Your Worcester Greenstar still heats the radiators but the taps run cold? Here are the usual causes, the checks you can safely do yourself, and the points where you need a Gas Safe registered engineer.
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.
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.
Pressure keeps dropping? A loop that needs topping up repeatedly usually means a leak somewhere in the system. Don't keep refilling indefinitely — have it investigated, as repeated overfilling can cause its own problems.
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.
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.
If there's a fault code showing, press the reset button on the front of the boiler once and wait a minute. A one-off lockout will often clear. Our guide on how to reset a Worcester Bosch boiler walks through this step by step. If the fault returns immediately, stop — don't keep pressing reset, and book an 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:
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 |
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.
See what a cover plan would include for your Worcester Bosch boiler, from a selected panel of UK providers.
Compare boiler coverThis pattern usually points to the diverter valve sticking on the heating position, the hot-water comfort/preheat setting being changed, or a faulty DHW sensor. Check the comfort setting first; if that's correct and pressure is normal, book a Gas Safe registered engineer to check the diverter valve.
No. The diverter valve sits inside the sealed boiler and reaching it means removing the casing, which is gas work. It must be handled by a Gas Safe registered engineer.
Around 1 to 1.5 bar when cold, rising to roughly 2 bar when hot. Below 1 bar is low and can cause a lockout. You can top it up via the filling loop, but if it keeps dropping you likely have a leak that needs investigating.
It can be, especially in hard-water areas and if the hot water has weakened gradually rather than stopped suddenly. Scale builds up in the heat exchanger over time. A Gas Safe registered engineer can assess whether a flush or descale is needed.
Once. If the fault clears, great. If it returns straight away, stop resetting and call an engineer — repeated resets won't fix an underlying fault and can mask it.
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.