Why does my heating work but I have no hot water?

On a combi boiler this is the classic sign of a stuck diverter valve. A combi has one heat exchanger and a diverter valve that decides whether the heat goes to your radiators or to the hot-water side when you open a tap. If the diverter sticks in the heating position — usually because London's hard water has furred the brass with limescale, or the internal rubber diaphragm has worn — the radiators keep getting hot but the taps stay cold or only lukewarm. On a system or conventional boiler with a hot-water cylinder, the same symptom instead points to a failed hot-water motorised zone valve, a dead cylinder thermostat, or the hot-water channel of the programmer not calling for heat. Either way it's a specific failed component, not the whole boiler, so it's normally an economic repair.

What is a diverter valve and why does it stick?

The diverter valve is the part inside a combi boiler that routes hot water from the single main heat exchanger either to the radiators (for heating) or to the plate heat exchanger that warms your tap water. Hot water has priority: the moment a flow sensor detects you've opened a hot tap, the diverter swings over to send all the heat to the taps, then swings back when you close the tap. Because it moves constantly and sits in the path of mains water, it's vulnerable to limescale build-up in hard-water areas like much of north and east London, and to wear of its internal rubber seal or its motorised actuator. When it can no longer move freely it jams in one position, which is what causes hot water without heating, or heating without hot water.

My hot water is only lukewarm — is that the diverter valve too?

Not usually. If the hot water still flows but never gets properly hot — or if it runs hot for a minute then goes cold — the more likely culprit on a combi is a scaled-up plate heat exchanger rather than the diverter. The plate heat exchanger is a stack of thin metal plates that transfers heat from the boiler's water into your tap water; in hard-water London it furs up with limescale and loses its ability to transfer heat, so the water comes through under-heated. A diverter fault is more of an all-or-nothing symptom (hot water or none), whereas a scaled heat exchanger gives you weak, fluctuating heat. An engineer can tell them apart in a few minutes on site by checking flow temperatures, and either flush or replace the exchanger as needed.

Can I fix a stuck diverter valve myself?

No — diverter valve work means opening up a gas appliance, draining part of the system and working on sealed water-side components, which is Gas Safe registered work by law. Attempting it yourself is both unsafe and likely to void your boiler warranty. There are a few safe things you can try first that occasionally free a marginally stuck valve or rule out simpler faults: make sure the programmer is calling for hot water, check the boiler pressure is between 1 and 1.5 bar when cold, turn the hot-water and cylinder thermostats up, and reset the boiler once using its reset button. If hot water doesn't return after that, the diverter, heat exchanger or zone valve needs an engineer. Don't keep resetting repeatedly — that won't fix a mechanical valve fault.

How long can a landlord leave a tenant with no hot water in London?

Loss of hot water or heating is treated as an urgent repair. Under Section 11 of the Landlord and Tenant Act 1985 and the Homes (Fitness for Human Habitation) Act 2018, a London landlord must keep the installations for heating and hot water in working order and repair them within a reasonable time — and for a total loss of hot water or heating, especially in cold weather or where vulnerable or elderly tenants or young children are involved, 'reasonable' is generally interpreted as around 24 hours. Most reputable letting agents and managing agents operate to a same-day or next-day standard for no-heat and no-hot-water faults precisely because the legal expectation is so short. A combi diverter or a cylinder zone valve is normally a same-visit repair, so there's rarely a good reason for a long delay.

How much does it cost to fix no hot water in London?

It depends on the part. In London during normal hours, a diverter valve repair or cartridge swap is typically £150–£350, and a full diverter valve replacement £200–£450 including parts and labour. A scaled plate heat exchanger is around £200–£400 to replace, or less if it can be flushed. On a system boiler with a cylinder, a motorised zone valve (S-plan or Y-plan) is about £150–£280, a cylinder thermostat £90–£160, and a heating programmer £120–£250. A diagnostic call-out is £60 weekday daytime or £90 out-of-hours, which is usually rolled into the repair if you go ahead. Out-of-hours rates add roughly 30–50%. An engineer confirms the fixed figure before starting work.

No Hot Water but Heating Works? The London Diverter-Valve and Cylinder Diagnostic

Hot water gone but radiators warm — or the other way round? Here's how a London engineer diagnoses a stuck combi diverter valve, a failed motorised zone valve and a dead cylinder thermostat.

It's one of the most disorientating boiler faults there is: the radiators are warming up nicely, the house feels fine — but you go to run a bath and the tap gushes cold. Or the exact opposite: endless hot water at the taps, yet the radiators stay stone cold no matter how high you push the thermostat. People assume the worst and start pricing up a new boiler. They almost always shouldn't. When heat reaches one part of your system but not the other, the boiler itself is usually working perfectly — what's failed is the single component that decides where the heat goes. Find that component and you've found the repair.

First, Work Out Which Boiler You Have

The diagnosis splits cleanly down one line, so this is the first question any London engineer asks on the phone: do you have a hot-water cylinder, or not?

A combi (combination) boiler makes hot water on demand, straight from the mains, with no storage tank or cylinder anywhere. If you have no airing-cupboard cylinder and no loft tank, you almost certainly have a combi. These dominate London's converted flats and smaller terraces because they save the space a cylinder would take.
A system or conventional (regular) boiler heats a separate hot-water cylinder — the tall insulated copper or steel tank usually in an airing cupboard. Conventional systems also have a cold tank in the loft; sealed "system" boilers and unvented (Megaflo-type) cylinders don't. These are common in larger Victorian and Edwardian houses across north and outer London.

That one distinction changes everything, because a combi and a cylinder system route heat in completely different ways — and so they fail in completely different ways. If you're not sure which you've got, our guide to combi vs system boilers in a London terrace walks through how to tell them apart.

Combi Boilers: The Diverter Valve

A combi has just one main heat exchanger doing two jobs, and a clever bit of engineering decides which job at any moment: the diverter valve. Most of the time the diverter sends the boiler's heat round your radiators. But hot water has absolute priority — the instant a flow sensor detects that you've opened a hot tap, the diverter swings over and channels all the heat to the plate heat exchanger that warms your tap water, pausing the radiators for as long as the tap runs. Close the tap and it swings back. It does this dozens of times a day, every day, for years.

That constant movement, in the path of pressurised mains water, is exactly why the diverter valve is one of the most common combi failures — and why the symptom is so distinctive:

Stuck towards heating → radiators hot, taps cold. The diverter can't swing over to the hot-water side when you open a tap, so the heat never reaches the plate exchanger. Heating looks perfect; hot water is cold or barely warm.
Stuck towards hot water → taps hot, radiators cold. The diverter is jammed on the hot-water path, so calling for heating does nothing — the radiators stay cold even though the boiler fires and the taps are scalding.

What jams it? Three things, in roughly this order across London: limescale furring the brass internals in hard-water areas; a worn rubber diaphragm or seal inside the valve that no longer moves cleanly; and, on motorised types, a failed diverter actuator or micro-switch. The fix is either a diverter repair kit (new cartridge, rubber and seals) or a full valve replacement, depending on the make and how badly it's scaled. It's a contained, same-visit job on most boilers — Worcester Bosch, Vaillant, Ideal and Baxi diverters are all routine parts our engineers carry. A new boiler it is not.

Combi: Lukewarm or Hot-Then-Cold Water

There's a related combi symptom that often gets blamed on the diverter but usually isn't: hot water that flows but never gets properly hot, or runs hot for a minute and then turns cold. That's the signature of a scaled-up plate heat exchanger, not a stuck diverter.

The plate heat exchanger is a compact stack of thin metal plates that transfers heat from the boiler's internal water into your incoming tap water. In hard-water London it slowly furs with limescale until the plates can't pass enough heat across, and the water comes through under-heated. The tell-tale difference: a diverter fault is all-or-nothing (full hot water, or none), while a scaled exchanger gives you weak, fluctuating warmth. An engineer distinguishes them in minutes by reading the flow and return temperatures while a tap runs, then either descales/flushes the exchanger or replaces it. If your home is on a hard-water main, fitting a scale reducer at the same visit slows it happening again — the same limescale problem we cover in why boilers keep losing pressure.

System Boilers: Zone Valves and the Cylinder Stat

If you do have a hot-water cylinder, there's no diverter valve at all — so a combi diagnosis sends you down the wrong path. Here the heating and hot-water circuits are separated by motorised zone valves, and the symptom "heating works but the cylinder's cold" points to that side of the system:

A failed hot-water zone valve. Most London cylinder systems are wired as S-plan (two separate 2-port motorised valves — one for heating, one for hot water) or Y-plan (a single 3-port mid-position valve serving both). When the hot-water valve's actuator head burns out or the valve sticks, hot water from the boiler can't reach the cylinder coil — so the radiators heat normally but the cylinder stays cold. The Honeywell/Drayton actuator heads on these valves are a classic, well-known failure point, and a routine swap.
A dead cylinder thermostat. The cylinder stat is a small sensor strapped to the side of the tank that tells the system when the water's hot enough. If it fails, it may never call for the cylinder to be heated, leaving the water cold while the heating runs fine.
The programmer's hot-water channel. A programmer that's lost its hot-water output (or simply isn't set to come on) will heat the rooms but not the tank. Always worth checking before assuming a valve.

These are heating-controls faults rather than boiler faults, which is why a new boiler would do nothing to fix them. Our central heating service covers zone valves, thermostats and programmer faults across all these older London systems.

The Reverse: Hot Water but No Heating

Flip the symptom and the logic flips with it. Hot water fine, radiators cold on a combi usually means the diverter is jammed on the hot-water side (above), or occasionally that the central-heating pump has seized or the system pressure has dropped too low to circulate. On a cylinder system, it means the heating zone valve has failed, the room thermostat or the heating channel of the programmer isn't calling, or the pump has stopped.

One quick self-check for the cylinder-system case: if some radiators are warm and others stone cold, that's a balancing or sludge problem (a power flush job), not a zone valve — a zone-valve failure kills the whole heating circuit at once, not just a few rads. And any time you've got no heat anywhere in genuinely cold weather, treat it as urgent, especially for elderly or vulnerable occupants.

What to Check Before You Call

None of the actual repairs here are DIY — opening a combi or working on gas-side controls is Gas Safe registered work by law. But there are a handful of safe checks that occasionally rule out a simpler fault or even free a marginally stuck valve, and they save you a call-out if one of them is the answer:

Is the programmer/timer actually calling? Make sure the hot-water (and heating) output is switched on and not sitting in an "off" or timed-out period. It sounds obvious; it's a surprisingly common cause.
Check the boiler pressure. It should read 1–1.5 bar on the gauge when the system is cold. Too low and the boiler may not circulate properly — top it up via the filling loop (see how to repressurise a boiler).
Turn the thermostats up. Push the room thermostat and, on a cylinder system, the cylinder thermostat well above the current temperature, in case a setting has drifted.
Reset the boiler once. A single press of the reset button can clear a lockout. If hot water or heating doesn't return, stop — repeated resetting won't fix a mechanical valve and can mask a fault.

If you've done all four and the symptom persists, it's a diverter, heat exchanger, zone valve or thermostat — and that's the point to call an engineer.

Why This Happens So Often in London

Two London realities make these faults especially common. First, hard water. Much of north, east and outer London — the Lee Valley boroughs like Enfield, Haringey and Waltham Forest in particular — sits on a hard-water supply that lays down heavy limescale. That scale is the number-one killer of combi diverter valves and plate heat exchangers, which is why we see far more of these faults in EN and N postcodes than in the soft-water pockets. Second, system age and mix. London's housing runs from 1930s semis with original Y-plan cylinder systems in the airing cupboard to renovated terraces with brand-new combis crammed into a kitchen cupboard — so the same "no hot water" phone call can mean a £150 actuator head on one street and a diverter cartridge on the next. Getting the diagnosis right before anyone quotes is what separates an honest repair from an unnecessary boiler-replacement sales pitch.

Landlords: Your Repair Obligation

If you let a property in London, loss of hot water or heating is not a "next week" job. Under Section 11 of the Landlord and Tenant Act 1985 and the Homes (Fitness for Human Habitation) Act 2018, you must keep the heating and hot-water installations in working order and repair faults within a reasonable time — and for a total loss of hot water or heating, "reasonable" is generally read as around 24 hours, tighter still in cold weather or where there are elderly tenants, young children or anyone vulnerable. The good news is that the faults in this guide — a diverter valve, a zone valve, a cylinder stat — are almost all same-visit repairs, so meeting that standard is rarely a problem if you call promptly. For managed portfolios we run a planned and reactive maintenance service with a no-heat/no-hot-water priority response.

2026 London Cost Guide

Indicative London prices, normal weekday daytime hours, parts and labour included. Out-of-hours adds roughly 30–50%; the diagnostic fee is usually rolled into the repair if you go ahead.

Diagnostic call-out: £60 weekday daytime, £90 out-of-hours (covers travel + first 45 minutes)
Combi diverter valve repair / cartridge: £150–£350
Combi diverter valve full replacement: £200–£450
Plate heat exchanger (replace; less if flushed): £200–£400
Motorised zone valve — S-plan or Y-plan (actuator or full valve): £150–£280
Cylinder thermostat: £90–£160
Heating programmer / time clock: £120–£250

Against the £2,000–£3,500 cost of a new combi in London, almost every fault in this guide is a fraction of replacement — which is exactly why the diagnosis matters. If an engineer's first move on a "no hot water" call is to quote you a new boiler without checking the diverter, the heat exchanger and the zone valves, get a second opinion. A genuine emergency — a gas smell, water pouring from the boiler, or no heat at all for a vulnerable person in winter — is what our 24/7 boiler repair team is for; everything else is a planned, fixed-price repair.