Decision
Combi vs system vs regular boiler
Combi heats hot water on demand. System heats a stored cylinder. Regular heats a stored cylinder fed by gravity from loft tanks. The right answer depends on bathroom count, mains pressure, and how much space you can give up.
At a glance
Three boiler types compared
| Combi | System | Regular | |
|---|---|---|---|
| How it heats hot water | On demand from mains | Stored cylinder, mains-fed | Stored cylinder, gravity-fed |
| Cylinder? | No | Yes | Yes |
| Loft tanks? | No | No | Yes (header + F&E) |
| Best for | 1 to 2 bath homes | 2 to 4 bath homes | Low-pressure areas, older homes |
| Indicative installed cost | £1,800 to £3,500 | £2,200 to £4,500 | £2,000 to £4,000 |
| Hot-water flow | 10 to 17 LPM (one outlet at a time) | Multiple outlets, no drop | Multiple outlets at gravity pressure |
| Space needed | Minimal (wall unit) | Cylinder (airing cupboard) | Cylinder + loft tanks |
| Efficiency | 94 percent ErP A | 94 percent ErP A | 92 to 94 percent ErP A |
Plain English
How each one works
Skip the cylinder talk. Here is the one-paragraph version of each boiler type.
Combi
A wall-hung unit that heats hot water on demand straight from the cold mains. No cylinder, no tanks, all in one box. Hot water runs at the speed the boiler can heat it, so flow is the cap.
System
A wall-hung unit that heats a separate hot-water cylinder. Cylinder is mains-fed and pressurised. Hot water is stored, so multiple outlets work in parallel without losing pressure.
Regular
Sometimes called heat-only or conventional. The boiler heats a cylinder fed by gravity from cold-water tanks in the loft. Old-school, but tolerant of low mains pressure.
Strengths and weaknesses
Pros and cons of each
Combi
+ Pros
- +Lowest install cost
- +Smallest footprint
- +Instant hot water, no waiting
- +No legionella risk
- Cons
- -One outlet at a time, really
- -Mains-pressure dependent
- -No backup if it fails
System
+ Pros
- +Multiple outlets work in parallel
- +Strong shower flow
- +Compatible with solar thermal
- +Pressurised cylinder, no loft tanks
- Cons
- -Higher install cost
- -Needs cylinder cupboard space
- -Hot water can run out at peak demand
Regular
+ Pros
- +Works with low mains pressure
- +Cylinder stores hot water
- +Cheaper repairs (older tech)
- +Immersion heater backup
- Cons
- -Needs cylinder + loft tanks
- -Largest footprint
- -Slightly less efficient
- -Tank freeze risk in unheated lofts
Tool
Which type is right for you?
Answer four questions about your home and demand pattern. The tool returns the highest-fit boiler type with reasoning.
Decision tool
How many bathrooms?
Simultaneous hot water use
Mains water pressure
Space for a hot-water cylinder
Recommended type
Combi boiler
Heats hot water on demand. Saves space, simpler system, lowest install cost.
FAQs
Common questions
Is a combi cheaper to run than a system boiler?v
Slightly, in a 1 to 2 bathroom home. There is no cylinder standing loss. In a 3+ bathroom home, a system boiler with good cylinder insulation is similar or cheaper because the boiler can run longer steady-state cycles instead of repeated quick fires.
Can I switch from a combi back to a system boiler?v
Yes, but it costs £2,000 to £3,500 because the cylinder, cold-feed pipework, and hot-water distribution all need to be put back in. Most homeowners who realise their combi is undersized swap to a higher kW combi rather than going system.
Do all UK homes need a combi?v
No. Combi is the cheapest install for small to medium homes with one or two bathrooms. Larger homes, low-mains-pressure areas, and households that run multiple outlets at once are usually better served by a system boiler with a stored cylinder.
What is the difference between a system and a regular boiler?v
A system boiler is mains-fed and uses a pressurised cylinder. A regular (heat-only) boiler uses gravity from loft tanks to feed an unpressurised cylinder. System is more compact and modern, regular is more tolerant of low mains pressure.
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