Conventional versus Combination Boilers - The Pros and Cons:

A Conventional boiler is purely a heating appliance – there will be an external hot water cylinder, control valves and radiators.

Worcester-Bosch Ri conventional boiler

Why would you want one?

  • More than one hot water outlet can be supplied simultaneously (dependant on a separate storage cylinder)
  • You desire a longer service life for a boiler than 10-12 years.
  • You would prefer to have a stored hot/cold water supply in the house
  • You may have recently invested in a new hot water cylinder/tanks
  • You want an electrical immersion heater as hot water backup in case of boiler failure
  • The water flow rate (sometimes erroneously described as pressure by plumbers) is too low for a combination boiler (but see low water mains pressure solutions as there are solutions available)
  • All conventional boilers are now condensing technology, mandatory in the UK
  • Perceived reliability advantage.

A Combination boiler contains all the necessary controls and equipment to provide instantaneous hot water and also central heating – apart from radiators and pipe, everything else is in the boiler.  There are now over 7 million households in the UK equipped with Combination boilers.

Worcester-Bosch CDi combination boiler

Why would you want one?

  • Convenience. Hot water is always available, 24 hours a day
  • Economy. Only heat the hot water you use.  Ideal if you are at home infrequently.
  • Consecutive hot water demands can be made without running out
  • You don’t require to run two hot water taps simultaneously
  • No water tanks in the roof or hot water cylinders are required.
  • It’s a one box compact solution, ideal for small properties
  • All combination boilers are condensing versions, now mandatory in the UK
  • It offers mains pressure showering without a pump (ideal for flats or loft conversions)
A note about reliability:

Combination boilers are considered by some to be less reliable than Conventional units.

It’s important to remember that the Combination boiler does the equivalent of a conventional system, but in one box. It removes the need for these external parts:

  • Central Heating Pump
  • Hot water Cylinder
  • Water tanks (two)
  • Motorised Valves
  • Hot water thermostat

All of which would otherwise be fitted (and occasionally need repair) in a conventional system.

Providing a combination boiler is connected to a properly cleansed system (see ‘Powerflush’) that is dosed with corrosion inhibitor there should be no discernable difference in reliability.   Your decision should be made on the features and benefits.

It is a general rule with boilers that you get what you pay for.  We only recommend products we have found to be reliable and which are well made.

Condensing (High efficiency) – what you need to know

Condensing High Efficiency Boilers – an explanation:

Older boilers emit flue gases which are very hot (in excess of 160 degrees). This is rather wasteful because most homeowners want a boiler to heat their house, rather than the surrounding countryside.

With older boilers between 20-35% of every £1.00 spent on gas, goes out of the flue (chimney) as heat into the atmosphere.

For some years, high efficiency boilers have been available for industrial and large domestic systems, but their use has not been widely publicised. These units recirculate their hot exhaust gases until more useful heat has been extracted into the boiler, before expelling a much cooler exhaust. The cooler exhaust contains particles of water, (or condensate, as it is described in the trade) which create a visible ‘steam like’ plume.   The condensation is deliberately encouraged within the boiler because it helps transfer more heat into the radiators.

Boilers which recycle most of the otherwise wasted flue heat are therefore called ‘condensing’.  All boilers described in the UK as ‘high efficiency’ can be assumed to be condensing boilers.

Condensing boilers are available for Gas, LPG and Oil fuels, and come in ‘conventional’ and ‘combination’ types.  They can be floor mounted, wall hung – and since 2017 condensing combustion technology for warm air systems is mandatory too.  We’ve got a separate page for these.

Government mandate:

The UK Government signed an international agreement (Kyoto Agreement) committing us all to reducing energy losses. As a result, from 2005, it became mandatory for UK homeowners to install high efficiency ‘condensing boilers’ when fitting a new gas unit.

Oil boilers became subject to this ruling later in 2007. See ‘exceptions’ below.

The Government class these units as having an efficiency of SEDBUK of grade ‘A’, which is top marks in laymans terms (see for more information).    Similar markings are available for domestic appliances, light bulbs etc.

In October 2010 boilers with the SEDBUK B rating were dropped from the list of acceptable fitments, raising the bar of efficiency a little further.

Further steps can be taken to further increase efficiency of condensing boilers, such as ‘exhaust gas recuperators’ and ‘weather compensation’.

Condensing boiler reliability:

Unfortunately, in the early 2000> period certain domestic boiler manufacturers exhibited undue haste in releasing their condensing boilers onto the market, before they had been fully tested. There have been some expensive failures, and it was not uncommon to have to scrap a condensing boiler only a few years old.

Whilst they are more complex, there is no reason why a new quality condensing boiler should be any less reliable than a standard unit.

The key is to buy from a volume manufacturer with a large Research & Development budget and a good reliability track record to keep; which brings us back to Worcester Bosch, Viessmann,  Vaillant & Grant* (*oil boilers only).  

Exceptions to the ‘condensing only’ Building Regulations:

Many of our European cousins have been successfully running condensing boilers for years. However, European installations tend to use ‘vertical flues’ where they protrude through the roof. The condense pluming is therefore far less visible and irritating than in the typical UK installations, which tend to use horizontal flues (ie: wall mounted).

It is recognised that there will be situations, particularly in high density housing, where the condense plume from a horizontal flue will be unacceptable to residents, or where the flues are shared within a central duct in a block of flats which may not be practically viable to alter.

Therefore, there are exceptions to the Government mandate, some non-condensing boilers will continue in production to cater for this market.  Over the past few years we have found very,very few homes that could be classed as exempt, although some high rise modern dwellings could qualify.

We can’t see why anyone would want to use more fuel than necessary and achieve an exemption.

Our advice:

If you want reliability, specify a good boiler, whatever your preferred choice of system, rather than a budget unit. We only fit the former!

For more advice on boiler installation in Surrey, Sussex, Kent and South London please contact us.

Condensing technology, more background:

The high efficiency boiler was introduced in the UK by Archie Kidd. Whilst today’s media suggest the idea is the latest thing, you could have bought a condensing boiler from Mr Kidd in 1982, who decided to commercially market his 1959 design, and chances are, you’d still be using it now.

Today’s condensing boilers work on a similar principle of transferring as much heat from the fuel being burnt, to the water in your pipes. The last 10% of usable boiler efficiency requires the (gas or oil) fuel’s internal water content to condense inside the boiler, against the water jacket. The ‘latent heat of evaporation’ gives away more energy when the water in the fuel is encouraged to turn from a gaseous to liquid form.

With all the extra efficiency, the flue gases emitted from the boiler to outside are of a much lower temperature; this way your boiler wastes less valuable fuel heating the garden. A disadvantage is your flue tends to look a bit like a steam train’s funnel in winter.

There’s been a lot of negative publicity in the press about condensing boilers. It’s true that a significant number of boilers from 2000 onwards – culprits being predominantly British and Italian manufacturers – have already been scrapped by their owners.

Some manufacturers fretted about being left behind, desperately rushed through models which had not been comprehensively tested or funded – these boilers are the ones you have read negative comments about.

Our Installation:

Our installation, specification and design is second to none.  For actual examples of our work please refer to our Blog section.

If you’d like to know what our customers think, see our Which? Trusted Trader reviews.



Customer Testimonials

Excellent Work

Excellent work , the workers were very efficient, considerate and tidy. They worked with the least disruption possible for an elderly lady. Any unexpected problem was dealt with safely and professionally. The finished heating is to a high standard. K.T. Kent

Jennie 05/05/2017