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How to Use a Combi Boiler Efficiently

Nick Geary
By: Nick Geary
Updated: 6th July 2022

Modern condensing combi boilers are very efficient. But if you're not using it efficiently then you might not see the expected drop in your energy bills.

Learning how to use a combi boiler efficiently will reduce energy waste, your carbon footprint and your heating bills. In this article, we'll tell you how to use a combi boiler efficiently.

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How does a combi boiler work?

Combi boilers have become a popular choice for homeowners across the UK and are now the boiler of choice in new build installations. Mainly thanks to their high levels of efficiency and small size.

Unlike system boilers and regular boilers, that store domestic hot water in a cylinder, combi boilers heat water directly from the mains as and when needed. This means that if you turn on the hot water tap, the combi will ignite and begin heating the water to the desired temperature.

Heating hot water on demand allows you to enjoy mains water pressure, although the pressure will weaken if more taps and showers are turned on at the same time. For this reason, combi boilers are best suited to smaller homes with 1-2 bathrooms.

Find out more in our guide to how combi boilers work.

What is a condensing combi boiler?

Any boiler installed in the UK, whether it's a combi, system or regular unit must be condensing. Condensing boilers have a Flue Gas Heat Recovery System, not found in non-condensing units, which captures heat from waste gases to preheat cold water from the mains as it enters the boiler.

In non-condensing units, this heat from waste gases would be lost, so by making the most of this heat, condensing units use less energy, become more efficient and should help to keep your energy bills down.

You don’t have to choose between a combi or condensing boiler. A combi boiler is a type of boiler and is made condensing by the way it recovers heat that would otherwise go straight out the flue pipe.

Learn more in Condensing vs Combi Boilers.

How efficient are combi boilers?

Combi boilers are designed to be highly efficient and only units with a high efficiency rating of 92% can now be installed in the UK.

This efficiency rating is reflective of how much of every £1 actually being spent on heating your home. When the boiler burns gas or oil, waste gases are produced that are released into the atmosphere, and with them heat is lost.

So, a 92% efficiency rating means that only 8p of every £1 being spent on heating your home is wasted. That's a considerable saving when you consider that older non-condensing boiler were only 60-70% efficient.

How to use a combi boiler efficiently

Having the most efficient combi boiler will only help so much as you will also need to use it efficiently too.

Don't keep the central heating on all the time

First and foremost, don't keep the central heating on all the time. Instead, set the heating to only come on as and when you need it.

The domestic hot water, on the other hand, can be kept on all the time, as combi boilers provide this to taps and showers on demand.

Turn the central heating off when you're out

Heating an empty home is a waste of energy and will lead to your heating bills rising. If your schedule changes regularly and you're not sure when you'll be home, then you might want to invest in a smart thermostat that has geolocation and learning features.

Install a smart thermostat

A thermostat gives you greater control of your central heating to help keep your energy bills to a minimum. In recent years, thermostats have advanced and smart thermostats have become a popular choice for homeowners.

With a smart thermostat, you can control your central heating from anywhere using a mobile or tablet. Plus, over time, depending on the model, a smart thermostat will learn about your central heating habits and adapt to heat your home in the most efficient way.

Find out which models came out on top when we compared the Best Smart Thermostats.

Check your thermostat

If you already have a thermostat, check the temperature it's set to as it may be heating your home more than it needs to be doing. Turning your thermostat down by a single degree could potentially reduce your annual energy bill by up to £65.

Adjust the boiler controls

Combi boilers have controls on the unit which allow you to set the temperature of the water that will be circulating around the central heating system and out of the taps.

The best setting for combi boilers is often recommended as 75°C for central heating and 60°C for domestic hot water. Turning down the temperature of the domestic hot water is particularly helpful as it can greatly reduce any risk of scalding.

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Arrange an annual service

Having your boiler serviced annually by a Gas Safe registered engineer is vital for a number of key reasons:

  • Maintain the safe running of the boiler
  • Ensure the boiler is operating efficiently
  • Keep the manufacturer’s warranty valid

A gas boiler service will typically cost between £60-£120 but could potentially save you much more as they can bring to light potential faults before they happen and keep energy bills down for longer.

Fit radiator valves

When you turn the central heating on, it's unlikely that you'll always need it to come on in every room and this is where radiator valves can help. A radiator valve is fitted to the side of a radiator and allows you to control how hot you'd like it to get and whether you want it to come on at all.

The main types of radiator valves are manual radiator valves and thermostatic radiator valves, which work slightly differently:

  • Manual radiator valves can be turned to increase and decrease the temperature of the radiator or turn it off altogether.
  • TRVs give you the ability to set the exact temperature you'd like the room to reach, once the TRV senses that the room is at the desired temperature, it will stop the flow of water into the radiator.

While TRVs are the more advanced option, either type of radiator valve offers increased control and will allow you to turn radiators off and have individual rooms at different temperatures so that energy isn't being wasted.

If you’re planning to upgrade the valves on your radiators then you’ll need to know how to change a radiator valve.

Make sure your property is well insulated

Without sufficient insulation, even the most efficient boiler will have to work harder to heat up a home. Insulation, whether in the form of double glazing, cavity wall insulation, floor insulation or loft insulation prevents heat from escaping to keep your home warmer for longer.

Draught-proofing a property by filling any gaps and cracks, which can be done with some insulating tape, is also very beneficial as it will keep draughts from getting into your property.

Find out more in How to Make a Cold House Warm.

Make use of the controls on the combi boiler

On the combi unit, you'll find that you can set the temperature that the boiler should heat the water to before it's circulated around the central heating system and delivered to the taps.

Combi boilers heat water on demand as and when needed. This means that when you turn the hot water tap on, it can take up to 20 seconds for you to actually get hot water. A useful tip to remember when it comes to filling a sink is to put the plug in and catch the cold water as well as the hot water that follows, so that the water is at a comfortable temperature when the sink is full.

Switch energy supplier

If you haven't seen an improvement in your energy bills since installing a modern combi boiler then, ultimately, it could come down to your energy tariff.


OFGEM, Switching energy tariff or supplier

Nick Geary

About the author

Nick Geary

Nick has a wide range of experience writing about conventional and renewable heating solutions, and is always increasing his knowledge by researching any new heating technologies on the horizon.

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