Combi or system boiler for a Telford semi?

The real question is simultaneous demand

A combi heats water as it flows, so it is brilliant at one outlet and struggles at two. A system boiler with a cylinder has the hot water already stored, so two showers at once is no problem until the cylinder runs out.

So the question is not how big your house is. It is how often two hot outlets run at the same time, and whether anyone would notice if they could not.

Choose a combi if

• One bathroom, or an en suite that is rarely used at the same time
• You want the airing cupboard and the loft space back
• Your incoming mains flow and pressure are good, which is measured rather than assumed
• You would rather not heat a cylinder of water you might not use

Choose a system boiler and cylinder if

• Two bathrooms in regular simultaneous use, or a busy family with a morning queue
• You want a strong shower without relying on a pump
• Your mains flow is weak, which is a genuine constraint on any combi
• There is somewhere sensible for the cylinder to live

What a conversion involves

Going from an old regular boiler with loft tanks to a combi is more than a boiler swap. Tanks come out, pipework changes, the cylinder goes, and there is making good afterwards where things used to be. It is usually two days rather than one, and it is worth knowing that up front rather than finding out on day two.

One more option

An unvented cylinder, sometimes sold as a megaflo style system, gives mains pressure hot water at several outlets at once, which is the best of both if you have the flow and the space. It costs more and it needs its own annual attention, so it suits bigger houses more than a standard semi.