Boiler Not Heating Water: What to Check First

Step one: is the heating working?

This one question halves the list of suspects. Turn the room thermostat up a few degrees and give the boiler five minutes. Then feel the pipe going into a radiator, or the radiator itself.

If the radiators get hot, the boiler can burn gas, pump water and control itself. The fault is in whatever sends heat to your taps, and you can skip to the section for your type of system below.

If the radiators stay cold as well, the boiler is not running at all, or it is locked out. The hot water is just the first thing you noticed. Work through the next section before anything else.

Nothing heating at all: whole-boiler checks

These are all outside the casing and safe to do yourself. Do them in this order, because each one rules out a cause that would make the next check misleading: a boiler with no power shows no pressure fault, and a boiler with no gas can look like it has locked out for another reason. Note what you find at each step. If you do end up calling someone, telling them the display was lit, the gauge read 1.2 bar, the hob lit and the code was the same after one reset saves time on the visit.

• Power: is the display lit? Check the fused spur next to the boiler is on and the fuse board has not tripped.
• Pressure: the gauge should read 1 to 1.5 bar when cold. If it has dropped near zero, see boiler pressure at zero.
• Gas: does the hob light? On a prepayment meter, check the credit and the emergency credit.
• Condensate pipe: in freezing weather, the white plastic pipe running outside can freeze and block, and many boilers lock out when it does. Pouring warm, not boiling, water along it often clears it; then reset the boiler once.
• Fault code: write it down, then press reset once. If it comes back, stop there.

If you smell gas, do not reset anything. Open windows, turn the gas off at the meter if you can reach it safely, keep away from switches, then leave the property and call the National Gas Emergency Service on 0800 111 999.

Combi boiler: heating works, water stays cold

If there is no hot water tank in the house, you have a combi, and a working heating circuit means the fault is inside its hot water side: the flow sensor, the diverter valve or the plate heat exchanger. First check the hot water temperature setting on the boiler has not been turned right down or switched off. Then run the tap test in heating on but no hot water, which tells you which of those three parts has failed.

Boiler with a cylinder: heating works, tank stays cold

With a system or regular boiler, the boiler heats a hot water cylinder, usually in an airing cupboard. When the radiators work but the cylinder stays cold, check the controls in this order.

The programmer: many have separate hot water and heating channels. Make sure the hot water channel is on, or press its boost or advance button. A power cut can reset the clock so the hot water periods fall at the wrong time of day.

The cylinder thermostat: a small dial strapped to the side of the cylinder, about a third of the way up. Health and Safety Executive guidance for controlling legionella is for stored hot water to be kept at 60°C or above, so it should normally be set around 60. If it has been knocked down, the boiler will stop heating the tank early.

The zone valve: a small motorised valve near the cylinder, often a two-port valve on the hot water pipe or a three-port mid-position valve feeding both circuits. When its motor fails, the valve stays shut to the cylinder while the heating carries on. Many have a manual lever on the side that an engineer can use to hold it open temporarily. Replacing the motor head is a common repair.

Some homes, especially flats, have no gas boiler at all and heat their water with an electric cylinder or a point-of-use water heater under the sink. If that is your setup, check the switch or fused spur that feeds it and the fuse board. Most electric water heaters also have a thermal cut-out that trips if the water overheats, and it stays off until reset. Resetting it involves removing a cover over live parts, so leave that to an electrician or engineer.

If the boiler will not heat the tank, an immersion heater can usually keep you going. If the immersion does not work either, see immersion heater stopped working.

On an unvented cylinder, water running out of the tundish (the small funnel on the pipework) means a safety valve has opened. Switch off the boiler and immersion heater and do not adjust the valves. Unvented cylinders fall under Building Regulations Part G3, and any work on them needs a qualified installer. See water running into the tundish.

What the repair usually involves, and cost

An engineer confirms the fault with the boiler’s diagnostic mode or by testing the controls, then replaces the failed part. On a combi that is usually the flow sensor, diverter valve or plate heat exchanger. On a cylinder system it is usually a zone valve motor, cylinder thermostat or programmer.

Typical market ranges, parts and fitting, are roughly £120 to £300 for a flow sensor, £150 to £450 for a diverter valve, £250 to £600 for a plate heat exchanger, £100 to £250 for a zone valve motor head, and £80 to £200 for a cylinder thermostat.

Our no hot water repair visit starts with the £60 call-out at any hour, and the repair price is agreed with you on site before any work starts. Gas work is carried out by Gas Safe registered engineers. For cylinder faults, see the hot water problems hub.