⚖️ Combi vs System Boiler — At a Glance
| Feature | Combi Boiler | System Boiler |
|---|---|---|
| Hot water cylinder needed? | ❌ No | ✅ Yes |
| Hot water storage? | ❌ On demand only | ✅ Large stored volume |
| Multiple showers at once? | ⚠️ Can struggle | ✅ No problem |
| Installation cost | 💰 Lower | 💰💰 Higher |
| Running cost | Similar | Similar |
| Space needed | ✅ Compact | ⚠️ More space |
| Water pressure | Mains pressure | Mains pressure |
| Best for | Flats, small homes | Large homes, families |
🔥 Combi Boiler — Pros and Cons
✅ Advantages of a combi boiler
- Compact — no hot water cylinder or cold water tank needed, saving significant space
- Instant hot water — hot water on demand, no waiting for a tank to heat up
- Lower installation cost — simpler system means less pipework and labour
- No heat loss from cylinder — only heats water you actually use
- Ideal for flats — perfect where space is limited
❌ Disadvantages of a combi boiler
- Flow rate limitations — struggles to supply more than one shower at a time in larger homes
- Dependent on mains pressure — poor pressure affects performance
- No backup — if the boiler fails you have no hot water at all
- Higher servicing complexity — more components than a system boiler
🏠 System Boiler — Pros and Cons
✅ Advantages of a system boiler
- High flow rates — multiple showers and taps running simultaneously with no drop in pressure
- Large hot water volume — cylinder stores 150-300 litres, ideal for large families
- Immersion heater backup — if the boiler fails, the immersion heater can still provide hot water
- Better for larger homes — more radiators, multiple bathrooms
- Compatible with solar thermal — solar panels can pre-heat the cylinder
❌ Disadvantages of a system boiler
- Space requirements — hot water cylinder needs an airing cupboard
- Higher installation cost — more components and pipework
- Running out of hot water — if the cylinder empties you have to wait for it to reheat
- Heat loss from cylinder — insulated cylinders lose heat even when not in use
🏡 Which Boiler Is Right for Your Home?
| Your Situation | Best Choice | Reason |
|---|---|---|
| Studio or 1-bed flat | ✅ Combi | Limited space, low demand |
| 2-bed flat or small house | ✅ Combi | Good match for demand |
| 3-bed house, 1 bathroom | ✅ Combi or System | Either works well |
| 4+ bed house, 2+ bathrooms | ✅ System | Multiple simultaneous users |
| High demand / large family | ✅ System | Stored volume handles peaks |
| Replacing combi with no space | ✅ Combi | Like for like is simplest |
| Have solar panels / renewable | ✅ System | Compatible with solar thermal |
Not sure which is best for your home? Call FE Maintenance on 020 8445 1303 for a free no-obligation assessment. We'll recommend the right boiler for your specific needs and budget.
💷 Boiler Installation Costs in London 2026
| Installation Type | Typical Cost | Time |
|---|---|---|
| Like-for-like combi replacement | £1,800 – £2,500 | 1 day |
| Combi with new pipework | £2,000 – £3,000 | 1–2 days |
| System boiler + new cylinder | £2,500 – £3,500 | 1–2 days |
| Convert system to combi | £2,200 – £3,200 | 1–2 days |
| Convert combi to system | £2,500 – £3,500 | 2 days |
All FE Maintenance installations include: removal of old boiler, full installation, commissioning, gas safety check, 1-year workmanship guarantee, and manufacturer warranty registration.
Frequently Asked Questions
A combi (combination) boiler heats water on demand directly from the mains — no storage cylinder needed. A system boiler heats water and stores it in a hot water cylinder. Combis are better for smaller homes, system boilers are better for large homes with multiple bathrooms and high hot water demand.
Running costs are similar for both types when modern A-rated boilers are compared. The main cost difference is installation — combis are cheaper to install as they don't need a cylinder. However system boilers can lose heat from the cylinder, making them marginally more expensive to run unless the cylinder is well insulated.
Yes — converting from a system boiler to a combi is a common job for FE Maintenance. We remove the old cylinder and tank, install the combi boiler and adapt the pipework. It typically takes 1-2 days and costs £2,200-3,200. Call 020 8445 1303 for a free quote.
Worcester Bosch is consistently rated the most reliable boiler brand in the UK. Vaillant and Baxi are also excellent choices. All three offer 10-12 year warranties and have good parts availability. FE Maintenance installs all three brands — call 020 8445 1303 for a recommendation based on your budget.
Boiler sizing depends on the number of radiators, bathrooms and bedrooms. A typical 3-bed house needs a 24-28kW combi. A larger 4-5 bed house with 2 bathrooms needs a 30-35kW combi or a system boiler. FE Maintenance will calculate the correct size for your home as part of our free survey.
📚 Related Guides
Need a Gas Safe Engineer in London?
FE Maintenance — trusted by hundreds of London homeowners and landlords. Same-day appointments available.
Gas Safe Reg. No. 587248 · info@femaintenance.com