GasBoilerCost.com / UK 2026
Boiler types / Page 02

Combi vs system vs conventional. Which type do you need?

Choosing the wrong type can cost you thousands. Here is the side-by-side comparison, the decision framework, and the conversion costs if you want to switch.

Combi boiler
£1,800 to £3,500
installed in 2026
Output
24 to 35 kW
Lifespan
10 to 15 years

Best fit: 1 to 3-bed homes, one bathroom, decent mains pressure.

Pros

  • Compact: fits in a kitchen cupboard
  • No cylinder or tank, frees loft and airing space
  • Hot water on demand, no waiting
  • Cheapest install of the three types

Cons

  • Pressure drops if two showers run together
  • Struggles in homes with poor mains pressure
  • High demand on flow rate may exceed unit capacity
System boiler
£2,000 to £4,500
installed in 2026
Output
18 to 32 kW
Lifespan
12 to 15 years

Best fit: 3 to 5-bed homes, two or more bathrooms.

Pros

  • Hot water cylinder serves multiple taps at once
  • Compatible with solar thermal and high-output showers
  • Better hot water flow rate than a combi
  • Most components are inside the unit, simpler than conventional

Cons

  • Needs space for an unvented or vented cylinder
  • Hot water can run out if cylinder is undersized
  • Slightly higher install cost than combi
Conventional (regular) boiler
£1,700 to £4,000
installed in 2026
Output
12 to 30 kW
Lifespan
15 to 20 years

Best fit: Older homes with traditional layouts, low mains pressure, very large properties.

Pros

  • Works with low or fluctuating mains pressure
  • Excellent flow rate from gravity-fed system
  • Longest typical lifespan of the three
  • Simple, robust components

Cons

  • Needs cylinder plus a cold-water feed tank in the loft
  • Slowest to heat water from cold start
  • Most space-hungry and most pipework

The decision in three boxes

The fastest way to choose. Most UK homes match exactly one of the three boxes below.

Choose a combi if

  • Your home has 1 bathroom
  • 1 to 3 bedrooms
  • Mains pressure is at least 1.5 bar
  • Space is tight

Choose a system if

  • You have 2 or more bathrooms
  • Two showers run together
  • You want solar thermal compatibility
  • Cylinder space exists

Choose a conventional if

  • Mains pressure is poor or variable
  • You have a heritage gravity system
  • Loft tank and cylinder still serviceable
  • Property is very large

Conversion costs (switching type)

FromToTotal costNotes
ConventionalCombi£2,500 to £4,000Cylinder and tank removed, all pipework reroutes through the new unit.
ConventionalSystem£2,200 to £3,800Tank goes, cylinder usually replaced for unvented.
CombiSystem£3,000 to £5,000Adds cylinder, hot water main, and unvented kit. Disruptive.
SystemCombi£2,500 to £4,000Removes cylinder. Only sensible if hot water demand has dropped.
Like-for-like swapSame type£1,800 to £3,500Cheapest path. Often a 1-day job for an experienced engineer.

Conversion costs more than like-for-like because of additional pipework, redundant component removal, and the extra labour of changing the system layout. Based on indicative quotes April 2026.

Lifespan and signs of failure

Combi

10 to 15 years

Heat exchangers and diverter valves are the typical first failure points.

System

12 to 15 years

Pumps and expansion vessels typically wear before the boiler itself.

Conventional

15 to 20 years

Cast-iron heat exchangers can last decades. Thermostats and pumps wear out first.

Common signs you are nearing replacement: rising bills despite normal use, banging or kettling noises, repeated lockouts, leaking from internal seals, and parts that are no longer manufactured. If you have spent more than £400 on repairs in 12 months, replacement is usually the better economic call.

FAQ

  • Can I switch from a conventional to a combi?
    Yes. A regular-to-combi conversion typically costs £2,500 to £4,000. The cold-water tank and hot-water cylinder come out, and pipework reroutes through the combi unit. Make sure your mains pressure (at least 1.5 bar) and flow rate (12 litres per minute or better) can support a combi before committing.
  • Which boiler type is cheapest to run?
    For a small home with low hot-water demand, a combi is cheapest because it heats only the water you draw. For larger homes, a well-sized system boiler with a well-insulated cylinder can match a combi on running cost and comfortably outperform it on flow. Conventional boilers tend to be slightly less efficient because of standing heat losses from the cylinder and feed tank.
  • Does a system boiler need a hot water tank?
    It needs a hot water cylinder, but not a separate cold-water feed tank in the loft. That is the main practical difference between a system and a conventional boiler. The cylinder is usually unvented, taking pressure direct from the mains.
  • How do I know what boiler I currently have?
    Look in the loft. A cold-water tank and a hot-water cylinder in the airing cupboard means conventional. A cylinder but no loft tank means system. No tank and no cylinder, just a single wall-mounted unit, means combi. The Gas Safe certificate from the last service or installation will also state the type.
Pricing

Full pricing breakdown

Type, tier and total installed cost on the homepage table.

Bills

Annual running costs

What each type actually costs to run at the Q2 2026 cap.

Looking for a deeper dive on combi specifically? See our sister site CombiBoilerCost.com for combi sizing, brand comparison and conversion guidance.