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Boiler Information and Advice.

When replacing or buying a boiler, it's important to know what is right for you. Boilers like many other products have advanced a great deal in recent years, and come packed with many new features.

It is also worth noting that if you buy a new boiler it will by law (As of 2005 for gas and 2007 for oil) have to be a condensing boiler. What does this mean? Simply put, they are more efficient as they can recover the heat held in waste gases.

So, how do you go about choosing a boiler?

Here we have a little info to get you started.

Most people will choose a combi (combination) boiler which are Britain's best-selling type of boiler, probably because they are:

  • Compact and easy to install
  • Provide heat for your radiators and hot running water on demand.
  • They don't need a water storage cylinder making them ideal for smaller properties.
  • Can provide mains pressure hot water.

For larger households or for if you would like the convenience of being able to run hot water from more than one tap at a time. You may wish to go with a system boiler.

For a standard single-coil cylinder, you should allow approx. 25 litres per person per day. It is, however, recommend that new installations have a twin-coil cylinder, to futureproof the home in the event a solar thermal or renewable heating system needs to be fitted.

If you have a gravity-fed system (where pressure is generated from a cold water tank, often placed in the loft) it would be worth while considering a four-pipe system instead. Usually in the UK systems have two pipes, which are providing hot water and heating concurrently, however Viessmann's set-up uses four pipes to deliver hot water separately to both heating and hot water. Which means, the boiler prioritises hot water providing immediate availability whilst heating water temperature is reduced so the boiler fully condenses most of the time. This leads to a lower output, more cost effective boiler that uses less energy.


Types of Boiler.

Combi (combination) Boiler

How do they work?
Combi boiler animation

Combination or combi boilers combine the central heating with domestic hot water (DHW) in one device. When DHW is used, a the boiler stops supplying water to heating and diverts all the boiler's power over to DHW. Some combi boilers have small internal water storage vessels combining the energy of the stored water and the gas or oil burner to give faster DHW at the taps or to improve the DHW flow rate.

Combination boilers are rated by the DHW flow rate. Typically the kW ratings for domestic units are in the range of 24 kW to 54 kW, giving flow rates of 9 to 23 litres approx. Anything larger would be more commonly used in commercial / institutional applications, or for multiple-unit dwellings. High flow-rate models can simultaneously supply two showers.

Advantages: Combis are quicker, easier and cheaper to fit than system boilers, as well as space saving due to the lack of a cylinder or cistern. Water is delivered at mains pressure, so you can enjoy a better shower than with gravity fed systems.

Disadvantages: It's a priority system, so it only really deals with one heating need at a time. While fine for small families with one bathroom, larger families will experience poor flow rates when multiple outlets are used at once. Performance is also dependant on the diameter of the pipe entering the property: if it's less than 22mm, then a combi is a bad choice.


System Boiler.

How they work:
System boiler animation

System boilers are fitted to sealed heating systems, but unlike combis they store hot water in a cylinder, so they are able to provide for several outlets at once at mains pressure. There's no need for a cistern in the loft and the expansion vessel is built in.

Advantages: Ideal for larger homes with higher demands, and as they have most of their major components built in (i.e. expansion vessel and pump), installation is quicker, cheaper and neater. Flow rates are usually high as water is delivered at mains pressure, and hot water is instantaneous.

Disadvantages: Will run out of hot water if overused. Some installers claim they are more complex and prone to problems than regular boilers, such as pressure loss.


Regular/Conventional Boiler.

How they work:
System boiler animation

Regular boilers are most commonly purchased as replacements for homes with an open-vented heating system. Like system boilers, they store water and require a separate hot water cylinder.

Advantages: The water out of the taps will be at a good flow rate (not pressure) and hot water is instantly available. This is perfect if you wish to use a �power' shower, which requires a cold water feed from the cistern and a separate electric pump.

Disadvantages: They tend to cost more to install, needing more components and pipework, and they take up alot more space. If you cant get the cistern high enough they can suffer from low pressure, which means you may need additional shower boosters. Hot water can run out.

Note: Sealed or Open? In a sealed circuit, the system is filled to approx 1 bar pressure and then sealed. Unlike an open-vented system, there is no cistern, so an expansion vessel handles excess water. Sealed systems are more efficient.