Short answer: If the heating works but there is no hot water, a combi may have a stuck diverter valve, low flow or a scaled heat exchanger. With a cylinder, the cause is often a failed immersion heater, thermostat or motorised valve. Check settings and the immersion first. Any work on the gas boiler itself needs a Gas Safe registered engineer.
Warm radiators and cold hot taps is a confusing combination. The boiler is clearly working, so why is there no hot water? The answer depends on what kind of system you have. This guide explains the water side causes a plumber can deal with, and where you need a Gas Safe registered engineer instead.
Which system do you have?
| System | How to recognise it | How hot water is made |
|---|---|---|
| Combi boiler | No hot water cylinder, boiler on the kitchen wall or in a cupboard | Heated on demand as you open a hot tap |
| System or regular boiler with cylinder | A hot water cylinder in an airing cupboard | Stored in the cylinder, heated by the boiler and often an immersion heater |
| Electric only | Cylinder with immersion heaters, no gas boiler | Heated by electricity |
| Communal system | No boiler in the flat, a heat unit or cylinder fed from the building | Heat comes from a central plant room |
If you are in a block with a communal system, our guide to no hot water in a mansion block is a better place to start.
Combi boilers: the common causes
Low water flow
A combi needs a certain flow of cold water before it fires up for hot water. If flow is low, the boiler may not switch on at all, or may give lukewarm water. Low flow can come from a part closed stop tap, a blocked filter on the cold inlet, or low mains pressure. Our guide to low water pressure in a flat covers that in more detail.
The diverter valve
Inside a combi, a diverter valve switches the heat between the radiators and the hot water. If it sticks in the heating position, the radiators get hot and the taps stay cold. This is part of the boiler. A diverter valve repair should be done by a Gas Safe registered engineer, because it means opening the boiler.
Limescale
Thames Water says all the water in its region is hard. In a combi, hard water passes through a small plate heat exchanger every time you run a hot tap. Over the years, scale builds up inside it. Signs are hot water that goes hot and cold, a boiler that cuts out on hot water, or hot water that takes longer to come through. Thames Water suggests fitting an electric descaler or softener on appliances to reduce scale, and recommends any softening unit is inspected regularly by an approved plumber.
Cylinder systems: the common causes
The immersion heater
Many cylinders have an immersion heater as well as being heated by the boiler. If the boiler side has stopped heating the water, switching the immersion on can give you hot water while the cause is found. If the immersion also does nothing, the element or its thermostat may have failed.
The cylinder thermostat
A cylinder thermostat tells the boiler when the water is hot enough. If it fails, the boiler may never be asked to heat the water.
The motorised valve
In many systems, a motorised valve opens to send hot boiler water to the cylinder. If it sticks shut, the radiators work but the cylinder stays cold. You may see a small lever on the valve head, which can be used to open it manually as a test.
Programmer settings
It sounds obvious, but check the hot water timer and programmer first. After a power cut or a clock change, the hot water channel may be off or set to the wrong times.
Wandsworth houses and conversions
In Wandsworth, 22.2% of households live in terraced houses and 17.7% in converted houses, according to the 2021 Census (Nomis, TS044). Balham, Tooting and the streets between the commons are full of Victorian terraces where cylinders still sit in airing cupboards, sometimes fed by a cold water tank in the loft. In the conversions, many flats have had combis fitted on kitchen walls instead.
That mix means both sets of causes come up. In older cylinder systems, motorised valves, cylinder thermostats and immersion heaters are common faults. In combi flats, low flow from an old shared supply pipe and scale in the heat exchanger are the usual suspects.
What a plumber can do, and what needs a Gas Safe engineer
| Job | Who |
|---|---|
| Checking stop taps, valves and cold water flow | Plumber |
| Immersion heater element or thermostat | Plumber or electrician |
| Motorised valve on the pipework | Plumber, if it does not involve the boiler |
| Cylinder faults and leaks | Plumber |
| Anything inside a gas boiler, including diverter valves and heat exchangers | Gas Safe registered engineer |
Gas work in the UK must be done by a Gas Safe registered engineer. Ask to see their ID card and check it on the Gas Safe Register. If you ever smell gas, stop and call 0800 111 999.
If you rent
Under section 11 of the Landlord and Tenant Act 1985, landlords must keep the installations for heating water in repair and proper working order. Report no hot water to your landlord in writing. Our no water or no hot water page explains what a plumber will check on a visit.
Quick questions
Is no hot water an emergency?
Usually not on its own, unless someone vulnerable needs it or it comes with a leak. It is worth fixing soon, especially in winter.
Why does my combi give hot water only sometimes?
Low flow, a sticking diverter valve or scale in the hot water side are common causes. A plumber can check flow and pressure. Gas side faults need a Gas Safe engineer.
Can I use the immersion heater if the boiler is not heating water?
If you have a cylinder with an immersion heater, switching it on can give you hot water while the boiler side is looked at.
Last updated 23 October 2026. Written for homes in London. Rules can change, so check the linked sources for the latest position.