A combination or combi boiler is a high-efficiency water heater and central heating boiler in a single compact appliance.
The major difference between this and other types of boiler is that it provides domestic hot water (DHW) directly from the mains when you turn on a tap or shower. As you’d expect, it also offers a constant, controllable temperature for central heating when you need it.
With domestic hot water supplied instantly and on demand, one noticeable advantage of a combi boiler is the fact that it frees up the space normally needed to accommodate a cylinder and/or cold water storage tank in the attic.
As a result, this kind of appliance is ideal if you want to make the most of an area previously used as an airing cupboard or if you’re tackling a loft conversion where you want to maximise every inch of available space.
Discrete wall-hung models, such as Worcester, Bosch Group’s Greenstar Si and CDi compact gas-fired combi boilers feature an ultra-compact heat exchanger, resulting in a very small appliance that doesn’t compromise on output or efficiency.
These models are popular for smaller properties with just one bathroom and take up little more room than a kitchen cabinet. There are also larger, more powerful models, such as Worcester Bosch’s HighFlow floor-standing series and larger wall-hung combis, meaning they can power homes with two bathrooms or more.
Using the mains supply to deliver hot water has the additional advantage of boosting pressure throughout the home, so you get a more powerful shower without needing a separate pump.
As you only pay for the energy you actually need for heating and DHW, combis also prove highly cost-effective. What’s more, as this type of boiler doesn’t need a cold water storage feed, the risk of frozen pipes or tanks – one of the main causes of leaks during winter months – is reduced.
Case Study: New combi boiler for renovated bungalow
|Owner: Sue Pennington|
Project: Renovation of a five-bedroom masonry home
Product: Greenstar CDi classic combi boiler
Eager to find a home to accommodate her family of four, Sue Pennington was thrilled when she discovered a property in her ideal location. However, the house didn’t meet the family’s needs so Sue decided to seize the opportunity to do a complete renovation.
When it came to planning the heating system, Sue wanted to install a setup where hot water was available on demand and two showers could be used at the same time, if needed. She opted for a wall-hung combi boiler from Worcester Bosch’s Greenstar range to support the house’s radiators, and paired it with a Wave smart control system to achieve maximum efficiency.
“We’ve had combis in most of our previous homes and we like the fact that you never run out of hot water, which isn’t always the case with cylinder-based setups,” says Sue. The system cost approximately £1,600, including the boiler, control system and installation. “We’re really pleased with the arrangement. We’ve supplemented it with a log-burning stove in the lounge which is useful if we don’t want to heat the whole house.”
A combi boiler typically requires less pipework than a system or regular appliance. Because fewer components are needed installation is often quicker and cheaper. Consider what controls you will use for the boiler and bear in mind that the right setup could boost your appliance’s efficiency. Modern control systems, such as Worcester Bosch’s Wave, can help you get the best possible output from your boiler.
Combi appliances require the same level of upkeep as other boiler setups. An annual service ensures the system is clear and any issues are corrected before they cause a problem. As with other heating arrangements, it’s recommended that a system filter is fitted to keep contamination to a minimum and ensure your central heating runs effectively.
Specifying sustainable tech as part of your heating system can save you money, but like any arrangement, it needs to match your home and your project objectives. Solar thermal panels, ground source and air-to-water heat pumps all require a cylinder to store the water heated by renewable sources, so may not be ideal if you’re trying to save space.
A number of Worcester’s combi boilers will now accept pre-heated water, so they can be combined with a range of renewables to ensure you have DHW all year round.
If you are undertaking a home extension project, an air-to-air source heat pump that works independently of your property’s heating system will be able to warm the additional space without putting extra load on your combi boiler.
What type of boiler is right for my home?
|Q. I’m renovating a four-bedroom property for myself and my husband, and we often have guests over to stay. The dwelling is quite large and although we’re upgrading the insulation, it’s still quite draughty. We have a regular boiler at the moment, but would a combi model suit?|
The heat output of a combi unit is typically around 20-25kW, so it’s likely that this would be large enough to cater for the size of your home. It’s important to take into account how big your radiators are – in older properties there’s a strong chance that you’ll have a combination of 1kW and 3kW double panel units. Check that the combined total output of all your radiators doesn’t exceed the heat output your boiler is able to generate, or seek advice from a heating engineer.
Q. I’m self building an airtight three-bedroom house for myself, my partner and our two children. We lead a busy lifestyle and have an irregular routine – especially when it comes to showering times. What sort of boiler would you recommend?
For homes with up to two bathing zones and an average occupancy level, the largest of Worcester’s combi models is likely to be a good fit. If your home has a high occupancy and more than two bathrooms, a traditional storage cylinder with a system boiler may be a better fit to meet the hot water demand.
Q. My wife and I are about to downsize to a two-bedroom 1950s bungalow where space is limited. We’re looking for a boiler that caters to our needs on demand, and possibly something that would work alongside renewable technology. What would you suggest?
A combi boiler would be perfect in this case, as it would allow you to maximise available space and run your hot water and central heating as and when you need it. You could combine this with solar PV panels to reduce your electricity needs, or possibly a woodburning stove to decrease your dependency on gas.
Q. I’m planning the heating scheme for a large barn conversion with a spacious open-plan living area and five bedrooms. Our four children have already moved out, but our six grandchildren regularly come to stay over, so it will be a full house and we get through a lot of hot water. What kind of boiler would suit in this case?
Your household is too large for a combi boiler to make sense – but there are plenty of alternatives you can opt for. If the main structural work has already been completed and the floors fitted, a system or regular boiler with a hot water storage cylinder and conventionally-sized radiators would be the least disruptive setup for you to install.
However, if a guest bathroom is further than 10m away from the cylinder it might be worth fitting an additional vessel just for this zone, or even having secondary hot water circulation from one main storage tank. If you install a twin-coil cylinder here, you will also have the option to supplement the property’s DHW supply with solar thermal panels.
Martyn Bridges began his career as a heating and plumbing apprentice and has a strong technical knowledge of boilers and renewable heating systems. He is now the director of marketing and technical support at Worcester, Bosch Group. For further information on Worcester’s high efficiency boiler range visit www.worcester-bosch.co.uk or call 0330 123 9339.
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