What is the ideal temperature for my boiler?

15 November 2023

The ideal temperature for a boiler can vary depending on factors such as the type of system, personal preferences and energy efficiency considerations. Let’s break down the pros and cons of having a boiler set too hot or too cold for hot water and central heating, and provide an example of a good temperature range for both systems.

Hot water

Hot water starts its journey as mains water, and gets fed through the boiler to be heated up. Exactly what happens next depends on the type of boiler you have.

With a combi boiler, there’s a sensor that detects when you have turned on a hot tap (or shower if it’s fed from the boiler). That turns on the boiler flame, and circulates water over the flame and through a heat exchanger. The water from the mains doesn’t pass directly over the flame – it is heated up by that circulated hot water inside the boiler, which is used over and over again. If the boiler was already on when the tap was turned on (i.e. if you were using central heating), the boiler redirects the water to heat up mains water instead. Your central heating will be temporarily disabled.

With a traditional boiler setup, the boiler heats the water in the same way, but instead of being on-demand, it fills up a hot water cylinder, which stores the hot water until it’s needed. This usually happens overnight, but it can be timed to fill at any time, and can be boosted manually via the programmer if you need hot water in the near future. Water can be kept hot or even be heated by using an electric immersion heater in the cylinder. It works like a kettle, but can use quite a lot of energy.

Hot water too hot?

Setting the boiler temperature too high for hot water can lead to scalding and potential burn accidents, especially for children or the elderly. It also increases the risk of wasting water by needing to mix in cold water to reach a safe temperature. It also means you’ll be wasting gas, as the amount of gas you burn directly affects the temperature of the water. If it’s hotter than you need it, you’ve probably wasted energy.

However, a cylinder can only hold so much water, so in some cases it’s worth having the water hotter than needed on the understanding that it will be diluted with cold water. An example would be if several people need a bath one after the other.

Hot water is too cold

If the temperature is too low, hot water may not be at a comfortable temperature for showers, washing dishes or other such uses. Temperature also has a disinfectant effect, too. The hotter your water, the better it is at killing germs and removing grease and fat. If you are washing your dishes in a kitchen sink, that matters. Dishwashers will not be affected though – they heat up cold water independently.


For hot water, a recommended temperature range is typically between 55 and 60 degrees Celsius. This temperature range balances safety, energy efficiency and comfort. 55 degrees C will still feel quite hot to the touch, but probably won’t scald the average person. At 60 degrees, however, the water will feel very hot, and could cause scalding if it’s used undiluted in a bath or washing up bowl.

Anything above 60 degrees would generally be too hot for practical use from a tap – if you need actual boiling water (i.e. 100 degrees C), you should use a kettle. Most items on your plumbing will have a maximum temperature (Shower valves, pumps etc and this is normally around 65 degrees)

Some modern tap systems can deliver boiling water instantly, and have a purpose built tank under the sink. They have extra safety measures in place to prevent users from accidentally turning them on. These are not fed by a boiler, however. Most boilers are maxed out at around 65 degrees for tap water.

Central heating

With central heating, the water is circulated around the system time and again – no new water enters the system. That’s good for efficiency, as once the heating is up and running, the “cold” water returning to the boiler will still actually be pretty warm, so won’t need much energy to re-heat it to the desired temperature.

Central heating works pretty much the same for combi, system and heat only boilers. However, it should be noted that the temperature of the water and the temperature you set at the thermostat are two different things. The circulating water will probably be around 65 degrees C, but you certainly wouldn’t want your home that hot! Most people’s homes are in the 18–22 degree range.

Central heating too hot?

A boiler set too hot for central heating can result in overheating of the radiators, leading to discomfort, wasted energy and potentially damage to the heating system. It may also cause excessive expansion and noise from expanding pipework.

What if it’s too cold?

Setting the boiler temperature too low for central heating may not provide sufficient warmth and thus result in a colder home. The boiler will be fired up constantly as the thermostat will never reach its desired temperature. Counterintuitively, having the water temperature set too low could therefore increase your gas bills.


For central heating, a common recommended temperature is around 65 degrees Celsius, although this can vary based on the specific system and insulation of the home. It is advisable to adjust the thermostat to achieve the desired indoor temperature rather than solely relying on the boiler temperature.

It is important to note that these temperature ranges are general guidelines, and you should also consider individual preferences, energy efficiency goals and safety aspects specific to your household. 

Correctly designed systems will have been sized and designed for a certain flow temperature so if you have documentation supporting this then we would recommend sticking with this design temperature. 

How to change the temperature

All boilers are different, so refer to the instruction manual that came with your model. If you can’t find yours, go online or contact the manufacturers to get a manual. Generally, however, there will be a control panel on the boiler, and you will have to go through some sort of menu to arrive at the temperature settings. There should be two separate settings – one for hot water, and another for radiators.

If you feel like your temperature adjustments are making no difference to the water, there could be a problem with your boiler, such as the pump, a sensor or a valve. If you’re in the Milton Keynes area, give us a call or contact us online so you can book an appointment. One of our engineers should be able to diagnose the problem.