The basic difference
A combi boiler heats water directly from the mains, on demand — no separate hot water cylinder or tank. A system boiler works with a separate hot water cylinder, which stores heated water ready to use. Both are common across Birmingham homes; which one you've got usually comes down to your property's age and hot water demand.
Why it matters for repairs
On a combi, a diverter valve fault is a classic cause of losing hot water while heating still works (or vice versa) — see our no hot water page. A system boiler doesn't have a diverter valve in the same way, so that specific fault simply doesn't apply; instead, issues are more likely to involve the cylinder itself, or the separate pump and controls that move water between the boiler and the cylinder.
Pressure behaves differently too
Combi systems are sealed and pressurised, so pressure loss (see our pressure guide) is a genuinely common combi fault. Older system boilers paired with a tank in the loft work quite differently and don't develop the same pressure-loss symptoms in the same way.
Why this is worth knowing before you call
Telling us which type you've got — or just the make and model — helps us turn up with the right expectation of what's likely going on, rather than starting from scratch. If you're not sure which you have, that's fine too; it's one of the first things we'll confirm on arrival.