There are many different variants of central heating boilers, and we can categorize them several ways. The easiest way to group them is by the way in which they operate. We have combination, or combi boilers, where typically everything is encased inside one box on the wall. And then we have conventional boilers, sometimes referred to as system boilers, (however the term "system" can also be used when there is no tank in the loft for the central heating to draw its water from).

 

Combination boiler

 

This is the best-selling boiler type for gas central heating in Britain. Over half of all boilers fitted now are combination boilers. A combination - or 'combi' - boiler provides heat for your radiators and domestic hot water on demand. They have been around for the last 30 years or so, some of the earlier models carry a few horror stories for reliabilty, however modern technology has come on leaps and bounds, especially in the last 10 years.

The name comes from the fact it combines all the functions of a traditional boiler, hot water cylinder and cold water tank set-up. All the major parts are in one unit, which makes them convenient to install.

There are different types, but they all work by taking cold water straight from the mains supply and heating it as you need it. When you turn on a tap, water is drawn from the mains, heated in the boiler and comes straight out of your tap nice and hot

This means there's no need for a separate hot water storage cylinder or cold water feed cistern. But there's a small time lag between turning on the tap and getting hot water, and the flow rate is limited by how quickly the boiler is able to heat up the water.

The advantage is they require less space than heat-only boilers (because there is no storage cylinder), and avoid heating water that's not used immediately. If you have adequate pressure, you can also run a nice power shower from a combi boiler. You can also put your combination boiler up into the loft space, as it's a sealed system, gravity has less of an effect on the way it works, (whereas a tank fed system has to have the tank at the highest point of the system)

On the down side, the flow rate limits how much water you can use at one time – no filling the kitchen sink while running a bath, for instance. Also, they can only provide heating, or hot water, not both at the same time, so if you have a large family who showers early on a winters morning, be prepared that your heating will cool down for as long as you're in the shower. Some areas also have very poor mains water flow, so a combination boiler may be least preferred over a tank fed system.

Combination boilers may not be suitable for boiler changes on an old system. They exert a higher water pressure on your central heating system opposed to a tank fed system,, and if you have 'suspect' pipes, valves or radiators, you may get a little wet!!

You might also miss using the (wasted) heat from your hot water cylinder in your airing cupboard. When upgrading from a conventional boiler to a combination boiler, please be aware that the gas pipe may not be large enough. Provision may have to be made to increase the gas pipe, the gas meter may be the other side of the house, this may be a deciding factor in staying with a conventional boiler.

Conventional heat-only open-vent boiler

 

Heat-only boilers are mainly fitted in larger houses, and came as standard in many new houses built during the seventies, eighties and nineties. They are fitted in some new builds, however combination boilers are starting to buck the trend for the spacesaving element. They provide heat for radiators directly, and produce hot water which is stored in a separate hot water cylinder. You normally have two tanks in the loft, one for the domestic hot water feed and one to feed the central heating.

This type of boiler can be wall mounted, floor standing, or even behind your fire, as a combined back boiler fire unit. These arrangements take up more space than a combi boiler system, but because there is a large tank of stored hot water you can use several taps at once. This makes this type of system more suitable for larger houses, and for those with families.

Please note that with a standard vented cylinder, you are restricted by the amount of water stored in the cylinder, so larger users may wish to choose their set up wisely. For instance you might run a bath, then need to wait for the boiler to heat up the water in the cylinder again before you can run another. You can overcome this by installing an unvented hot water cylinder, or a thermal store cylinder with a mains coil.

System heat-only boilers

 

A system boiler, just like an open-vent boiler, works excatly as a conventional boiler above, however the need for a feed and expansion tank is removed, and we commonly use the term "sealed system" .There is a pressure gauge on the boiler, and a method of letting water into the system normally via a filling tap. Some system boilers also have the pump built in, so there are less components elsewhere on the system.

Generally speaking, a "sealed system" tends to suffer from airlocks a lot less (whereby awkward pipe runs cause air to stop the water flowing to certain radiators), and also radiators tend to warm up quicker. Once again, because there is no tank in the loft, one can install a system boiler at the highest point of the house

Coupled with a high recovery cylinder (a cylinder with a very fast warm up time), or an unvented cylinder (mains pressure water storage), it is the ideal choice for the family home which needs every inch of space for storage and has a high demand for hot water.