A Guide To Combi Boiler Systems
A combi boiler heating setup does not require attic tanks or hot water cylinders. It produces hot water and powers the central heating from within the single boiler unit.
What do we mean by a Combi Boiler?
This may be confusing because there are three main types of boiler configuration:
- Open Vent/Conventional/Regular Boiler – Requires tanks in the attic for storing cold water, and cylinders (normally in an airing cupboard) to store hot water.
- System Boiler – Requires a cylinder to store hot water. Doesn’t require attic tanks.
- Combination (or Combi) Boiler – Does not require any additional tanks or cylinders. Hot water is created on-demand and uses the pressure of the incoming mains water feed to power the water through your home.
Condensing or Combi Boiler?
This is a question we sometimes get, especially if someone needs to upgrade from an old back boiler.
Any modern boilers manufactured today, or since 2005 condensing boilers. This includes combi boilers. This is due to a government change to building regulations as condensing boilers are so much more efficient than non-condensing boilers.
Condensing technology is simply a way to extract more heat from exhaust gases that used to be wasted in older non-condensing boilers.
You can see how the technology works in this diagram below:
How a combi boiler works diagram.
Essentially, the hot gases created by burning natural gas, oil or LPG is used to heat incoming water. Essentially it allows the main natural gas burn to do less work overall.
How does a combi boiler work?
Combi is short for combination. A combi boiler system combines central heating and hot water within the same compact unit. They house two heat exchangers:
- A primary heat exchanger, for central heating. The primary heat exchanger heats and recycles the water that goes through the pipes into the radiators.
- A Secondary plate heat exchanger, for hot water. The secondary plate heat exchanger heats up the hot water that goes into your taps.
For a more detailed look at combi boiler pipes read our quick guide >
What’s inside a Combi Boiler
Primary Heat Exchanger for Central Heating
The radiators in your house get warmed up by the hot water coming from the boiler, heated by the primary heat exchanger inside a combi boiler system. Once this hot water has warmed up the radiators, it travels and recycles back into the boiler.
The boiler keeps firing to reheat that water and keeps it at your preset temperature, normally around 60 to 85 degrees Celsius. This water can get dirty over time because it picks up dirt and debris from the radiators and pipes.
Secondary Plate Heat Exchanger for Hot Water
The water that travels through the radiators and recycles back to the combi boiler can be dirty so it’s best not to use it for hot water taps or showers.
To give you clean hot water every time you turn on a hot water tap in your house, the combi boiler system pauses your central heating (if it’s running) and then opens a diverter valve which lets the heating water enter the secondary plate heat exchanger.
This hot central heating water runs through a heat exchanger to warm up your clean water for hot water taps. This is why you might hear the combi boiler clicking when you open the taps. It’s the diverter valve.
Reliability of a Combi Boiler System
Are combi boilers good?
Yes, depending on the make and model.
Combi boilers come in all different sizes and powers and are good for the smallest flats and homes to quite large homes – depending on the hot water requirement. These boilers provide hot water and heating on-demand to the property. They don’t require hot water cylinders and can be kept in a kitchen cupboard.
However, there are different variations available. For example, Viessmann offer a storage combi boiler which does store a bit of hot water to help with slightly higher demand.
In short; they’re great and can handle most properties.
Are combi boilers cheaper to run?
It all depends on your hot water usage. A combi boiler doesn’t use energy to heat water that may not be used, as a system boiler does. But, a combi boiler will need to run water along pipes to your (for example) shower whilst it’s still warming up and this probably won’t be used…then when the shower is turned off the hot water will sit in the pipes from the boiler to the shower.
Are combi boilers cheaper to run?
It all depends on your hot water usage. A combi boiler doesn’t use energy to heat water that may not be used, as a system boiler does. But, a combi boiler will need to run water along pipes to your (for example) shower whilst it’s still warming up and this probably won’t be used…then when the shower is turned off the hot water will sit in the pipes from the boiler to the shower.
Are combi boiler being phased out?
No. The government have a boiler upgrade scheme that allows certain people to receive a grant of £7,500 for a new air source heat pump. But these heat pumps are not a viable option for everyone so we don’t believe natural gas will be phased out in the next 15-25 years. Plus, most boiler brands are now 20% hydrogen ready which will help reduce reliance on gas.
Are combi boiler gas or electric?
The most popular combi boilers in the UK are gas-burning. Electric combi boilers are available, but they’re very expensive and most require an upgraded 3-phase electrical supply into your home.
Can a combi boiler cope with two showers?
Yes, if it’s powerful enough and you have good enough cold water flow rate to the boiler. If you don’t have good cold water flow you could consider a storage combi boiler. These are a mix of combi and system boilers. You can read more about storage combi boilers here >
