Best Gas Boilers 2021 | Our 'Pepsi Challenge'
Which is the best gas boiler for my home?
We have undertaken a fully independent review of the vast majority of domestic gas boilers available to buy in the UK. Most 'best buy' blogs are written by boiler installation companies and are biased in favour of the boilers sold by that company. Even Which? do not assess the technical merits of the boiler, relying rather on customer service criteria and market opinions, not technical facts. This means consumers never get a clear view of which is actually the best boiler for them and their budget.
We are completely independent and impartial with no affiliations with boiler manufacturers. Our assessment of UK boilers is based first and foremost on the technical merits of the boiler. We did the 'Pepsi challenge' for boilers, which is not to say we were blindfolded, thankfully, rather we ignored brand strength, marketing blurb and the number of boilers sold.
Our discovery
Following our review, we have found that the UK boiler market is made up of what we have termed 'low-tech' and 'high-tech' boilers. Low-tech boilers present few options for adjusting the boiler to your heating system to make it run efficiently whilst high-tech boilers offer the latest technology and the most potential to run efficiently.
Our most surprising discovery however is that price is absolutely no indication of whether a boiler is low-tech or high-tech. We found some very expensive low-tech boilers from familiar brands and very cheap high-tech boilers from manufacturers you may never of heard of.
Warning label
We want to answer the question of 'which is the best boiler' honestly and provide some meaningful advice on this topic. But our recommendations come with some heavy caveats and warning labels, as follows:
1) A boiler is only as good as the installer that fits it.
2) A boiler is only as good as the heating control it is paired with.
So that even 'the best' boiler will perform poorly if setup incorrectly and paired with a basic control. 99% of installers have not been trained to setup condensing boilers to run at their A-rated efficiencies and basic controls are not banned under building regulations, so most boilers are B-E rated in the home. (You can find out more about our work to recruit the top 1% of installers and shortlist high efficiency controls with the right boilers, go to Expert Panel.)
Award summary
We have grouped together boilers by type (combi, heat-only and system boilers) and then grouped them again by price. The tables show the boilers we have reviewed and the winners are given for each section.
Best heat only boilers
- Vaillant EcoFit Pure
- Worcester 8000
- Glow Worm Energy
Best system boiler
- Viessmann 200
- Ravenheat CS
- Intergas Xclusiv
Best combi boilers
- Ravenheat HE80
- Intergas Xclusive and Vaillant EcoTec Pure
- Viessmann Vitodens 200
The best combi boilers
In the tables below we have presented the smallest combi boilers in each range, by which we mean with the lowest hot water performance. Larger boilers that give a better hot water performance are available in all models but remember that the higher hot water performance, the less efficient they tend to be for heating. If you need a combi with a big hot water performance see the section at the end of this page.
The best entry level combi boiler - WINNER! Ravenheat HE80
As well as a great price and very decent 5-year warranty, the most impressive aspect of this boiler is its minimum output. All boilers operate within a range of maximum and minimum outputs. The lower the boiler can go the more efficient it will operate all year round. To find a boiler for this price that can operate at 3.9kW is very impressive!
Entry Level Boilers
| Manufacturer/Model | Code | Price | Standard warranty | Free Extended Warranty | Max heating output | Min heating output | Litres per minute hot water |
|---|---|---|---|---|---|---|---|
| Main Eco Elite | 25 | £525 | 5 | 5 | 24kW | 8.4kW | 10.2 |
| Ideal Independent | 24 | £670 | 5 | 5 | 24kW | 4.8kW | 9.9 |
| Alpha Evoke | 28 | £670 | 3 | 7 | 27kW | 4.3kW | 12.1 |
| Ravenheat HE80 | 25 | £649 | 5 | 5 | 21kW | 3.9kW | 10.2 |
| Potterton Titanium | 24 | £715 | 7 | 7 | 21kW | 7.4kW | 9.8 |
| Glow Worm Energy | 25 | £700 | 3 | 3 | 19kW | 5.3kW | 10.1 |
The best mid-range boiler - JOINT WINNERS! Intergas Xclusive and Vaillant Ecotec Plus
This is a crazy category. Prices vary between £738 at the lower end and £1,145, with no real consistent explanation as to the price difference. The cheapest and the second most expensive have the same lower output on the boiler – 7.4kW – the same efficiencies and the same hot water performance. Only the warranty is longer on the Baxi Platinum.
The winners are two great all-rounders. The Intergas Xclusive and Vaillant Ecotec Plus. The Intergas is a great price with a 10-year warranty. The Intergas has the edge with a lower minimum output and coupled with their sophisticated controls it is one of the most efficient boilers on the market. Vaillant are a more familiar brand with a strong customer service back up. Remember, the lower the boiler can go, the more efficiently it can operate all year round.
Mid range boilers
| Manufacturer/Model | Code | Price | Standard warranty | Free Extended Warranty | Max heating output | Min heating output | Litres per minute hot water |
|---|---|---|---|---|---|---|---|
| Potterton/Titanium | 24 | £715 | 7 | 7 | 21kW | 7.4kW | 9.8 |
| Worcester 2000 | 25 | £778 | 5 | 10 | 20kW | 6.7kW | 10.0 |
| Ideal Logic Plus | C24 | £825 | 7 | 7 | 25kW | 7.1kW | 9.9 |
| Vaillant Ecofit Pure | 825 | £910 | 2 | 7 | 19kW | 5.3kW | 10.4 |
| Baxi Ecoblue Advance | 24 | £1,020 | 10 | 10 | 21kW | 4.9kW | 9.8 |
| Intergas Xclusive | 24 | £969 | 10 | 10 | 19kW | 3.6kW | 9.8 |
| Worcester Greenstar Si | 25 | £1,045 | 5 | 10 | 24kW | 8.1kW | 9.8 |
| Ideal Vogue | C26 | £1,005 | 10 | 10 | 19kW | 4kW | 10.6 |
| Viessmann | 100-W | £1,070 | 5 | 7 | 26kW | 4.7kW | 8.9 |
| Baxi Platinum | 24 | £1,110 | 10 | 10 | 24kW | 7.4kW | 9.8 |
| Vaillant Ecotec Plus | 825 | £1,145 | 5 | 10 | 20kW | 3.8kW | 10.5 |
Best high-end combi boiler - WINNER! Viessmann Vitodens 200-W
At the upper end we have the ground breaking Viessmann 200 boiler. With the lowest output on the market at an impressive 1.9kW, this is one of the most efficient boiler available to buy.
We’d also like to give a special mention to the Intergas Xtreme. Still impressive at 3.6kW at the lower end and the cheapest of the group, this is an incredible boiler with some of the most advanced heating controls available.
High end boilers
| Manufacturer/Model | Code | Price | Standard warranty | Free Extended Warranty | Max heat output | Min heat output | Litres per minute hot water |
|---|---|---|---|---|---|---|---|
| Intergas Xtreme | 24 | £1,209 | 7 | 7 | 20kW | 3.6kW | 11 |
| Intergas Xclusive | 30 | £1,029 | 10 | 10 | 23kW | 3.6kW | 12.2 |
| Worcester 8000 | 30 | £1,275 | 5 | 10 | 30kW | 3kW | 10.7 |
| Viessmann Vitodens 200 | W | £1602 | 3 | 10 | 25kW | 1.9kW | 15.7 |
| Vaillant ecoTEC Green iQ | 835 | £1,465 | 5 | 10 | 26kW | 3.4kW | 14.5 |
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The best heat only boilers
We have researched the majority of heat only boilers to bring you our pick of the best priced and best-performing heat only boilers for 1-3 bed houses and larger homes. REMEMBER, the vast majority of UK homes do not need anything bigger than a 6-8kW boiler, 10kW max! Most of us will need a boiler from the 1-3 bedroom table, but you must factor in hot water production.
Best heat only boiler for 1-3 bedroom properties
The average UK home does not require more than 6-8kW of heat on a very cold day. Therefore the smallest boilers available to buy (commonly 12-13kW) will comfortably heat most homes. The most important figure to note in the tables below is not the maximum capacity – which will always be enough – but the lowest capacity. The lower the minimum output the more efficient the boiler will operate all year round.
REMEMBER - the hot water must be set up correctly for smaller output boilers. Smaller boilers will take longer to heat up a cylinder of water and that must be factored in. If you have a 12kW cylinder however then a 12kW boiler will suffice.
If you have a larger hot water cylinder then go to one of the larger models below (also see next table down). The important thing to note is the minimum output of the boiler. The Worcester 8000 has a big output of 30kW but can go right down to 3kW. This will give you 30kW of power for hot water and reduce down to 3kW for the heating.
We have ranked boilers in order of the lowest output. This means that the Worcester 30 boiler, which can output at 30kW, is actually the most efficient for smaller homes; it is also the most expensive. We have given our best boilers based on price and all-round good value:
| Boiler | Code | Price | Standard warranty | Free Extended Warranty | Max heating output | Min heating output |
|---|---|---|---|---|---|---|
| Worcester 8000 regular | 30 | £1,275 | 5 | 10 | 30kW | 3.0kW |
| Worcester Greenstar Ri | 12i | £810 | 5 | 10 | 12kW | 3.4kW |
| Vaillant ecoFIT pure regular | 412 | £740 | 2 | 7 | 12kW | 4.5kW |
| Glow Worm Energy Regular | 12 | £740 | 7 | 7 | 12kW | 4.5kW |
| Vaillant ecoTEC Plus Regular | 412 | £805 | 5 | 10 | 12kW | 4.5kW |
| Ideal Logic Heat + | 12 | £705 | 7 | 7 | 12kW | 5.1kW |
| Intergas HRE | 18 | £759 | 10 | 10 | 18kW | 6.1kW |
| Viessmans Vitodens 100-W Regular | 26 | £960 | 5 | 5 | 26kW | 7.2kW |
| Baxi 200 | 12 | £600 | 3 | 3 | 13kW | 7.5kW |
Best priced heat only winner for 1-3 bed homes - WINNER! Vaillant EcoFIT Pure
A great all-round purchase, particularly when it comes with the 10-year warranty (it must be fitted by an accredited installer to get this).
Best efficiency heat only boiler for 1-3 bed homes - WINNER! Worcester 8000 30kW
A little pricey at £1,253, but it offers the lowest output in the market and with a 10-year warranty.
The best heat only boilers for larger properties
Remember that most properties in the UK need less than 10kW on a very cold day, which includes a lot of larger properties. For example, a 4 bed home will need 4-8kW of heat depending age and insulation levels. For heating systems, the smallest boilers will be sufficient, i.e. 11-13kW, but larger properties are likely to have larger hot water requirements. Therefore you will need to consider a bigger output boiler that goes down really low on the heating side.
Again we have ranked boilers in order of their lowest output and again the Worcester Bosch 8000, a 30kW boiler, has the lowest output, making it the most efficient.
NOTE: It is really important not to increase your boiler size ‘just in case’ as the next size up boiler will have a higher minimum output and this will reduce efficiencies. Only increase the size of the boiler if you need more power for hot water.
| Boiler | Code | Price | Standard warranty | Free Extended Warranty | Max heating output | Min heating output |
|---|---|---|---|---|---|---|
| Worcester 8000 | 30 | £1,275 | 5 | 10 | 30kW | 3.0kW |
| Glow Worm Energy Regular | 15 | £755 | 7 | 7 | 15kW | 4.5kW |
| Vaillant ecoFIT pure regular | 415 | £775 | 2 | 7 | 15kW | 4.5kW |
| Vaillant ecoTEC plus regular | 415 | £880 | 5 | 10 | 15kW | 4.5kW |
| Worcester Bosch Greenstar Ri | 15 | £830 | 5 | 10 | 15kW | 4.6kW |
| Ideal Logic + Heat | 15 | £740 | 7 | 7 | 15kW | 4.9kW |
| Main Eco Compact | 15 | £685 | 5 | 5 | 16kW | 5.4kW |
| Intergas HRE | 18 | £759 | 7 | 7 | 18kW | 6.1kW |
| Viessmans Vitodens 100-W Regular | 16 | £855 | 5 | 5 | 16kW | 7.2kW |
| Baxi 200 Heat | 15 | £630 | 3 | 3 | 16kW | 7.5kW |
Best priced heat only boiler for larger houses - WINNER! Glow Worm Energy
At £722, it’s one of the cheapest and with the second lowest minimum output.
Best efficiency heat only boiler for larger houses - WINNER! Worcester 8000 30kW
Again a pricey £1,253 but it offers the lowest output in the market.
The best system boilers
We have reviewed the technical datasheets of all system boilers to find the most technologically advanced and best value. Again the lower the minimum output the more efficient the boiler will operate all year round. Our research has revealed some surprising winners.
The best system boiler for 1-3 bed houses, small hot water cylinder
All boilers operate within a range of maximum and minimum outputs - the lower the boiler can go the more efficient it will operate all year round. Remember that most UK homes only need 6-8kW on a very cold day, therefore the smallest system boilers available to buy (9kW-12k) will comfortably heat most homes.
However you must factor in hot water requirements. A 12kW boiler will take longer to heat a cylinder than a 30kW boiler. If you need a high output boiler for a large hot water cylinder, pick one that goes down really low on the heating side.
We have reviewed all of the system boilers on the market to bring you boilers with low minimum outputs.
| Manufacturer/Model | Code | Price | Standard warranty | Free Extended Warranty | Max heating output | Min heating output |
|---|---|---|---|---|---|---|
| Viessmann Vitodens 200-W | 11 | £1,375 | 5 | 10 | 11kW | 1.9kW |
| Worcester Greenstar system | 9i | £903 | 5 | 7-10 | 9kW | 3.0kW |
| Ideal Vogue system | 15 | £905 | 10 | 10 | 15kW | 3.2kW |
| Vaillant Ecotec Plus | 824 | £1,107 | 2 | 10 | 20kW | 7.1kW |
| Baxi System | 624 | 1015 | 7 | 7 | 25kW | 4.9kW |
| Vaillant ecoTEC Plus | 612 | £910 | 2 | 7 | 12kW | 4.7kW |
| Worcester 8000 system | 30 | £1,310 | 5 | 10 | 30kW | 3.0kW |
| Ravenheat CS system boiler | 24 | £545 | 5 | 5 | 26kW | 4.0kW |
| Worcester Greenstar system | 12i | 900 | 5 | 10 | 12Kw | 3.1kW |
| Vaillant ecoFIT pure | 612 | £865 | 2 | 7 | 12kW | 4.7kW |
| Glow Worm Energy | 12 | £690 | 7 | 7 | 12kW | 4.5kW |
| Viessmann Vitodens 100-W | 19 | £1,035 | 5 | 5 | 19kW | 4.3kW |
| Ideal Logic + system | 15 | £795 | 7 | 7 | 15kW | 5.1kW |
Best priced system boiler - WINNER! Ravenheat CS System 24
An incredible low £546 with an impressive 4kW minimum output. A great economy boiler, but just 2 years of warranty cover.
Best efficiency system boiler - WINNER! Viessmann Vitodens 200-W
Viessmann's latest boiler goes down to an incredible 1.9kW at the lower end - the lowest on the market. A little more expensive than the Worcester 8000 but well worth it for the efficiency gains.
Best system boilers for large houses, large hot water cylinder
We have found that many boiler manufacturers do not offer boilers under 15kW, hence why some 15kW or bigger appear in the 1-3 bedroom table above. The important figure to note is the minimum output in the right hand column. If that is low then it will be more efficient all year around.
Remember that most properties in the UK need less than 10kW on a very cold day, which includes a lot of larger properties. For example, a 4 bed home will need 4-8kW of heat depending age and insulation levels. For heating systems, the smallest boilers will be sufficient, i.e. 11-13kW, but larger properties are likely to have larger hot water requirements. Therefore you will need to consider a bigger output boiler that goes down really low on the heating side.
Again we have ranked boilers in order of their lowest output and again the Worcester Bosch 8000, a 30kW boiler, has the lowest output, making it the most efficient.
NOTE: It is really important not to increase your boiler size ‘just in case’ as the next size up boiler will have a higher minimum output and this will reduce efficiencies. Only increase the size of the boiler if you need more power for hot water.
| Manufacturer/Model | Code | Price | Standard warranty | Free Extended Warranty | Max heating output | Min heating output |
|---|---|---|---|---|---|---|
| Viessmann Vitodens 200-W | 11 | £1,470 | 5 | 10 | 19kW | 1.9kW |
| Worcester 8000 | 30 | £1,310 | 5 | 10 | 30kW | 3.0kW |
| Ideal Vogue system | 15 | £905 | 10 | 10 | 15kW | 3.2kW |
| Ravenheat CS system boiler | 24 | £546 | 2 | 2 | 26kW | 4.0kW |
| Vaillant ecoFIT pure | 612 | £865 | 2 | 7 | 12kW | 4.7kW |
| Baxi 600 system | 15 | £905 | 7 | 7 | 16kW | 4.9kW |
| Worcester Greenstar system | 12i | £900 | 5 | 10 | 12Kw | 3.1kW |
| Viessmann Vitodens 100-W | 19 | £1,035 | 5 | 10 | 19kW | 4.3kW |
| Ideal Logic + system | 15 | £905 | 10 | 10 | 15kW | 3.2kW |
Best prices system boiler for larger homes WINNER! Ravenheat CS System 24
Very low price of £546 with an impressive 4kW minimum output. A great economy boiler, but on 2 years of warranty cover.
Best efficiency system boiler for larger homes WINNER! Viessmann Vitodens 200-W
Incredible low minimum output of 1.9kW making it efficient all year round.
The best combis for big hot water performance
We have reviewed all the big hot water combis and compared them against their lowest outputs for heating. If you have two or more bathroom in use simultaneously then some of the larger combis may meet your requirements.
The downside of some larger combis is that the minimum heat requirement increases as the hot water performance increases.
It is essential that the boiler is not oversized as they are almost all too big for UK homes. Many 2-3 bathroom homes still only need 6-10kW of heat on a very cold day. Make sure the boiler’s maximum output is reduced down to meet this. We celebrate the manufacturers that have achieved great hot water flow rates alongside low minimum outputs. The top four on our table are all high-performance, high efficiency combis. Pair with a compensation control to achieve the best possible efficiencies.
Best combi for hot water WITH low minimum output - WINNER Viessmann 200-W
is the clear winner with a low output of 1.9kW and flow rate of 15.7 litres per minute. Although at the upper end price wise.
Best priced combi for hot water WITH low minimum output - WINNER! Intergas Xtreme
A decent low output and at the lower end price wise. The heating controls for Intergas are amongst the most efficient on the market so you would definitely benefit from further fuel savings with this boiler.
| Manufacturer/Model | Code | Price | Standard warranty | Free Extended Warranty | Max heating output | Min heating output | Litres per minute hot water |
|---|---|---|---|---|---|---|---|
| Viessmann Vitoden 200 | 25 | £1,602 | 3 | 5 | 25kW | 1.9kW | 15.7 |
| Worcester 8000 | 45 | 1,904 | 5 | 10 | 34kW | 3.5kW | 16.1 |
| Intergas Xtreme | 36 | £1,389 | 7 | 7 | 27kW | 3.6kW | 14.5 |
| Vaillant ecoTEC Green iQ | 843 | £1,688 | >5 | 10 | 33kW | 4.3kW | 17.8 |
| Ideal Vogue | 40 | £1,318 | 10 | 10 | 34kW | 6.1kW | 16.4 |
| Baxi Ecoblue advance | 40 | £1,364 | 5 | 5 | 34kW | 8.4kW | 16.4 |
| Baxi Platinum | 40 | £1,328 | 5 | 5 | 34kW | 12.1kW | 16.4 |
What size gas boiler do I need?
The average heat requirement for a UK home is just 6-8kW on a very cold day. As you can see from the boilers above, most boilers are oversized for the properties they are installed in. There is also often a temptation is to oversize the boiler, just in case. Oversized boilers are in fact less efficient. A boiler should be sized accurately based on your house construction, i.e. insulation levels and hot water requirements. For more achieving efficient systems read our guide: Why oversized boilers are bad for your bank balance and the planet.
The good news is that modern boilers can be 'range rated' down, which means the maximum output can be reduced to match the property's maximum heat requirement, for example 6-8kW.
The problem is that for combi boilers the minimum output often exceeds the maximum heat requirement of the property. Many of the smallest combi boilers will only reduce down to 7-8kW at their lowest. For the majority of the year the boiler will need less than 6-8kW and so the boiler will not run as efficiently as possible.
This is made worse when boilers sizes are increased in order to improve hot water performance; the better the hot water performance, the higher the minimum output. Only at the higher budget end do we find boilers with a good hot water performance alongside a low minimum output, such as the Viessmann 200, Worcester 8000 and Vaillant ecoTEC Green iQ.
If you want great hot water performance with a low heating output to maximise efficiencies, you can also consider a system boiler with high performing hot water tank.
Sizing a combi boiler
The table below gives you a rough indication based on 1) the size of your property and the number of bathrooms and 2) how well insulated it is. Property ages are given across the top and they assume standard insulation measures, such as double glazing and loft insulation, have been made. If your levels of insulation are very high in an older property then you may wish to move a column to the right. If you are planning on adding bathrooms you will need to factor in hot water flow rates (see below).
Sizing for Hot Water - Our Top Tips
The hot water flow rate of a combi boiler, expressed as ‘litres per minute’, indicates how quickly a combi boiler can heat hot water. This is important for houses with multiple bathrooms because the hot water flow rate will be halved if two showers are running simultaneously. We have grouped combi boilers together according to the heating output of the boiler which starts at 18kW. Most houses only require 6-8kW of heat at peak times therefore even the smallest combi will aways be oversized. In nearly all cases the gas engineer will need to reduce the output of the boiler on the heating side down to the heat requirement of your property.
Your choice of combi boiler really then comes down to other factors, for example the hot water flow rate and perhaps cost and warranty. For two or more bathrooms you may wish to consider a system boiler and high-performance hot water tank.
| Age of property | pre 1900 | 1920s-1930s | 1950-1980s | 1990s | 2000 onwards |
|---|---|---|---|---|---|
| Insulation levels | Poorly insulated | Some insulation | Moderately insulated | Reasonably insulated | Well insulated |
| No of bedrooms and bathrooms | |||||
| 1 bed + 1 bath | 7kW/9-12LPM | 7kW/9-12LPM | 6kW/9-12LPM | 6kW/9-12LPM | 4kW/9-12LPM |
| 2 bed + 1 bath | 8kW/9-12LPM | 8kW/9-12LPM | 7kW/9-12LPM | 6kW/9-12LPM | 5kW/9-12LPM |
| 2 bed + 2 bath | 8kW/12-16LPM | 8kW/12-16LPM | 7kW/12-16LPM | 6kW/12-16LPM | 5kW/12-16LPM |
| 3 bed + 1 bath | 8kW/9-12LPM | 8kW/9-12LPM | 7kW/9-12LPM | 7kW/9-12LPM | 6kW/9-12LPM |
| 3 bed + 2 bath | 8kW/12-16LPM | 8kW/12-16LPM | 8kW/12-16LPM | 8kW/12-16LPM | 6kW/12-16LPM |
| 4 bed + 1 bath | 10kW/9-12LPM | 10kW/9-12LPM | 8kW/9-12LPM | 8kW/9-12LPM | 7kW/9-12LPM |
| 4 bed + 2 bath | 10kW/12-16LPM | 10kW/12-16LPM | 8kW/12-16LPM | 8kW/12-16LPM | 7kW/12-16LPM |
| 5 bed + 2 bath | 12kW/12-16LPM | 12kW/12-16LPM | 10kW/12-16LPM | 10kW/12-16LPM | 9kW/12-16LPM |
Sizing a heat only boiler
It is more straightforward to size a heat only boiler than a combi as you are only sizing the heat production, not the hot water. In the table below you can get a rough idea of what size boiler you need based 1) the size of your property and 2) how well insulated it is. Property ages are given across the top and they assume standard insulation measures, such as double glazing and loft insulation, have been made. If your levels of insulation are very high in an older property then you may wish to move a column to the right.
| Age of property | pre 1900 | 1920s-1930s | 1950-1980s | 1990s | 2000 onwards |
|---|---|---|---|---|---|
| Insulation levels | Poorly insulated | Some insulation | Moderately insulated | Reasonably insulated | Well insulated |
| Number of bedrooms | |||||
| 1 | 7kW | 7kW | 6kW | 6kW | 4kW |
| 2 | 8kW | 8kW | 7kW | 6kW | 5kW |
| 3 | 8kW | 8kW | 7kW | 7kW | 6kW |
| 4 | 10kW | 10kW | 8kW | 8kW | 7kW |
| 5 | 12kW | 12kW | 10kW | 10kW | 9kW |
| 5+ | 15kW | 15kW | 14kW | 12kW | 10kW |
Sizing a system boiler
Like a heat only boiler, a system boiler can be sized for heating only. Use the table below to find the right size based on 1) property size and 2) property age factoring in insulation levels. If you house has seen standard levels of improvements, such as loft insulation or double glazing, then go with its age bracket. If it has been heavily improved then consider moving one column to the right. Also like heat-only boilers, all the system boilers available to buy are too big for the majority of UK homes and must be range rated down by a gas engineer to better match the heat requirement of your home.
| Age of property | pre 1900 | 1920s-1930s | 1950-1980s | 1990s | 2000 onwards |
|---|---|---|---|---|---|
| Insulation levels | Poorly insulated | Some insulation | Moderately insulated | Reasonably insulated | Well insulated |
| Number of bedrooms | |||||
| 1 | 7kW | 7kW | 6kW | 6kW | 4kW |
| 2 | 8kW | 8kW | 7kW | 6kW | 5kW |
| 3 | 8kW | 8kW | 9kW | 7kW | 6kW |
| 4 | 10kW | 10kW | 8kW | 8kW | 7kW |
| 5 | 12kW | 12kW | 10kW | 10kW | 9kW |
| 5+ | 15kW | 15kW | 14kW | 12kW | 10kW |
