When radiators only heat up while the hot water is running, but stay cold when the heating is called for on its own, the system is failing to direct hot water to the correct part of the circuit. This is a specific and well-understood fault in UK heating systems, and it almost always points to one of two components depending on the type of boiler installed. Understanding how heating and hot water share the same boiler makes the cause immediately clear and points directly to the appropriate fix.
If the heating has been unreliable in other ways alongside this symptom, or if some radiators are working while others are not, the house cold diagnostic will help establish whether the diverter or zone valve fault is the only issue or whether circulation and balance problems are also present.
How UK boilers divide heat between radiators and hot water
Most UK homes with a system boiler or conventional boiler and a hot water cylinder use a motorised zone valve to direct hot water from the boiler to either the radiators, the hot water cylinder, or both simultaneously depending on which demand is active. The most common configuration is a three-port mid-position valve, often called a Y-plan system, where a single motorised valve has three positions: radiators only, hot water only, or both together. When the programmer calls for heating, the valve motor moves to the heating position. When hot water is called for, it moves to the hot water position. When both are called for simultaneously, it sits in the mid-position and supplies both.
When this valve fails, it typically fails in one position. In the majority of cases it sticks in the hot water position, which means the boiler fires when hot water is called for and hot water circulates normally, but the valve cannot move to supply the radiator circuit. The symptom that results is exactly what many homeowners describe: the hot water works perfectly, the radiators do nothing when heating alone is called for, but if hot water and heating are called for simultaneously the radiators warm up because the valve is already open and the boiler is running.
The three-port valve: the most common cause in system and conventional boiler homes
The three-port valve is typically located in the airing cupboard next to the hot water cylinder, connected to the pipework by three pipe connections. The motorised head sits on top of the valve body and contains the motor and actuator that physically moves the valve between positions. When the motor weakens, the actuator gears wear, or the valve body becomes stiff with age, the valve can no longer complete the movement to the heating position reliably. It may work occasionally, or only when the motor warms up after a period of running, or it may have stuck permanently.
A straightforward first check is to listen to the valve when heating is called for independently of hot water. A functioning valve motor produces a faint hum and the valve body moves slightly as the actuator operates. A failing valve may produce a clicking or buzzing sound as the motor tries but fails to move the valve, or may be completely silent if the motor has failed entirely. Feeling the pipework connections while heating is called for is also informative: in a healthy system the pipe leading to the radiator circuit becomes warm when heating is active. If only the pipe leading to the hot water cylinder warms, the valve is stuck on the hot water side.
Replacing the motorised head on a three-port valve is a manageable task that does not require draining the system. The head is typically secured by two screws and an electrical connector and can be replaced without disturbing the valve body or the pipework. If the valve body itself has become stiff or corroded internally, the entire valve needs replacing, which does require the system to be drained and is a job for a plumber or heating engineer.
The diverter valve: the equivalent fault in combi boilers
Combi boilers do not use a separate external zone valve. Instead they contain a diverter valve within the boiler casing that performs the same function: directing hot water to the central heating circuit or the domestic hot water heat exchanger depending on which demand is active. In a combi boiler, domestic hot water takes priority by design, so when a tap is opened the diverter valve switches to the hot water position and the heating is temporarily suspended.
When the diverter valve in a combi boiler fails, it most commonly sticks in the hot water position. The result is that the boiler fires normally when a hot tap is run but cannot direct water to the heating circuit when the programmer calls for radiators alone. Radiators may warm slightly when the hot tap is running because some heat bleeds through the stuck valve, but they do not heat properly under a standalone heating demand.
Diverter valves in combi boilers switch positions many times daily throughout a normal heating season, which makes them one of the more wear-prone components in the system. Replacing a combi diverter valve requires access to the boiler internals and is a job for a Gas Safe registered engineer. The part itself is relatively inexpensive but the labour cost reflects the technical access required. If the boiler is older and the diverter valve has failed, it is worth asking the engineer whether other internal components are showing similar wear before deciding whether repair or replacement is the better decision.
When the thermostat or wiring is causing the same symptom
A room thermostat that is not sending a demand signal to the boiler produces a symptom that looks identical to a stuck zone valve: the heating does not come on when called for, but if the hot water programmer activates the boiler fires and radiators may warm incidentally. The difference is that in a thermostat fault the valve itself is functioning correctly but the boiler is not receiving the instruction to open the heating circuit.
Checking thermostat behaviour is straightforward. Turn the thermostat to its maximum setting and listen for the boiler to fire within a minute or two. If the boiler fires and radiators begin to warm when the thermostat is at maximum but not at normal settings, the thermostat is not reaching the boiler at its working temperature, either because the battery is low, because the thermostat is positioned in a location that reads warmer than the rest of the house, or because the wiring between the thermostat and the programmer has a fault. Replacing the battery is the first check. If the problem persists with a fresh battery, the thermostat placement or wiring is worth investigating before condemning the zone valve.
A programmer that has lost its schedule, or whose heating and hot water channels have been accidentally swapped, can produce the same apparent symptom. Checking that the programmer is set correctly and that the heating channel is active for the expected periods is worth confirming before any investigation of the valve or thermostat.
When circulation problems make the symptom worse
In some cases the zone valve or diverter valve is partially functional rather than completely stuck, and the symptom of heating only working when hot water is on is produced by the interaction between a marginal valve fault and poor system circulation. The valve opens partially for heating, allowing some flow to the radiator circuit, but that flow is so weak that radiators barely respond. When hot water is called for simultaneously, the boiler runs at higher output and for longer, and the additional circulation is enough to push some heat through to the radiators despite the valve only partially opening.
In this scenario, fixing the valve may reveal that the system also has circulation problems that were previously masked. Radiators that had appeared to work when hot water was running may now be found to be slow or uneven when the system is running properly. Checking for trapped air, confirming system pressure is within the normal range, and assessing whether the system needs balancing after the valve repair addresses these underlying issues. Bleeding and balancing after any significant valve or control work is covered in how to balance radiators, and if specific radiators remain slow after the valve is fixed, why radiators take so long to heat up covers the residual circulation causes.
Why this fault increases running costs if left unfixed
A heating system operating with a stuck or failing zone valve runs inefficiently in several ways. The boiler fires when hot water is called for regardless of whether heating is also needed, and any incidental warming of radiators during hot water cycles is uncontrolled and wasteful. The boiler cannot be scheduled to run heating and hot water independently, so the homeowner typically runs both together to get any radiator output at all, heating the hot water cylinder more frequently than necessary and increasing fuel consumption in the process.
Fixing the valve restores independent control of heating and hot water, allows the programmer and thermostat to function as intended, and typically produces a noticeable reduction in how long the boiler runs to maintain both comfort and hot water. How boiler control efficiency connects to the broader picture of home running costs is covered in the complete guide to keeping a UK home warm for cheap.
Start with the simplest checks: thermostat battery and settings, programmer schedule, and listening to the zone valve when heating is called for independently. These confirm whether the fault is in the control circuit or the valve itself before any component is replaced.