Genuine, unbiased consumer advice on combi boilers
- What is a combi boiler?
- How does a combi boiler work?
- Advantages and disadvantages
- Is a combi right for me?
- Sizing your hot water requirements
- What size combi do I need?
- Combi boiler prices
- Combis for larger homes
- The pitfalls of very large combi boilers
- Combi boiler efficiencies
- Best combi boilers
- Combi v system boilers
COMBI BOILER GUIDE
1) What is a combi boiler?
A combination boiler combines heating and hot water production into a single, usually wall hung, unit. Unlike a regular or system boiler that work with a hot water cylinder, a combi boiler heats water directly from the cold mains.
2) How does a combi boiler work?
Fresh water for drinking and washing (known in the trade as potable water) from the water mains is heated via an integral heat exchanger and sent straight to the taps and bathrooms. In order words, it provides instantaneous hot water rather than storing it in a cylinder. The heat exchanger has fresh water on one side and 'system' water on the other. The boiler heats the system water and the heat exchanger transfers the heat through metal plates to the fresh water.
3) Advantages and disadvantages
The pros and cons of a combi boiler will depend upon your home and existing heating system. If you have poor incoming pressure of multiple bathrooms a combi may not be suitable.
Advantages of a combi boiler
- Creates space when removing an old hot water cylinder
- Lower maintenance costs - the integral heating and hot water functions are covered by the boiler manufacturer's warranty/guarantee
- No long wait times for hot water
Disadvantages of a combi boiler
- They do not work well in properties with poor flow rates and/or poor incoming water pressure
- Hot water flow rates are reduced when providing hot water to two or more outlets simultaneously, although this is really only a problem in houses with two or more showers.
- Big hot water capacity combi boilers can be inefficiency for heating systems (see The pitfalls of very large combi boilers below).
4) Is a combi boiler right for me?
Deciding whether to keep your existing boiler or to switch to a combi depends on many factors. It comes down to the age of the system, your water pressure/internal flow rates and hot water requirements. Moving from a heat-only or system boiler to a combi boiler is more expensive than a straight swap. You will need to consider the advantages of a combi, e.g. better warranty cover and more space, against the additional installation cost. If you have a combi boiler already then now is a good time to review your heating and hot water requirements to make sure you pick the right combi boiler in order to maximise efficiencies and the lifespan of the boiler.
- Age of the system: If you have old radiators, or pipework buried in screed, there is a risk of leaks when installing a combi boiler. Open vented systems, with a Feed and Expansion (F&E) tank in the loft, will need to be converted to a ‘sealed system’, which puts the system under pressure and can cause leaks. You may wish to retain your heat only boiler on an open vented system.
- Water Pressure/Flow Rates: A combi boiler requires good incoming water pressure and good internal flow rates in order to operate. Some areas in the UK will have poor incoming water pressure and a combi boiler will not be suitable. Some properties will have very convoluted pipework which will also make it difficult for the boiler to operated. Combi boilers need at least 10 litres per minute flow rate to operate. If you have less then you'll either need a mains pump (which can increase it to 12 litres per minute) or a pressure accumulator tank. Check with your neighbours to see if they have a combi boiler. The surveying engineer will also test your water flow rates to see if it is suitable.
- Multiple bathrooms: Combi boilers can give flow rates between 10 and 25 litres per minute, which will cover the vast majority of homes with one or two bathrooms. If you have a large property, lots of bathrooms, you are adding bathrooms or if you and your family have lots of showers at the same time - then even the biggest domestic combi boiler might not cope. When lots of hot water outlets are in use at the same time - two showers for example - then the hot water flow rate of even the biggest boilers is halved. It might be worth considering a system boiler with unvented hot water cylinder (if the water pressure is good). Even the biggest combi boilers can only do so much. For better simultaneous hot water flow rates, a system boiler and high-performance hot water cylinder may be a better option.
5) Sizing your hot water requirements
The hot water flow rate, expressed as litres per minute (LPM), indicates the speed at which the boiler can heat water supplied to the hot water outlets (taps, showers etc). Hot water flow rates on combi boilers can vary from 9.8LPM to 25LPM. Our tables below show hot water flow rates in the right hand column. Boilers are grouped together according to hot water performance. To calculate your hot water requirements you need to consider how many bathrooms you have and how often they are in use at the same time.
A single bathroom property will only require a small combi with a flow rate of around 10LPM. In properties with two or more bathrooms it is important to opt for a higher hot water flow rate as it will be halved if, for example, two showers are in use at the same time. However a system boiler with hot water cylinder is really much more suitable and will give better flow rates using a smaller boiler.
You will notice that the flow rates can vary quite a lot and boilers that appear quite large can have a comparatively low hot water flow rate. Even the cost of the boiler is no indication as to its hot water performance. For example, the Worcester Greenstar Compact 36CDi has a hot water flow rate of 12.9 LMP and costs £1,358 compared with the Viessmann Vitodens 50 35 with a hot water flow rate of 14.5LMP costing just £906.
6) What size combi boiler do I need?
As all combi boilers are sized for hot water, they are nearly always too big for the heating side of the system. The average UK home only needs 6-8kW of heat on a very cold day. The outputs for combi boilers start at around 18kW and get bigger. No matter which combi boiler you buy, it is very important that your installer reduces the output of the heating down to match your heat requirement or else the system will not operate efficiently.
Deciding whether to keep your existing boiler to switch to a combi depends on many factors. It comes down to the age of the system, your water pressure/internal flow rates and hot water requirements. Moving from a heat only or system boiler to a combi boiler is more expensive than a straight swap. You will need to consider the advantages of a combi, e.g. better warranty cover and more space, against the additional installation cost.
7) Combi boiler selection
There are more than 100 models of combi boiler available to buy in the UK of varying quality, efficiency, price and warranty length. We review boilers from 11 of the main manufacturers in our Guide to combi boiler prices.
8) Combis for larger homes
The truth is even the smallest combi boiler will heat the majority of larger homes, it just might not meet your hot water requirements. The question is not which is the right boiler for a larger home, but which is the right boiler to meet my hot water requirements. The biggest combi boilers can offer about 20 litres per minute hot water flow rate, providing the incoming mains pressure can support that. We guide you on the careful selection of combi boilers in our Guide to combi boilers in larger homes.
9) The pitfalls of bigger combis
The popularity of combination boilers since their inception has driven manufacturers to develop products that cater for bigger hot water requirements. The reality is that big combis that give high hot water flow rates can make the heating side of the system inefficient. The biggest combi boilers can give hot water flow rates of 25 litres per minute, requiring 50kW of input. This makes everything bigger and more expensive: the boiler, the components and the gas pipe.
For most UK properties, the heating side of the system needs 6-10kW at most. A much more effective system set up is to use a smaller combi boiler with a small cylinder that provides a store of water at temperature ready to draw off a high flow rate. Whilst being drawn off, the combi boiler heats fresh water entering the cylinder at the same rate it is being drawn off so that it does not run out.
As well as cost savings from buying a smaller boiler, many of the components and pipework will also be smaller. Better still the as heating output is much more closely matched to heat requirements of the home, the boiler can spend much spend more time in ‘condensing mode’ which increasing efficiencies. Size your boiler with our Guide to boiler sizing.
10) Combi boiler efficiencies
Before condensing boilers, boilers were around 60-80% efficient, which means 20-40% of the heat produced by the boiler is wasted via the flue. Condensing boilers are at least 90% efficient. The extra efficiency over non-condensing boilers comes from recovering the heat lost via the flue, turning it back into water and using it to preheat cold water as it enters the system.
In order for a boiler to operate in ‘condensing mode’ the return temperatures to the boiler from the heating system must be around 54 degrees or lower. The problem with many heating systems, particularly those fitted with oversized boilers, is that the return temperature is higher than 54 degrees, which reduces the time the boiler can spend condensing; reducing its efficiency enormously.
For a boiler to condense the boiler output must match the property’s heat requirement, not exceed it! Which is why it is so important to accurately size your boiler. For how to size your heat requirement see our Guide to sizing your boiler. To achieve even better efficiencies, a modulating control is needed to vary the output according to outside temperatures. For more see our Guide to Advanced Controls.
11) Best combi boilers
There are lots of best boiler guides available to read, very few are impartial or look beyond a few boiler manufacturers. We impartially review combi boilers from 11 manufacturers to bring you the best combi boilers based on value for money, warranty length and efficiency. For more see our Guide to the best combi boilers.
12) Combi v system boilers
For homes with large hot water demands it is worth considering a system boiler over a combi boiler. We explore the advantages and disadvantages of both set ups in our blog Combi vs system boilers.