Best Oil Combi Boilers

Overview

From manufacturer Worcester Bosch comes the ‘Greenstar Heatslave II’, a range of Oil Combi Boilers with a smart contemporary design, eco friendly features and most importantly the Which? ‘Best Buy’ seal of approval.

Is the Greenstar Heatslave II the best Oil Combi Boiler for my home?

The ‘Greenstar Heatslave II’ is available in internal and external models and for those homeowners interested in internal only then you’ll be pleased to know that the ‘Greenstar Heatslave II’ is one of the quietest boilers in their class making them ideal for light sleepers.

The ‘Greenstar Heatslave II’ series caters for a wide range of homes with varied heating demands thanks to the three models available (12/18kW, 18/25kW and 25/32kW). The heating requirements of your home will determine your boiler size, typical small sized homes with 1-3 bedrooms and one bathroom would be an ideal fit for the 12/18kW and 18/25k models, whilst homes that require lots of hot water and have many bedrooms and bathrooms would be best suited with the 25/32kW model.

The ‘Greenstar Heatslave II’ reaches efficiency highs of 90% and comes with an eco hot water mode, helping to save you energy and cut down those household bills—it’s also compatible with a solar thermal system and has several options available for heating controls including the latest wireless and remote control technology.

Size

The ‘Greenstar Heatslave II’ internal Oil Combi Boilers are suitable for kitchen installs only and will be right at home underneath a kitchen worktop, remember these are floor-standing boilers and will require sufficient space meaning it must be easily movable for servicing (see specification below for dimensions).

Verdict

The ‘Greenstar Heatslave II’ is the UK’s most popular Oil Combi Boiler and is an ideal choice for homeowners looking for a powerful Boiler to fit in their kitchen. The only real downfall is the two years parts and labour warranty, which falls on the short side when compared with rival offerings.

With that said the ‘Greenstar Heatslave II’ is a great option for oil heated homes hence we consider it as one of Best Oil Combi Boilers on the market for homeowners in 2026.

Did you know? Worcester Bosch was one of only two brands to receive the Which? coveted five star score for build quality, reliability and how likely engineers are to recommended the brand, the ‘Greenstar Heatslave II’ also received the ‘Which? Best Buy’ seal of approval.

Specification

Please select a model below to view its specifications.

• Internal 12/18
• Internal 18/25
• Internal 25/32

Overview

From Grant, the VortexBlue range of oil combi boilers was designed to improve on traditional oil boiler technology by delivering cleaner combustion, high efficiency, and reliable performance.

The VortexBlue range demonstrated how modern oil combi boilers could meet increasingly strict emissions standards while still offering the strong heating and hot water performance required in off-grid UK homes.

What makes the VortexBlue different?

Unlike conventional oil boilers that produce a yellow flame, the VortexBlue uses blue flame combustion technology, made possible by the compact Riello Blue Flame burner.

This results in:

• Cleaner fuel combustion
• Lower emissions compared to older oil boilers
• Improved efficiency and reliability

The blue flame technology is based on proven Grant Vortex engineering, refined to deliver a more environmentally considerate oil boiler without compromising performance.

Is the Grant VortexBlue right for my home?

The VortexBlue range is available in internal and external models, offering flexibility depending on available space and installation requirements.

For internal installations, one of its standout features is low operating noise, with levels as low as 53.7 dB, making it well suited to utility rooms or locations close to living and sleeping areas.

The range includes three outputs:

• 21kW
• 26kW
• 36kW

Smaller homes with 1–3 bedrooms and a single bathroom are typically well matched to the 21kW or 26kW models, while larger homes with higher hot water demand may be better suited to the 36kW version.

As with any boiler, correct sizing is essential and should always be confirmed by a qualified heating engineer.

Efficiency and performance

Thanks to the Riello blue flame burner and condensing technology, the Grant VortexBlue range offers:

• Seasonal efficiency figures typically between 90% and 94%, depending on model
• ErP A-rated performance
• Reduced fuel consumption compared with older non-condensing oil boilers

This can help lower running costs while also reducing emissions.

Size and installation

The Grant VortexBlue boilers share similar dimensions with other Grant floor-standing models, making them a straightforward replacement for older Grant oil boilers.

As floor-standing units, they require adequate space — most commonly a utility room, garage, or external location for outdoor models.

Warranty and reliability

Grant boilers are well regarded for build quality, and the VortexBlue range benefits from:

• High-quality internal components
• A strong reliability record
• Up to a 10-year manufacturer warranty (subject to installer and servicing conditions)

Verdict

The Grant VortexBlue remains a high-quality oil combi boiler, particularly for homeowners looking for a quiet, efficient, and well-engineered solution for off-grid heating.

While newer Grant models have since built upon this technology, the VortexBlue still represents a solid and dependable option, especially where availability and pricing make it attractive. Its combination of blue flame combustion, strong efficiency, and long warranty ensures it continues to justify its place among the best oil combi boilers available today.

Did you know? The Grant VortexBlue range has previously received recognition from Which?, earning Best Buy status for its efficiency, performance, and reliability.

Specification

Please select a model below to view its specifications.

• Internal 21kW
• Internal 26kW
• Internal 36kW