Question 1

What hot water flow rate can I expect from a combi boiler?

Accepted Answer

Hot water flow rate from a combi boiler depends on the boiler's output size (measured in kW) and your mains water pressure. A 24kW combi boiler typically delivers around 10–11 litres per minute; a 30kW boiler around 14–15 litres per minute; and a 35kW boiler around 16–18 litres per minute. Adequate mains pressure (ideally 1.5–3 bar dynamic pressure) is essential for good performance.

Question 2

Is a combi boiler suitable for a large home?

Accepted Answer

Combi boilers can be suitable for larger homes if the mains water pressure is good and hot water isn't required simultaneously in multiple locations. For homes with 3+ bathrooms where simultaneous hot water use is common, a system boiler with a large unvented cylinder is usually a better solution, as it can deliver higher flow rates to multiple outlets at once.

Question 3

What pressure should my combi boiler be at?

Accepted Answer

Your combi boiler's pressure gauge should read between 1 and 1.5 bar when the system is cold. When the heating is running, pressure may rise to 1.5–2 bar, which is normal. If pressure is below 0.5 bar the boiler will likely lock out; if it regularly exceeds 2.5–3 bar this indicates a fault with the pressure relief valve or expansion vessel.

Question 4

What is the difference between a combi and a system boiler?

Accepted Answer

A combi boiler heats water on demand directly from the mains — there is no storage cylinder. A system boiler heats water and stores it in an insulated hot water cylinder. System boilers are better suited to homes with high hot water demand or multiple bathrooms, as the cylinder provides a ready supply. Combi boilers are more space-efficient as they eliminate the need for a cylinder.

Question 5

How much space does a combi boiler installation require?

Accepted Answer

A combi boiler unit itself is compact — roughly the size of a kitchen cupboard unit (typically 700mm × 400mm × 300mm). They can be installed in a kitchen cupboard, utility room, or airing cupboard. The key space consideration is adequate clearance around the unit for servicing and the flue exit point. Unlike a regular boiler, no loft tank or airing cupboard cylinder is needed.

Question 6

Can I run a bath and a shower simultaneously with a combi boiler?

Accepted Answer

Running a bath and shower simultaneously from a combi boiler will typically result in reduced flow to both outlets, as the boiler is limited to a single heat exchanger. Whether this is acceptable depends on the boiler size and your mains pressure. For households that frequently need simultaneous hot water use in multiple locations, a system boiler with unvented cylinder is a more suitable solution.

Question 7

Is a combi boiler suitable for a home with multiple bathrooms?

Accepted Answer

A combi boiler can work well in homes with two bathrooms if simultaneous use is infrequent and mains pressure is good. For homes with three or more bathrooms where the household regularly uses hot water in multiple places at the same time, a system boiler with a larger unvented cylinder will provide a more satisfying experience.

Question 8

Why does my combi boiler run cold water before the hot water comes through?

Accepted Answer

This is a common characteristic of combi boilers known as 'cold water slug.' When hot water is demanded, the boiler fires up and heats water in the heat exchanger, but any water already in the pipes will be cold until the newly heated water reaches the tap. The delay depends on pipe length. Some high-specification combi boilers have a small internal hot water store that eliminates this delay.

Combi Boiler FAQs