No Hot Water But Heating Works: Causes and Fixes for London Homes

Combi boiler: diverter valve fault

The majority of London flats and houses built or refurbished after the mid-1990s have a combination boiler — a combi — which heats both radiators and domestic hot water directly from the gas supply without a separate cylinder. The combi is what makes a loft conversion or a compact Victorian terrace bathroom feasible: no cold water tank in the roof space, no copper cylinder in the airing cupboard.

But that convenience comes with a mechanical dependency. The combi boiler uses a single heat exchanger for both jobs, and a component called the diverter valve (also called a mid-position valve inside the boiler) determines which job it does at any given moment. When you turn on a hot tap, the control board instructs the diverter valve to redirect hot water from the heating circuit to the domestic hot water plate heat exchanger. Radiators pause. Taps flow hot. When the tap is off, the valve flips back to heating mode.

Over 8 to 12 years — the typical service life of the diverter in London's hard water conditions, where Thames Water delivers water at around 320 mg/L of calcium carbonate — limescale and mechanical wear cause the valve to stick. The most common failure mode is it getting stuck in the heating position. The boiler fires, the radiators heat up, but when you run a hot tap the valve cannot switch, so the heat goes to the radiators rather than the hot water. The tap stays cold or barely warm.

Which boilers are most affected?

The two most widely installed combi boilers in London are the Vaillant ecoTec range (particularly the ecoTec Plus 24kW and 28kW) and the Worcester Bosch 24i and 28i. Both appear in tens of thousands of London properties and both have known diverter valve failure rates that peak between years 8 and 14. Baxi Platinum, Ideal Logic, and Potterton Titanium combis are also common in inner-east and south London estates, and share similar failure profiles.

If your combi boiler is over eight years old and you have suddenly lost hot water while the radiators are working perfectly, stop there. The diverter valve is the primary suspect.

Signs the diverter valve has failed

• Heating works normally but hot taps run cold or only reach lukewarm (around 30-35°C rather than 55-60°C).
• The boiler fires when you open a hot tap, but the water does not get properly hot.
• Intermittent hot water — some taps work for a few seconds then go cold.
• Hot water works briefly when the heating has been running but fails quickly.

What the repair involves

Replacing a diverter valve is a standard boiler repair. The engineer will isolate the gas supply, drain down the relevant circuit, remove the valve assembly (usually accessible from the front of the boiler after removing the casing), fit a new valve, refill, repressurise, and test hot water and heating across a full cycle. On a Vaillant ecoTec or Worcester Bosch, the job typically takes 90 minutes to 2.5 hours. For more details on what boiler repair in London involves, see our service page.

System boiler: cylinder and motorised valve faults

Victorian terraces in Hackney, Islington, and Lewisham. Mansion blocks in Kensington, Maida Vale, and Bloomsbury. Ex-council properties across Southwark, Tower Hamlets, and Lambeth. These properties — a substantial portion of London's housing stock — are far more likely to have a system boiler with a separate hot water cylinder rather than a combi boiler.

In this setup, the boiler heats the central heating circuit and also sends hot water through a coil inside a copper cylinder (usually located in an airing cupboard). The cylinder stores 100-210 litres of domestic hot water ready to use. A separate set of components — motorised zone valves, thermostats, and a programmer — controls whether the boiler's heat goes to the radiators, the cylinder, or both.

When heating works but hot water does not in this type of system, the boiler itself is rarely at fault. The fault is almost always upstream of the cylinder.

Motorised zone valve stuck closed

The most common cause. A motorised zone valve (typically a Honeywell V4043 or similar) opens to allow hot water to flow through the cylinder coil when the hot water programme is calling for heat. When the valve's motor or actuator fails — again, limescale and age are the main culprits in London — it can stick closed. The boiler fires for heating (via the separate heating zone valve), the radiators warm up, but no heat reaches the cylinder coil. The stored water stays cold.

You can sometimes hear a motorised zone valve fail: a buzzing or grinding sound from the airing cupboard when the hot water programme should be calling for heat is a strong indicator.

Cylinder thermostat (cylinder stat) failure

The cylinder thermostat is a sensor strapped to the side of the hot water cylinder, typically set to 60°C. It tells the system when the cylinder is up to temperature and when it needs more heat. If the cylinder stat fails in a way that makes it read the cylinder as already hot — even when it is cold — the system never calls for heat to the cylinder. The heating circuit operates fine because the room thermostat and heating programmer control that separately.

Cylinder stats are inexpensive components (£15-£30 for the part), and replacing one is a straightforward job for a Gas Safe engineer.

Programmer or timer fault

If your cylinder programmer has lost its schedule — common after a power cut or a battery failure in older mechanical programmers — it may simply not be calling for hot water at the right times. Check the programmer display: if it is blank or flashing, reset it and re-enter the hot water schedule. Many London rental properties have old Horstmann or Landis & Gyr programmers that lose their settings when the immersion circuit trips. A simple reprogram often resolves the fault with no parts required.

Immersion heater faults

Some London properties — particularly converted houses in Brixton, Stoke Newington, and Peckham, or older mansion block flats — have hot water cylinders with an immersion heater fitted as either the primary or secondary heat source. An immersion heater is an electric element (similar to a kettle element) that sits inside the cylinder and heats the water directly from the electricity supply, with no boiler involvement.

Immersion heater elements fail for the same reason most heating elements fail: scale buildup in London's hard water coats the element, causing localised overheating, and eventually the element burns out. The thermostat on the immersion can also fail, either preventing the element from switching on or — less safely — preventing it from switching off.

Signs the immersion element has failed

• The immersion switch is on but the water remains cold after 90 minutes.
• The circuit breaker for the immersion keeps tripping.
• You can hear the element sparking or making unusual noise when switched on (switch it off immediately if this happens).

Immersion element replacement

Replacing an immersion element requires draining the cylinder, removing the element (usually a 2.25-inch threaded fitting at the top or side of the cylinder), fitting a new element with a fresh sealing washer, refilling, and testing. The work is plumbing rather than gas work, so any competent plumber can carry it out — it does not require a Gas Safe certificate. Typical cost in London in 2026 is £100-£200 including parts and labour.

Thermostatic mixing valve issues

A thermostatic mixing valve (TMV) blends hot water from the cylinder or boiler with cold water to deliver a safe, consistent temperature at taps and showers — typically capped at 48°C at the outlet to comply with building regulations. TMVs are fitted as standard in new-build properties across London and are retrofitted in many older properties during bathroom renovations.

When a TMV fails or becomes limescaled, it can default to a low-temperature position — delivering water that feels barely warm even though the boiler or cylinder is producing water at the correct temperature. This can closely mimic a boiler or diverter valve fault.

How to identify a TMV fault

Check a tap that does not go through the TMV — often the kitchen hot tap is plumbed directly without a TMV. If the kitchen tap runs properly hot but bathroom taps and showers are lukewarm, the TMV is the most likely culprit rather than the boiler.

TMVs can sometimes be restored by flushing them through (following the manufacturer's procedure), but in London's hard water conditions, a heavily limescaled TMV usually needs replacing. The valve itself costs £30-£80; labour adds £60-£120 depending on accessibility.

How to diagnose the fault yourself

Before calling a Gas Safe engineer, working through these steps will help you identify the likely fault category and give the engineer useful information when they arrive — potentially saving diagnostic time.

When to call a Gas Safe engineer

Most of the faults described in this guide require a Gas Safe registered engineer — not because all the work involves gas, but because the boiler components (diverter valve, DHW sensor, PCB, motorised zone valves) require someone familiar with the specific model and the associated safety isolation procedures.

Call an engineer immediately — do not attempt further investigation yourself — in any of these situations:

• You smell gas at any point. Leave the property, do not use electrical switches, call the National Gas Emergency Service on 0800 111 999.
• The boiler is displaying a lockout error code that does not reset after one manual reset attempt.
• There is water leaking from or around the boiler casing.
• The boiler is making a loud banging or kettling sound when firing (this can indicate scale buildup and is a separate issue — see our guide on boiler service cost in London for what a service covers).
• The pressure relief valve is discharging water continuously.

For most no-hot-water situations that are not emergencies — where heating is working and there are no gas smells, leaks, or lockout codes — you can schedule a repair visit within 24-48 hours. If you need an urgent same-day visit, our emergency plumber London service covers all 33 boroughs around the clock.

Repair costs London 2026

Pricing below reflects current London labour rates for Gas Safe registered engineers in 2026. Parts prices vary by boiler manufacturer and whether genuine OEM parts or quality equivalents are used.

Our standard call-out fee is £60, which covers the engineer attending, diagnosing the fault, and providing a fixed quote before starting any work. The call-out fee is waived when the job runs over 2 hours — which most diverter valve, zone valve, and PCB repairs do.

If your boiler is over 12 years old and the repair cost approaches or exceeds £400, it is worth discussing replacement with the engineer. A new Worcester Bosch, Vaillant, or Ideal boiler comes with a 10-year manufacturer guarantee and is eligible for Gas Safe certification on installation. Our boiler service cost guide covers what annual servicing costs and what it includes. If the boiler runs but hot water runs out quickly rather than being absent entirely, see our guide on hot water running out quickly in London.

When a repair is not worth it

A PCB replacement on an older boiler is often the point at which replacement makes more economic sense. PCBs are boiler-specific, can be hard to source for discontinued models, and cost £200-£450 just for the part. If your boiler is a Baxi Duo-tec, an older Vaillant VUW, or a Worcester Bosch 24CDi (pre-2012 models), PCB availability may be limited. An engineer can advise on parts availability before committing to a repair.

Frequently asked questions