Combi vs system vs regular: which suits your house

The first decision in any boiler install isn’t which brand. It’s which type. Combi, system and regular boilers all heat your home, but they suit very different houses. Here’s how to tell which one is right for yours.

Combi (combination)

A combi boiler heats both your central heating and your hot water on demand: there’s no tank, no cylinder, just the boiler unit. When you turn on a hot tap, the boiler fires up and heats the water as it flows through.

Suits homes that have:

• One bathroom, or two with rare simultaneous use
• Limited cupboard space: no room for a cylinder
• Decent mains pressure (above 1.5 bar, which is most of Cardiff)
• A combi already, and the household hasn’t grown

Doesn’t suit:

• Homes where two showers run at once
• Low-pressure mains supply (some older Cardiff streets, especially CF11 and CF24)
• Larger families needing a bath drawn while a shower runs

Sizing: most 1–3 bedroom Cardiff homes need a 28–30 kW combi. Two-bathroom semis usually need 32–35 kW. Anything larger gets into system territory.

System

A system boiler heats your central heating and feeds an unvented hot-water cylinder (a Megaflo, Ariston Pro Eco, or similar). The cylinder stores hot water at mains pressure, so multiple taps and showers can run at full pressure simultaneously.

Suits homes that have:

• Two or more bathrooms, especially with simultaneous use
• Good cupboard space for a cylinder (typically 60×60×170 cm)
• A demand profile with peaks (school runs, evening showers)
• High-flow showers like rainfall heads that need 12+ L/min

Doesn’t suit:

• Homes with very limited cupboard space
• Single-occupancy flats where on-demand combi is more efficient

Sizing: most system boilers we fit are 24–32 kW. The cylinder size matters more than boiler kW for most households, typically 210–250L for 4–5 person homes.

Regular (heat-only / conventional)

A regular boiler is the traditional setup: boiler in the airing cupboard, cold-water tank in the loft, expansion tank in the loft, hot-water cylinder downstairs or in the airing cupboard. The boiler heats the cylinder and the radiators; the cold tank gravity-feeds the cylinder.

Suits homes that have:

• An existing regular setup that works fine, so no point redesigning the plumbing
• Loft tanks already in place with sound insulation
• High demand with no mains pressure (rural / older properties)
• Older radiator systems that struggle with mains-pressure water

Doesn’t suit:

• New-build or renovated properties (system is more efficient)
• Cardiff homes with good mains pressure (no reason for the loft tanks)
• Properties where the loft is being converted

Sizing: similar to system, 24–32 kW depending on home size.

Worked examples

A 2-bed flat in Cardiff Bay (CF10), one bathroom, two adults → Combi, 28 kW. Worcester Greenstar 30i or Vaillant ecoTEC plus 825. Fitted in a day, no cylinder needed.

A 4-bed Victorian semi in Cyncoed (CF23), two bathrooms, family of four → System boiler, 28 kW + Megaflo 250L cylinder. Vaillant ecoTEC plus 832 with a Megaflo Eco. Fitted in 1.5 days. Bathroom-pressure issues solved at the same time.

A 1930s 3-bed in Pontcanna (CF11), one bathroom, with original loft tanks → It’s a judgement call. The existing regular setup works fine; replacing it like-for-like with a Worcester Greenstar Ri 30 keeps the cost down (£1,995 fitted). Or convert to combi (no loft tanks) for £2,495 (cleaner long-term, more disruption now).

A 2-bed mid-terrace in Roath (CF24), one bathroom, low mains pressure → Combi sized for the pressure (24 kW with thermostatic mixers). Or, if the budget allows, system + cylinder which gives consistent shower flow regardless of mains. We’d flag the choice and let the customer decide.

What “good” looks like in a quote

A quote for a boiler install should always say:

• The boiler type: combi, system or regular
• The kW rating: chosen by heat-loss calculation, not guess
• Why this type: one paragraph on why it suits this specific house
• Optional alternatives: at least one alternative type with cost difference

If the quote just says “Worcester Greenstar combi, fitted £1,895” without explaining why a combi, why that size, why Worcester, get a second opinion. The choice should be deliberate, not default.

We do all our boiler installs this way: survey first, type second, brand third, model last. The order matters.