Hot Water But No Heating? Why Your Radiators Stay Cold

If you have hot water but no heating, the boiler is almost certainly lighting and making heat, because it managed to heat the water at the tap. What has failed is the part of the system that sends heat to the radiators. In order of likelihood that is a control problem (thermostat, programmer or a flat battery), a diverter valve stuck in the hot water position on a combi boiler, a motorised zone valve that will not open on a system or regular boiler, or a circulating pump that is too weak to push water round the radiators. The first group you can check yourself in ten minutes. The rest need an engineer.

Start With the Controls: Five Minutes of Checks

A surprising share of "hot water but no heating" call-outs end at the thermostat. Before you assume a fault, work through these:

• Room thermostat. Turn it up at least three or four degrees above the current room temperature. If it is set to 18°C and the room is already 19°C, the boiler has no reason to heat the radiators.
• Batteries. Wireless and smart thermostats run on batteries or a receiver unit near the boiler. A dead battery or a receiver that has lost its pairing looks exactly like a heating fault. If the receiver has a status light, check it is on.
• Programmer or timer. Many programmers control heating and hot water separately. Check heating is set to on, or that the current time falls inside a heating period. After a power cut the clock may have reset.
• Boiler heating dial. Most combis have two dials, one for hot water and one for heating. If the heating dial has been turned right down or to a summer setting, the radiators will not come on.
• Radiator valves. If just one or two radiators are cold, check their valves are open. If every radiator is cold, the valves are not the cause.

If the controls are all calling for heat and the radiators still stay cold, you are looking at a mechanical fault. Which one depends on your type of boiler. Our guide to telling a combi from a system or regular boiler helps if you are not sure.

Combi Boilers: The Diverter Valve

A combi boiler has one burner and two jobs. A diverter valve inside the casing decides which job it is doing: sending hot water round the radiators, or through the plate heat exchanger to your taps. When you are not running a tap, the valve should sit in the heating position.

If the valve sticks in the hot water position, or its motor or actuator fails, you get the exact pattern you are seeing: taps run hot, radiators stay cold. Sometimes the radiators get slightly warm and no more. Sometimes the heating works only while a hot tap is running, which points the same way. We explain the part in more detail in what a diverter valve does and why it fails.

Diverter valves sit inside the sealed boiler casing, so testing or replacing one is work for a Gas Safe registered engineer. Some can be freed and serviced; many are simply replaced.

System and Regular Boilers: The Motorised Zone Valve

If you have a hot water cylinder, the job of switching between heating and hot water is done outside the boiler, by motorised valves on the pipework, usually in the airing cupboard next to the cylinder. There are two common layouts:

• Two separate two-port valves, one for heating and one for hot water. If the heating valve's motor has burned out, the hot water side keeps working and the radiators get nothing.
• A single three-port (mid-position) valve that sends water to the cylinder, the radiators or both. When its motor or spring fails, it often stays in the hot water position.

A tell-tale sign is a small lever on the valve head. On many valves, if the lever moves freely with no resistance, the motor has failed. Do not wedge it open as a permanent fix. It hides the fault and can stop the boiler switching off properly. The motor heads are usually replaceable without draining the system, but the wiring runs back to the boiler and controls, so it is a job for an engineer.

A Weak or Failing Pump

The pump pushes heated water round the radiator circuit. On a system with a cylinder, the short run to the cylinder coil can still get enough flow from a pump that is starting to fail, while the longer radiator circuit gets almost none. You may hear the boiler fire, run hot and then shut off quickly because the heat is not being carried away.

Other signs of a pump problem are a humming or grinding noise from the pump body, the pump casing being very hot to the touch, or radiators that warm up only near the pipes. If that sounds familiar, see our page on central heating pump repair.

Air, Sludge and Low Pressure

If the heating comes on but the radiators stay lukewarm, rather than stone cold, the cause is often in the water itself. Air trapped at the top of radiators leaves them cold at the top. Sludge settling at the bottom leaves them cold at the bottom. Our article on radiators that heat unevenly goes through those patterns one by one.

Low system pressure usually stops the boiler doing anything at all, so it is less likely to cause hot water without heating. Still, check the gauge. If it reads below about 1 bar with the system cold, top it up following the boiler manual. If you have to do that more than once or twice a year, read why a boiler keeps losing pressure.

What an Engineer Will Check

A proper diagnosis follows the same order every time, cheapest and most likely first:

1. Confirm the controls are sending a call for heat, by checking the thermostat and programmer outputs at the boiler or wiring centre.
2. Confirm the boiler fires when heating is called for, and look for any stored fault codes.
3. On a combi, test the diverter valve's movement and actuator. On a system or regular boiler, test each zone valve's motor and its end switch.
4. Check the pump runs, has proper flow, and is not seized or airlocked.
5. Check flow and return temperatures across the radiator circuit for blockages.

Working in that order means you are not paying for a new pump when the real problem was a zone valve motor, or for a new diverter valve when the receiver had lost its pairing.

When to Call Us

If the thermostat, programmer and boiler dials are all set correctly and the radiators still will not warm up, the remaining causes are internal or wired into the heating controls. Our Gas Safe registered engineers diagnose these across London every day. You can book a diagnostic visit, and if the house is cold with vulnerable people in it, see our no heating repair service for the quickest route. The opposite fault, where the radiators work but the taps run cold, is covered in no hot water but the heating still works.