If you're replacing a boiler in 2026, there's a good chance you've been told you "should" go combi. Most installers will steer you that way by default — they're cheaper, they take less space, and they're what 80% of UK households end up with.
But "most" doesn't mean "best for you". A combi boiler in the wrong house produces lukewarm showers and cold dish-rinsing on mornings when the kids are getting ready and the dishwasher's running. A system boiler in the wrong house wastes money and floor space.
Here's the honest decision tree we walk through with every customer in South Yorkshire.
What's actually different between them
A combi boiler does two jobs from one unit. It heats your radiators, and it heats hot water on demand — straight from the cold mains, the moment you turn a tap. No tank.
A system boiler has a hot-water cylinder (usually in an airing cupboard). The boiler heats the water in the cylinder; the cylinder stores it ready to go. When you open a tap, you draw from a reservoir of pre-heated water.
That single difference — on-demand vs stored — drives every other consideration.
A combi boiler in the wrong house produces lukewarm showers and cold dish-rinsing on the mornings when the kids are getting ready and the dishwasher's running.
The three questions that decide it
1. How many bathrooms run at once?
Combi boilers serve hot water through one pipe at mains pressure. If two showers run at once, the flow rate splits between them and both feel weak.
A 30 kW combi can comfortably handle one shower or one sink at a time. A 35 kW or larger combi can manage two outlets — but only just, and only at decent mains pressure.
If your household regularly has two showers running simultaneously (think two adults getting ready for work), or you have a family bath plus a downstairs shower, a system boiler with a cylinder is the better answer. The cylinder just dispenses what's already heated; it doesn't care how many taps are open.
2. What's your incoming water pressure?
Combi boilers depend entirely on your mains supply. If your incoming pressure is below about 1.5 bar, or your flow rate is under 12 litres per minute, a combi will underperform no matter the kW rating.
We test pressure and flow on every survey. If your supply is borderline, we'll tell you straight — fitting a combi to a weak main is one of the most common reasons customers end up unhappy with a new boiler.
A system boiler bypasses the issue. The cylinder feeds at gravity or pumped pressure, so a slow mains doesn't bottleneck your hot water.
3. Where would the cylinder go?
System boilers need somewhere to put the cylinder — typically a 150–250 litre unvented unit, around the size of a thin wardrobe. If you have an airing cupboard or loft space, this is fine. If you're in a one-bed flat where every cupboard is precious, removing the cylinder is exactly why combis exist.
If you're converting from a heat-only (regular) boiler to a combi, you'll get the cylinder cupboard back as usable storage. That's a real benefit families with growing kids notice immediately.
What about cost?
Like-for-like, a combi installation is roughly £400–£800 cheaper than a system installation, mainly because there's no cylinder to fit and less pipework to rework.
But "cheaper to install" doesn't always mean "cheaper to live with". An undersized combi in a busy household leads to one of three things: complaints, an upgrade within a year, or a household quietly avoiding running two taps at once. None of those are wins.
Our honest recommendation
We fit both. Roughly 70% of our installs in South Yorkshire are combi swaps — semi-detached and terraced houses, one or two bathrooms, decent mains pressure. They're the right call most of the time.
The other 30% are system installs, mostly in:
- Larger detached homes with three or more bathrooms
- Properties with low mains pressure that we can't economically boost
- Households where two people genuinely shower at the same time most days
- Customers who already have a cylinder cupboard and just want reliability over saving £600
If you're not sure which camp you're in, book a free survey and we'll measure your flow, count your outlets, and give you a straight answer — including which scenario your house actually falls into. No upsell, no hidden margin on one option versus the other.
The wrong boiler is the one you regret every winter. The right one is invisible — and that's the one we're trying to fit.
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