In the UK the most popular type of boiler is the combination boiler, commonly known as a combi boiler. These boilers come as a single unit and provide both hot water and heating. They are space saving and highly efficient and are suited to properties that don’t have high water output requirements, such as flats or small houses.
They take up very little space and can often fit in a kitchen cabinet. There is no need for a hot water cylinder as they produce hot water on demand. No cold water tanks in the loft either, as they run off the mains. When you open a hot tap, the flow switch in the boiler detects the flow of water and fires the boiler up. This is highly efficient as you only heat the water that you use.
So how do you choose the right one?
Start off by thinking about how much hot water you’ll need. Combination boilers can only provide either the heating or the hot water at any one time. As it takes a lot of energy to quickly heat cold water, this is generally the larger requirement of the two. The hot water requirement will determine the kW output of the boiler you’ll need.
Combination boilers normally range in output from 24kW up to around 40kW. A 24kW is capable of producing 9.8 litres per minute at a 35 degree rise (if the incoming cold water is at 15 degrees centigrade, it can heat it to 50 degrees). A normal shower uses approximately 8 litres per minute. So a 24 kW boiler is adequate for the provision of one shower. In a two bathroom property however, you wouldn’t be able to use both showers simultaneously.
A 40kW boiler can produce approximately 16 litres per minute, so it’s suitable for a 2 bathroom house. Any more bathrooms than that and you’ll need a hot water cylinder. However, be very careful here. There is no point buying the more expensive high-output boiler if your incoming water supply pressure and flow are not up to the job.
A pressure gauge and a water flow meter can be used to test this prior to choosing a boiler. You can buy these from any plumbers’ merchants. This is, however, something that your chosen installer should do for you.
If you want to fully utilise a 40kW boiler, you need to see a water flow rate, prior to the boiler, of more than 16 litres per minute. Around 20 will normally suffice. This is because you can expect to lose a few litres a minute as the water passes through your boiler, due to restrictive pipe diameters slowing the flow. How much is slows will depend on the water pressure (i.e. the strength with which the water is being pushed through your pipes.)
If you have a large heating requirement but a low hot water requirement, you will need to calculate the heating requirement instead. To do this, the dimensions of each room in the house will need to be measured individually. There is a free tool for calculating the requirements from that here: www.radcalcs.com. When you have worked out all the rooms, simply add them together and this will give you your total heating requirement. Give yourself a few kW to spare, so if your heating requirement is 24kW, then go for something like a 27kW boiler.
Now that you know the kW output of the boiler you require, it is a matter of choosing between the different brands. In this writer’s opinion, Vaillant and Worcester Bosch are the two highest quality manufacturers of boilers in the UK. Not surprisingly, they are also the most expensive.
If you want to go down a cheaper route then it is always worth looking at the length of the guarantees currently being offered by the various manufacturers. The standard guarantee for boilers is 2 years. Be very suspicious of any guarantee for a time period less than this. From time to time manufacturers even offer 5 year guarantees, giving you an extra level of security.
About the author: Mike Genner is the Managing Director of Gas Safe London, a family-run Gas Safe registered company in London, who can provide a gas safe certificate for any kind of gas appliance. He writes articles concerning all aspects of their installation, service and repair. Gas Safe London are experts in landlords gas safety certificate matters, and offer discounted checks anywhere in the Capital. You can follow the company on Google+.