Worcester Bosch Boiler: No Hot Water (Causes & Fixes)

A Worcester Bosch Greenstar losing hot water is one of the most common combi complaints we see, and the cause is often something simple. The telling clue is whether your heating still works. If the radiators warm up but the hot taps stay cold, the boiler is firing — so the problem usually sits with the hot water side specifically, not the boiler as a whole.

Below we walk through the likely causes, starting with the things you can check yourself, then the parts that need an engineer. None of the homeowner checks here involve gas, the flue, or removing the boiler casing — and they never should.

Worcester's Greenstar range spans combis (the Greenstar i, CDi and CDi Classic, and the newer Greenstar 4000 and 8000), as well as system and regular boilers.

On a combi, the boiler makes hot water on demand, so a hot-water-only fault usually points to the burner, the diverter or a sensor inside the appliance.

On a system or regular boiler the hot water comes from a separate cylinder, so the cause can sit outside the boiler entirely (see the section on external valves below).

Common causes of no hot water on a Greenstar

• A frozen condensate pipe — in a cold snap the external condensate pipe can freeze and block, shutting the boiler down. This is the single most common winter cause and one of the few you can safely fix yourself (see below).
• Low system pressure — if pressure drops below about 1 bar, the boiler may lock out and refuse to heat water or radiators.
• A stuck diverter valve — the valve that switches flow between heating and hot water can stick on the heating setting, so you get warm radiators but cold taps.
• Air locks in the system — trapped air can stop water circulating properly, especially after a refill or repair.
• A faulty DHW (hot water) flow sensor or thermistor — if the boiler can't sense that a tap has opened, it won't fire for hot water.
• Ignition / no-ignition — if the boiler isn't lighting at all you may see a flame-failure code (such as EA). This is strictly an engineer job — never touch the gas valve, ignition leads or anything on the combustion side.
• Limescale or sludge — scale in the heat exchanger (common in hard-water areas) reduces hot-water performance over time.
• The preheat / Eco-Comfort setting — on many Greenstar models this controls how quickly hot water arrives; it's easy to knock out of the setting you expect.
• A fault code on the display — Greenstar models show codes such as EA, E9 or A1 that point straight to the issue (see the fault-code table below).

Safe checks you can do yourself

1. Thaw a frozen condensate pipe (winter)

If it's been cold and the boiler has stopped, a frozen condensate pipe is the most likely cause and the easiest to fix. The condensate pipe carries acidic waste water from the boiler to a drain, and where it runs outside it can freeze and block in icy weather.

The tell-tale signs are a gurgling noise from the boiler, a stoppage after a sudden cold snap, and on a digital display an EA fault code. Worcester Bosch confirms these as the typical symptoms.

The pipe is usually a white or grey plastic pipe (commonly around 32mm on the external run) running down an outside wall to a drain. It tends to freeze at the most exposed point — the open end, a bend or elbow, or a dip where condensate collects. To thaw it safely, Worcester Bosch's own guidance is to:

• Pour warm — never boiling — water along the outside of the pipe, close to the likely blockage. Boiling water can crack the pipe or scald you.
• Or hold a hot water bottle, a microwaveable heating pack or a cloth soaked in hot water against the pipe near the blockage.
• Take care carrying hot water if the ground is icy.

Once the pipe is clear, reset the boiler once and give it a couple of minutes to restart (see the reset step below). If it freezes again, an engineer can re-route or insulate the pipe to stop it recurring. Our guide on a frozen condensate pipe covers the method in more detail.

2. Check the pressure gauge

Find the pressure gauge on the front of the boiler (or the digital pressure reading on newer models). Cold, it should sit at roughly 1 to 1.5 bar, rising towards about 2 bar when hot. If it's below 1 bar, low pressure may be the cause.

You can usually top it up yourself using the filling loop — the braided silver hose underneath the boiler — opening both valves slowly until the gauge reaches around 1.5 bar, then closing them again. Your Worcester manual shows the exact filling loop for your model.

3. Check the comfort / preheat setting

On Greenstar combis the hot-water "comfort" or preheat control (often shown as Eco / Comfort, or a tap symbol on the control knob) affects how fast and how reliably hot water reaches the taps.

If someone has changed it — or it reset after a power cut — switching it back to your usual position can restore normal hot water. This is a front-panel control only; it's perfectly safe to adjust.

4. Check the power and the programmer

Make sure the boiler has power and that any wireless thermostat or programmer hasn't run flat or dropped its settings. A simple flat thermostat battery or a tripped fuse can stop the boiler responding. Replacing a thermostat battery or checking a fuse is homeowner-safe.

5. Bleed the radiators if you suspect an air lock

If circulation seems poor — for example the boiler fires but heat doesn't move around the system, or you hear gurgling in the radiators — there may be trapped air. Bleeding the radiators is a homeowner-safe job; our guide on how to bleed a radiator walks through the method.

Keep an eye on the pressure gauge afterwards, as bleeding can drop the pressure and you may need a small top-up. Anything on the sealed side of the system — the pump, the internal pipework or the expansion vessel — is an engineer's job, not a DIY one.

6. Try one Worcester reset

If there's a fault code showing, reset the boiler once. On most Greenstar models you press and hold the reset button for about 3 seconds (some Greenstar i models need around 5 seconds) until the display clears, then wait a couple of minutes for the boiler to restart.

A one-off lockout will often clear. Our guide on how to reset a Worcester Bosch boiler walks through this step by step. Reset only once. If the fault returns straight away, stop — don't keep pressing reset, and book a Gas Safe registered engineer.

When you need a Gas Safe registered engineer

If the safe checks above don't fix it, the cause is likely internal — and these are not DIY jobs. Anything involving the gas valve, the sealed combustion circuit, the flue, or removing the casing must be done by a Gas Safe registered engineer. Book one for:

• A stuck or failed diverter valve — heating works but hot water doesn't, with normal pressure and the right comfort setting.
• A faulty DHW sensor or thermistor — the boiler doesn't fire when you open a hot tap.
• Limescale or a partially blocked heat exchanger, which may need a power flush or descale.
• An ignition or no-ignition fault — anything on the gas valve, ignition leads, fan or combustion side is strictly engineer-only.
• A fault code that won't clear after one reset.

You can confirm any engineer is qualified at the Gas Safe Register. If you ever smell gas, leave the property and call the National Gas Emergency line on 0800 111 999.

Symptom	Likely cause	Who fixes it
Cold taps and cold radiators, low gauge	Low pressure / lockout	You can top up; engineer if it recurs
Warm radiators, cold taps	Diverter valve or comfort setting	Check setting; engineer for the valve
No hot water when tap opens	DHW flow sensor / thermistor	Gas Safe engineer
Weak hot water, hard-water area	Limescale / scaling	Gas Safe engineer
Fault code that won't clear	Internal fault	Gas Safe engineer

System and regular boilers: the external diverter or zone valve

If you have a Greenstar system or regular (heat-only) boiler rather than a combi, your hot water comes from a stored cylinder, and the flow between heating and hot water is controlled by an external motorised valve wired into the heating system — typically a mid-position valve on a Y-plan setup, or two zone valves on an S-plan setup.

These valves sit on the pipework near the cylinder, not inside the boiler.

When one of these external valves or its actuator (the motorised head) fails or sticks, you can get heating but no hot water — or hot water but no heating — even though the boiler itself is fine. This is a different part from the internal diverter valve in a combi.

Replacing the actuator is usually a plumber/heating-engineer job rather than gas work, but anything that involves the boiler's gas or combustion side still needs a Gas Safe registered engineer.

If you have a system or regular boiler and your stored hot water has gone cold, mention the motorised valve when you book the visit.

Worcester Greenstar fault codes for no hot water

Greenstar boilers show a short code on the display when they fault. The codes below are the ones most likely to appear when hot water stops — with what they typically mean and who should deal with them.

Codes can vary slightly between models, so always check the meaning against your own boiler's manual or the Worcester Bosch error code guide. Verify anything safety-related on the Worcester Bosch website before acting.

Code	Typically means	What to do
EA	Flame not detected (no ignition). Common causes are a frozen condensate pipe, the gas being off, or a blocked flue.	Check the gas is on and thaw the condensate pipe if frozen, then reset once. If it returns, Gas Safe engineer.
E9	Overheat — the safety temperature limiter on the heating flow has tripped, often from poor circulation.	Check pressure and bleed radiators. If it recurs, Gas Safe engineer (pump/sensor).
A1	Poor circulation / pump or low-pressure related — water isn't moving round the system as expected.	Check pressure is 1–1.5 bar cold and bleed air. If unresolved, engineer.
F0	Internal fault detected by the boiler's control board (PCB).	Reset once. If it returns, Gas Safe engineer — internal/electronics.
Low-pressure warning	System pressure has dropped below the safe minimum (the boiler may show a low-pressure symbol or a model-specific code).	Top up via the filling loop to ~1.5 bar. If it keeps dropping, investigate for a leak.

How boiler cover helps

Diverter valves, flow sensors and heat exchangers can be among the pricier combi repairs once you add parts and labour. A boiler cover policy spreads that cost into a fixed monthly amount and gives you a number to call when the hot water goes.

If you're weighing it up, our guide on whether boiler cover is worth it sets out the trade-offs, and you can compare boiler cover from our selected panel to see what's included. Check the excess and any limescale or pre-existing fault exclusions before you buy.

For the wider picture beyond just Worcester models, see our general guide to no hot water from a boiler, which covers system and heat-only boilers too.

FAQ

This article is general information, not advice, and reflects a selected panel of providers rather than the whole market. Prices and ranges are indicative for 2026. Always use a Gas Safe registered engineer for gas work.