The Worcester Bosch Greenstar Highflow 550CDi is a floor-standing gas combi with a built-in thermal store, and it was the most powerful hot water combi Worcester made. It gives up to 25 litres per minute at mains pressure.
That made it a favourite for big family homes with two or three bathrooms that didn’t want a cylinder. The Highflow CDi range has now been discontinued, and Worcester replaced it with the Greenstar 8000 F floor-standing combi in late 2025. If yours is due for replacement, you’ll find what it does well, where it falls short and what to look at next below.
What Are The Worcester Highflow 550CDi’s Key Specs?
The 550CDi gives up to 30.6kW of heating, 41.1kW of hot water power and 25 l/min at a 35°C rise. It’s discontinued.
| Specification | Greenstar Highflow 550CDi |
| Boiler type | Floor-standing condensing combi with integral thermal store |
| Central heating output | 9.7kW to 30.6kW |
| Hot water output | 41.1kW |
| Hot water flow rate | 25 l/min at a 35°C rise |
| ErP rating | A for heating (92% seasonal efficiency), B for hot water |
| Dimensions (H x W x D) | 850mm x 600mm x 600mm |
| Dry weight | Around 112kg |
| Mains pressure needed for full flow | 1.7 bar |
| Fuel | Natural gas, with an LPG version |
| Status | Discontinued. Replaced by the Worcester Bosch Greenstar 8000 F |
What Is The Worcester Bosch Highflow 550CDi?
It’s the larger of the two Highflow CDi boilers, sitting above the Highflow 440CDi.
Both are floor-standing combis built to fit under a kitchen worktop. Each has a small internal thermal store that the boiler keeps hot. That heat passes through a plate heat exchanger to warm the mains water on its way to your taps.
The difference is on the hot water side. The 550CDi’s 25 litres per minute is a big step up from the 440CDi’s 20, and well beyond most wall-hung combis. In real homes, two showers can run together with far less of the pressure drop you’d get from a standard combi.
Heating output is similar to the smaller model at 30.6kW. Paying extra for a 550CDi was always about hot water, not radiators.
What Features Does The Highflow 550CDi Have?
Its standout feature is 25 l/min of mains-pressure hot water, backed by an integral thermal store.
- 25 l/min hot water: one of the highest flow rates of any gas combi made for the UK market.
- Integral thermal store: stored heat backs up the burner when demand peaks.
- Mains-pressure delivery: no loft tank and no separate cylinder.
- Under-worktop footprint: 850mm x 600mm x 600mm, the size of a standard kitchen base unit.
- Room-sealed fanned flue: the compartment doesn’t need extra ventilation.
- LPG version available: handy for rural homes. Our LPG and oil boilers page covers off-grid options.
How Efficient Is The Highflow 550CDi?
It’s A-rated for space heating at 92% seasonal efficiency, but only B-rated for hot water.
Combis with a hot water store tend to score lower on hot water because some energy goes into keeping the store hot. Most current instantaneous combis are A-rated here.
Day to day, the 550CDi burns a small amount of gas to hold its store at temperature, even when nobody’s running a tap. Big families who use lots of hot water get the benefit of that readiness. Smaller households end up paying for heat they rarely need.
Its rated heating efficiency is close to a modern boiler’s. Once a 550CDi is over 8 years old, though, efficiency has usually dropped and faults become more common. Replacing it is then often better value than paying for repairs. On a younger one, you’ll usually save more with good controls, a lower flow temperature and a clean system. Our smart thermostats page explains the controls we recommend.
What Are The Pros And Cons Of The Highflow 550CDi?
The big pro is 25 l/min of hot water without a cylinder. The cons are its B hot water rating, its high mains pressure needs and the fact it’s discontinued.
Pros
- 25 l/min of mains-pressure hot water
- Handles two showers at once far better than a standard combi
- No cylinder or loft tank needed
- A-rated for space heating
- Fits under a kitchen worktop
- Natural gas and LPG versions were produced
Cons
- Discontinued, so you can’t buy one new
- B-rated for hot water because of standing losses from the store
- Needs 1.7 bar of mains pressure to deliver its full flow
- Heavy and bulky, at around 112kg
- The direct replacement doesn’t quite match its peak hot water figure
What Size Home Does The Highflow 550CDi Suit?
It suits four and five-bedroom houses with up to three bathrooms, where lots of hot water is needed at busy times like morning showers.
Your water supply sets the limit. Worcester’s figures call for 1.7 bar of incoming mains pressure for the 550CDi to reach its rated output.
Below that, flow drops. Below 0.5 bar, the boiler won’t run its hot water at all.
If your home has good mains pressure but more than three bathrooms, a system boiler with a large unvented cylinder is usually the better fit. Our guide to combi, system and conventional boilers compares the options.
How Much Does It Cost To Replace A Highflow 550CDi In 2026?
Swapping a 550CDi for a Greenstar 8000 F 50kW floor-standing combi typically costs £4,800 to £6,200 fitted.
These are guide ranges only. Your quote will depend on the boiler you choose, access, the flue route, gas supply and the state of your system.
| Type Of Job | Typical Installed Price Range (Guide Only) |
| Swap to a Greenstar 8000 F 50kW floor-standing combi | £4,800 to £6,200 |
| Replace with a large wall-hung combi | £3,200 to £4,700 |
| Convert to a system boiler and unvented cylinder | £4,500 to £6,500+ |
Our new boiler cost guide explains what drives the price. If you’d like to spread the cost, pay monthly finance is available.
What Are The Best Alternatives To The Highflow 550CDi?
The Worcester Bosch Greenstar 8000 F 50kW is the direct replacement, though a cylinder system can suit very busy homes better.
- Worcester Bosch Greenstar 8000 F 50kW: Worcester’s floor-standing replacement for the Highflow CDi, in the same 850mm x 600mm x 600mm size. It’s A-rated for heating and hot water and listed at around 19.8 l/min at a 35°C rise. That’s less than the 550CDi’s 25 l/min, so check how much hot water your household really uses before assuming a like-for-like swap.
- Worcester Bosch Greenstar 8000+ Life 50kW: a wall-hung combi that frees up the kitchen unit. See our Greenstar 8000 Life and Style review.
- A system boiler with an unvented cylinder: for homes where several outlets run together every day, this gives the most consistent hot water. Compare options in our best system boilers guide.
Is The Worcester Bosch Highflow 550CDi Worth It?
Yes, if yours is under 8 years old, working well and serviced every year. Once it’s over 8 years old, replacing it is usually better value than paying for repairs.
It was a strong answer for large homes that wanted combi simplicity with near-cylinder levels of hot water. When you do replace it, though, think carefully about what comes next.
A 50kW floor-standing combi is the easiest swap. For a busy household, a cylinder system may serve you better.
Choose A 550CDi-Style Floor-Standing Combi If:
- You have a large home with two or three bathrooms
- Your mains pressure is 1.7 bar or higher
- You’d rather not fit a hot water cylinder
Look Elsewhere If:
- Your incoming water pressure or flow is weak
- Your hot water use is light and standing losses would be wasted
- You need three or more showers running at once
Get A Free Boiler Quote From Clever Energy Boilers
Clever Energy Boilers carries out new Worcester Bosch boiler installations for homeowners and landlords across the North West, Yorkshire and the Midlands. As Worcester Bosch accredited installers, we can test your water supply first. Then we’ll tell you whether a floor-standing combi, a wall-hung combi or a cylinder system is the right replacement for your Highflow.
Every boiler we fit can come with a long, market-leading warranty arranged through Clever Energy.
Get a free, fixed-price boiler quote online in a few minutes.
Frequently Asked Questions
Is The Worcester Highflow 550CDi Still Available?
No, the Highflow 550CDi has been discontinued. Worcester replaced the Highflow CDi range with the Greenstar 8000 F floor-standing combi in late 2025.
What Is The Flow Rate Of The Highflow 550CDi?
It’s up to 25 litres per minute at a 35°C rise. The boiler needs 1.7 bar of mains pressure to reach that figure.
What Is The Difference Between The Highflow 440CDi And 550CDi?
The 550CDi gives more hot water, 25 l/min against 20 l/min, and has slightly more heating output. Both share the same floor-standing casing size.
What Replaces A Worcester Highflow 550CDi?
The Greenstar 8000 F 50kW floor-standing combi is Worcester’s direct replacement. Its listed hot water flow is lower than the 550CDi’s, so some homes may be better served by a system boiler and cylinder.
Why Is The 550CDi B-Rated For Hot Water?
The likely reason is the energy it uses to keep its internal thermal store hot between uses. Its space heating rating is A, at 92% seasonal efficiency.