Summary

Replacing a boiler like-for-like with the same brand is always the simplest option, but it is not always the best value or the right choice for the customer. This guide maps equivalent models across the four major UK manufacturers so you can confidently recommend an alternative when an existing boiler is discontinued, unavailable, or when the customer wants to switch brand. All models listed are current production as of early 2026. The tables are organised by output rating so you can quickly find the nearest match. Always verify against the manufacturer's current datasheet before quoting -- ranges and model numbers can change mid-year.

Key Facts

  • All four manufacturers produce condensing boilers that meet the minimum 92% ErP efficiency required under Boiler Plus (England, April 2018 onwards)
  • Flues are brand-specific -- you generally cannot reuse an existing flue when switching manufacturer (see detailed guidance below)
  • Pipe connection positions vary between brands. Budget time for pipework modification on any cross-brand swap
  • Worcester Bosch and Vaillant dominate the premium end; Ideal and Baxi compete strongly on value and installer-friendliness
  • Warranty length depends on installer accreditation: Worcester Accredited Installers can offer up to 12 years; Vaillant Advance up to 12 years; Ideal Max Accredited up to 12 years; Baxi offers 10 years without requiring scheme membership
  • All current models listed are hydrogen-blend ready (20% H2 mix) in line with UK gas network transition plans
  • The Vaillant ecoTEC plus range was refreshed in 2024/25 -- the 825 is now the 826, the 835 is now the 836, etc. Footprint and connections are identical to the outgoing models
  • Ideal's "Logic" and "Logic Max" lines are distinct: Logic is the budget tier (7-year warranty); Logic Max is the mid-range (10-year warranty)

Combi Boiler Cross-Reference (2025/2026 Current Models)

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Premium / Mid-Range Combi Boilers

Output (kW) Worcester Bosch Vaillant Ideal Baxi Typical Use
24-26 kW Greenstar 4000 25 ecoTEC plus 826 Logic Max C24 800 Combi 2 24 1 bathroom, up to 10 radiators
28-30 kW Greenstar 4000 30 ecoTEC plus 832 Logic Max C30 800 Combi 2 30 1-2 bathrooms, up to 15 radiators
35-36 kW Greenstar 8000 Life 35 ecoTEC plus 836 Logic Max C35 800 Combi 2 36 2 bathrooms, up to 20 radiators
38-40 kW Greenstar 8000 Life 40 ecoTEC plus 840 Vogue Gen2 C40 -- 2-3 bathrooms, larger properties
45 kW Greenstar 8000 Life 45 -- -- -- Large property, high DHW demand
50 kW Greenstar 8000 Life 50 ecoTEC plus 940 -- -- Very large property, multiple bathrooms

Budget / Value Combi Boilers

Output (kW) Worcester Bosch Vaillant Ideal Baxi Typical Use
24 kW Greenstar 2000 25 ecoTEC pro 24 Logic+ C24 600 Combi 2 24 1 bathroom, up to 10 radiators
28-30 kW Greenstar 2000 30 ecoTEC pro 28 / pro 30 Logic+ C30 600 Combi 2 30 1-2 bathrooms, up to 15 radiators
35-36 kW -- -- Logic+ C35 600 Combi 2 36 2 bathrooms, up to 20 radiators

Key Dimensions Comparison (Combi Models, approx.)

Brand Model H x W x D (mm) Weight (kg)
Worcester Bosch Greenstar 4000 724 x 400 x 310 34.6
Worcester Bosch Greenstar 2000 724 x 400 x 300 ~32
Vaillant ecoTEC plus 826/832/836 720 x 440 x 338 36-38
Ideal Logic Max C24/C30/C35 700 x 395 x 278 28.5
Baxi 800 Combi 2 715 x 393 x 278 ~30

Note: Ideal Logic Max and Baxi 800 are the most compact and lightest -- important when the existing boiler sits in a tight kitchen cupboard.

System Boiler Cross-Reference

Premium / Mid-Range System Boilers

Output (kW) Worcester Bosch Vaillant Ideal Baxi Typical Use
12 kW Greenstar 4000 12 ecoTEC plus 612 -- -- Small flat, low heat demand
15 kW Greenstar 4000 15 ecoTEC plus 615 Logic Max S15 -- Small house, 1 bathroom
18 kW Greenstar 4000 18 ecoTEC plus 618 Logic Max S18 800 System 2 18 2-bed house, 1 bathroom
24 kW Greenstar 4000 24 ecoTEC plus 624 Logic Max S24 800 System 2 24 3-bed house, 1-2 bathrooms
30 kW Greenstar 4000 30 ecoTEC plus 630 Logic Max S30 -- 4-bed house, 2+ bathrooms
37 kW -- ecoTEC plus 637 -- -- Large property, high heat load

Budget System Boilers

Output (kW) Worcester Bosch Vaillant Ideal Baxi
15 kW Greenstar 2000 S15 -- Logic+ S15 --
18 kW Greenstar 2000 S18 -- Logic+ S18 --
24 kW Greenstar 2000 S24 -- Logic+ S24 600 System 2 24
30 kW Greenstar 2000 S30 -- Logic+ S30 600 System 2 30

Detailed Guidance

What do I need to check when replacing one brand with another?

  1. kW output -- match the existing output or upsize if the property has been extended. Never downsize without a heat loss calculation.
  2. Flue diameter and type -- concentric 60/100mm or 80/125mm. Most domestic combis use 60/100mm but some higher-output models require 80/125mm.
  3. Flue terminal position -- check distance to openable windows, air bricks, and boundaries per BS 5440-1.
  4. Gas connection position -- bottom-centre on most brands but offset on some. Measure before ordering.
  5. Water connection positions -- flow, return, DHW flow, DHW return, and condensate. These vary significantly between brands.
  6. Physical dimensions -- will the new boiler fit in the same space? Check the depth especially if it sits in a cupboard.
  7. Electrical supply -- all four brands use a standard fused spur but check the cable reach.
  8. Controls compatibility -- if the customer has a Vaillant VRT or Worcester Wave, switching brand means replacing the controls too.

Can I reuse the existing flue?

Short answer: No, in most cases you cannot.

  • Flue systems are brand-specific and covered by the manufacturer's installation instructions. Using a non-approved flue voids the warranty and may not comply with Building Regulations.
  • The internal diameters (60/100mm concentric) are nominally the same across brands, but the boiler-spigot connection, seals, and locking mechanisms differ.
  • Exception: If the existing flue is a generic concentric system (not a branded boiler-specific kit) and the new boiler's installation manual permits generic flues, reuse may be acceptable. This is uncommon with current models.
  • Never reuse a flue from a non-condensing boiler on a condensing replacement. The condensate will corrode the old flue material rapidly.
  • Budget for a new horizontal flue kit on every cross-brand replacement. Typical cost: 40-80 GBP trade.

What about mounting position and pipe connections?

Pipe positions are the biggest practical headache on cross-brand swaps:

  • Worcester Greenstar 4000: Connections are bottom-mounted, roughly centred. Flow/return spacing is relatively wide.
  • Vaillant ecoTEC plus: Bottom connections, slightly offset to the left when viewed from the front. Wider overall casing means connections may not align with Worcester or Ideal pipework.
  • Ideal Logic Max: Narrowest casing of the four (395mm). Connections are tightly grouped at the bottom centre. Often the easiest brand to fit into a tight space.
  • Baxi 800: Very similar connection layout to Ideal -- bottom-centre, compact. If swapping between Ideal and Baxi, pipework modification is usually minimal.

General rule: Allow 1-2 hours of additional pipework modification time when switching between any brand pairing. Worcester-to-Vaillant swaps typically require the most rework due to the dimensional differences.

Which brands offer the best installer support?

Factor Worcester Bosch Vaillant Ideal Baxi
Installer scheme Worcester Accredited Vaillant Advance Max Accredited Baxi Works
Max warranty (via scheme) 12 years 12 years 12 years 10 years
Warranty without scheme 7 years (Greenstar 4000) 5 years 7 years 10 years
Parts availability Excellent -- huge installed base Excellent Good Good
Technical helpline Strong reputation Strong reputation Good Good
Training centres Nationwide Nationwide Regional Regional
Scheme requires annual service No No No No
Included accessories Flue (on some packs) Filter + controls (on packs) System filter Filter + thermostat + flue

Notable: Baxi 800 packs include an Adey Micro2 filter, Salus wireless thermostat, and horizontal flue as standard -- strong value proposition on a like-for-like basis. Ideal Logic Max also ships with a system filter included.

Frequently Asked Questions

A customer has a 10-year-old Worcester Greenstar 30i. What is the closest current Worcester replacement?

The Greenstar 30i was replaced by the Greenstar 4000 30. It is the direct successor with similar dimensions and connection positions. If the customer wants to stay with Worcester, this is the simplest swap. The flue spigot position is in the same location, so reusing the existing Worcester flue may be possible -- check the 4000's installation manual for approved flue kits.

The customer wants the cheapest option. Which brand and model?

At the 30 kW combi level, the Baxi 600 Combi 2 30 and Ideal Logic+ C30 are typically the lowest trade price. Both carry 7-year warranties. The Baxi 600 pack often includes a filter and thermostat, which can save 100-150 GBP in sundries. The Greenstar 2000 is Worcester's budget entry but usually costs 100-200 GBP more at trade for a comparable output.

Can I swap a combi for a system boiler (or vice versa) on a like-for-like replacement?

Technically yes, but it is not a like-for-like replacement and requires a new Building Regulations notification. Swapping from combi to system means adding an unvented or vented cylinder, potentially a feed-and-expansion tank (if vented), and replumbing the DHW circuit. Swapping from system to combi means removing the cylinder and capping off the old pipework. Both scenarios add significant cost and complexity -- only recommend this if there is a genuine performance reason (e.g. the property has grown and a combi can no longer meet DHW demand).

Do I need to notify Building Control for a boiler replacement?

Yes. All boiler replacements in England and Wales must be notified under Building Regulations Part L (conservation of fuel and power). If you are registered with a Competent Persons Scheme (e.g. Gas Safe self-certification), you can self-certify. Otherwise, the homeowner must apply for a Building Control completion certificate. This applies whether you are replacing like-for-like or switching brands.

Regulations & Standards