No hot water: what to check, and why the boiler might not be the problem

Running a hot tap and getting nothing but cold water is one of the most common reasons people ring a plumber, and one of the most unsettling. The good news is that no hot water rarely means the boiler has reached the end of its life. In most homes the cause is one of a handful of faults, and several of them you can rule out yourself in a few minutes without taking anything apart.

This guide works through those checks in a sensible order, then explains what is usually going on when the heating still works but the hot water does not.

First, know which system you have

The likely causes depend on how your home makes hot water, so it is worth being sure before you start.

• Combi boiler: there is no hot water tank anywhere. The boiler heats water the moment you open a tap. Most London flats and a large share of terraced houses have one.
• System or regular (heat-only) boiler: there is a hot water cylinder, usually in an airing cupboard, a loft or a utility room. The boiler heats the water in that cylinder, and the cylinder feeds the taps. A regular boiler also has a cold water tank in the loft; a system boiler does not.
• Unvented cylinder: a sealed cylinder fed straight from the mains, usually a tall white or grey tank with a set of valves and a small open funnel (a tundish) on the pipework. These have safety controls that only a qualified engineer should touch.

Checks you can safely do yourself

None of these involve removing the boiler casing or working on anything connected to the gas. If a check needs more than looking, pressing a reset button or turning a control, stop and leave it to an engineer.

• Is it every tap, or just one? If the kitchen runs hot but the bathroom basin does not, the fault is in that tap, its pipework or a mixing valve, not the boiler. A thermostatic shower that has gone cold while the taps are fine usually points to the shower cartridge.
• Look at the boiler display. A fault code or a flashing light is the most useful clue you have. Note it down and look it up in the manual; many codes point straight at the cause, and an engineer will want to know it.
• Check the pressure gauge. On a combi or system boiler, a gauge that has dropped into the red, or below roughly 1 bar when the system is cold, will often stop the boiler from firing at all. Our guide to low boiler pressure covers how to top it up.
• Check the controls and power. Make sure the boiler has power, the programmer or smart controls are set to give hot water (many let you switch hot water off separately from the heating), and the clock has not reset after a power cut.
• Check the gas. If the hob will not light either, the problem is the gas supply rather than the boiler. On a prepayment meter, check the credit.
• In a cold snap, think about the condensate pipe. Modern condensing boilers drain water through a plastic pipe that often runs outside. If it freezes, the boiler locks out, sometimes with a gurgling noise. Pouring warm (not boiling) water along the outside section usually thaws it, after which you can reset the boiler.
• Open the tap fully. A combi only fires once enough water is flowing. A tap opened just a little, or a very low mains pressure, may not be enough to trigger it.

Heating works but there is no hot water: combi boilers

When the radiators heat up fine but the taps run cold, the boiler itself is clearly capable of burning gas and heating water. What has failed is the part that sends that heat to your taps instead of the radiators. The usual suspects are:

• The diverter valve. This switches the boiler between heating and hot water. If it sticks in the heating position, the radiators get warm and the taps never do. It is one of the most common combi faults and a routine repair.
• The flow sensor. This tells the boiler that a tap has been opened. If it fails, the boiler simply does not know you want hot water.
• The plate heat exchanger. This is where heat passes into the water going to your taps. London water is hard, and over time limescale builds up inside it. The typical sign is water that is lukewarm, or that swings between hot and cold, rather than none at all.
• A temperature sensor on the hot water side giving the boiler a false reading.

Heating works but there is no hot water: homes with a cylinder

With a system or regular boiler, the boiler running is no guarantee that the cylinder is being heated. That is controlled separately, so the fault is usually somewhere between the boiler and the tank.

• The programmer. Check the hot water channel is on and not in a holiday or off setting.
• The cylinder thermostat. This is a small dial strapped to the side of the tank. If it is set too low or has failed, the cylinder will not call for heat.
• The motorised valve. A zone valve, usually near the cylinder or the boiler, opens to let hot water from the boiler into the cylinder coil. If its motor fails, the valve can stay shut on the hot water side.
• The pump. A failed or airlocked pump can affect hot water before you notice any difference in the heating.

If your cylinder has an immersion heater, switching it on is a useful stopgap while you wait for a repair. If the immersion heater does not heat the water either, and the boiler is working, the problem is in the cylinder itself rather than the boiler side.

Hot water, but only lukewarm

Water that comes through warm but never properly hot is usually a settings or scale problem rather than a breakdown. Check the hot water temperature setting on the boiler or the cylinder thermostat before anything else. Stored hot water in a cylinder should be kept at 60°C or above, which is the level that controls legionella bacteria, so turning the cylinder right down to save energy is not a good idea. On a combi, lukewarm water that gets worse over months usually points to scale in the plate heat exchanger.

When to stop and call an engineer

• If you smell gas, turn off the gas at the meter if you can safely reach it, open windows, do not use switches or flames, and call the National Gas Emergency Service on 0800 111 999.
• If water is running out of the tundish on an unvented cylinder, or from the copper discharge pipe outside, the safety controls are doing their job and the cause needs a qualified engineer.
• If the boiler shows the same fault code again after one reset, keep resetting it and you risk masking the fault. Leave it and book a boiler repair.
• Any job that means taking the casing off a gas boiler must be done by a Gas Safe registered engineer. That is the law, not a sales line.

Most of the faults above are specific parts that can be repaired or replaced, not a sign the whole boiler needs changing. An annual boiler service is also a chance for the engineer to spot a sticking valve or scaling heat exchanger before it leaves you with cold taps. If you have a cylinder that keeps giving trouble, our page on hot water cylinders explains the options. And if your household has no hot water at all and it cannot wait, our emergency plumbing line is open around the clock on 020 3576 5825.