Combi Boiler FAQ

What are the advantages and disadvantages of a combi boiler?

The combi, or combination, boiler is now the bestselling boiler type for gas central heating in Britain, but they are not for everyone. What are 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 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 up to 18 litres of water a minute at 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.

My new boiler has a Sedbuk energy rating. What does it mean?

The Sedbuk label is the boiler equivalent of the energy label found on domestic appliances such as washing machines, fridges and dishwashers.Sedbuk stands for 'seasonal efficiency of a domestic boiler in the UK', and has lettered ratings from A to G.The rating represents the average annual efficiency achieved in typical domestic conditions, making reasonable assumptions about how it's used, the climate, controls, and other influences.

How Its Calculated.

It's calculated from the results of standard laboratory tests together with other important factors such asboiler type and the kind of fuel used. This index, which was developed under the UK Government's Energy Efficiency Best Practice Programme with the help of boiler manufacturers, enables you to compare different models of boiler fairly.

What each letter means.

Each letter rating represents a range of percentage efficiencies with A (efficiency of more than 90%) being the best and G (efficiency of less than 70%) being the worst. Conventional boilers are around 66-81% efficient, while condensing boilers are between 85% and 91% efficient.

New building regulations.

New building regulations require that only gas boilers with Sedbuk A rating can be installed, unless it's too difficult or expensive, in which case an exception procedure must be applied for.
Click here to see the Sedbuk ratings of current boilers on the Boiler efficiency databse.

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. 
The  table below illustrates just how much a modern condensing boiler could cut your gas bills, suggesting savings of between £160 and £360 per year depending on the size of your house.
Boiler type Efficiency Flat Bungalow Terraced Semi-detached Detached
Old gas heavy weight 55% £267 £341 £354 £397 £550
Old gas light weight 65% £231 £293 £304 £340 £470
New non-condensing 78% £197 £249 £258 £289 £396
New condensing 88% £178 £224 £232 £259 £355
Annual fuel costs with different boiler types (source: Sedbuk)

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Contact Details

Simon Mason
mobile - 07783146897
landline - 01733 755667 
Energy Efficiency Qualified Heating Engineer - Part L
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