Understanding boiler specs

Whether you're looking for a new boiler, reading up on the heating system in your new home, or just trying to compare your existing boiler with others available, the facts and figures contained in boiler specs are important. That’s because they will help you work out the most efficient and safest ways you can heat your home and its water. Have a look at our easy-to-follow guide to boiler specs below. 

Main types of boiler

There are mainly three types of gas boilers in the UK and each one works a little differently. So, whether you’re looking at a combi, system, or conventional boiler, you’re likely to encounter slightly different specifications. Here, we’ll cover some of the key technical specs and what they mean. We’ll look at:

  • Carbon monoxide emissions
  • Boiler efficiency
  • Boiler power
  • Gas and water pressure
  • Water flow rate

Specs that matter

Carbon monoxide emissions

Find your ErP rating

Knowing your boiler is safe is probably your most important consideration. All gas boilers emit carbon monoxide when heating your home or hot water. This is usually nothing to worry about as this harmful gas is channelled out of your home through the boiler’s flue.

The lower the carbon monoxide emissions listed in the specifications, the better. This is because, should the boiler malfunction or a flue or vent become blocked, less carbon monoxide will be released into the home. You’ll find carbon monoxide emissions included in the specifications of any boiler.

To reduce the risk of carbon monoxide emissions in your home, it’s worth getting your boiler serviced regularly to make sure it is working properly. It’s also a good idea to put an alarm in your home to detect carbon monoxide. The gas can’t be seen, smelt or tasted so an alarm is the only means of learning if it’s escaping into your home and not going up the flue as it should.

To find out more about how to protect yourself from excess carbon monoxide and what to do if you suspect a carbon monoxide leak, see our dedicated article on carbon monoxide.

Boiler efficiency

Another very important spec is all new boilers come with an ‘Energy Related Products’ rating, or ErP rating. It’s a standardised system to help customers understand efficiency and compare products. You might recognise it from the sticker on your fridge, oven, or other appliances in your home. Put simply, the highest rating for a boiler is ‘A’ and the lowest is ‘G’, with A-rated boilers likely to save you money on your heating bills because they’re more energy efficient.

For example, if you live in a detached house in England, Scotland or Wales, and you upgrade from a D-rated gas boiler to an A-rated condensing gas boiler, the Energy Saving Trust estimates you could save around £170 a year on your central heating bill.

For combi boilers, there’s an ErP rating for both heating the space in your home and your water. That’s because combi boilers power your central heating and hot water independently. In contrast, conventional and system boilers just have one ErP rating for heating.

To find out more about boiler efficiency and how it’s calculated, see our handy guide to boiler efficiency

How to understand your boilers specs

Boiler power

What are your boiler specifications

Your boiler’s power is another key specification you need to familiarise yourself with. Normally shown in kilowatts (kW), it determines how much hot water and heating can be supplied to your home (we'll come back to flow rate later). In short, a more powerful boiler will be able to heat more radiators and provide more hot water, which is helpful if you’ve got multiple bathrooms or a larger home.

It’s important to work out how powerful your boiler needs to be to meet your needs. This is best done by looking at a few key factors including: the size of your home, insulation (loft and cavity), the number of occupants, and your energy usage habits.

Two key things to note about boiler power are 'input' and 'output'. Both are measured in kW and, depending on the boiler type, can refer to just your central heating (CH) or both your heating and your domestic hot water (DHW).


This is how much energy your boiler can consume to heat your home and hot water, and it’ll be reflected in your energy bill.


This is the amount of energy that the boiler can actually put into heating your home and hot water — and this number will always be lower than the input because of wastage.


For gas boilers, if you dig a little deeper into their technical specs, you’ll find a series of figures related to pressure. These can refer to either gas pressure or water pressure, and both have an important role to play in boiler health. The specifications you see are typically in units called 'bars', which are also used to measure pressure in car tyres, scuba diving equipment, and weather fronts.

Gas pressure

'Minimum inlet pressure' describes the lowest pressure required for gas entering the boiler in order for your boiler to operate safely and efficiently. It’s the most common specification you’ll see and it’s typically measured in 'millibars'. Each millibar is 1/1000th of a bar. British Gas can recommend what’s suitable for you.

How to check your boiler gas pressure

Water pressure

You might also come across minimum and maximum ranges of pressure for the water in your system and boiler. These measurements help you, and our engineers, keep an eye on pressure levels, to make sure your boiler is working properly and that water isn’t being lost through leaks. To find out more about boiler pressure, and how to maintain it, take a look at our dedicated guide.

How to find your water flow rate

Water flow rate

Dangers of carbon monoxide and why you need a co2 detector

Another important feature — specific to combi boilers — is the water flow rate through your taps. Essentially this determines how much hot water your boiler can send to your taps or showers.

This is usually measured in litres per minute (l/min) and equates to the volume of water coming from your plumbing fixtures. This means that the more taps, showers and other water-bearing fixtures you have in your home, the higher hot water 'flow rate' you will need, especially if you want to run them at the same time. To give you an idea of flow rates in your home, the UK government states that a water efficient kitchen tap should have a flow rate of between 4-6 l/min and showers no less than 6 l/min.

The output specifications for a boiler will often show the maximum flow rate that the boiler can provide at a specific temperature. For example, you might see a flow rate of 12 l/min at a temperature of 40 degrees centigrade. This is how much water it can send out to the taps in your home at that temperature. Put into useful context – if you’re running two showers and a tap at the same time, all requiring 40-degree water, you’ll likely see a drop in water flow rate.

The more powerful your boiler, generally speaking, the higher your water flow rate. It’s worth remembering however, that the amount of hot water from your taps and showers will be determined by the water flow rate into the boiler from the mains. So, even if your boiler is capable of heating a flow rate of 8 l/min, it’ll only be able to deliver 2 l/min if that’s the flow rate from your mains.

Hopefully, you now feel better equipped to evaluate the boilers you research. But if all this information still feels a little overwhelming, don’t worry, you can set up a free, no obligation appointment with one of our heating advisers. They’ll visit your home, complete an assessment, then talk through your options and answer any questions. You can book an appointment online in less than five minutes, here.