Radiators are warm, the heating’s clearly firing, but you turn the shower on and get nothing but cold. It’s an odd one, and it trips people up because it feels backwards. Surely if the boiler’s working, hot water should just… happen?
No hot water, but heating works is one of the most common combi boilers faults we get called out for across Hackney, Islington and Archway, and the good news is it’s almost never as serious as a boiler that’s stopped working completely. Here’s what’s going on, what’s worth checking yourself before anyone comes out, and what tends to need an engineer.
If you’re dealing with the opposite problem instead (heating not working at all, or the system losing pressure repeatedly), our guides on boiler losing pressure and boiler making a banging noise cover those specifically.
The Most Likely Culprit: A Stuck Diverter Valve
If you’ve got a combi boiler and no hot water, but heating works, this is where to start. Inside the boiler there’s a diverter valve, and its whole job is deciding where the heat goes: to your radiators, or to your hot water. When you turn a tap on, the valve is supposed to switch over and send heat that way instead.
When it sticks in the heating position, which is genuinely one of the most common combi boilers faults we see across London, your radiators carry on working exactly as normal while your taps stay stone cold. It’s not something you can fix yourself; it means draining part of the system and working on internal boiler components, so this one’s an engineer job. But it’s also a well understood, usually straightforward repair, not a sign you need a new boiler. If yours is also flashing a fault code alongside the cold taps, our boiler error codes guide will help you work out what it’s telling you before the engineer arrives.
Before You Call Anyone, Check These Three Things
A decent chunk of “no hot water” calls turns out to be something you can sort in five minutes. Worth ruling these out first:
Boiler pressure. Look at the gauge on the front of the boiler. It should sit somewhere around 1 to 1.5 bar. If it’s dropped below 1, low pressure can stop the boiler heating water properly even though the heating side still limps along. Topping it up via the filling loop (check your boiler’s manual for the exact steps) sometimes clears the problem entirely.
Hot water settings. Sounds obvious, but it’s more common than you’d think: someone’s turned the hot water schedule off on the thermostat or programmer, often without realising, especially after a power cut resets things to default. Check the hot water setting is actually switched on before assuming there’s a fault.
Is it every tap, or just one? If only your kitchen tap runs cold but the bathroom’s fine, the problem isn’t your boiler at all, it’s that specific tap or its cartridge. That’s a plumbing fix, not a boiler one, and a much smaller job.
If you’ve checked all three and you’re still getting nothing, it’s time to call someone.
Other Causes, Beyond the Diverter Valve
An airlock. Trapped air in the pipework can block water flow to your taps without affecting the radiator circuit the same way. Usually shows up as spluttering taps or hot water that starts then cuts out.
A frozen condensate pipe. More of a winter issue, but worth knowing in a hard freeze, the pipe that carries condensation away from the boiler can freeze solid and trigger a shutdown, sometimes on the hot water side specifically. If it’s genuinely freezing outside and this has happened suddenly, this is worth ruling out before anything more complicated.
A failing PCB or thermostat. Less common, but the boiler’s internal controls can develop a fault that stops it firing for hot water specifically while heating carries on working. This one need proper diagnosis rather than a guess.
If you’ve got a hot water cylinder, not a combi, the cause list shifts slightly: a faulty cylinder thermostat, a failed immersion element, or a motorised valve that’s supposed to direct flow to the cylinder but isn’t. If you’ve got an unvented cylinder specifically, this needs a G3-certified engineer, not just a general Gas Safe one; we’ve written more on that in our unvented cylinder cost and repair guide.
Is This an Emergency, or Can It Wait?
Genuinely, it depends on your situation more than the fault itself. No hot water overnight in a house with kids or anyone vulnerable is a fair reason to want it sorted the same day. If it’s just you and it’s not urgent, a next-day appointment is usually fine, and you’ll often pay less for it than an out-of-hour call out.
Either way, it’s rarely a “turn the gas off and evacuate” situation. The exception is if you smell gas, hear the boiler banging or making unusual noises, or notice scorching anywhere near it: that’s not a “wait and see” fault, that’s turn it off, ventilate, and call the National Gas Emergency Service on 0800 111 999 straight away.
What an Engineer Actually Does to Diagnose It
Worth knowing before anyone comes out, so it doesn’t feel like guesswork: a proper diagnosis usually starts with checking pressure and settings (ruling out the easy stuff first), then testing whether the diverter valve is switching when a tap’s opened. That alone tells an engineer about most of what they need to know within the first few minutes on site, which is why this fault rarely takes a long time to identify, even if the actual repair takes a bit longer.
Got heating but no hot water in Hackney, Islington or Archway? Call 07888078885 for a same-day diagnosis, or book through our emergency call out form if it can’t wait until morning.
FAQ
Almost always a boiler-side fault rather than anything wrong with your radiators. The most common cause in combi boilers is a stuck diverter valve, which is meant to switch heat between radiators and hot water but has stopped switching properly.
Sometimes. Low pressure, an incorrect hot water schedule, or a single blocked tap are all things you can check and often fix yourself. A diverter valve, PCB, or thermostat fault needs a Gas Safe registered engineer.
It depends on your household. It’s rarely dangerous on its own, but if you’ve got young children, elderly or vulnerable people at home, or it’s happening overnight in genuinely cold weather, it’s reasonable to treat it as urgent and book a same-day visit.
Costs vary depending on the boiler to make and whether the valve needs repairing or replacing outright, but it’s a well understood, common repair rather than an expensive one. An engineer can give you an accurate price once they’ve confirmed that’s actually the fault.
