Boiler reviews: FAQs

What are the advantages and disadvantages of a combi boiler?

The combination – or 'combi' – boiler is now the bestselling boiler type for gas central-heating in Britain, but they are not for everyone. Here we outline their advantages and disadvantages.

Pros of combi boilers

Instant, unlimited hot water supply – you never have to wait for your hot water tank supply to be replenished.
An economical choice for smaller households with lower hot water demands. More efficient and cheaper to run for hot water supplies.
Smaller heating systems – combi boilers eliminate the need for a cold water tank and hot water cylinder.
Drinkable water at all taps because none of the cold water is stored in a tank.
Some smaller capacity combi boilers can be fit into cupboards.
Modern combi boilers can produce between 9 and 18 litres of water a minute when the cold water is heated up by 35°C.
Storage combi boilers are available if you want to have a small store of hot water.
No airing cupboard for the cylinder, so more space.
You can run a great shower off the bath taps with a combi boiler.

Cons of combi boilers

Standard combi boilers provide maximum pressure through only one tap at a time. If you have two taps running, the flow rate diminishes; flow can be interrupted if someone turns on a tap elsewhere in your home.
The flow rate for all the appliances in the house is limited by the capacity of the rising water main. Flushing a toilet may thus lead to the hot water temperature fluctuating, or even going cold.
There is a delay in getting hot water out of the tap – you need to wait for the combi boiler’s burner to warm up enough to get the water to the right temperature.
A combi boiler can’t be used with a power shower, because they can’t heat hot water fast enough to supply large volumes of water.
They are also slower to run a bath than hot water systems using stored hot water.
Lower water pressure than some other types of boiler.
Not suitable for big homes where multiple sources of water might be used simultaneously.
Delayed hot water in summer months with some models (not storage combis).
No airing cupboard for storage or drying clothes.
No immersion heater, so no back up if there is a problem with the boiler or gas supply.

What does the Sedbuk efficiency rating mean?

Sedbuk stands for 'seasonal efficiency of a domestic boiler in the UK' and is now given as a percentage score. This score is an estimate of efficiency when the boiler is installed under typical conditions in Britain, taking into account climate, housing conditions, occupancy patterns, and controls.

How Sedbuk ratings are calculated?

It's calculated from the results of standard laboratory tests together with other important factors such as boiler type and the kind of fuel used. You'll see it stated as 'SAP 2005' or 'SAP 2009' depending on which method was used. 'SAP' stands for standard assessment procedure, and describes how the boiler efficiency is measured.

This index, which was developed under the UK Government's Energy Efficiency Best Practice Programme with the help of boiler manufacturers, enables you to fairly compare different models of boiler.

Traditionally, conventional boilers ranged from around 66-81% efficient, while condensing boilers are between 85% and 91% efficient (SAP 2005).

Legislation now requires all newly-installed boilers to be at least 88% efficient (SAP 2009). Boilers with a score below 90% are still being sold but these will disappear over time as new models come to the market.

Sebuk ratings are now presented as a percentage - here's how the new percentages match up to the old-style ratings

What happened to the Sedbuk A-D energy labels?

The Sedbuk efficiency rating is now a percentage score and this score has replaced the old A-D labels for boilers. The letter labels have been withdrawn to avoid confusion with the European energy labels, which use similar ratings but are based on different principles. You can find the Sedbuk efficiency rating of all current boilers on the Boiler efficiency database.

To see how the new percentage scores relate to the old letter categories, see our chart (right).

How reliable are boilers?

When we surveyed our members in 2011 we discovered that 46% of owners who bought a boiler bought after April 2005 have experienced some kind of fault. Not all of these faults required a repair. In fact the most frequent problem, a blocked drainage pipe, suffered by 11% of owners is actually a problem caused by poor installations. See below.

However, as it's such a fundamental domestic appliance you might want to consider taking out a boiler maintenance contract.

My boiler's condensate pipe has frozen. What can I do?

To prevent freezing, boilers retain condensate water and then let it out in a gush. However, particularly cold weather can still cause the water in the condensate pipe to freeze. This is especially true if the condensate pipe is external.

If your condensate pipe is frozen up, your boiler will probably switch itself off - exactly the opposite of what you need in freezing weather. To get your boiler going again, try the following steps.

Use warm, but not boiling, water to unfreeze the pipe's contents. This is only a temporary fix if the weather remains cold.
Lag the pipe with insulating material.
If the pipe is at a shallow angle or very narrow, it may need to be re-installed. The larger and more vertical the pipe is, the better.

Could I save money on my fuel bill?

It's estimated that the average efficiency of boilers in UK homes is about 60%. This means they waste up to 40% of the heat they generate in the form of hot flue gases.

Using more of this escaping heat to warm your water, rather than the outside, would reduce the amount of gas you need to burn to keep your central heating piping hot. Using less gas will obviously save you money on your gas bill.

Our table below illustrates just how much a modern condensing boiler could cut your gas bills, suggesting savings of between £50 and £200 per year depending on the size of your house. It is worth noting that this table is based on a gas price of 1.63 pence per unit. Gas prices are now much higher.

Annual fuel costs with different boiler types (source: Sedbuk)
Boiler type	Efficiency	Flat	Bungalow	Terraced	Semi-detached	Detached
Old gas heavy weight	55%	£308	£393	£408	£458	£634
Old gas light weight	65%	£266	£338	£350	£393	£542
New non-condensing	78%	£227	£287	£298	£333	£457
New condensing	88%	£205	£258	£268	£299	£409

In this review

You may also be interested in