If you’re replacing an old boiler in London, the right choice usually comes down to three things: how many bathrooms you have, your water pressure, and whether you want extra storage space back. For most flats and smaller homes, a combi boiler is the best replacement. For larger houses with two or more bathrooms, a system boiler paired with an unvented cylinder often performs better. Regular (conventional) boilers are now mostly a like-for-like option for older properties that already have a loft tank and don’t want to change their pipework.
That’s the short answer. The rest of this guide walks through exactly why, using real property scenarios, hot water demand, and London-specific water pressure issues that many national guides ignore.
Quick Answer: Which Boiler Type Fits Your Home?
| Home Type | Best Boiler | Why |
|---|---|---|
| 1–2 bed flat, 1 bathroom | Combi boiler | No cylinder or tank needed, compact, fast install |
| 2–3 bed house, 1 bathroom | Combi boiler | Handles single-outlet demand well |
| 3–5 bed house, 2+ bathrooms | System boiler + unvented cylinder | Strong simultaneous hot water flow |
| Victorian/period property, low pressure | System or regular boiler | Doesn’t rely on mains pressure alone |
| Large family, high hot water use | System boiler | Stored hot water keeps up with demand |
| Replacing a regular boiler, keeping the tank | Regular boiler (like-for-like) | Avoids re-piping, cheaper install |
Combi vs System vs Regular Boiler: The Core Differences
A boiler replacement isn’t just swapping one metal box for another. Each boiler type heats and delivers water differently, and that difference is what determines whether your new boiler actually solves the problems your old one had.
Combi Boiler
A combi (combination) boiler heats water directly from the mains, on demand, with no tank or cylinder needed anywhere in the house.
- Best for: flats, smaller houses, 1–2 bathroom homes
- Pros: compact, no cold water tank in the loft, no hot water cylinder, constant hot water supply, quicker installation
- Cons: hot water flow drops when two outlets run at once (shower plus kitchen tap, for example), performance depends on mains water pressure
System Boiler
A system boiler heats water and stores it in a hot water cylinder, usually unvented, but most of the major components are built into the boiler itself rather than in the loft.
- Best for: homes with two or more bathrooms, higher hot water demand, larger families
- Pros: strong simultaneous hot water flow to multiple outlets, doesn’t rely purely on mains pressure, no loft tank required
- Cons: needs cylinder space (usually an airing cupboard), higher upfront cost than a combi
Regular (Conventional) Boiler
A regular boiler works with a separate cold water tank in the loft and a hot water cylinder, typically vented.
- Best for: like-for-like replacements in older properties that already have this setup and don’t want to change radiators or pipework
- Pros: works well with older heating systems, unaffected by mains pressure, good for large homes with existing tank infrastructure
- Cons: takes up the most space, slower hot water recovery than a system or combi, two tanks to maintain
Combi vs System vs Regular Boiler Replacement: Full Comparison Table
| Feature | Combi | System | Regular |
|---|---|---|---|
| Cold water tank in loft | No | No | Yes |
| Hot water cylinder | No | Yes | Yes |
| Simultaneous hot water use | Weak | Strong | Moderate |
| Ideal bathrooms | 1 | 2+ | 2+ |
| Install space needed | Low | Medium | High |
| Mains pressure dependent | Yes | Partially | No |
| Typical install time | 1 day | 1–2 days | 1–2 days |
| Good for period properties | Sometimes | Often | Often |
Best Boiler for Replacement by Property Type
Best Boiler for a London Flat
Most London flats have a single bathroom and limited space, which makes a combi boiler the practical choice. There’s no tank to find room for, and combis fit neatly into a kitchen cupboard. The one thing to check first is water pressure, since some older blocks of flats have weaker mains pressure that can affect combi performance during peak times.
Best Boiler for a Victorian or Period London House
Victorian terraces and period conversions often have older pipework and existing tanks in the loft. If the property already has a working tank-and-cylinder setup, a regular boiler replacement avoids costly re-piping. But if you’re already renovating or converting the loft, switching to a system boiler frees up that loft space permanently.
Best Boiler for a 2, 3, 4, or 5 Bedroom House
- 2 bedroom house, 1 bathroom: combi boiler is usually sufficient
- 3 bedroom house, 1 bathroom: combi boiler, though check hot water flow rate if the household is large
- 4 bedroom house, 2 bathrooms: system boiler recommended for consistent simultaneous hot water
- 5 bedroom house, 2+ bathrooms, high demand: system boiler with a larger cylinder, or regular boiler if the infrastructure already exists
Best Boiler for Multiple Bathrooms or Large Families
Combi boilers struggle when two showers, or a shower and a bath tap, run at the same time, because they can only heat one flow of water at a given rate. A system boiler stores hot water in advance, so it can supply two bathrooms without the pressure and temperature drop combis are known for. For a family of four or five with regular simultaneous water use, this is usually the deciding factor.
Boiler Replacement and London Water Pressure
London’s mains water pressure varies significantly by borough and even street to street, largely due to the age and layout of the water network. Low pressure areas can make a combi boiler underperform, especially on upper floors of converted properties or in parts of outer London served by older mains infrastructure.
If your area has low mains pressure:
- A system boiler with an unvented cylinder can boost pressure independently of the mains
- A combi boiler may still work but could struggle with strong shower flow
- A pressure test from a Gas Safe engineer before installation avoids a mismatch after the boiler is fitted
If your area has strong, consistent mains pressure, a combi boiler will typically perform well even in a two-bathroom home, though simultaneous use is still better handled by a system boiler.
Can You Change Boiler Type When Replacing an Old Boiler?
Yes, switching boiler types during replacement is common and often improves performance, but it does affect cost and installation time.
- Regular to combi: Requires removing the loft tank and cylinder, and may need new pipework. Frees up significant space.
- Regular to system: Keeps a cylinder but removes the loft tank. A moderate change, usually completed in one to two days.
- System to combi: Removes the cylinder, simplifying the setup, but reduces multi-outlet hot water performance.
- Like-for-like replacement: Fastest and cheapest option, since existing pipework and tanks are reused.
If your priority is minimising disruption and cost, a like-for-like swap is usually best. If your priority is solving a hot water problem you’ve had for years, changing type is often worth the extra work.
Step-by-Step: How to Choose the Right Replacement Boiler
- Count your bathrooms. One bathroom generally points to a combi. Two or more points to a system boiler.
- Check your household size and hot water habits. Larger families or households that shower back-to-back benefit from stored hot water.
- Assess your current setup. Note whether you already have a loft tank and cylinder, since this affects installation cost.
- Get your mains water pressure checked. A Gas Safe registered engineer can measure this in minutes.
- Factor in available space. Combis need the least space; regular boilers need the most.
- Get quotes based on boiler type, not just brand. Installation cost varies more by type change than by boiler make in most cases.
- Confirm sizing (kW output) with an engineer. Bigger isn’t always better; oversized boilers waste money and undersized ones underperform.
Frequently Asked Questions
Which is better, a combi or system boiler?
Neither is universally better. A combi suits homes with one bathroom and good mains pressure. A system boiler suits homes with two or more bathrooms or households that use hot water from multiple taps at once.
Should I replace my old boiler with a combi?
If you currently have a single bathroom and reasonable water pressure, a combi is usually a good upgrade, especially since it frees up loft or airing cupboard space.
Can I replace a regular boiler with a system boiler?
Yes. The loft tank is removed, but the hot water cylinder can often be reused or upgraded to an unvented model, giving better pressure and flow.
Can I replace my system boiler with a combi?
Yes, though this only makes sense if your household’s hot water demand has decreased, since you’ll lose the stored hot water advantage.
What boiler is best for a London flat?
A combi boiler is best for most London flats due to limited space and typically single-bathroom layouts, provided mains pressure is adequate.
How many bathrooms can a combi boiler handle?
Combi boilers work best with one bathroom. They can supply two, but hot water flow and pressure will drop noticeably when both are used simultaneously.
Is a combi boiler suitable for a Victorian terrace in London?
It can be, but many Victorian properties have older pipework or existing tank systems, so a regular or system boiler replacement is sometimes more practical and cost-effective.
Does London water pressure affect boiler choice?
Yes. Areas with low mains pressure perform better with a system boiler and unvented cylinder, since a combi’s output depends heavily on incoming mains pressure.
What size boiler do I need for a 2 bathroom house?
Most 2-bathroom homes need a system boiler in the 24–30kW range, though exact sizing depends on the cylinder capacity and number of radiators. An engineer’s heat loss calculation gives the most accurate figure.
Is boiler replacement without changing radiators possible?
Yes, in most cases. Boiler type changes affect the tank and cylinder setup, not the radiators, unless the new boiler’s output significantly differs from the old one.
Final Thoughts
Choosing between a combi, system, or regular boiler for replacement comes down to your property, your household’s hot water habits, and your local mains pressure, not just brand preference. Flats and single-bathroom homes are usually best served by a combi. Larger households with multiple bathrooms tend to get better results from a system boiler. And if your current setup already works, a like-for-like regular boiler replacement can be the most cost-effective route.
Before committing, get a Gas Safe registered engineer to assess your property, check your water pressure, and confirm sizing. That one step prevents most of the common regrets people have after a boiler replacement.