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If you’re comparing combi boilers for a flat or a smaller home, the Worcester Greenstar 25i will probably come up quickly. It’s a well-known name, it’s been around for years, and plenty of installers recommend it.
But is it right for your home? That depends on one thing most reviews skip: how much hot water your household uses at the same time. We’ll cover that below, along with the specs, real pros and cons, and what you should expect to pay.
The quick answer
The Greenstar 25i is a solid, no-fuss combi boiler for homes with one bathroom. It’s ErP A-rated, saves space because there’s no hot water tank, and Worcester Bosch parts are easy to find. If two showers often run at once, or you have two bathrooms, you’ll want something with a bigger hot water output.
Key specs at a glance
| Worcester Greenstar 25i | |
|---|---|
| Type | Gas condensing combi |
| Central heating output | 24kW |
| Hot water output | 25kW |
| Hot water flow rate | About 10.2 litres a minute (at 35°C rise) |
| Energy rating | ErP A (around 93% seasonal efficiency) |
| Typical home | 1 bathroom, small to medium size |
| Standard guarantee | 5 years, with longer cover possible via an accredited installer |
Figures can differ slightly between model versions, so check the data sheet for the exact unit being fitted to your home.
What kind of boiler is it?
The 25i is a combi, short for combination boiler. It does two jobs in one box: it heats your radiators and it heats water on demand straight from the mains.
That means no cylinder in the airing cupboard and no tank in the loft. For a lot of flats and terraced houses in London, that freed-up space is the main reason people go for a combi.
Is the Greenstar 25i energy efficient?
Yes, it’s a condensing boiler rated ErP A, and published listings put its seasonal efficiency at roughly 93%.
In plain terms, a boiler like this wastes far less heat than an old non-condensing model. If you’re replacing a boiler that’s 15 or more years old, you’ll likely notice the difference in your gas bill.
How much you actually save depends on your insulation, your radiators and how you use your heating. We’d rather tell you that than promise a number we can’t guarantee.
Who the 25i suits best
It’s a good match if:
- you have one bathroom
- your home is a flat, maisonette or small to medium house
- you want a straightforward boiler with a proven track record
- you’d like to get rid of a bulky hot water tank
- you’re happy to add a smart thermostat for better control
When we’d suggest something bigger
It’s worth thinking twice if:
- you have two bathrooms
- you get several people showering in the morning rush
- you want to fill a big bath quickly while another tap is running
A flow rate of around 10 litres a minute is fine for one shower and a kitchen tap. Ask for more at once, and you may notice weaker pressure or a drop in temperature. In that case, a higher-output Worcester combi is the safer choice.
What we like
- It’s simple. The design is well understood, so servicing and repairs are usually straightforward.
- It’s compact. It fits in most kitchen cupboards and utility rooms.
- It’s well supported. Worcester Bosch has a big engineer network and parts are easy to get.
- It works with modern controls, including Worcester’s own smart options.
What we’d flag
- The flow rate is modest. Great for one bathroom, limited for busy households.
- It’s from an older range. Worcester’s newer Greenstar 2000 and 4000 ranges have taken over in many price lists, so ask your installer what’s currently available.
- Longer guarantees come with conditions. Extended cover usually depends on installation by an accredited installer, correct registration and annual servicing.
How much does it cost to install?
Most customers can expect to pay around £1,775 to £2,275 for supply and installation.
Where you land in that range depends on:
- whether we’re swapping like for like or moving the boiler
- the flue route and position
- your choice of thermostat or smart controls
- whether your system needs a filter or a cleanse because of sludge
- the condition of your existing pipework
The best way to avoid surprises is a quote based on your actual home, not a general online figure.
What actually happens on installation day?
It’s less mysterious than it sounds. A Gas Safe registered engineer will:
- Look at your current setup, including pipework, radiators and where the boiler currently sits.
- Agree the best position, taking into account ventilation, access and safety rules.
- Fit the boiler and connect the gas, mains water and heating pipes.
- Test everything for leaks and safe operation.
- Commission it, which means setting it up properly and showing you how the controls work.
Only a Gas Safe registered engineer can legally install a gas boiler, so always ask to see the ID card.
A few checks worth doing before you buy
- Mains water pressure and flow. A combi can only deliver what comes into your home.
- Radiator and pipe condition. Old sludge shortens the life of any new boiler, so a magnetic filter is a smart addition.
- Controls. A good thermostat can make a bigger difference to comfort than people expect.
- Servicing. Book it every year. It keeps the guarantee valid and catches small problems early.
Greenstar 25i vs a bigger combi
| Greenstar 25i | Larger Worcester combi (30kW and up) | |
|---|---|---|
| Best for | 1 bathroom | 1 to 2 bathrooms |
| Hot water at busy times | Fine for a single outlet | Copes better with multiple outlets |
| Space and size | Compact | Similar footprint |
If you’re still comparing brands, our guide to Worcester Bosch vs Baxi vs Ideal is a good next read. You might also want to see our list of boiler brands to avoid in 2026.
Our verdict
The Greenstar 25i earns its reputation as a sensible, dependable boiler for a one-bathroom home. It isn’t flashy, and it isn’t meant for big families. But if it fits your household, it does the job well.
Not sure whether 25kW is enough for you? Tell us how many bathrooms you have and how many people live at home, and we’ll point you to the right boiler, even if that means recommending something other than the 25i.
Call us on 07877767776 for a free quote on a Worcester Bosch boiler for your home.
Frequently asked questions
For a one-bathroom home, yes. It’s efficient, reliable and well supported. It’s less suited to larger households with heavy hot water demand.
It carries an ErP A rating, with published seasonal efficiency of around 93%.
That depends on your home’s heat loss, but it’s typically suited to small to medium properties. A proper survey gives a definite answer.
No. It heats water directly from the mains.
Worcester’s standard guarantee is 5 years. Longer cover can be available when it’s installed by an accredited installer and registered correctly, so get the exact terms in writing.