As you may well know the newer condensing combi boilers are upto 93% efficient. They will ONLY be this efficient if they are run at maximim capacity all of the time. The new boiler will specify what KW output it has and the direct hotwater output DHW. This will be in litres per minute.

Choosing a boiler can be a very difficult propostion as you will need one which is large enough to cope with your hot water demands and small enough to still be efficicient. An oversized boiler will not bring the energy saving that you might be hoping.

This guide will help you to choose a suitable boiler for your hot water and heating demands by calculating the heat that is lost from your property and estimating your daily hot water requirements.

Hot water - your needs will depend on how many showers you have in the property. ie do you have a family bathroom and an ensuite which will likely be running at the same time? Do you have a large family who wake up the same time every morning and therefore place a great demand on hot water at certain times in the morning?

A £500 Glow-Worm Betacom 24KW will run radiators upto a maximum of 80,000 BTU's of output which is generally enough for a small to medium size property. However, the DHW is only 10 litres per minute. This will provide an adequate, but not great, shower and will take longer to fill a hot bath than most boilers.

A £1100 Vaillant EcoTEC Plus 837 which is just 28KW will run upto 14 radiators if installed correctly, however this combi boiler provides 15.2 litres per minute. I am sure you will apreciate that it will be a much more powerful shower and will be possibly to run 2 showers at once (just about).

The difference is £600 but you will be getting a better quality boiler with a longer warranty for parts and labour and you will have the added benefit of a more powerful heating system, In the long run the £600 will be paid back and more. You also need to consider the extra warranty the boiler gives you. For every extra year of warranty you wont have to pay for any HomeCare agreement with British Gas you might have which saves even more pounds.

Heating - If you are having new radiators installed, each and every room needs to be measured and the heat loss needs to be measured individually. There are free heat loss calculators available online which allow you to enter figures simply and easily and it will come out with a minimum figure in BTU's the radiator must be for that particular room. It takes into account such things as cavity wall or loft insulation, double glazing, whether its a bedroom, lounge or common area like a kitchen or hallway and how many outside walls that room has. Obviously each room will lose different amounts of heat during the day because the room make up is different so this means that radiators will need to be different sizes. In the past it was acceptable by some to guess radiator sizes but now a more professional approach has arrived.

There are lots of free online BTU calculators you can use but there is a quick and easy way. Measure the room in cubic feet and apply the following factors

Lounges and dining rooms - multiply cubic feet by 5
Bedrooms - multiply cubic feet by 4
Common areas and kitchens - multiply cubic feet by 3
For rooms facing north - add 15%
For French windows - add 20%
For double glazing - deduct 10%

This will give you the output of any radiator in BTU's (British Thermal Units) Adding the total for all the rooms in your property will give you the approximate demand in BTU's for the whole house. You should add another 20% to the total for a hot water circulating tank and 10% for general losses. This will then give you your boiler size and you will be in a better position to make a judgement.

What do I do next?

When you know your hot water and heating needs you need to choose the best boiler for your budget.

Things to consider will be:

Reliability of boiler, manufacturer warranty, ease of installation, service and repair, price, availability of parts and cost.