involves taking the case off a gas boiler is gas work and must be done by a Gas Safe registered engineer. That includes the diverter valve, the flow sensor and the plate heat exchanger. Check the card of whoever turns up.

When to stop and call

Call an engineer if the boiler locks out again after a reset, if the pressure keeps dropping after you have topped it up, if water is dripping from the boiler or the pipe underneath it, or if you have made all the checks above and the taps still run cold. If you smell gas, do not reset anything: turn the gas off at the meter, open windows and call the gas emergency line on 0800 111 999.

We have 20 engineers covering London and hot water faults are the core of what we do, so in most cases we can get someone out the same day. Call us or send an enquiry with the model name and any code on the display, and our rate is set out on the pricing page before anyone attends.