Boiler flow temperature – What should it be?

The temperature of the water in the supply pipe, commonly referred to as the flow pipe, in a heating system is called boiler flow temperature. 

The correct boiler flow temperature is essential to obtain optimum efficiency from the boiler.

The Heating and Hot Water Council (HHIC) report suggests that households can save 6.4% to 8% energy by reducing the heating system to the mean flow temperature of 50°C on their condensing combi boilers. 

Most boilers run 10% to 25% under their A-rated efficiency due to the higher flow temperature.

A right flow temperature not only reduces your energy consumption but significantly increases the boiler efficiency.

To understand this, first, we need to understand how flow temperature works and how to control it.

How does flow temperature work?

Most boilers have a standard 80/60 flow and return temperature which means that a flow temperature of 80 degrees centigrade is sent to the radiators of your home.

After travelling through all the radiators of your house it sends back 60-degree centigrade water to your heating system while giving 20 degrees centigrade to the house.

What type of boiler do I need? 

A vast majority of UK households have condensing combi boilers. If you have one then you can control the flow temperature on your boiler.

A combi boiler is able to provide both heating and hot water under a single unit. All the boilers fitted in the last 16 years are condensing boilers. 

What is an ideal flow temperature? 

Setting boiler flow at a high temperature can increase your bills considerably while decreasing your boiler efficiency by 10 to 25 per cent.

A right flow temperature helps to increase the boiler efficiency while keeping your house warm and cosy. 

Ironically, condensing gas boilers are designed to achieve 92 per cent efficiency but this cannot be obtained if the flow temperature is set at 80/60.

To get the 92 per cent efficiency from the boiler, you must set the flow temperature low, allowing it to run on condensing mode as much as possible.

In other words, the boilers are not automatically A-rated out of the box – they need to be set so that they can perform at the optimum level.

The recommended output temperature for the radiators is 75-degree centigrades and for water, it is between 50 degrees centigrade to 60 degrees centigrade.

How do I reduce flow temperature?

The flow temperature should not be confused with the room thermostat.

The temperature you will set on your boiler will not be the temperature of your room.

Typically, you can adjust your flow temperature from the front of your boiler. 

The display and the way of boiler settings are not the same in different brands. Let’s have a brief look at these.

Boilers with dials: 

Most combi boilers come with dials, one for heating and the other for hot water.

Some boilers also come with only one dial which is meant to control heating flow temperature. The dials are either numbered or have dashed lines.

The flow temperature in most boilers operates between 30  to 80 degrees centigrades.

If you work it out manually, 55 degrees centigrade will be halfway which will be number 3 on a dial that has numbers from 1 to 6.

Some boilers have ‘e’ on the dial which stands for economy. If you setting your boiler on the economy, you are setting it on 65 degrees centigrades. 

Dials with digital display: 

Some boilers have a digital display which shows the setting you are in.

Whatever setting you choose will appear on the screen which makes it easier to adjust the flow temperature.

In this case, turn the dial down until you reach 55-60 degrees centigrade. 

Digital display with buttons: 

The new boilers came up with the addition of the digital display boilers by introducing up and down buttons which makes the adjustment of flow temperature and hot water even easier. 

Best temperature for a Condensing boiler to condense

A condensing boiler recovers all the heat from the flue gases and uses it to heat your home. This makes it the most efficient type of boiler.

To help it work efficiently, the heat exchanger needs to be equal to or below the dew point temperature. 

A measurement that determines that humidity of air is called dew point temperature. It is the temperature at which water droplets form in the heat exchanger.

An ideal dew point temperature for a gas boiler is around 55 degrees centigrade or lower.

In simple words, the water in the return pipe must be 55C or lower otherwise your boiler will not perform at maximum efficiency and will not condense.

These days the boilers are coming with a mark on the screen that shows you the point at which the condensing mode will be turned on.

This feature put the guessing game to an end with users just having to turn the dial towards the marked point.

What if I have a hot water cylinder?

Unfortunately, the boilers with a hot water cylinder are not efficient.

According to the Centre for Sustainable Energy (CSE), a domestic hot water cylinder thermostat should be set at 60 to 65 degrees centigrade.

The higher thermostat will increase the risk of scalding as the water that comes out of the taps will be too hot. Installing a mixing valve for extra safety is a good idea.

CSE suggests that setting the thermostat to a high temperature will not help in the fast heating up of water.

It will simply mean that you have to add lots of cold water to the hot and will have wasted the energy spent in raising it to an unnecessarily high temperature. 

Can I set my thermostat lower than 60 degrees? 

Lowering the thermostat from 60 degrees in the water cylinder will not increase its efficiency but will increase the risk of contracting Legionnaires disease.

It is a form of pneumonia that’s caused by bacteria in water if the temperature of the water in your cylinder is not hot enough.