BOILER BUYING GUIDE
BOILER BUYING GUIDE
Buying a new boiler is probably one of the most significant investments a homeowner can make. But it can be tricky to know which one to get if you don’t know what to look out for.
In this boiler guide, we’ll show you how to choose a boiler system. We’ll cover the different types of boiler available (brands, sizes and energy efficiency), and help you understand when a boiler needs replacing, how much they cost to buy and install, and what you need to consider before purchasing a boiler.
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Key takeaways
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Types: The 3 main types include; combi boilers, heat only boilers and system boilers.
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Warning signs: If your boiler regularly breaks down, makes strange noises, loses pressure or the heating is unreliable - you may need to call a professional out, or consider replacing your boiler.
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Servicing: Having regular boiler services can help identify problems early and maintain your boilers overall health.
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Consider repair vs replacement: If you have an aging boiler, it may be more financially viable for a replacement instead of a repair.
Types of boilers
There are three main types of boilers; combi boilers, heat only boilers and system boilers. Each type works differently and are designed for different applications. Below outlines the differences between them all, their pros and cons and which home they are best suited for.
Combi boilers
A combi boiler is a boiler that draws water directly from the mains cold water supply and heats it on demand. For this reason, homes with a combi boiler don’t need a cold-water tank or a hot water cylinder.
Combi boiler pros:
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They take up less space than other boilers, and there’s no need for a cold water tank or hot water cylinder.
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They provide instant hot water, so you don’t have to wait for a water tank to heat up.
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They tend to be more efficient as the water is heated only when needed.
Combi boiler cons:
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You can only get hot water from one tap/shower at a time (unless you have a storage combi boiler).
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The mains water pressure limits them, so they don't work well in homes with poor water pressure.
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They contain moving parts, which means there’s more potential for breakdowns than with other boiler types.
Combi boilers are best for smaller homes or flats with only one bathroom and low water demand. Electric Combi boilers are a popular alternative to gas or oil boilers, where gas isn't available or a more sustainable approach is wanted.
Heat only boilers
A heat only boiler is a boiler that draws water from a cold-water feed tank, usually stored in the loft. The boiler heats the water and sends it directly to the radiators. It’s also connected to a hot water cylinder that heats and stores the water until it’s needed. Heat only boilers are also referred to as ‘regular’ or ‘conventional’ boilers.
Heat only boiler pros:
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They meet the water demand for larger households and can supply several taps/showers at the same time.
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They tend to have good hot-water flow for strong showers and baths.
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Usually compatible with existing systems (if replacing a conventional boiler).
Heat only boiler cons:
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If all the hot water in the storage tank is used up, you must wait for it to reheat before having hot water.
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They take up a lot of space because they require a cold-water tank and a hot water cylinder.
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They are less efficient than combi boilers.
Heat only boilers are best for larger homes with high water demand and multiple bathrooms that will be used at the same time.
System boilers
A system boiler is a boiler that draws water directly from the main cold water supply. But unlike a combi boiler, it doesn’t heat the water on demand. Instead, it heats water stored in a hot water cylinder. System boilers are also referred to as ‘closed vent’ or ‘sealed system’ boilers.
System boiler pros:
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They meet the water demand for larger households and can supply several taps/showers at the same time.
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They save space compared to a heat only or regular boiler because they don’t require a cold-water feed tank.
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Usually have good mains-water pressure as normally fed directly from the mains.
System boiler cons:
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If all the hot water in the storage tank is used up, you must wait for it to reheat before having hot water.
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You need space for a hot-water cylinder (e.g. an airing cupboard or utility).
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They are less efficient than combi boilers, as you’ll lose heat from the cylinder (despite insulation).
System boilers are best for larger homes with poor water pressure and a high water demand or multiple bathrooms that may be used at the same time.
Comparing combi boilers, heat only boilers & system boilers
When choosing the right boiler for you, it’s important to consider the key differences between them.
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Combi boilers |
Heat only boilers |
System boilers | |
|
Hot water heating |
On demand |
Stored cylinder |
Stored cylinder |
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Mains pressure dependence |
High |
Low |
Moderate |
|
Space required |
Low |
High |
Moderate |
|
Multiple showers |
Limited |
Great |
Great |
|
Energy efficiency |
High |
Moderate |
Moderate |
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Best suited to |
Small houses |
Large houses |
Medium or larger houses |
Which boilers are the most energy-efficient?
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Combi boilers: Very efficient for smaller households, as hot water is only heated when needed.
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Heat only boilers: Can be efficient, but some heat loss.
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System boilers: Can be efficient, but some heat loss.
All new boilers sold today are condensing boilers. Their energy efficiency is measured with an ErP (Energy-related Products) rating, which runs on a scale (as of September 2021) from A-G).
To get the most efficient boiler, choose the highest rating possible for your budget. But it's also essential to get one that's the right size in kilowatts for the property. A heat only boiler, for example, is efficient in a larger property. But its power will be too high for a small property.
What size boiler do I need?
When choosing a boiler size, you need to consider its physical size and the size of its heating capabilities.
The right physical size depends on the property and how much space there is available for the boiler to go. A boiler’s heating size is measured in kilowatts (kW). Choosing the right size depends on the property, the number of people living in it, and the number of bathrooms and radiators. Use the table below as a guide to the correct boiler size for the property.
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Property size |
Number of bathrooms |
Number of radiators |
Boiler size (in kW*) |
|
1-2 bedrooms |
1 |
Up to 10 |
24-27kW |
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2-3 bedrooms |
2-3 |
10-15 |
28-34kW |
|
4+ bedrooms |
3-4 |
15-20 |
35-42kW |
*The boiler size range considers insulation efficiency. An older, poorly insulated home will need the largest size. In contrast, a new, well-insulated home will require the smallest size.
Signs you need a new boiler
It’s not always easy to know when to replace a boiler because faults can go unnoticed until it breaks down. Here are the signs to look out for:
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Radiators take a long time to heat up - can indicate a failing boiler.
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A bad smell - if you can smell gas; you must turn off the gas supply (if safe), ventilate the area properly and call a professional urgently. This is a safety issue that requires immediate attention.
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Increased energy bills - if there is no change in usage, it may be a sign that your boiler may be becoming less efficient.
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Leaking water - you will need to call a professional to take a look urgently.
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Unusual noises - if you can hear banging, gurgling, whistling or humming, there may be underlying issues.
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Regular breakdowns - repeated call outs can mean that a replacement is more economical than paying for continued repairs.
When replacement is worth considering
If your boiler is 15+ years old, then it can be very expensive to repair any faults. A replacement is usually more financially viable instead of putting money into an ageing unit. If your boiler is relatively young (and the problem is inexpensive), then repair may be the best option.
How much does a new boiler cost?
New boiler costs can vary significantly depending on the type of boiler, the brand, and model, and its size.
You then need to factor in installation costs. Again, these will vary depending on the type of boiler being installed and what system it is replacing, but you can typically expect to pay between £1000-£5000. The installation cost for a like-for-like replacement will be on the lower end of the scale. Whereas the installation of a different boiler and system requiring pipework or location changes will be on the upper end of the scale.
Who qualifies for a boiler grant?
Homeowners and tenants of privately rented properties who are in receipt of benefits and have a broken boiler can apply for a boiler grant. You can check if you’re likely to qualify on the Boiler Grants website.
How much is a boiler service?
Boilers should be serviced once a year by a registered Gas Safe Engineer and costs vary between different companies. According to Which? you can expect to pay between £60-£100 for a one-off service. Or a package deal, that covers the cost of breakdowns and repairs, as well as an annual service, can cost £200-£300 per year. Read our boiler service guide for further information.
Key considerations when getting a new boiler
It’s always important to do your research before getting a new boiler to ensure you get the right product for the property.
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Existing heating system - Often, the easiest and most cost-effective option is to replace a boiler based on the existing boiler and heating system so that it’s a like-for-like swap. It is possible to install a different type of boiler, but you’ll need to consider the extra costs (for parts and installation), whether there is enough space in the property to install a cold water tank or hot water cylinder (if necessary) and whether the new boiler system is sufficient to meet the heating and hot water demands of the household.
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Location of the boiler - Choosing the right location for a boiler is important because it could impact the type of boiler you can select. All gas and oil boilers must be installed onto an external wall so waste gases can escape through a flue outside the property. If you’re replacing like-for-like, you’ll be able to put the new boiler in the same place as the current one. But if you’ve chosen a different boiler, you need to consider its physical size. For example, if it’s bigger than the existing boiler, it may need a new location. It’s also worth bearing in mind that any change to the location of the boiler will increase the installation costs.
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Location of the flue - If the boiler location is being changed, then the flue location will need to be changed too. The new site must be 30-60cm away from any opening windows or doors to prevent gases from re-entering the property. They shouldn’t be placed too close to any material that hot gasses could damage, such as plastic guttering. Also, consider the neighbours and don’t place the flue where the steam will be a nuisance.
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Budget - For most people, the deciding factor on the make and model of the boiler they buy is their budget. So, setting a budget straight away will help with the decision-making process. When setting the budget, factor in the cost of the boiler, the installation cost, plus any other parts, such as a new cold-water tank or hot water cylinder. Always buy the best boiler possible for the budget. A top-of-the-range boiler may seem pricey, but it’s likely to be more energy-efficient and have fewer breakdowns than a lower-priced model. It may also last longer - plus, all these factors mean that they’ll cost less in the long term.
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Warranty and manufacturer guarantees - Most boilers come with a manufacturer's guarantee that covers the cost of parts and repairs should the boiler break down within a certain period. The guarantee length varies depending on the brand and the specific model. Generally, the longer the guarantee, the more reliable the boiler is.