What is a heat-only gas boiler?

Heat-only gas boilers provide central heating and stored water and most often use an airing cupboard storage cylinder and a cold-water storage tank plus an expansion tank in the loft.

They are also commonly referred to as ‘conventional’, ‘regular’ or ‘open-vent’ boilers and, of the many different terms used, ‘open-vent’ probably best describes how the system works. That is to say, there is a boiler condensate pipe that runs from the hot water cylinder to the expansion tank and it is this boiler condensate pipe that releases any excess pressure caused by water expanding when it is heated up. If this pressure build-up is not controlled, your pipes and pumps may be damaged.

How does it work?

Heat-only/open-vent gas boilers are very common in traditional heating systems throughout the UK and offer many benefits for larger homes with a high demand for hot water. Not only do they enable your household to run multiple appliances and use hot water taps simultaneously without a significant drop in pressure, but they are also a cost-effective solution if your property previously had an older heating and hot water system, and existing pipework is still in place.

More space is needed for heat-only/open-vent gas boilers but as they are most popular for larger properties, this is not usually a problem. Requiring three tanks, this system typically has:

  • A cold-water cistern in the loft which is filled with water from the mains supply and feeds the storage cylinder
  • A small feed and expansion cistern to maintain the appropriate level of water in the central heating system
  • An insulated cylinder to store hot water (most often located in an airing cupboard)

What size do I need?

A good starting point is to count the number of radiators and bathrooms that you have. You can then review your boiler size options which are referred to by kW output rating. This rating tells you how much power a boiler provides to a heating and hot water system. Bear in mind that you can also look at the kW size of your current boiler as, in most cases, a straight swap is likely to be sufficient.

A Gas Safe registered engineer will be able to conduct a detailed home survey, help you choose the best size model to suit your property and hot water demand, and advise on current heat-only boiler prices. As part of this process other factors will be taken into account such as the number of people living in your home, how well insulated it is and the age and type of building concerned.

Choosing a new boiler

Looking for a new or replacement boiler? Our guide will help you understand the best option for you.

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