No hot water: what causes it & how to fix it

There are few plumbing problems more disruptive to daily life than having no hot water in the house. Having a bath is off the cards, and probably so too is taking shower. Doing the dishes isn’t an option (unless you have a dishwasher), and who wants to wash their hands in cold water? In this article, we’ll go through the most common reasons why the hot water isn’t working in your home, and what you can do to fix it.

No hot water – the basics

Before we look at the possible reasons why there’s no hot water, there are a few really basic things to look at first:

  • Make sure that the power supply to the relevant appliance is switched on
  • Try resetting the boiler using its reset switch, or turn the power on and off again. This may clear any redundant fault codes which prevent the boiler from operating.
  • Check that you have selected the correct program on the boiler or thermostat.
  • If your hot water depends on a clock or timer, make sure that the time is set correctly, especially at the times of year when the clocks change.
  • Make sure that the fuel supply is open – e.g. the gas main.
  • Make sure that you actually have fuel – e.g. by checking the gauge on your oil tank.
  • In older boilers, check to see that the pilot light is lit.

Health & Safety

Before you attempt to diagnose the problem, there are a few health and safety considerations:

  • The loft – If you are going into the loft, you need to be using a suitable ladder. Once in the loft space, you will need a source of light, such as a torch. Be extremely careful with older lamps – a hot bulb could be a serious fire hazard – and be careful where you tread. Keep an eye out for nails and other sharp objects protruding from the rafters and other carpentry work.
  • Water – Working with hot water and hot pipes comes with a risk of scalding. The water in your central heating circuit is dirty, and contains anti-corrosion and biocidal chemicals. It’s likely to stain objects and textiles.
  • Electricity – Some fittings, such as motorised valves and immersion heaters, are supplied with mains electricity. You should never work on such components if you don’t know what you’re doing. Always isolate a circuit before working on it, and always use a pair of electrical testers to confirm that a circuit is dead.
  • Gas – It’s illegal for an individual who is not on the Gas Safe register to work on a gas appliance.

If you have no hot water and a combi boiler

Low boiler pressure

If you have a combi boiler, one of the most common reasons for having no hot water is low boiler pressure. In order to operate, the system must be pressurised to approximately 1 – 1.5 bar when cold. Without adequate pressure, the boiler won’t operate, neither for central heating nor hot water.

If the pressure gauge indicates less than 1 bar when the system is off, then low pressure may be the culprit. Topping up the system via the filling loop should solve this problem – click here to learn how to do this. If the boiler keeps losing pressure, your system has a leak somewhere. The leak may be either on the pipework/central heating circuit, or because of a faulty boiler component. Click here for more information on this topic.

Faulty diverter valve

This is a component of a combi boiler which controls how heat is used. Combi boilers will typically prioritise the domestic hot water, so if someone opens a hot tap while your central heating is on, the diverter valve will react to that, and heat will be used to heat water for the taps instead of water for the radiators. Close the hot tap, and the boiler will go back to heating your home.

A fault diverter valve is very likely if you have a combi boiler and central heating but no hot water. This will have to be replaced by a qualified engineer.

If you have no hot water and a hot water cylinder

If you have a hot water cylinder, then water from either the mains or a cold water storage cistern in the loft supplies the cylinder with fresh water. The water is then heated by the boiler via water in the central heating circuit, which is pumped through a coil of pipework inside the cylinder. This is known as an indirect system. On a direct cylinder, there is no coil – the water is heated by two electric immersion heaters.

Before you read on, it’s worth noting that the average hot water cylinder holds around 160-230 litres of water. Think back – have people in the household been taking more baths or showers? If so, then you may have no hot water simply because of high demand. Use the boost feature on the immersion heater to get hot water back again. Don’t do this too often though, as it will have an impact on your energy bills. If there is a greater demand for hot water overall, then reprogram the timer/thermostat in order to accommodate it.

Check the supply of cold water

Is the gate valve on the cold supply pipe to the cylinder open? If it’s closed, you’ll have no hot water. That’s because hot water exits the top of the cylinder due to gravity acting on the cold water cistern in the loft.

For the same reason, there also needs to be water in the storage cistern. In some houses, the storage cistern also supplies the bathroom cold taps. If you have no hot water but the kitchen cold tap is working, then the storage cistern may be empty. This is almost certainly the case if the bathroom cold taps don’t work either. A quick visual inspection will confirm the state of the cistern. Has the ball valve got stuck? Moving the arm up and down may free the valve. If the cistern starts to fill, congratulations – you have solved the problem.

Faulty motorised valve

On an indirect system, a valve known as a motorised valve diverts water from the boiler to flow through either the hot water cylinder coil or through the radiators. A faulty motorised valve is often the reason for having no hot water, especially if you have heating but no hot water, or hot water but no heating.

Blocked primary circuit

The coil in the hot water cylinder forms part of what is known as the primary circuit. Hot water from the boiler – known as the ‘flow’ – enters the coil at the top, transfers its heat, and then exits at the bottom via the ‘return’.

A blockage in the circuit may stop the boiler from heating the water in the cylinder. This could easily be the case if the boiler starts up but stops shortly after. The boiler does this because the heat it’s producing is not being transferred to the cylinder. As a result, an internal thermostat shuts the boiler off in order to protect it from damage.

You may also find that the ‘return’ pipe adjacent to the cylinder is cool. If the ‘flow’ pipe is hot, then it’s very likely that a blockage is preventing water from flowing through the coil properly.

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