The Ultimate Buyer’s Guide: How to Choose the Right New Boiler for Your London Home

A Gas Safe registered engineer explains what actually matters when choosing a new boiler, what does not, and how to avoid the most common mistakes London homeowners make.

Replacing a boiler is one of the biggest home decisions you will make. Get it right and you will have reliable heating and hot water for years. Get it wrong and you are looking at poor performance, cold showers, and avoidable callbacks.

This guide is written by engineers who install boilers every day across London. We are going to tell you what actually matters, what is marketing nonsense, and how to make a decision you will not regret.

Do You Actually Need a New Boiler?

Before you replace a boiler, make sure you actually need to. We see plenty of boilers with years of life left in them, and we also see people throwing repairs at boilers that are clearly at end of life.

Signs You Probably Need to Replace

• Age: Your boiler is older and repairs are becoming frequent
• Efficiency: It is a non-condensing boiler (typically older models) wasting a meaningful chunk of gas
• Parts: The manufacturer has discontinued key spare parts
• Reliability: Multiple breakdowns in a short period
• Safety: Your engineer has flagged it as “At Risk” or “Immediately Dangerous”

Signs You Might Not Need to Replace

• One breakdown: A single repair, even a major one, can still be the smarter move
• Age alone: A well-maintained boiler can have life left even if it is not new
• Pressure issues: Often fixable without replacing the whole boiler
• Noisy operation: May need a power flush or system work rather than a new boiler

If you are unsure, read our Repair or Replace Guide for the full decision framework.

Combi vs System vs Regular: Which Type Do You Need?

There are three main boiler types. Choosing the wrong one is the most expensive mistake you can make because it can force redesign later.

Combi Boilers

Best for: Flats, small to medium houses (typically up to 3 bedrooms, 1 bathroom)

A combi heats water directly from the mains when you turn on a tap. No hot water cylinder, no cold water tank in the loft. Compact, efficient, and by far the most popular choice in London.

Pros:

• No tank or cylinder needed (saves space)
• Hot water on demand
• Less pipework, less to go wrong
• Great for flats, terraces, and kitchens with limited cupboard space

Cons:

• Water pressure and flow can drop if multiple outlets run at once
• Limited by your incoming mains pressure and flow
• Not ideal for homes where two showers are used at the same time, every day

System Boilers

Best for: Larger houses (3+ bedrooms, 2+ bathrooms), homes with high hot water demand

A system boiler heats water and stores it in a cylinder. The cylinder means you can run multiple taps and showers with strong, consistent delivery (assuming the system is designed correctly).

Pros:

• Better for multiple bathrooms
• Stable supply to several outlets
• Compatible with solar thermal setups
• No cold water tank in loft required

Cons:

• Needs space for a hot water cylinder
• Hot water can run out if demand exceeds cylinder recovery
• More components means more design decisions (done right it is brilliant, done wrong it is pain)

Regular (Conventional) Boilers

Best for: Older properties with existing tank systems, low mains pressure areas

A regular boiler works with a hot water cylinder and a cold water storage tank (often in the loft). This is common in older London properties where the system was built around a tank.

Pros:

• Works well where mains pressure is weak
• Can support high hot water demand with the right cylinder and tank setup
• Often easiest like-for-like upgrade in older homes

Cons:

• Takes up the most space (cylinder plus loft tank)
• More pipework and components
• Not usually the best choice if you want to free up space

For a deeper comparison, see our Combi vs Hot Water Tank Guide.

Getting the Size Right

Boiler output is measured in kilowatts (kW). Get this wrong and you will either struggle with hot water, or waste efficiency by oversizing and short-cycling.

Combi Boiler Sizing (Hot Water Focus)

Property Size	Bathrooms	Typical Output Range
1 to 2 bed flat	1	24 to 27 kW
2 to 3 bed house	1	28 to 32 kW
3 to 4 bed house	2	32 to 38 kW
4+ bed house	2+	Consider a system boiler

System Boiler Sizing (Heating Load Focus)

System boilers are sized differently because the cylinder handles most hot water demand. The boiler should match your heating load and the cylinder should match your hot water usage.

Property Size	Radiators	Typical Output Range
3 bed house	8 to 12	12 to 18 kW
4 bed house	12 to 16	18 to 24 kW
5+ bed house	16+	24 to 30 kW

The real calculation (what good installers do)

These tables are rough guides. A proper sizing assessment considers:

• Total radiator output and heat loss
• Property insulation and exposure
• Number of external walls and window quality
• Incoming mains temperature and flow rate (especially for combis)
• How many showers and outlets are used at the same time

Any installer quoting you should do a proper assessment. If they just eyeball it, get another quote.

Which Brand Should You Choose?

There are dozens of boiler brands in the UK. The brand matters, but not as much as the installation quality and long-term parts support.

What matters when choosing a brand

• Parts availability: How quickly parts can be sourced in the UK
• Warranty support: How the manufacturer handles claims and engineer support
• Installer competence: A “great” boiler installed badly is still a problem
• Serviceability: Access, common faults, and how straightforward repairs are

Common reliable choices (London reality)

Worcester Bosch

• Strong UK presence and support
• Good parts availability
• Solid warranty pathways when installed and registered correctly

Vaillant

• Very well built and efficient when commissioned properly
• Good support and parts network
• Consistently rated highly by engineers

For a detailed comparison, see our Vaillant vs Worcester Showdown.

Other solid performers

• Viessmann: Excellent build quality, very capable products
• Ideal: Good mid-range option with a wide installer base
• Baxi: Long-standing UK presence and commonly installed
• Glow-worm: Popular budget-friendly choice with broad support

If you want brand-specific fault support later, see our Boiler Manufacturer Help Hubs.

Features That Matter (and Those That Do Not)

Features worth prioritising

• Good modulation range: Helps efficiency and reduces cycling
• Clear display and diagnostics: Fault info helps faster diagnosis
• Service access: A boiler that is easy to service gets serviced properly
• Frost protection: Important in unheated cupboards, loft spaces, and some conversions
• Quiet operation: Important for kitchen installs and flats

Features that sound good but rarely matter

• WiFi connectivity: Nice-to-have, but most users stop using apps after the novelty
• Weather compensation: Can be excellent, but only when installed and configured properly
• “Ultra-high efficiency” claims: Real performance depends on system setup and commissioning

What actually improves efficiency

• Correct boiler sizing and commissioning
• Proper system balancing after installation
• TRVs on radiators (thermostatic radiator valves)
• Decent room thermostat and sensible scheduling
• A clean system (no sludge) – see our Power Flushing Guide

What Good Installation Looks Like

A boiler is only as good as its installation. A premium boiler installed poorly will give you more trouble than a mid-range boiler installed properly.

What your installer should do

• Site survey: Visit your property before quoting (phone-only quotes are red flags)
• Proper assessment: Confirm boiler type, sizing, and hot water demand
• Gas Safe registration: Non-negotiable, check the engineer is registered
• Building Regulations notification: Required after installation
• System clean: Ensure system water is clean before connecting a new boiler
• Magnetic filter: Fit one to protect the new boiler
• Benchmark commissioning: Complete the log book properly
• User demonstration: Show you how to use controls and repressurise safely

London-specific installation realities (what good installers plan for)

• Flats and access: Parking, permits, lift access, and working hours in managed buildings
• Flue constraints: External wall rules, balconies, conservation areas, and safe termination positions
• Cupboard installs: Service access and ventilation clearances done properly
• Older pipework: Microbore and older valves need honest assessment
• Leaseholder rules: Some buildings require method statements and scheduled works

Red flags

• Quotes given without a site visit
• Significantly lower quotes with vague scope and missing details
• Pressure to decide immediately
• No clear plan for system cleanliness and protection
• No mention of Building Regulations notification

Read more in our Guide to Choosing a Heating Professional.

What Drives Boiler Quotes in London? (Without the Fluff)

If you are comparing quotes, do not compare “headline numbers”. Compare scope. In London, two quotes can look similar but include very different levels of work and protection.

1. Like-for-like replacement vs changing the system type

• Like-for-like: Usually the simplest route if your current setup is right for your home
• Changing type: Converting regular to combi or combi to system can involve major pipework and design changes

2. Boiler location and flue complexity

• Short, straightforward flue routes are simpler
• Long flue runs, awkward termination positions, and access constraints increase complexity
• In flats and conservation areas, flue positioning can be a real constraint

3. System condition (sludge, scale, and protection)

• If your system water is dirty, the installer should plan cleaning before commissioning
• A magnetic filter and inhibitor are not optional “extras”, they protect the boiler
• Skipping protection leads to kettling, circulation faults, and early component stress

4. Controls and commissioning quality

• Correct thermostat setup and commissioning is where efficiency is won or lost
• Balancing radiators is often skipped, but it matters for comfort and efficiency

5. Building and access realities in London

• Parking, congestion, permits, lift access, and working-hour restrictions affect planning
• Managed buildings may require paperwork and timed access

Getting Quotes Without Getting Scammed

What a proper quote should include

• Make and model of boiler
• kW output and why it is suitable
• Warranty length and terms
• What is included (system cleaning approach, filter, inhibitor, controls)
• What is not included (for example: making good, decoration)
• Estimated installation time
• Building Regulations notification
• Warranty registration process

Questions to ask every installer

1. Can I see your Gas Safe registration card?
2. Are you registered with the boiler manufacturer for warranty purposes?
3. Will you complete the Benchmark commissioning checklist?
4. How will Building Regulations be notified?
5. What happens if something goes wrong shortly after install?
6. Do you carry public liability insurance?

Warranty fine print

Manufacturer warranties commonly require:

• Annual servicing by a registered engineer
• Completed Benchmark log
• Registration within the required timeframe
• Correct system protection (filter and inhibitor)

If any of these are missed, your warranty can be void. Make sure your installer handles registration and leaves paperwork completed.

Common Questions

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