Unlike a combi gas boiler system, an open-vented heat only gas boiler system (with a water tank) has components like the circulation pump housed externally from the boiler. This type of system will also usually require cold water, and feed and expansion tanks in the loft. With a heat only boiler (also known as a conventional boiler system), the hot water is stored in a hot water cylinder or hot water storage tank. You will also normally have an on / off switch, so you can heat water in the storage tank when you know you are going to need it, and leave it switched off when not in use. Heat only boilers – pros and cons Heat only boilers are better suited to larger households where several people frequently need to use hot water at the same time. Unlike a combi gas boiler, once the storage tank runs out of hot water, you will have to wait for the water to heat up again: if you run a bath for example, you may have no hot water for a while afterwards. If you’re interested in getting a heat only boiler, see our boilers review for the latest models, including test scores and Best Buys. Our gas boiler brands satisfaction results also show how Which? members rate different boiler manufacturers.