Need help understanding boiler specifications?
When you’re shopping for a new boiler, it can be hard to know where to start.
If you’ve taken your boiler search online, you were probably surprised by how much choice there is!
Although boilers essentially do the same job of keeping your home warm, and your baths hot, there’s a vast array of different makes and models to choose from.
And although boilers might all look very similar (to the untrained eye!) – don’t be fooled – not all boilers are created equal!
To find the most cost-effective and efficient way to heat your home, you need to make an informed decision, not one based solely on aesthetics or price.
This is where boiler specifications come in. The technical information found in a boiler specification is important. It will help you to find the most cost-effective and efficient way to heat your home.
But what does it all mean?!
The facts, figures, and general jargon contained in a boiler specification can be confusing. So, we’ve put together this handy guide to help you understand what you’re looking at.
The three types of boilers
Before we get into the nitty gritty, let’s start with the types of boilers that are available.
There are three main types of boilers for you to choose from: combi boilers; system boilers; and heat-only boilers. Since they all work differently, you’ll come across slightly different boiler specifications for each.
We’ve dedicated entire blogs to each of these boiler types, but we’ll summarise the key differences again here:
Combi boilers
Combination or ‘combi’ boiler is the most common type of domestic boiler in the UK. They provide a combination (hence the name!) of heating and hot water from one compact unit. They don’t require any external tanks or cylinders because they heat water instantaneously on demand.
Read their pros and cons here.
Heat-only boilers
Heat-only boilers, also known as traditional or conventional boilers, are usually fitted to an open-vented system, which require cold water and feed and expansion tanks in the loft. Since they store hot water in a cylinder, they can meet high demand for hot water, and are also perfect for homes with low mains pressure.
Read their pros and cons here.
System boilers
System boilers operate on a sealed system. So they don’t have tanks in the loft, but they do store hot water in a cylinder. As with heat-only boilers, this makes them well suited to homes with a high demand for water.
Read their pros and cons here.
Understanding boiler specifications
Now let’s look at some of the key terms you may come across in a boiler specification:
Understanding boiler specifications: DHW
DHW stands for Domestic Hot Water. This refers to the water that comes from a hot tap or is stored in a hot water cylinder (with a heat-only or system boiler).
With a combi boiler, the central heating and DHW are combined into one appliance. When you turn on a hot tap, a combi boiler prioritises the DHW, and stops pumping water to the heating circuit.
Understanding boiler specifications: CH output
CH stands for central heating. As well as providing your hot water for your taps and showers, your boiler also heats your home. It does this by heating water which is pumped around your radiators, before returning to the boiler for reheating.
Understanding boiler specifications: Boiler efficiency (ErP)
Thanks to legislation from the EU, all new boilers up to 400kW come with an ErP rating.
ErP stands for Energy-Related Products. It is a rating system which rates an appliance’s energy efficiency from A-G, with A-rated products being the most efficient.
You’ll also see this rating system on other household appliances, such as your fridge, washing machine and oven.
Combi boilers have two ErP ratings, one for heating your home, and one for heating water for your hot taps. This is because combi boilers power your central heating and hot water independently.
If you have a heat-only or a system boiler, you’ll just have one ErP rating for heating.
The purpose of ErP ratings
The purpose of the ErP rating system is to ensure that consumers are aware of how energy efficient (or not!) their appliances are.
In turn, this helps governments to reduce carbon emissions, while helping homeowners to reduce their energy bills.
All new boilers must be 92% efficient (or ‘A-Rated’ on the ErP label).
If you’re upgrading from a boiler with a lower efficiency rating, you’ll save money on your energy bills each year because efficient boilers waste less energy.
To find out more about this, you should check out our related blogs: What is an efficient boiler, and why should I want one? and What is Boiler Plus?
According to the Energy Saving Trust, if you swap a D-rated gas boiler to an A-rated condensing gas boiler, you could save around £170 a year on your central heating bill. The savings will be even greater if swapping from G-rated boiler to an A-rated one.
And, with gas prices soaring, the savings are likely to be substantially higher than that now!
Understanding boiler specifications: SEDBUK
Although SEDBUK was effectively replaced by the Energy related Products (ErP) directive in 2015, you’ll often still see SEDBUK appearing on boiler specifications.
SEDBUK stands for: ‘Seasonal Efficiency of Domestic Boilers in the UK’. SEDBUK was a UK initiative which provided a basis for fair comparison of the energy efficiency of different models of boilers.
The SEDBUK scale labelled boilers with a letter from A – G, with A being the most efficient boilers.
On the SEDBUK scale, boilers classed as A and B were highly efficient, whilst bands C to G were classed as having ‘standard efficiency’.
Understanding boiler specifications: Boiler power (kWs)
Another key boiler specification you need to be aware of is boiler power.
This can be confusing, because you’re more likely to have heard of people referring to boiler power as boiler ‘size’.
But when we’re talking about a boiler’s ‘size’, we’re not talking about its physical dimensions, but rather, its power.
Boiler power is measured in kilowatts (kW). The more power the boiler can generate in kWs, the bigger it is in terms of ‘size’.
At its simplest, a more powerful (or ‘bigger’) boiler will be able to heat more radiators and provide more hot water. But this doesn’t mean that bigger is always better!
Many people fall into the trap of choosing a boiler that’s too big (or too powerful) for their needs, thinking that it’s better to have too much power, than too little.
The problem with oversizing boilers
But oversizing your boiler is an expensive mistake because it leads to ‘boiler cycling’ and excessively high heating bills.
But too little power isn’t good either, because your boiler will struggle to provide enough heating and hot water for your needs.
So, you need to choose a boiler that’s power output can precisely meet your requirements. Whilst a lot of online ‘calculators’ say you can do this by counting your radiators – and this really grinds our gears! – this isn’t true.
The accurate way to size a boiler (and industry standard under Part L of the Energy Efficiency Regulations) is through a heat-loss calculation.
Rest assured, that The Heating People conduct heat-loss calculations as standard as part of your free survey. We will calculate and recommend the correct size of boiler for your home.
But so that you can understand this better, let’s look at what the Kw numbers mean…
What the numbers on the specification mean…
When looking at a boiler specification, you might see two numbers by boiler power. One is the power input; the other is power 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).
Input refers to how much energy goes into the boiler to make it work, and it’ll be reflected in your energy bill.
Output tells you how much of that energy is converted into heat for your radiators and hot water for your taps. This number will always be lower than the input because no boiler is 100% efficient. Some energy will be lost in the heating process.
What is a kW?
A kilowatt is a measure of power, equal to 1,000 watts. Watts measure power generated per second.
So, a boiler with a maximum output of 30kW will produce 30,000 joules of energy per second when in operation. This energy is used to heat your home.
The higher the kW rating of a boiler, the more powerful it is, and the more radiators and taps it can supply at one time.
Why do combi boilers have higher power outputs?
You may notice that combi boilers often have larger power outputs than heat-only or system boilers. This is because combi boilers operate differently, by heating water instantly on demand.
Heat-only and system boilers don’t have to heat water instantly, they can heat it up more slowly because they store it in a cylinder for later use.
What is a kilowatt-hour (kWh)?
You might have come across kilowatt-hours on your energy bill, and be wondering how this differs from a kilowatt.
A kilowatt (kW) is a measurement of power. A kilowatt hour (kWh) measures how much energy an appliance could use if it was on for one hour.
For example, a 30 kW boiler needs 30 kWh of energy for each hour that it’s heating your home. So, if it’s operating for 3 hours then it will use 90 kWh of energy to do its job.
Understanding boiler specifications: Boiler modulation
You’ll see some specifications talking about boiler modulation. But unless you’re in the heating industry, this probably won’t mean much to you!
So, what does boiler modulation mean, and why is it important?
Boiler modulation refers to the ability of the boiler to automatically reduce its output to suit the demand for heat.
Let’s say you only need 15 kw of heat for your home for the next couple of hours. But you have a 30kw boiler.
Instead of your boiler outputting 30kw and then cycling on and off, your boiler would reduce its output by turning down its flame by 50%.
The advantage to this is that it lowers the amount of gas being used by your boiler. This saves energy, and therefore money on your energy bills.
It also creates a more consistent environment and helps to prolong the life of your boiler by preventing on/off cycling.
What are modulation ratios?
Boiler manufacturers state the ability of a boiler to modulate as a modulation ratio.
The fraction refers to a boiler’s minimum output in relation to its maximum output. The wider the modulation range, the better the ratio.
Let’s look at some examples to help explain this:
Boiler A
Boiler A has a maximum output of 30kW and has a 5:1 ratio. This means that the maximum power it can output to heat your home is 30kW, and it can modulate this down to 6kW.
Boiler B
Boiler B has a maximum power output of 30kW and a 10:1 ratio. This means that the maximum power it can output to your home is 30kW, and it can modulate this down to 3 kW.
Boiler C
Boiler C has a maximum output of 30kW and an extremely impressive 17:1 ratio. This means that the maximum power it can output to your home is 30kW, and it can modulate this down to 1.7 kW.
So, which of the boilers in our examples is the most efficient?
Boiler C is the most efficient boiler because it can modulate its output from 30kW to 1.7kW with a total of 17 different heat outputs.
Lower power output means less fuel burned, and more money saved on your bills.
To put this in perspective… If you only need a very small increase in heat, Boiler A would fire at 6Kw, but Boiler C would fire at a lower (and more efficient) 1.7Kw.
A non-modulating boiler would fire at its maximum output of 30Kw regardless of the heat demand, consuming unnecessary energy and overheating the home.
Another advantage is that there will be less boiler cycling, and so there will be a lower level of stress placed upon the boiler, potentially extending its lifespan.
If you’re interested in boiler modulation, you might also be interested in our related article on the benefits of low temperature heating.
Understanding boiler specifications: Water flow rate
An important feature specific to combi boilers, is the water flow rate.
Measured in litres per minute, the water flow rate shows how much hot water your boiler can send to your taps in one minute.
Sometimes the output specifications for a boiler will state the maximum flow rate that the boiler can provide at a specific temperature. For example, you might see a flow rate of 15 l/min at a temperature of 40 degrees centigrade.
Generally, the more taps and showers you have in your home, the higher water flow rate you need.
However, you could be wasting your money if you choose the boiler with the maximum flow rate, if the mains supply to your house is lower than the flow rate of the boiler.
If you were to buy a boiler with a flow rate of 12 litres per minute (l/min), but the flow rate of your mains water is only 8l/min, your boiler won’t be able to reach 12 l/min.
If your mains water isn’t coming in fast, it won’t come out of your tap fast either!
Understanding boiler specifications: Pressure
Technical specifications for boilers often include figures related to gas and water pressure, and are typically measured in bars.
‘Minimum inlet pressure’ describes the lowest pressure required for gas entering the boiler in order for your boiler to operate safely and efficiently.
There also might be a minimum and maximum rate for water pressure.
Conclusion
A boiler is a big investment, and you’ll want to get it right. After all, it’s going to be tasked with keeping you and your family warm and comfortable for at least the next decade!
So, we hope that reading this guide has left you feeling better equipped to research your new boiler.
But if you still feel overwhelmed, don’t worry, The Heating People are here to help! Contact one of our friendly team to set up a free, no obligation appointment with one of our heating specialists.
They’ll visit your home, complete a survey, then talk through your options and answer any questions.
Thinking of replacing a
First off, it’s important to establish what your existing heating system is, and what you’re looking for.
Heat-only boilers are often thought of as the traditional way of heating a home. And out of the three types of boiler available (heat-only, system and combi) they take up the most space.
Since heat-only boilers store large quantities of hot water in a cylinder; they can meet a high demand for hot water.
If you’re thinking about switching to a combi boiler, you’re probably already aware of the drawbacks of a heat-only boiler.
Combi boilers are currently the most popular type of domestic boiler installed in the UK. We’ve dedicated a whole blog to what they are, which you can read in full 
Combi boilers are very popular in the UK because they offer a lot of benefits:
Thanks to the 
If you’ve currently got a heat-only boiler; you might have had occasions where you’ve used all of the hot water in the cylinder; and have had to wait for it to refill.
It sounds counterintuitive to say that a combi will never run out of hot water; and then say that it can’t meet high demands for hot water – but stay with us!
Installation
The 
If you’ve decided that you want a combi boiler, your next challenge is 
We hope you’ve found this guide useful in helping you to decide whether switching from a heat-only to a combi boiler sounds like a good option for you.
8 Interesting heating facts… Yep, we did say interesting.
To borrow a line from Monty Python, “Apart from sanitation; medicine; education; wine; public order; irrigation; roads; a fresh water system; and public health, what have the Romans ever done for us?” …
Mr Franz San Galli, a Polish-born, Russian inventor; has been credited with inventing the first heating radiator – famously calling it a ‘hot-box’.
So, the Romans came up with the first central heating system. And the Russians and Americans invented the first radiators. Us Brits must have come up with the first 
Despite their misleading name, radiators heat our homes via convection, rather than radiation.
If you’ve read our blog: 
The key to keeping your home as warm as possible, and your energy bills as low as possible, is insulation!
According to the Energy Saving Trust (2020), heating and hot water accounts for approximately 55% of your household’s energy bills.
According to Which? 1 in 5 boiler breakdowns is due to inadequate boiler maintenance, or in other words, not booking in for that 
What is power flushing?
If you’ve noticed that your 
If you’re finding that your hot water is fluctuating between hot and cold, or the water flow is reduced to a trickle, this could indicate problems such as corrosion in the heat exchanger, and limescale in the pipes.
The length of time needed for a Power flush typically depends on your system’s size and condition, but it usually takes about a day.
If you’re installing a new boiler, it may be necessary to flush your system to ensure that your new boiler can heat your home effectively, and won’t be damaged by sludge that is already in your radiators and pipes.
No. Power Flushing machines are designed so that there is no increase in pressure in your system. It is a high flow rate that makes power flushing effective, not high pressure.
There are lots of factors that will affect the price of a power flush such as size and complexity of your heating system, and whether any other issues or faults come to light.
What are hydrogen boilers? Even prior to 
In a word, no.
Under the Paris Agreement, the UK has a legal commitment to reach net zero carbon emissions by 2050. By achieving this, we can hopefully limit global warming to 1.5C, and thereby avoid catastrophic effects of climate change.
So, we’re likely to see a mix of low-carbon technologies such as air-to-water or ground-to-water heat pumps, heat networks, and potentially switching the natural gas in the grid to low-carbon hydrogen to run hydrogen boilers.
You might be wondering how hydrogen – which is a gas – is ‘greener’ than our 
As we’ve mentioned Hydrogen is much ‘greener’ than our natural gas, and it’s also more abundant. It’s about 1,800 times more plentiful than oxygen! So, why haven’t we been using it before now?
As we’ve mentioned, the government isn’t planning to make any firm decisions on hydrogen heating until 2026, when timescales and technical details will hopefully be ironed out.
The next stage would be introducing a blend of natural gas and 20% hydrogen into the UK mains supply.
Hydrogen Boilers are very similar to our traditional gas boilers and could provide a like-for-like replacement should we switch the gas network to hydrogen gas.
Hydrogen boilers are similar to traditional gas boilers that we’re familiar with. They’re built and function in much the same way.
Currently, there are no 100% hydrogen boilers available. However, Viessmann, Baxi and Worcester Bosch have produced prototype “hydrogen-ready” boilers capable of burning either natural gas or 100% hydrogen.
Emissions from hydrogen boilers contain zero carbon
A big advantage of using hydrogen for our heating is that it wouldn’t require any behaviour change from us. A hydrogen boiler would look the same and provide the same delivery of comfort as our traditional boilers.
There has been much discussion in the media about the need for a ‘Fabric First’ approach before 
As we’ve mentioned, hydrogen doesn’t exist in a pure form, it has to be separated from other elements first. To do this in an environmentally friendly way isn’t cheap.
Hydrogen has the National Fire Protection Association’s highest rating of 4 on the flammability scale. It’s flammable when mixed even in small amounts with ordinary air; and it has a lower ignition energy than gasoline or natural gas. This means it can ignite more easily.
What’s clear is that to meet Net Zero virtually all heat in buildings will need to be decarbonised. The government has set the ambition of phasing out the installation of new natural gas boilers from 2035.
