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Worcester Bosch Greenstar Heatslave II External

The Worcester Bosch Greenstar Heatslave II External is an energy efficient oil-fired combi boiler that is designed for outdoor installation.

If you live off the gas grid and are looking for a reliable oil boiler for your small, medium or large home, the Worcester Bosch Greenstar Heatslave II External will not disappoint. It is designed to be installed outside so that it won’t take up any valuable space indoors and comes complete with a green-coloured cabinet to blend in with your garden. Not only that, but this model is smart and solar compatible, achieves an A rating for efficiency and has even been awarded Which? Best Buy.

How Much Does this Boiler Cost?

The Worcester Bosch Greenstar Heatslave II External is usually priced between £2,400 and £2750 because it is available in three central heating outputs of 18kW, 25kW and 32kW. The higher the output you need, the more you can expect to pay. Prices also vary from one supplier to another, so it’s important to shop around before purchasing this model. Then there’s the cost of installation, which depends on certain factors, such as your chosen installer and how much work is required.

How will I know if this Boiler is Suitable for My Home?

This oil-fired combi must be fitted outside and doesn’t require any loft tanks or a separate cylinder for storing hot water, making it a great choice if your home is short on space. Like all combi boilers, the Worcester Bosch Greenstar Heatslave II External takes water from the mains to heat your radiators and provide hot water for your taps on demand. Because it comes in three central heating outputs, you need to make sure you select the correct size for your home.

As a rule of thumb, the bigger your property and the greater your demands for heating and hot water, the higher the kW output you’ll need to choose. However, you must play it safe and ask an OFTEC registered engineer to help you work out the right size of boiler to ensure it can run efficiently whilst meeting your heating and hot water requirements.

Does this Boiler have a Good Range of Controls and Features?

Just like the internal model, the Greenstar Heatslave II External has some excellent controls and features, such as an Eco Hot Water Mode that saves energy, inbuilt frost protection, and one-piece baffle for easy servicing. The controls are sealed in a drawer and the boiler itself comes in a green powder coated cabinet to ensure it’s protected from the elements and doesn’t look out of place in your garden. This model can be used with water that is heated by green sources or be connected to a range of optional energy efficient controls to reduce your fuel bills even further. For ultimate control over your boiler, you can purchase the Bosch EasyControl smart thermostat that lets you monitor and manage your central heating and hot water from a smartphone or tablet.

How long is the Warranty on this Boiler?

Worcester Bosch’s Greenstar Heatslave II External comes with a 2-year parts and labour warranty as standard. Prior to purchasing this boiler, check the Worcester Bosch website to see if there are any existing promotions or offers on oil-fired boiler warranties.

Greenstar Heatslave II External By Size and Price

ModelTypeDimensionsMaximum Flow RateOutputEfficiency RatingTypical Price
Greenstar Heatslave II External 18CombiH – 940 mm
W – 640 mm
D – 770 mm
22 l/min18 kWA 90% and above£2400
Greenstar Heatslave II External 25CombiH – 940 mm
W – 640 mm
D – 770 mm
22 l/min25 kWA 90% and above£2550
Greenstar Heatslave II External 32CombiH – 940 mm
W – 640 mm
D – 770 mm
22 l/min32 kWA 90% and above£2750

Summary of Important Information

  • Priced between £2,400 and £2750
  • Heating output of 18kW, 25kW or 32kW
  • Hot water flow rates of 15, 18 or 22 litres per minute
  • Dimensions of 950mm (H) x 640mm (W) x 770mm (D)
  • Uses oil as its fuel source
  • A-rated for efficiency
  • Smart compatible
  • Solar thermal compatible
  • 2-year warranty as standard

Pros and Cons

It’s hard not to fall in love with the Worcester Bosch Greenstar Heatslave II External, which just happens to be one of the UK’s most popular oil-fired combi boilers. Below, you can find a list of all its pros and cons.

  • Awarded Which? Best Buy with a score of 87%
  • Outdoor installation – frees up space indoors
  • Suited to small, medium and large homes without a mains gas supply
  • Weatherproof cabinet
  • No need for separate cylinder or loft tanks
  • Simple boiler controls and clear temperature display
  • Eco hot water mode – saves energy
  • Provides maximum energy efficiency – reduce fuel bills and carbon emissions
  • Compatible with solar thermal – additional savings
  • Compatible with Bosch EasyControl – can tailor hot water times and heating temperatures from your smartphone or tablet, even when you’re not at home
  • 5-year guarantee on the primary heat exchanger


  • 2-year warranty – would prefer a longer standard guarantee
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