No hot water but the heating still works? What to check first

This one confuses people because the boiler is obviously working. The radiators get hot, the display looks normal, and yet the kitchen tap runs cold however long you leave it. The good news is that when heating works and hot water does not, the boiler is getting gas and firing, so the fault sits in the part of the system that makes or delivers hot water. That narrows it down a lot.

First, which kind of system do you have?

The checks depend on how your house makes hot water. If there is no hot water cylinder in an airing cupboard and hot water comes straight from the boiler when you open a tap, you have a combi. If there is a cylinder, the boiler heats that tank and the taps draw from it, which means a system or regular boiler.

Checks for a combi boiler

1. 1Open a hot tap fully. Combis need a minimum flow before they fire for hot water, so a tap barely cracked open can leave the water cold.
2. 2Try a different hot tap. If one tap is cold and the others are fine, the problem is that tap or its pipe, not the boiler.
3. 3Check the cold water pressure. If the cold taps are also weak, the combi may not be getting enough water to fire. That is a supply problem, and worth checking with neighbours.
4. 4Look at the boiler display for a fault code and look it up in the manual. Note it down either way.
5. 5Check the hot water setting on the boiler has not been turned right down or switched to heating only.

If all of that checks out, the classic cause on a combi is the diverter valve. It is the part that switches the boiler between heating the radiators and heating your tap water. When it sticks in the heating position you get exactly this: warm radiators and cold taps. It is a common repair and it does not mean the boiler is finished.

Checks for a boiler with a hot water cylinder

1. 1Check the programmer or timer. Hot water often has its own schedule, and after a power cut the clock can reset or the hot water channel can be switched off.
2. 2Check the cylinder thermostat, usually a small dial strapped to the side of the tank. If it has been knocked right down, the tank will not heat.
3. 3Feel the pipe going into the cylinder from the boiler when hot water is set to on. If it stays cold, the water is not being sent to the tank.
4. 4If you have an immersion heater, try switching it on for an hour. If the water gets hot, the cylinder is fine and the fault is between the boiler and the tank.

The usual culprit on these systems is a motorised valve, which opens to let hot water from the boiler into the cylinder coil. When it fails shut, the heating keeps working and the tank goes cold. Again, it is a part, not a new boiler.

When to ring someone

• You have been through the checks and the water is still cold.
• The boiler is showing a fault code that the manual says needs an engineer.
• There is water leaking from the boiler, the cylinder or the pipework around them.
• Anyone vulnerable in the house is without hot water in cold weather.

Please do not take the boiler casing off to have a look. Anything inside the case is gas work, and it has to be done by a Gas Safe registered engineer. When you ring, tell me whether it is a combi or has a cylinder, whether the heating works, and any code on the display. That usually tells me what to bring.