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UK-based boiler manufacturer Worcester Bosch have become a household name for their high quality and efficient energy products. With over six decades of experience, Worcester Bosch have become a key player in the boiler market competing against the likes of Vaillant and Ideal.

Today we will be taking a closer look at the Greenstar 25Si Compact combi boiler, which is the compact alternative to Worcester Bosch's top-selling combi boiler, the Greenstar 25i. If you're tight on space and need a new boiler to fit in one of your kitchen cupboards, then look no further.

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Will the Worcester Bosch Greenstar 25Si Compact be suitable for my home?

With a heating output of 24kW and a hot water output of 25kW, the 25Si is the perfect fit for small to medium-sized homes with one bathroom. The flow rate works at around 10.2 litres per minute, which is suitable for your kitchen/bathroom taps and a shower. If you have a bath installed in your home, we'd recommend that you take a look at the 30Si, which has a higher flow rate and reduces the time you’ll have to wait for your bath to fill up.


The control panel on the front of the boiler offers all the same features of a standard combi boiler, with an on/off switch, temperature settings, and an LED to display fault diagnostics. Worcester Bosch offer a wide variety of boiler controls, which range from standard room thermostats, through to smart thermostats. The most basic control you could use is a mechanical room thermostat with a dial to set temperatures, although you won't have the accurate control over your heating and energy use. To ensure proper control of your heating, we’d recommend that you take a look at the Greenstar Comfort thermostat, or Worcester Bosch’s own smart thermostat, the Wave.

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With the Wave, you’ll be able to accurately control your heating and set schedules straight from your smartphone. The Wave is also a self-learning device, which alters your heating schedules based on your everyday use of the app. You will start to see enhanced energy efficiency and improved control over your heating almost straight away. We recently reviewed the Wave thermostat here. If the Wave isn’t for you, the Nest is another smart thermostat alternative. Like the Wave, the Nest is highly intelligent and learns from your everyday use of your heating and hot water. You can also operate the thermostat through Amazon’s Alexa, using voice commands to alter your heating settings. If you’d like more information about the Nest, we recently compared the Nest to British Gas’ Hive. Take a look at the article here.


Coming in at 690x390x280mm, the 25Si would easily fit into one of your kitchen cupboards, which is great if you're looking to save as much space as possible. You'll also benefit from the lack of hot water tank and cylinder (like any combi boiler) as the Worcester 25Si uses mains fed water on demand.

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Worcester Bosch offer a 5-year warranty with each of their Greenstar boilers, which can be extended by a further 5 years if a Worcester Accredited Installer performs the installation. An extended warranty will give you extra peace of mind in case of a breakdown or if your boiler needs a repair at any point (which is highly unlikely).

Energy Efficiency

If you’re looking for a “green” boiler, you’re in safe hands with the Greenstar 25Si. With an energy-efficiency rating of 89.8% (an A grade overall), you can expect a high-performance boiler which will save you money in the long-term. It’s now more important than ever for your boiler to be as efficient as possible as we combat rising energy prices, climate change and carbon emissions.

The introduction of the ErP Directive has meant that boiler manufacturers within the EU need to have their products tested for performance and efficiency. In order for their products to be sold to the wider public, their products need to pass certain criteria and achieve the highest possible level of efficiency. The ErP Directive will actively step in to improve the design of boilers if it's found that they're under-performing and wasting too much energy. You can read more about the ErP Directive here.

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The Greenstar 25Si Compact may be small but it packs a serious punch. For a small to mid-sized properties, this boiler is perfectly suitable for one bathroom and up to 3 bedrooms. Knowing that the Greenstar boiler range has won the Which? Best Buy award for 7 years running, which is a great sign of quality and promise from the Worcester Bosch brand itself, and with such a high energy efficiency rating, you have the peace of mind that it won’t break down anytime soon and that you can start to save on your energy bills from the moment it’s on the wall.

Now, if space isn’t an issue and you’re looking for a slightly more powerful boiler, the Worcester Bosch Greenstar 25i is a safe bet. You can also take a look at the entire Worcester Bosch combi boiler range here.

 Worcester Bosch Greenstar 25Si Compact
ErP rating (energy efficiency)A (89.8%)
Size (HxWxD in mm)690x390x280
Heating output (kW)24
How water output (kW)25
Flow rate (litres/min)10.2
Standard warranty (years)5 (extended warranties available)
Other available sizes (kW)30

Which? review each boiler on the market to find the most (and least) reliable boilers according to a survey completed by thousands of boiler owners. In each review, Which? supply a brand score percentage and star ratings for each category. Take a look at the Worcester Bosch Greenstar 25Si Compact ratings below: