Boiler reviews: Features explained

Condensing boilers

Nearly all the new boilers being installed are condensing boilers. Condensing boilers are far more energy-efficient than older boilers, as they cleverly reuse heat that would otherwise be wasted.

We've reviewed hundreds of condensing boilers. To read more about them visit our boiler reviews.

How does a condensing boiler work?

A simplified condensing boiler in action

Boilers contain a combustion chamber that the hot gases created by burning gas or oil pass through.  This is surrounded by a heat exchanger which transfers heat from the gases into the water inside, heating it up.

In condensing boilers a second, or larger primary, heat exchanger removes even more of the heat from the flue gases. It pre-warms water coming back into the boiler from a heating system, so you burn less gas to heat the water up.

By using this otherwise wasted heat, condensing boilers use around 90% of the heat they generate.

Because the waste gas loses some of its heat to the secondary heat exchanger it cools down into an acidic water called condensate and water vapour. The cooler, heavier flue gases are blown out of the boiler by a fan.

Condensate

A liquid called condensate forms in the condensing boiler's heat exchanger and flue. It's slightly acidic and must be drained from the boiler, via a pipe directing the condensate to a drain.

Where possible, connections should always be made to internal drains (stack pipe or waste pipe). External condensate pipes are more likely to become blocked by freezing, leaves or general debris.

We've discovered that 11% of new boiler owners experience a problem with their condensate pipes freezing in cold weather, which stops the boiler working. 

Find out how to deal with a frozen boiler condensate pipe.

The plume produced by a condensing boiler on a cold day

Condenser boiler plumes

Flue gases leaving a condensing boiler are cool and produce a noticeable mist or plume of water vapour as they leave the flue. 

The boiler installer should consider the location of the flue during installation. It shouldn't be close to neighbouring properties or windows, doors and paths regularly used in the winter.

Types of condensing boiler

There are three main types of condensing boiler: combi boilers, heat-only and system boilers. We cover all three types in our boiler reviews.

Combi boilers

These are the bestselling boiler type for gas central heating in Britain. A combi boiler provides heat for your radiators and domestic hot water on demand. 

The name comes from the fact it combines all the functions of a traditional boiler, hot water cylinder and cold water tank set-up. All the major parts are in one unit, which makes them convenient to install.

When you turn on a tap, water is drawn from the mains, heated in the boiler and comes straight out of your tap piping hot. Storage combi boilers are available if you want to have a small store of hot water.

Pros of combi boilers

• Instant, unlimited hot water supply – no waiting for the hot water cylinder to refill.
• An economical choice for smaller households with low hot water demands.
• Smaller heating systems – combi boilers eliminate the need for a cold water tank, hot water cylinder and airing cupboard.
• Drinkable water at all taps – it's supplied straight from the mains.
• Combi boilers can produce between 9 and 18 litres of water a minute (when the cold water is heated up by 35°C).
• You can run a powerful shower off the bath taps with a combi boiler.

Cons of combi boilers 

• Combi boilers can provide maximum water pressure through only one tap at a time. They're not suitable for big homes where multiple sources of water might be used simultaneously.
• There's a delay in getting hot water from the tap – you need to wait for the combi boiler's burner to warm up.
• Combi boilers can't heat hot water fast enough to supply a power shower and are slow to run a bath.
• No immersion heater, so no back up if there is a problem with the boiler or gas supply.

Read more about the features and efficiencies of the combination boilers on sale now with our combi boiler reviews.

Heat-only boilers

A heat-only boiler

Heat-only boilers are the heart of most old-fashioned heating systems. They provide heat for radiators directly and produce hot water which is stored in a separate hot water cylinder. The cold water is normally supplied by a cold water cistern, usually in the loft above the boiler (a gravity feed) and there is also a separate expansion cistern.

Pros of heat-only boilers

• There's a large tank of stored hot water – you can use several taps at once.
• Suitable for larger houses with higher hot water demand.
• Easy to replace in old-fashioned heating systems without too many pipework changes.

Cons of heat-only boilers

• Heat-only hot water systems take up more space than a combi boiler system. 
• Unless a cylinder is very well insulated it usually loses heat into its surroundings, thereby wasting energy. 
• The amount of hot water you can use is limited by the size of your cylinder.
• The pump for the heating and hot water systems is housed outside the boiler and takes up more space.

Take a shortcut to our heat-only boiler reviews.

System boilers

A system boiler, just like an open-vent boiler, works on the principle of stored hot water. However, many of the major individual components of the heating and hot-water system are built into a system boiler. The system boiler removes the need for a feed and expansion cistern, but there is still a cold water tank (unless you have a pressurised hot water cylinder).

It is the ideal choice for the modern family home which needs every inch of space for storage and has a high demand for hot water.

Pros of system boilers

• Because there is a large tank of stored hot water you can use several taps at once.
• No need for a feed and expansion tank in the loft. 
• Can be used with pressurised cylinders, so you don't need a cold water tank in the loft either.
• Suitable for larger houses with higher hot water demand.
• Fewer heating system components outside of the boiler means that installation is quicker, neater, easier and more efficient.
• The pump is included in the system boiler, resulting in a fast response and more economical running costs.

Cons of system boilers

• Hot water cylinder-reliant hot water systems take up more space than a combi boiler system. 
• Unless a cylinder is very well insulated it usually loses heat into its surroundings, thereby wasting energy. 
• The amount of hot water you can use is limited by the size of your cylinder. For instance you might run a bath, then need to wait for the boiler to heat up the water in the cylinder again before you can run another. 

We've reviewed system boilers by all the major manufacturers. Find out how system boilers compare.

Oil boilers

More than 1.5 million homes in the UK use oil for their heating, and this number is growing as more homes are built in the countryside, away from mains gas. Read more about the pros and cons of oil boilers in our boilers buying guide.

There's a wide range of external, cabin and internal (kitchen or utility) condensing oil-fired boilers on offer. Most condensing oil-fired boilers are floor standing, and many have extended flue options which help to site the flue terminal so that the plume of water vapour they produce will not be a nuisance or visual annoyance.

Most UK domestic oil-fired boilers use kerosene, and need to be plumbed in to allow acidic condensate liquid to drain away. 

Looking for an oil boiler? Compare oil boiler ratings, using our boilers product finder tool to filter results.

Boiler size

A boiler range usually offers different sizes to provide boilers with the power to suit small or large homes

Manufacturers often make each of their boilers in a range of different heat outputs to suit a range of different-sized homes, typically ranging from 12 to 40kW.

A higher number means a boiler can heat water more quickly, so it can supply hot water to more radiators or taps. 

As homes have vastly different numbers of radiators and bathrooms these days, it's difficult to generalise about what size boiler is needed for a flat, cottage or larger house. Your boiler will be sized according to the amount of heat and, in the case of a combi boiler, hot water you require. 

When buying a new boiler you'll need to get your hot water demand assessed individually by your plumber or installer. They should also take factors such as wall and loft insulation and double glazing into account.

More on this...

• Get answers to common boiler questions with boilers FAQs
• Read our guide to the boiler manufacturers
• Find out who makes the most reliable boilers