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A combi boiler is similar to a conventional boiler with a second heating circuit which instantaneously produces hot water.
Often overlooked is the fact that the operating / engineering differences between gas and oil fired combi boilers is significantly different.
Oil fired combi’s, unlike gas, do not have modulating burners to cope with additional heating demand when a hot tap is opened.
This problem is addressed in two ways:
When the heat store is exhausted, hot water flow of 10-13ltrs / minute @ 35oC rise above mains temperature is around the norm. To fill a bath (112ltrs) will take about ten minutes, provided no other taps are opened.
Second heating circuit for hot water
High life-time costs...unlike a gas combi, water is stored in the boiler at circa 80oC, components subjected to this constant residual temperature will fail. Oil fired combi boilers are complex, have bespoke components; they will be more expensive to maintain than a conventional system. Good business for boiler manufacturers, not necessarily the best option for the homeowner!
So what's the solution?
Keep it simple! In so many cases a conventional boiler with a pressurised domestic hot water cylinder such as a Megaflow is a far better option.