Choosing a New Oil Fired Boiler

March 25, 2011 | Boilers/Agas/Rayburns

If your are thinking about replacing your oil-fired boiler what questions do you need to ask yourself? A new boiler is not cheap but should be more efficient at running your heating system.

Even among oil-fired boilers, the choice is bewildering. Choosing the right boiler is an important decision. If it is too small then it will not meet your needs while if it is too big it will have an impact in your wallet. The correct size of boiler is essentially determined by two broad factors: the volume of your house and how well it is insulated. Boiler output is usually measured in kW. As a general rule: 1kW is required per 10m3

If you are replacing an existing boiler and your current boiler has been okay, then you should go with a similar size boiler but if you are starting from scratch, then you will need to consider:


Hot Water Usage

  • How many showers?
  • How many sinks?
  • How many baths do you have in your house?


Size of House

  • How many rooms need are heated?
  • What size are the radiators in these rooms?


Heat Losses

  • The type of house: terraced, semi-detached, detached.
  • Does your house have cavity wall insulation?
  • Do you have loft insulation?
  • Do you have double glazing?


Aesthetic Considerations

  • Do you want to hide the boiler in a cupboard?


Condensing Boilers

Condensing boilers are more efficient than normal boilers.  As their name suggests, they use the hot water return to condense the gases in the flu exhaust to recover to heat  the water. This type of boiler can be very efficient, around 90% + saving you money on your heating oil bill. Condensing boilers also reduce the amount of CO2 going into the atmosphere. It is estimated that around 60% of the carbon dioxide generated by domestic use comes from boilers. If every British household with central heating installed a high-efficiency boiler, more than 13million tonnes of CO2 would be kept from reaching the atmosphere. If there is a disadvantage, it would be that to squeeze every last measure of efficiency, condensing boilers are more complicated and so there is more to go wrong. However,  most are very reliable these days.

There are grants from the government for those upgrading boilers and improving the insulation in their homes. Find out if you are eligible at the Energy Saving Trust.



The efficiency of a boiler is given in terms of its SEDBUK (Seasonal Efficiency of a Domestic Boiler in the UK) rating. The scale goes from A-G, with boilers in the A range being 90%-91.3% efficient. If you want to save money on your heating oil then you should choose the most efficient boiler you can get. New boilers will be either A or B. You can find out more about SEDBUK Ratings at

In a drive to cut carbon emissions and improve energy efficiency, all new properties must be fitted with a condensing boiler by law. The building regulations state for oil-fired boilers:

  • All oil-fired boilers fitted after the 1st April 2007 have to be condensing boilers and again this applies to new installations and replacement boilers.
  • All condensing boilers fitted after the 1st April 2005 have to have a SEDBUK efficiency rating of either A or B.


Hot Water Tank or Combination?

Combination boilers provide both water for heating and an almost instant supply of hot water, hence the name. They have become increasingly more popular. In fact, combination boilers now account for well over half of all the new domestic boilers installed in Britain every year. The advantages are, there is only a very small reservoir of water that needs to be heated at any one time and no hot water tank is required so this could be an opportunity to save space although, oil-fired combination boilers are generally slower to provide hot water than some gas combi-boilers. Combination boilers may not be suitable for large houses where several people could be using the hot water at the same time.

Your heating and hot water requirements are individual to you so it is always a good idea to seek advice from an expert when choosing a boiler but hopefully this has made you aware of the relevant questions to think about when choosing your next boiler.



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  1. Boilerjuice website states “All oil-fired boilers fitted after the 1st April 2007 have to be condensing boilers and again this applies to new installations and replacement boilers”. Howver, this is not entirly correct because Oil Fired Boilers that are part of a range cooker do not, at the moment, have to be condensing. Although i understand that this will change in the future. I recently brought a Heritage Range cooker with an entergrated 96,000 BTU boiler, the boiler is not condensing and it does comply with the curent regulations regarding this.

  2. If replacing an existing oil fired boiler in my house in NORTHERN IRELAND must it be a condensing one?

    Do building ni allow fo r an A or B rating non- condensing boiler to be installed as a direct replacement?

  3. We are looking to replace our boiler which is a Worcester Danemoor 35/50 located in our garage. Bearing in mind we have never set the temp more than 2.5 out of a possible 5, the radiators are then only touchable for a couple of seconds. Do we need such a powerful boiler esp. as it will be a condensing type?

  4. Richard, if you follow the link above, to the SEDBUK site, you will get a page called “boiler efficiency database. You will see an option for ” recommended boiler size” Good luck

  5. It is highly recommended that you have your boiler serviced every year to help maintain it whilst identifying and fixing any issues – Just like you do for your car and the annual MOT.

    Also, almost all new boilers require an annual service to keep your warranty valid.

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