Why Have I Got No Hot Water? A Quick Diagnosis

Question 1: does water come out of the hot tap?

Open a hot tap fully. If water flows but stays cold, the supply is fine and the water is just not being heated, so carry on with the questions below. If only a trickle or nothing comes out while the cold taps run normally, the hot supply is blocked rather than unheated: an empty loft tank, an airlock, a frozen pipe or a blocked strainer. That has its own guide, no hot water coming out of the taps.

If the cold taps are dead too, the water supply to the house is off. Check with neighbours before assuming a fault in your home.

Question 2: is the heating working?

Turn the thermostat up and feel a radiator after ten minutes. This one test splits the problem in half.

Heating works, hot water does not: the boiler is fine. The fault is in the part that only serves hot water. On a combi that is the flow sensor, diverter valve or plate heat exchanger. With a cylinder it is the programmer hot water channel, the cylinder thermostat or the hot water zone valve. See heating on but no hot water, or water heater working but nothing hot at the taps for the same checks on electric heaters.

Neither works: the boiler itself is off, locked out or starved of power, gas or pressure. Go to question 3.

Radiators warm up when you run a hot tap with the heating off: the combi diverter valve is sending heat the wrong way.

Question 3: what does the boiler display show?

Go to the boiler and look before touching anything; if your flat has no boiler, only a metal box in a cupboard fed by a communal system, work through no heating or hot water from a heat interface unit instead. A blank display means no power: check the fused spur switch beside the boiler and the consumer unit. A fault code or a flashing light means the boiler has locked itself out, and the code tells you why. A normal display with nothing happening means it is not being asked to heat, or it cannot detect the demand.

• Pressure gauge: on a sealed system it should read about 1 to 1.5 bar cold. Below roughly 0.5 bar many boilers lock out, though the exact cut-off varies by make. Low boiler pressure is one of the most common causes and often a quick fix.
• Gas: if you have a gas hob, light it. If that fails too, check the gas meter and any prepayment credit.
• Settings: check the hot water temperature has not been turned right down, and that a programmer or smart control has not switched hot water off.
• Reset: after fixing any of the above, press the reset button once. If the same code returns, stop and note it for the engineer.

If you smell gas, do not reset or relight anything. Open windows, turn the gas off at the meter if you can reach it safely, avoid switches, then leave the property and call the National Gas Emergency Service on 0800 111 999. Never remove the boiler casing.

Question 4: what happened just before it stopped?

The timing is often the biggest clue, and it is one of the first things an engineer will ask.

• After a power cut: programmers lose their clock or reset to off, and some boilers need a reset. Check the time and the hot water channel.
• During a hard frost: the condensate pipe outside a condensing boiler may be frozen. Pour warm (not boiling) water along the outside section, then reset once.
• After work in the street or a water company repair: grit can block the inlet filter on a combi or the strainer on an unvented cylinder, leaving the hot weak or dead.
• Faded over weeks: hot water getting cooler bit by bit points at limescale in a combi plate heat exchanger, or a failing immersion or cylinder thermostat.
• After a loud bang, dripping or a smell of burning: switch the boiler off and call out an engineer rather than trying a reset.

Question 5: do you have a hot water cylinder?

If there is a cylinder in an airing cupboard, you have two ways to heat water: the boiler through a coil inside the cylinder, and often an electric immersion heater as a backup.

If the boiler heats the radiators but not the cylinder, and the hot water is on at the programmer, the likely parts are the cylinder thermostat, a small dial strapped to the side of the cylinder, or the motorised zone valve that lets boiler water into the coil. The thermostat is normally set around 60°C, which matches HSE guidance that stored hot water should be kept at 60°C to control legionella. If it has been knocked down to its lowest setting, turn it back up.

If you rely on the immersion and it has stopped, check its switch and the fuse or breaker first. Some immersions have a thermostat with a safety cut-out under the cover, but that cover hides live terminals, so switch off at the consumer unit before any cover comes off, or leave it to someone qualified. Immersion heater stopped working goes through it in order.

While the boiler side is being fixed, switching the immersion on is the quickest way to get hot water back tonight.

When to stop checking and call

Call an engineer once the simple checks are done and the answer is still no hot water: a fault code that returns after one reset, a boiler that fires but gives no hot water, a cylinder that stays cold with the controls calling, or any smell, leak or noise from the boiler. If you rent, the landlord is responsible for keeping hot water working, and tenant no hot water rights sets out what to do.

An engineer follows the same order you have just used, then tests the parts: gas pressure, flow sensor, diverter, thermostats and valves. 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.