There’s no right or wrong temperature for hot water from the taps. Some people like it very hot so they can clean dishes with it; others prefer it to be a bit cooler so they lower the risk scalding and can fill a bath without needing much cold water. The temperature of your hot water depends on what kind of boiler you have, how it’s adjusted and whether everything’s in good working order. In this article we’ll be dealing with bringing your water temperature down safely.

Confirm the water temperature

One thing that’s useful to do is to check the temperature before you make any adjustments, so you can compare it after any changes. Poking your finger into water flowing from the tap isn’t accurate. The water might not be up to temperature, and if your hand is cold, the water will feel hotter than it would if your hand’s warm.

Turn the hot tap on and leave it running for about 20 seconds, then fill a large glass. A pint glass or measuring jug are good – anything that won’t lose heat quickly. Then, dip in a thermometer to get the temperature. Thermometers are easily available from cooking shops and baby & child shops. Note the temperature.

What kind of boiler do you have?

Now we need to know what type of boiler you have.

  • If it’s a combi boiler, hot water goes directly from it to the taps.
  • A system or conventional heat-only boiler feeds its water in pipes around the hot water cylinder, which warms up the fresh water inside. The water that comes from the tap has not been through the boiler. Boiler water is circulated again and again.

Adjusting a combi boiler

Combi boilers have some sort of control to set the temperatures. They could be manual, i.e. a dial that you physically rotate or a slider; or they could be controlled digitally, from a panel. If it’s digital, consult your boiler’s instructions to find the settings, but it should be quite straightforward.

Either way, you will have two separate controls: tap and radiator. Adjusting one should not adjust the other. Turn the temperature of the tap control down a bit, then perform the thermometer test again. You should be able to do it straight away. Keep making small adjustments until you’re happy with the temperature.

Remember, a combi boiler heats water as and when it’s needed, so the results should be instant.

There could be a third control, which is the temperature of the circulating water, or pre-heat. This will be higher than the other two. Adjusting this will not have much of an effect on the hot water, as the boiler will always try to match the tap temperature you set. Turning it down might make your boiler greener and cheaper to run, however, but if you have it too low, it will limit the temperature you can get from your taps.

Adjusting a system or heat only boiler

Adjusting the temperature of a system or conventional boiler can be a bit more tricky, as the thermostat and adjuster are often on the boiler itself. Remote control devices are available, but for most people they are unnecessary as the temperature is probably adjusted rarely.

The temperature setting on the cylinder might be visible, or it could be behind a panel. Once you’ve located it, it could be a manual control (a knob, dial or a slot for a screwdriver) or digital. Check your manual for details, but you should be able to lower the temperature at one of these controls.

What you are doing here is telling the cylinder to start or stop the flow of hot water through the heat exchanger from the boiler. Once the cylinder reaches your desired temperature, the flow will be switched off. If the temperature is lower than your setting, and there are no timers in the “off” position, the cylinder will start warming up.

Unlike lowering the temperature in a combi boiler, cylinders will take time to cool down if they are at maximum temperature already. How long it takes depends on how well your cylinder is insulated, but once a few hours have passed, or if the hot taps have been run a few times, the temperature should settle to your new setting. Test again with the glass of water.

Immersion heaters

If you have immersion heaters in your cylinder, they will have their own thermostats. If they are set too hot, they will override the boiler-heated indirect water heating system. You might need to remove a cover to access the thermostat, and it’s often one that you adjust with a screwdriver. Again, the effect will not be instant – you need to wait for the water to cool down, or you can empty the tank’s hot water by turning on a hot tap. Then test the temperature again with a thermometer.

Room temperature is not water temperature

Don’t confuse the temperature on your room thermostat with the water temperature – they are two different things. The room thermostat tells the boiler whether to pump water through the radiators or not. Hotter water might make the room heat up faster, but the eventual temperature will be the same.

Radiator temperature is not tap temperature

Similarly, the temperature of your radiators in not the same as your hot tap water temperature. They both run through different systems, and although your cylinder might be on the radiator system, the water that you wash with is refreshed constantly – it’s just warmed by separate loop circulating through the boiler.

Legionnaire’s disease

Finding the right temperature is a balance between being efficient and avoiding scalding, but there’s a third consideration, too – legionnaire’s disease. This type of pneumonia is caused by the Legionella bacteria, which thrives in water that’s between 20 °C and 45 °C. Although it’s quite rare, especially in water that is regularly changed, like in a cylinder, the risk should not be ignored. It is killed by temperatures above 60 °C, so health organisations like the HSE recommend that hot water should be stored at no less than 60 °C.

Possible faults

It’s also possible that your water is running too hot because of a mechanical or electric failure in your boiler or system. Water flowing too slowly over the heat exchanger could cause it to overheat, as could a faulty thermostat. If manually turning down the temperature doesn’t work, it’s time to call out a Gas Safe engineer. Use our “Find an Installer” service to make your booking.

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