Combi vs System vs Heat-Only Boiler: Which Is Right for Your Home?
Three boiler types cover almost every Cornwall replacement: combi, system and heat-only. Each suits a different household. We explain sizing, flow rates, conversion costs, and where homeowners get over-specified into something that short-cycles.
The three boiler types in plain English
Three mains-gas boiler types account for almost every replacement we fit in Cornwall:
- Combi (combination) — heats radiators and provides instant hot water from one unit. No hot water cylinder or cold tank.
- System — heats radiators and feeds a pressurised unvented hot water cylinder. No cold tank in the loft.
- Heat-only (regular) — heats radiators and feeds a vented cylinder, with a cold tank in the loft. The traditional setup in older Cornwall properties.
When a combi is the right choice
Combis make sense for 1 and 2 bathroom homes with decent mains pressure. They save cupboard space and cost less to install because there is no cylinder. The flip side is they cannot serve two hot taps at once at full flow, and their performance is entirely dependent on mains pressure and flow rate.
If your mains flow rate is below about 12 litres per minute, a combi will underperform regardless of the boiler brand. We measure mains flow before quoting.
When a system boiler makes more sense
System boilers suit 3+ bedroom homes, properties with two or more bathrooms, and anyone who runs multiple hot taps at once. The unvented cylinder provides full mains-pressure hot water to every outlet. Recovery is fast, and you can specify a cylinder that will future-proof for a heat pump swap later.
System boilers are our most commonly installed type across Cornwall for homes with existing cylinders.
When heat-only still wins
Some older Cornwall properties have large open vented systems that are perfectly serviceable. If the loft tank is sound, pipework is generous, and the homeowner prefers simplicity, a modern condensing heat-only boiler paired with the existing cylinder can still be the most cost-effective replacement. Do not let anyone pressure you into an unnecessary system conversion just because the install is newer.
Sizing and flow rate
Combis are rated in kW for hot water (e.g. 24, 30, 35 kW). System and heat-only boilers are rated in kW for heating. Both types need to be sized to the house, not overspecified. A common mistake we see is a 35kW combi fitted to a 90m² bungalow that would have been properly served by a 24kW — the result is short cycling, worse efficiency and earlier failure.
Costs and getting a quote
Indicative fully-installed prices in Cornwall in 2026:
- Combi like-for-like swap — £2,400 to £3,500
- System boiler swap — £2,800 to £4,200
- Heat-only swap — £2,400 to £3,500
- Conversion (combi to system or vice versa) — £3,500 to £5,500
All prices include Gas Safe commissioning, flushing and registration. See our boiler replacement guide, or compare against a heat pump if you are thinking longer term. Contact us for a fixed quote.
Need personalised advice?
Our MCS certified engineers can answer your questions and provide a free, no-obligation assessment for your property.