Combi vs System Boiler: Which Should You Choose?
The combi-versus-system question answers itself with three facts about your home: how many bathrooms run simultaneously, what your mains flow rate actually measures, and whether you have (and want to keep) cylinder space. Combis heat water on demand with no stored volume; system boilers charge an unvented cylinder that serves multiple outlets at once. Everything else — brand talk, installer preference, what the neighbours have — is noise around those three facts.
How each system works
A combi (combination) boiler does both jobs from one wall-hung unit: central heating, and hot water heated instantly as it flows through the boiler when you open a tap. No cylinder, no loft tanks, no stored hot water to run out — and no stored hot water to draw on either, which is the whole trade. Hot water output is capped by the boiler's instantaneous capacity, quoted as a flow rate at a temperature rise.
A system boiler heats the same radiators but charges an unvented cylinder with hot water held at mains pressure. Multiple showers and taps draw from the stored volume simultaneously without the boiler mattering in the moment; the cylinder recharges afterwards. The costs of that capability: the cylinder's floor or cupboard space, a modest standing heat loss, and a slightly larger installation bill.
The three deciding facts
Bathroom arithmetic: one bathroom (or two never used at once) suits a combi cleanly. Two-plus bathrooms with morning overlap — two showers at 7.30 — is the system boiler's home ground, because a combi splits its instantaneous output between simultaneous draws and both showers feel it. This single fact settles most households.
Mains flow rate: a combi can only heat the water your main delivers — measure it (litres per minute into a bucket at the kitchen cold tap) before anyone recommends anything. Below ~10–12 l/min, a big combi is a brochure fiction and either the main gets upgraded or the system route (or an accumulator) enters the conversation. Unvented cylinders also want decent mains flow; genuinely poor mains points at keeping vented storage or fixing the supply. Cylinder space is the third fact: no space, and the system route needs to find some; generous space, and the combi's tank-removal appeal shrinks.
Costs and conversions
Like-for-like swaps are cheapest in both directions: combi-for-combi £1,800–£2,800; system-for-system similar plus cylinder work if it's renewed. Conversions cost the difference in kind: cylinder-system to combi £2,500–£4,000 (removing cylinder and tanks, rerouting, often upsizing the gas pipe); combi to system-plus-cylinder £2,800–£4,500 (finding cylinder space and pipework). Converting against your home's facts — a combi into a three-bathroom house — buys disappointment at full price.
Running costs split subtly: combis waste nothing on storage but cycle for small draws; cylinders lose a little heat continuously (modern insulation makes it genuinely little) but let the boiler run efficient charging cycles and pair with cheap-rate strategies. For most homes the running-cost difference is marginal against the capability difference — decide on capability.
Verdicts by household
Choose a combi: flats and one-bathroom houses, decent mains flow, space at a premium, households that value simplicity and never queue for showers. Choose a system boiler and unvented cylinder: family homes with simultaneous bathrooms, good mains, cylinder space available, and anyone with future heat pump intentions — a cylinder-based system converts to heat pump service far more gracefully than a combi household starting from scratch.
Keep a heat-only (conventional) boiler with tanks: the honest third answer for some older gravity systems where wholesale conversion isn't worth it this cycle, or where mains flow can't support unvented pressure. And in every case: make the installer measure the flow rate and count the bathrooms before quoting — a recommendation that arrives before those numbers isn't a recommendation, it's a reflex.
| Factor | Combi | System + cylinder |
|---|---|---|
| Simultaneous showers | Weak — output splits | Strong — stored volume |
| Space needed | Minimal | Cylinder cupboard |
| Typical install (swap) | £1,800–£2,800 | £2,800–£4,500 with cylinder |
| Mains flow dependency | High | High (unvented) |
| Heat pump future-fit | Poor | Good — cylinder in place |
| Best for | 1-bath homes, flats | 2+ bath family homes |
The questions that settle it
Whichever way you're leaning, three questions stress-test the decision: what does my property actually support (measured, not assumed)? What's the ten-year cost of each option — capital, running and servicing — rather than just the install price? And what happens at failure: parts availability, engineer familiarity, disruption? The option that survives all three is usually right, even when it wasn't cheapest on day one.
Frequently asked questions
Can a big combi serve two bathrooms?
High-output combis exist, but simultaneous draws still split one instantaneous capacity — and only if your mains flow feeds it. Two genuinely concurrent showers are cylinder territory; sequential showers a large combi can manage.
How do I measure my mains flow rate?
Time the kitchen cold tap into a measuring jug or bucket: litres collected × 6 ÷ time in ten-second units gives litres/minute — or simply litres caught in 30 seconds doubled. Below ~10–12 l/min, big-combi promises deserve scepticism.
Which is cheaper to run?
Marginal either way for most homes: combis avoid storage losses but cycle on small draws; modern cylinders lose little and charge efficiently. Choose on capability fit — the running-cost gap won't rescue a wrong-sized choice.
What happens to my loft tanks with a combi or unvented cylinder?
Both routes are sealed/mains-pressure systems: the loft tanks and their pipework are removed, which frees the loft and removes a freeze risk. It's part of the conversion cost — and one of its quiet pleasures.
If you'd rather hand this over
Plenty of readers reach this point, weigh the afternoon involved, and sensibly hand it to someone who does it weekly. No judgement — that's what the network exists for. Every engineer is Gas Safe registered (plus OFTEC for oil and MCS for heat pumps), insured and locally based; prices are agreed up front; and genuine no-heat emergencies are prioritised. Describe the symptoms you've identified here and you'll get an accurate price rather than a guess — informed customers reliably get better quotes.
Carbon monoxide is odourless and colourless. Our CO safety guide covers the symptoms, where alarms belong and the emergency response.