02 February 2020
How to buy the right gas boiler

How to buy the right gas boiler

So you’re replacing your old gas boiler and want to make sure you choose the perfect replacement? We’re here to help with some impartial expert advice.

A new boiler is one of the biggest household purchases you’ll make. The good news is you should save money because you will have a more efficient boiler. The efficiency of your new boiler will determine how much you’ll save in the years to come. Typically, good boilers should last more than ten years if you choose the right one.

Why does boiler efficiency matter?

Your gas boiler will account for around 55% of your home's CO2 emissions*. So choosing the right boiler becomes a real priority if you want to cut your energy bills and your carbon footprint.

Replacing an old boiler can improve efficiency by up to 30%. As a result, you can see significant savings on your monthly energy bill.

*Source: Energy Saving Trust.

What should you think about when buying a boiler?

There’s a lot to think about when choosing a suitable boiler for your home. Here are just some of the things you’ll need to consider:

  • The type of boiler
  • The number of rooms in your home and how big they are
  • How much hot water you use
  • How cold it is where you live
  • The heat output capabilities of the boiler
  • The heat loss of your home (which depends on how good your insulation is)
  • Your budget

We’ll cover all of these in this article.

It’s worth taking the time to make the right choice now so you can save money on gas and repairs in the next decade or more.


Let’s start by defining some common gas heating terminology.


We measure domestic boiler size in terms of heat output. Heat output typically ranges from 5 to 35kW (kilowatts).

  • Small boiler: Under 27kW
  • Medium boiler: 28-34kW
  • Large boiler: Over 34kW

“Heating system”

An entire heating system includes more than just the boiler. It also includes:

  • a thermostat
  • heating controls
  • pipes
  • radiators
  • hot water tank (for some systems)

“Heating controls”

There are four types of heating controls:

  1. Manual room thermostats that you must physically set and continually adjust for each room in your home
  2. Programmable room thermostats with built-in timers
  3. Thermostatic radiator valves (TRVs) which attach directly to your radiators
  4. Smart heating controls – internet-enabled devices that allow you to manage, schedule, and adjust your heating and hot water using an app or web browser

Let us know if you come across any other terminology you think we should add.

What type of boiler do you need?

If you have a central heating system with a gas boiler, you will choose between one of three types of boiler for your replacement:

Combi (combination) boiler

A combi boiler provides heat for your radiators and domestic hot water when you need it. This means it doesn’t waste energy when hot water is not required, saving you money.

This type of boiler is a very efficient source of hot water for smaller homes and flats. However, a combi boiler is not ideal for delivering hot water to more than one source at the same time, for example, a shower and the kitchen tap simultaneously. If you want a combi-boiler but need the shower to work while you are running hot water in another location, you would benefit from an electric shower.

Combi boilers are the most compact type of boiler, taking up the least amount of space.

Heat-only (conventional) boiler

Heat-only boilers need a cold water storage tank, usually stored in a loft, and a hot water cylinder which is typically stored in an airing cupboard.

A heat-only gas boiler can store gallons of hot water. It can supply this to more than one source at a time. However, it can run out of hot water if not programmed to heat enough water to meet the demand of the house.

Heat-only boilers are ideal for big homes and families that need plenty of hot water at the same time, for example, when filling a bath while someone else is doing the dishes or having a shower.

System boiler

A system boiler is similar to a heat-only boiler. However, it takes up less space and doesn’t need a cold water storage tank. Instead, it preheats and stores hot water in a cylinder. System boilers use a pressurised system.

System boilers can store large quantities of hot water. This is ideal for supplying hot water on demand to more than one source at once.

This type of boiler is best for homes with weak water pressure or limited space for a cold water storage tank. They can be slightly pricier than a heat-only boiler.

Getting the right size

There are three factors to consider when it comes to the size of a boiler:

  • How much hot water and heating capability you need
  • The physical size of the boiler and whether it can fit into the space you have for it
  • The loss of heat from your house

Hot water demand

One of the most critical factors in choosing a new boiler how much water you will need. This will depend on the size of your home, how many radiators you have, and how many people will be using hot water during the day.

It’s important not to underestimate how much hot water you will need now and in the future. If you have a young family, consider how much more water your children will use in the future. Or if you plan to expand your home by converting a loft, a garage, or by adding an extension, think about how much additional heating will be required for those rooms.

This is something a heating engineer can help you understand and get right.

Physical size

The space you have available for your new boiler will be a determining factor in what boiler you can buy. Measure the height, width, and depth of the space where your boiler will be installed.

You may find that your replacement boiler is smaller than your current boiler if it’s quite old.

You also have the option of relocating your boiler. You may be considering moving it from your kitchen to your garage, for example. Or maybe you have plans to convert your garage, moving your boiler from there to your kitchen. This obviously costs more but can be worth it to free up the space.

Heat loss

Heat loss is a calculation that measures the heat your house loses on the coldest day of the year. The heat loss of your home takes into account things like:

  • The total floor area
  • The number of radiators
  • The number of doors and windows in your property
  • How well your doors and windows prevent drafts
  • The quality and effectiveness of your insulation

These are all factors that directly affect how well your heating system can perform. For example, poor insulation will mean your boiler will have to work harder to maintain the temperature you program through your heating controls. It will use more gas which will cost you more money.

A home heating expert can perform a heat loss survey for your home. This information is used to calculate the correct boiler size for your home.

What's the best boiler for your size of home?

Homes come in different shapes and sizes so we have made some recommendations for common types of homes:

Gas boilers for flats

Smaller properties usually have limited storage space. This rules out the possibility of a separate hot water tank. Combi-boilers tend to be the most popular choice in the UK for this reason.

Verdict: Combi-boiler

Enquire about a gas boiler for your flat

Gas boilers for 2-3 bedroom homes

A mid-sized family home will probably have a steady demand for hot water. If you have space for a hot water cylinder and a cold water storage tank, a small heat-only boiler is recommended. This will deliver hot water to more than one tap at the same time. Otherwise, a mid-sized combi boiler is your best bet.

If your home has a single bath and shower, we recommend a combi-boiler with an output of around 24-30kW. If you have an en-suite as well, you should consider going with a larger 30-35kW boiler.

Verdict: A mid-sized combi boiler or a small heat-only boiler

Enquire about a gas boiler for your 2-3 bedroom home

Gas boilers for 4-5 bedroom homes

A heat-only boiler is the best choice for larger homes. Typically, houses of this size will have multiple bathrooms. This means that hot water will need to be supplied at the same time on demand.

You will need space for a hot water storage cylinder and a cold water feed tank (usually located in the loft or an upstairs cupboard).

Verdict: A heat-only boiler

Enquire about a gas boiler for your 4-5 bedroom home

Gas boilers for homes with low water pressure

If you live in an area with weak water pressure or have a larger home but no space for a cold water tank, choose a system boiler.

A system boiler uses a pressured system. It doesn’t rely on high water pressure being available to your home. A system boiler doesn’t require a cold water feed tank either.

Verdict: A system boiler

Enquire about a gas boiler for your home with low water pressure

Gas boilers for homes with old heating systems

Usually, it’s better to retain your current heating system (the pipes, radiators, etc). This is better for you in terms of cost and disruption. However, for older homes, replacing the heating system may simply be necessary. A new heating system, together with a new boiler, will be more efficient in the long-term.

Verdict: Consider a new heating system but consult a heating engineer first

Enquire about a new heating system

Boiler cost

Of course, a deciding factor when selecting a new boiler will be how much it costs in light of your budget. The costs for replacing a boiler will vary, but a straightforward gas boiler replacement plus thermostatic radiator valves will typically cost about £2,300 excluding radiators.

However, you could save up to £300 per year when replacing an old gas boiler with a new A-rated condensing boiler with a programmer, room thermostat and thermostatic radiator controls.

The amount you can save depends on factors like the size of your home, how efficient your old boiler was, and your insulation.

Other factors

We have covered the main factors for choosing the right gas boiler for your home. There are other factors to consider like these:

  • Reliability, maintenance, and warranties
  • Insulation
  • Installation
  • Heating controls and automation

We will cover in detail as part of a future article soon.

If in doubt, ask an expert engineer

An experienced heating engineer will assess your needs and recommend the right size of boiler that best fits your requirements.

We hope this guide will provide all the information you need to make an informed choice about your new boiler.

If you still need some advice or want to double-check your choice, you can email us or call our friendly team on 0800 3899 463 (Mon to Fri, 9am to 5pm) to arrange for one of our knowledgeable engineers to visit your home and help.

Written by The Kingdom Gas Team