Understanding boiler specs

From pressure to power and flow, here’s everything you need to know about boiler specs…

Getting to know your boiler

In the market for a new boiler? Or just trying to understand your heating system a bit better? Boilers are pretty complicated bits of kit, so there’s a lot to get your head around when it comes to boiler specs. But once you do, it will help you work out the most efficient and safest ways to heat your home and its water.

There are three main 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, but here are the main ones you need to know about.

1. Carbon monoxide emissions

Most important of the boiler specs you should know about are those relating to safety. All gas boilers emit carbon monoxide, a poisonous gas that has no smell or taste, when heating your home or hot water. Usually, this is nothing to worry about as the boiler’s flue channels it out of your home. If something goes wrong however, and this doesn’t happen, carbon monoxide can build up in your home. Breathing it in can make you very unwell - it can even be fatal.

You’ll find carbon monoxide emissions in your boiler specs. The lower they are the better because, should the boiler malfunction or a flue or vent become blocked, less carbon monoxide will be released into your home.

As well as checking your boiler specs, it’s a good idea to have a carbon monoxide detector alarm installed. Regular boiler services will also help to ensure it’s working properly and you’re not at risk of carbon monoxide poisoning.

Learn more about protecting yourself with the British Gas carbon monoxide alarm.

2. Energy efficiency rating

Among your boiler specs, you’ll find the ‘Energy Related Products’ rating, or ErP rating. All new electrical appliances, from your boiler to your light bulbs, come with this important labelling, and it’s a standardised system to help people understand efficiency and compare products.

When you see it on your boiler specs, you might recognise it from the sticker on your fridge, oven, or other appliances in your home. The highest rating 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 semi-detached house in England, Scotland or Wales, and you upgrade from an old G-rated gas boiler to an A-rated condensing gas boiler with a programmer, thermostat and thermostatic radiator valves, the Energy Saving Trust estimates you could save around £380 per year on your central heating bill.

If you have a combi boiler, your boiler specs will include separate ErP ratings for heating your home and your water because they power your central heating and hot water independently. In contrast, conventional and system boiler specs just have one ErP rating for heating. Learn more about the different types of boilers.

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

3. Your boiler’s power

Normally shown in kilowatts (kW) in your boiler specs, your boiler’s power tells you how much heating and hot water it can supply your home. The more powerful the boiler, the more radiators and hot water it can heat – so it’s an important part of your boiler specs if you live in a large house or have multiple bathrooms.

To work out how powerful your boiler needs to be, you’ll need to consider your home’s size and insulation, as well as the number of people in it and their energy use habits.

Your boiler specs will show you both input and output boiler power. Depending on your boiler type, it can refer to just your central heating (CH) or both your heating and domestic hot water (DHW).


This is the amount of energy your boiler can use to heat your home and hot water, and is reflected in your energy bill.


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

4. Your boiler’s pressure

The figures related to pressure in your boiler specs are important indicators of its health and can refer to either gas pressure or water pressure. They’re typically in units called ‘bars’, the same as those used to measure pressure in car tyres, scuba diving equipment and weather fronts.

Gas pressure

'Minimum inlet pressure' describes the lowest pressure needed for gas entering the boiler for it 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.

Water pressure

You might also come across minimum and maximum ranges of pressure for the water in your system and boiler in your boiler specs. 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.

5. Water flow rate

If you have a combi boiler, your boiler specs will include the water flow rate. Usually measured in litres per minute (l/min), it tells you how much hot water your boiler can send to your taps or showers. The more taps, showers, and other water-bearing fixtures you have in your home, the higher the hot water ‘flow rate’ you’ll need, especially if you’re running them at the same time. According to the UK government, a water efficient kitchen tap should have a flow rate of 4-6 l/min, and a shower no less than 6 l/min.

Your boiler specs will often show the maximum flow rate that your boiler can provide at a specific temperature, for example 12 l/min at 40 degrees centigrade. This shows how much water it can send to your taps at that temperature. What it means is that if you’re running two showers and a tap at that temperature at the same time, the water flow rate is likely to drop.

Generally, more powerful boilers will have higher water flow rates. It’s worth remembering though that the water flow rate from the mains to your boiler will also determine the amount of hot water from your boiler to your taps. So even if your boiler is capable of heating at 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.

Still bamboozled by boilers? Don’t worry if all this information still feels a little overwhelming, you can set up a free, no obligation appointment with one of our heating advisers. They will complete an assessment then talk through your options and answer any questions. You can book an appointment online in under five minutes.

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