The “Combi” boiler is probably one of the most popular on the market, this gives you instant hot water without the need to wait on a hot water tank heating up. These come in various outputs with hot water delivery at around 9 ltrs from a 24kw unit to around 16 ltrs @ around 35c a minute from a 42kw unit.
The “system” boiler tends to be combined with an unvented hot water tank, these boilers are also sealed systems and do not require a header tank in your attic. When combined with an unvented hot water tank this will produce the better flow rate of around 22ltrs a minute, these systems are better for larger families in larger properties, you can also add solar thermal panels to reduce your gas bills and help the environment.
Heat only boilers are more of the traditional type of units, these are ideal if it’s only a direct replacement the customer is after, these tend to be fitted to an open vented system which retains both the header tank and cold water storage tank within the attic.