Boiler Systems

What type of boiler system do I need?

With the constant development of building technologies, there is a greater range of options than ever before when considering the source of heat you will use to provide hot water and heating to your property. The various different options can be bewildering at times and include all sorts of options from air or ground source heat pumps, through gas boilers to oil, electric or even solid fuelled boilers. The easy access to mains gas in South-West London, however, means that the best option is invariably a gas powered boiler.

Broadly speaking, there are three types of gas boiler - combination (often called combi) boilers, closed vented (mains pressure) and open vented (gravity) system boilers. So in this blog article let's try and work out the pros and cons of these three options.

Open-vented (gravity) system boilers

Open-vented system boilers are the traditional system with tanks of water in the loft, a hot water cylinder (usually in an airing cupboard or similar) and a separate boiler. Unless the system has been installed recently then this will almost certainly be the existing system in your home.

If you are replacing an existing regular boiler which has broken down then this will clearly be what to install. If, however, you are installing a new heating / hot water system we would generally advise one of the other options in order to maximise space. The tanks in the loft clearly inhibit loft conversions and (if poorly insulated) pipes have been known to freeze in winter. Even if space can be made for the tanks there will not be an option for plumbing / heating in the loft.

Another downside to open-vented systems is that they require a difference in height between the bottom of the water tank in the loft and the hot water outlet to generate pressure using gravity. This distance is known as the head of water and if it is less than 1 metre, which would happen if you were to install a shower at loft level, then you might get nothing more than a trickle from the shower head. This is why showers in houses with open-vented systems often have to have their own pump to generate enough pressure to take a shower.

A regular boiler may be the best option for replacing an existing boiler if the property has an older radiator system, as it might not be able to cope with the higher water pressure that is delivered by system or combi boilers that work off mains pressure. This issue can be alleviated, however, by pressure-testing the existing system to identify any leaks at an early stage.


Combi Boilers

A combination or 'combi' boiler is both a high efficiency water heater and a central heating boiler in a single compact unit. Combi boilers heat water directly from the mains when you turn on a tap, so you won't need a hot water storage cylinder or a cold water storage tank in the roof space.

They are also very cost-effective and energy-efficient as water is heated instantly rather than being heated and stored in a cylinder. An added benefit is that hot water is delivered at mains pressure, which means that you could get a powerful shower without the need for a separate pump.

The only significant downside to a combination boiler is that when more than one hot water source is run at the same time then the flow rate can be affected since the boiler can only heat so much water at a time.

These factors mean that combination boilers are ideal for homes where space is at a premium and where only one bathroom is likely to be in use at any one time.


Closed vented (mains pressure) system boilers

Closed vented system boilers require a pressurised cylinder for storing hot water, however the major heating and hot water system components are built into the boiler itself. In addition, there is no need for a tank in the loft, so it can be an option in a home with little or no loft space or where the space is earmarked for a conversion.

As the water is coming into the hot water cylinder at mains pressure you also don't have the issue of poor pressure in the loft shower for example.

Additionally, the fact that there is a ready supply of hot water at all times makes this system ideal for homes where multiple sources of hot water are required simultaneously - family homes with more than one bathroom, for instance.

Finally if you are planning on fitting water underfloor heating to your house it would be wise at this point to consider replacing an open vented system boiler with a mains pressure system boiler as the former probably won't have been specified to take the extra load of an underfloor heating system on top of the hot water and central heating demands.system_boiler_diagram


There is much to consider when choosing what sort of boiler and heating / hot water system you would like to install but as a general rule of thumb - if you have more than one bathroom which are likely to be in use simultaneously and you have sufficient space for the hot water cylinder then a closed vented system boiler would most likely serve your needs best. If space is at a premium and you have limited requirement for hot water at any one time then a combination boiler might be more appropriate.

We hope this short article has been of use to you. If you need further advice and are considering a renovation yourself then please do contact us and we would be glad to meet to discuss your requirements and provide a free no-obligation quote.

If you have any questions relating to this article, why not ask our Build team?

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