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With so many different systems available, it can be difficult to choose the right central heating system for your house. Except in a few exceptional circumstances, new boilers need to be of the condensing type. This means you will either be fitting a normal condensing boiler or a combination (combi) boiler.
Non-condensing boilers work by burning gas to produce heat which is then transferred through a heat exchanger to the water. The exhaust gas leaves the boiler via a flue. In a normal boiler, all of the energy in this exhaust gas is wasted.
A condensing boiler uses the heat from the exhaust gas (which is still considerable) in a secondary heat exchanger to increase the temperature of the central heating water returning to the boiler. This then reduces the temperature of the exhaust gas further, causing condensation to form in the exhaust gas. This is why these boilers are called condensing boilers.
Condensing boilers also supply a separate hot water cylinder with hot water. If there is no hot water cylinder then it is actually a combination boiler you have.
For these boilers to work at their optimum efficiency, the temperature of the water in the return pipework as it reaches the boiler must be kept below 55 degrees C. This means that there is still enough heat in the exhaust gas to warm the water further in the secondary heat exchanger.
Unfortunately most old central heating systems are designed to operate with flow and return pipes temperatures around 80 degrees C. Following this train of thought, older systems have smaller radiators to heat the rooms as there is a larger temperature difference between the radiator and the room (this means a small radiator running at 80 degrees can give out as much heat as a large radiator running at 55 degrees).
To get the most from your new boiler install, radiators should be resized at the same time as the boiler is installed.
If, after reading this, you decide to keep your old radiators, you must have the system power-flushed before the new boiler is installed. This cleans out all of the years of sludge and rust that will have accumulated in the system. The pipes through the heat exchangers in modern boilers are far narrower than in old boilers and not power-flushing your old radiators will lead to the new boiler blocking up. Nearly all boiler manufacturers will not honour their warranty unless your old heating system is power-flushed before the boiler is fitted.
Combination boilers are also condensing boilers in that they utilise heat from exhaust gases in a secondary heat exchanger as discussed above. They also heat hot water on demand. When a hot water tap is opened, the boiler senses the immediate pressure drop in the hot water system and begins to supply hot water which is heated on demand. As this uses a lot of energy they temporarily stop heating the central heating circuit, so hot water always has priority.
This setup means there is no hot water cylinder to keep warm, meaning energy bills are considerably lower. Combination boilers generally come in 4 power ratings - 24KW, 28KW, 35KW and 40KW. The more power they have, the higher the hot water flow rate. Whilst the 24KW and 28KW models will be more than adequate to supply enough water to a shower, if you turn on another tap or want to fill a bath their limitations become apparent. The hot water flow rate of these models is then found to be lacking!
The 35KW and 40KW actually supply plenty of hot water to allow you to run 2 hot taps at once. Baths run very quickly with these models. They cost anything up to £1,100 though, but this fairly high cost does mean you can lose the hot water cylinder in larger households. There are very few houses in which you need to turn on more than 2 hot water taps at once, but if you need to do this then a combination boiler is not for you and a condensing boiler would be a better choice.
These aren't really heating systems, but as they connect to your boiler we felt it was best to discuss them in this section. Condensing boilers can be connected to either a vented (low-pressure) or unvented (high-pressure) hot water cylinder. A vented cylinder will have a cold-water storage cistern feeding it, either above it in the airing cupboard or in the loft space of the property.
Vented hot water systems tend to be a legacy from older central heating systems. Any competent plumber can install one but their main disadvantage is that they are gravity fed from the cold-water storage cistern linked to them. This means the lack of hot water pressure can cause problems installing showers and other modern bathroom/kitchen fittings.
The low water pressure can be worked around by installing a water pump to increase the pressure of the hot water.
The better solution is an unvented hot water cylinder. These are pressurised hot water cylinders. They are not gravity fed - they are supplied at mains pressure. The main problem generated by these is that hot water increases in volume as it expands. As these are sealed systems this increase in volume is taken up by an 'expansion chamber'. Failure to set up these systems correctly can result in the cylinder exploding with potentially fatal results. These systems can only be installed by a plumber who has completed the BPEC Unvented Hot Water Course. They should be able and willing to show you their certificate on request.
These are a relatively new technology. They consist of a large unit, easily enough to fill most airing cupboards. They draw a huge amount of current - given that the average home will have a 100 amp fuse on the incoming mains, these can draw up to 65 amps of power. You can see that when drawing this amount of current, you need to use them on an off-peak tariff or they quickly cost a fortune to run.
Electric combination boilers work by using a large unvented hot-water cylinder as an 'energy store'. The most modern of these boilers work on the Economy 10 tariff. You require a special Economy 10 meter and usually 2 consumer units in your home to install these. Electric combination boilers draw power from the main during the cheaper off-peak times provided under the Economy 10 tariff. The stored energy in the cylinder then provide hot water and central heating during the peak times, thus saving the homeowner an expensive electric bill. The downside of these boilers is there high initial cost (around £2,500) but this benefit is they can be cheaper than gas to run, and usually around a third of the running cost of an electric flow boiler.
One word of caution, as they cannot provide as much power as a gas boiler, you need to ensure that whoever installs them does whatever they can to minimise heat (and thus energy) losses from the system. All heating pipes should be insulated wherever possible. The home should really have loft and cavity (if applicable) insulation - this means smaller radiators are necessary to heat the rooms. It is very important that radiators are correctly sized for the property - this is not usually a problem as these boilers are normally installed where there is no central heating already present.
Electric boilers have been around for a while now but running costs have traditionally always been higher than gas and so they only tend to be used where gas is not available.
As per gas boilers, electric boilers come in two types. The first type is a flow boiler and operates in a similar fashion to a condensing boiler. It requires a separate hot water cylinder. Flow boilers are initially quite cheap to buy (around £600 for a good model) but this cheap cost is soon offset by the fact that they cannot take advantage of off-peak electricity tariffs such as Economy 7 and Economy 10. These boilers are very expensive to run, around £1,800 to £2,000 for a 3 bedroom semi.