Hot water but no heating? The checks to run first

What hot water tells you

Hot water at the taps means the boiler has gas, ignites, and can heat water. That rules out most of the expensive faults at once. What is left is the part of the system that asks for heating and the part that sends hot water to the radiators instead of the taps. Several of those are checks you can make in ten minutes.

Six checks before you call anyone

• The programmer. Is the heating channel set to on or to a timed period that covers now? After a power cut, many timers reset to midnight or lose their programme.
• The room thermostat. Turn it up a few degrees above the room temperature and listen for a click or watch for the boiler to respond. A wireless thermostat needs working batteries, and its receiver near the boiler usually has a light to show it is paired.
• The boiler's own setting. Many combis have a heating dial or a summer mode that gives hot water only. Check it has not been turned down or switched over.
• The pressure. Combi and system boilers will often still make hot water at a pressure too low to run the heating safely. The gauge should read 1 to 1.5 bar cold.
• The radiator valves. If every thermostatic valve has been turned to frost or off, the system has nowhere to send water.
• A fault code. Write down anything shown on the display. It is the first thing an engineer will ask for.

If the checks pass: it depends on the boiler

On a combi, a diverter valve sends the hot water from the boiler either to the taps or round the radiators. When it sticks in the hot water position, you get exactly this fault. It is a part inside the boiler, so replacing it is a job for a Gas Safe registered engineer, and it typically costs £200 to £400.

On a system or regular boiler with a hot water cylinder, motorised valves on the pipework near the cylinder open the route to the heating or to the cylinder. The motor head on top of a valve fails more often than the valve body, and many heads can be changed without draining the system. A manual lever on the side of some valves will open it by hand as a temporary measure, which tells you whether that valve is the cause.

Heating that works when the hot tap is off

A combi gives hot water priority. While a tap is running, the radiators cool off, and they come back once it is shut. That is how the boiler is designed, not a fault. If the radiators stay cool after the tap closes, the diverter valve is the likely cause.

What a repair costs

Most firms credit the diagnostic visit against the repair if you go ahead. Have the boiler make and model ready when you call, from the label under the front panel or on the side, because it decides whether the engineer can bring the part on the first visit.

If there is no hot water either

When neither heating nor hot water works, the boiler itself is not firing, and the list changes. Check that other gas appliances such as the hob work, and that a prepayment meter has credit. Check the pressure. In freezing weather, look at the plastic condensate pipe where it leaves the house: if it has frozen, the boiler locks out, and pouring warm, not boiling, water along it usually clears it. Then reset the boiler once. If it locks out again with a fault code, that is a job for a Gas Safe registered engineer.

What not to do

Do not keep resetting a boiler that locks out with the same fault code. The lockout is a safety response, and repeated resets can hide a problem the engineer needs to see. Do not take the boiler casing off. And in a cold spell, if the household includes a baby or someone elderly or unwell, say so when you call: no heating in freezing weather is treated as urgent by most firms even though nothing is leaking.