New Combi Boiler Guide

Quick guide summary

What is a combi boiler? Is a combi right for me? What size do I need? How much does a new combi cost?

A combi boiler is a single unit combing heating and hot water production. In this guide we explain combi boilers and the importance of hot water flow rates when sizing your boiler. We advise when a combi is right for your home and provide easy comparison tables that show the best combi boilers on the market by property size, boiler price, warranty length, efficiency and hot water flow rates. Prices range from £600 - £2,000 depending on the model and the size. We help you find the right combi in your price bracket. How else can we help? We can give you guidance on the different ways you can pay for a new boiler. For a bespoke installation cost, try our Online Boiler Quote Calculator, results appear instantly on screen - we do not require an email address or any other personal information.

Overview of Combi Boilers

What is a combi boiler?

A combination boiler (or “combi”) is a compact, single unit that generates all the heating and hot water needed for the household. There are no hot water tanks.

Is a combi right for me?

We help you weigh up space saving attributes and comprehensive boiler warranty cover against multiple bathrooms and poor incoming mains pressure.

What size combi do I need?

We show you how to size for the hot water requirements of your household.

Find a great value combi

Whether you are on a tight budget or you need a large, high performing combi boiler, we present the best boilers in their price bracket so you can find the best deal.

How to use this Guide

What is a combi?

Consider pros and cons

Find the right size & best model

Combis in larger properties

What is a combi boiler, what are the pros and cons?

What is a combi boiler and how does it work?

​A combination boiler combines heating and hot water production into a single, usually wall hung, unit. Unlike a regular or system boiler that works with a hot water tank, a combi boiler heats water directly from the cold mains. Fresh water 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 tank.

Advantages of a Combi boiler

  • Creates space when removing old hot water tanks
  • 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.

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. A single bathroom property in a 1 or 2 bedroom flat will only require a small combi with a flow rate of around 10LPM. In larger 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.

Our tables below show hot water flow rates in the right hand column. Boilers are grouped together according to property size. To calculate your hot water requirements you need to consider how many bathrooms you have and hot often they are in use at the same time. Most hot water flow rates are in line with the property sizes below, but you might need to make adjustments. For example, if you have three bathrooms that are in use at the same time in the mornings then opt for a boiler from the 5+ bed house range. If you have one bathroom but just want very good hot water performance, consider a boiler from the 3-bed house range (NB the heating output is usually the same for the small-medium sized boilers in a manufacturer's range). Also consider how far the hot water has to travel from the boiler. If your boiler is in the loft then opt for a bigger hot water flow rate. 

You will notice that the flow rates can vary quite a lot in each property size category. 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.

Do I stick with my current boiler or install a combi?

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.

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.

Water Pressure/Flow Rates: A combi boiler requires a 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.

Warranty cover and space: If there is little risk of system leaks and you have good water pressure, then installing a combi will be space saving and it will give you a comprehensive warranty cover from the manufacturer for both heating and hot water functions. It will also reduce maintenance costs long term as the old hot water tank, pump and valves will be removed when a combi is installed

Hot water demand: 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 tank (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.

Find the best boiler for your budget

Find your property size

  • 1-2 Bedroom Flat
  • 2 Bedroom House
  • 3 Bedroom House
  • 4-5 Bedroom House
  • 5+ Bedroom House

Factor in hot water flow rates (adjust as required -see above)

  • 1 bathroom - 10-12 LPM
  • 2 bathrooms - 11-14 LPM
  • 3 bathrooms - 14-16 LPM
  • 3+ bathrooms - 16-25 LPM

What is your priority?

  • Cupboard fit
  • Best overall boiler package
  • Best price
  • Best efficiency
  • Best hot water flow rate

Find your best boiler on price & warranty

  • £ - Entry level (cheapest)
  • ££ - Economy (one-up from entry level)
  • £££ - Mid-range
  • ££££ - Premium (the most expensive)

Top Tips

Find your property size and your best boiler

  1. Find your property size below and open the drop down box.
  2. If you have multiple bathrooms, you can opt for a larger boiler and get better hot water flow rates. For example if you have a 3 bed house with 3 bathrooms, that are often in use at the same time, you could consider one of the boilers in the 4 or 5+ bed category.
  3. If you require a cupboard fit then use the first column to find compact models
  4. Boilers are listed in budget order, low to high
  5. Find the best boiler for your budget and circumstances. For example, if you are moving in two years and on a tight budget, consider one of the budget boilers that still come with a 5 year warranty.
  6. Efficiency and hot water flow rates are listed at the end. Hot water flow rates are important if you have more than one bathroom. If you have 2-3 bathrooms that often in use at the same times then consider one of the larger boilers with a better hot water flow rate.

Find your best boiler according to your budget

1-2 bed flat
Price band   Cupboard fit Boiler cost Manufacturer Cover Warranty or Guarantee Efficiency Litres per minute hot water
£ Glow Worm Easicom 24 Y £ 570 4 G 90% 10
££ Baxi 400 424 Y £ 663 5 W 89% 9.8
££ Baxi Duotec 24 N £ 675 7 W 89% 9.8
££ Ideal Logic + C24 N £ 794 7 W 90% 9.9
£££ Vaillant EcoFIT pure 825 Y £ 864 5-10 G 89% 10.4
£££ Worcester Bosch Greenstar Pro 25i N £ 875 7 G 94% 9.8
£££ Baxi Ecoblue Advance 24 Y £ 920 10 W 89% 9.8
£££ Worcester Greenstar Compact 25Si Y £ 1,010 9 G 94% 9.8
£££ Viessman Vitodens 100 26 N £ 1,050 10 W 89% 8.9
££££ Worcester Greenstar Compact 28CDi Y £ 1,045 10 G 94% 10
££££ Ideal Vogue C26 N £ 1,056 10 W 91% 10.6
££££ Vaillant EcoTEC plus 825 Y £ 1,107 5-10 G 89% 10.5
2 bed house
Price band   Cupboard fit Boiler cost Manufacturer Cover Warranty or Guarantee Efficiency Litres per minute hot water
£ Glow Worm Easicom 28 Y £ 605 4 G 90% 11.7
£ Ideal Independent C30 Y £ 651 5 W 89% 12.3
£ Glow Worm Ultimate 2 30 N £ 694 5 G 89% 12.5
££ Baxi Duotec 28 Y £ 795 7 W 89% 11.5
££ Potterton Titanium 28 N £ 798 7 W 91% 11.5
£££ Vailliant EcoFITpure 830 Y £ 896 5-10 G 89% 12.4
£££ Baxi Platinum 28 N £ 955 10 W 89% 11.5
£££ Baxi Ecoblue Advance 28 Y £ 1,060 10 W 89% 11.5
£££ Viessmann Vitodens 100 30 N £ 1,100 10 W 89% 11.2
£££ Worcester Greenstar Compact 30Si N £ 1,115 9 G 94% 11.5
££££ Worcester Greenstar Compact 32CDi Y £ 1,232 10 G 94% 11.5
3 bed house
Price band   Cupboard fit Boiler cost Manufacturer Cover Warranty or Guarantee Efficiency Litres per minute hot water
£ Ideal Independent C35 Y £ 727 5 W 89% 14.5
£ Glow Worm Ultimate 2 35 Y £ 766 5 G 89% 14.4
££ Potterton Titanium 33 N £ 858 7 W 91% 13.5
££ Viessmann/Vitodens 50 35 N £ 906 7 W 89% 14.5
£££ Ideal Logic+ C35 Y £ 985 7 W 90% 14.5
£££ Vailliant EcoFITpure 835 Y £ 1,098 5-10 G 89% 14.4
£££ Viessmann/Vitodens 100 N £ 1,100 10 W 89% 13.5
£££ Baxi Ecoblue Advance 33 Y £ 1,175 10 W 89% 13.5
££££ Ideal Vogue C32 N £ 1,245 10 W 91% 13.1
££££ Vailliant EcoTEC plus 835 N £ 1,300 5-10 G 89% 14.3
££££ Worcester Greenstar Compact 36CDi Y £ 1,358 10 G 94% 12.9
££££ Vaillant EcoTEC Exclusive 835 with Green IQ N £ 1,634 5-10 G 94% 14.5
4 bed house
Price band   Cupboard fit Boiler cost Manufacturer Cover Warranty or Guarantee Efficiency Litres per minute hot water
£ Ferroli / Modena 38kW Y £ 764 5 W 89% 16.0
££ Potterton / Titanium 40 N £ 930 7 W 91% 16.4
££ Baxi Duotec 40 N £ 985 7 W 89% 16.4
££££ Baxi Platinum 40 N £ 1,225 10 W 89% 16.4
££££ Ideal Vogue C40 N £ 1,399 10 W 91% 16.4
££££ Vailliant EcoTEC plus 838 Y £ 1,570 7-10 G 89% 15.9
££££ Worcester Greenstar Classic 42CDi N £ 1,665 10 G 92% 15.0
5+ bed house
Price band   Cupboard fit Boiler cost Manufacturer Cover Warranty or Guarantee Efficiency Litres per minute hot water
£££ Vaillant ecoTEC plus 938 storage combi N £ 1,570 7-10 G 89% 20.0
££££ Vaillant ecoTEC Green iQ 843 N £ 1,979 7-10 G 94% 17.8
££££ Worcester Highflow 440CDi N £ 2,016 10 G 92% 20.0
££££ Worcester Highflow 550CDi N £ 2,447 10 G 92% 25.0

Combis in larger homes

The biggest combi boilers can offer about 20 litres per minute hot water flow rate, providing the incoming mains pressure can support that. If you have a large household who shower simultaneously then the flow rates will often be cut to 7 or 10 litres per minute, less if a hot tap is running somewhere too. For very large properties a combi boiler is not likely to be suitable, unless you install more than one of them. Instead consider a system boiler with high performing hot water tank. For more on system boilers and hot water tanks, read our guides (links below).

What would you like to do next?

Use our calculator to get an estimate of boiler installation cost


Boiler Types Explained – combi, regular and system boilers


Guide to Hot Water Tanks


Other useful guides

Book a survey with a local heating company


Guide to boiler warranties and guarantees


Manufacturer smart controls v Nest and Hive