The Worcester Bosch Greenstar 42CDi Classic ErP is the biggest wall-hung combi in Worcester’s old CDi Classic range. It gives 17.2 litres per minute of mains-pressure hot water, which was about as much as a wall-mounted combi could manage at the time.
Worcester no longer makes it. The CDi Classic combis were replaced by the Greenstar 8000 series, now sold as the Greenstar 8000+. If you’ve got a 42CDi on the wall, or you’re weighing up what to replace it with, you’ll find the specs, strengths, weak spots and current alternatives below.
What Are The Worcester 42CDi Classic’s Key Specs?
The 42CDi Classic has 30kW of heating, 42kW of hot water power and 17.2 l/min at a 35°C rise. It’s discontinued.
| Specification | Greenstar 42CDi Classic ErP |
| Boiler type | Wall-hung condensing combi |
| Central heating output | 9.4kW to 30kW |
| Maximum hot water output | 42kW |
| Hot water flow rate | 17.2 l/min at a 35°C rise (15 l/min at a 40°C rise) |
| ErP rating | A for heating (92% seasonal efficiency), A for hot water |
| Dimensions (H x W x D) | 760mm x 440mm x 360mm |
| Fuel | Natural gas, with an LPG version |
| Status | Discontinued. Replaced by the Greenstar 8000 range (now the Greenstar 8000+) |
What Is The Worcester Bosch 42CDi Classic?
It’s the top model in the Greenstar CDi Classic combi line-up, sitting above the 29CDi, 34CDi and 38CDi.
Every model in that family shares the same casing and the same 30kW heating output. The number in the name refers to hot water power, and the 42CDi had the most of it.
Its 42kW hot water burner can heat more incoming mains water than the smaller models. Day to day, you get a slightly stronger shower and a bit more headroom when someone opens a second tap. The gain over the 38CDi is real but modest: 17.2 litres per minute against 16.4.
We often found the 42CDi in larger houses where the owners wanted a combi rather than a cylinder. Sometimes that was to free up the airing cupboard. Sometimes they just didn’t want stored hot water.
Most of these boilers are now well over 8 years old, the age when efficiency usually drops and faults become more common.
What Features Does The 42CDi Classic Have?
Its headline feature is the highest hot water flow in the CDi Classic combi range. It also condenses in hot water mode and has a built-in filling link.
- Highest flow in the CDi Classic combi range at 17.2 l/min (35°C rise).
- Condenses in hot water mode, so it recovers heat from the flue gases whether it’s heating radiators or running taps.
- Modulates down to 9.4kW, which helps it avoid short cycling when heating demand is low.
- An integrated filling link makes topping up system pressure simpler.
- An LPG version was made for homes off the gas grid. Our page on LPG and oil boilers covers off-grid options.
How Efficient Is The 42CDi Classic?
It’s A-rated for both heating and hot water, with a seasonal space heating efficiency of 92%. Under the older SEDBUK 2009 method it was rated at 89.4%.
Those figures are close to what current Worcester combis achieve when new. After 8 years or more on the wall, though, real-world efficiency has usually dropped and faults become more common. That’s why replacing an older 42CDi is often better value than paying for repairs.
Running costs depend a lot on how the system’s set up. Three things help any condensing boiler get closer to its rated efficiency:
- a lower flow temperature
- weather or load compensation, where your controls allow it
- a clean system with a magnetic filter
If you’d like smarter control, our smart thermostats page covers the options we fit.
What Are The Pros And Cons Of The 42CDi Classic?
The big plus is the best hot water flow of any CDi Classic combi. The main minus is that it’s discontinued and only slightly ahead of the 38CDi.
Pros
- Best hot water flow of any CDi Classic combi
- Plenty of heating output for large homes
- A-rated for heating and hot water
- Fits the same wall space as the smaller CDi Classic models
- Natural gas and LPG versions were available
- Common enough that most engineers know it well
Cons
- Discontinued, so you can’t buy one new
- Only a small hot water gain over the 38CDi
- Still a combi, so showers running together share the flow
- Needs good mains pressure and flow to reach its rated output
- Older controls compared with today’s touchscreen models
What Size Home Does The 42CDi Classic Suit?
It’s a good match for a four-bedroom home with a main bathroom and one en-suite. The 30kW heating output copes with a high radiator count, so most large houses are fine.
Hot water is the limit. At 17.2 litres per minute, one strong shower plus a basin tap is comfortable, but two showers at once will both drop in flow.
If you’ve got three bathrooms, or a family that all wants a shower at the same time each morning, a system boiler and an unvented cylinder usually works better.
Your incoming water supply also sets a ceiling. A combi can’t deliver more water than the mains provides, so we always measure pressure and flow before recommending any large combi. Our guide to combi, system and conventional boilers explains the trade-offs.
How Much Does It Cost To Replace A 42CDi In 2026?
A straight swap to a current 40kW to 50kW Worcester combi typically costs £3,000 to £4,000 fitted. With the 42CDi out of production, that’s the figure most owners need.
These are guide ranges only. Prices vary with the boiler you choose, the flue route, gas pipe sizing and any work the system needs.
| Type Of Job | Typical Installed Price Range (Guide Only) |
| Straight swap to a current 40kW to 50kW Worcester combi | £3,000 to £4,000 |
| Swap with gas pipe upgrade, system flush and new controls | £3,500 to £4,700 |
| Switching to a system boiler and unvented cylinder | £4,500 to £6,500+ |
Large combis often need a bigger gas supply than smaller boilers, so allow for possible pipework upgrades. Our new boiler cost guide explains what drives the final price.
What Are The Best Alternatives To The 42CDi Classic?
The Worcester Bosch Greenstar 8000+ Life is the natural replacement, and its 50kW model gives more hot water than the 42CDi.
- Worcester Bosch Greenstar 8000+ Life: the current replacement for the CDi Classic, in sizes up to 50kW. The 50kW model is listed at around 20 l/min at a 35°C rise, a clear step up. See our Greenstar 8000 Life and Style review.
- Worcester Bosch Greenstar 8000+ Style: the same performance in a black glass-look casing.
- The 38CDi Classic: if you’re comparing old models, our Worcester 38CDi Classic review shows how close the two are.
- A system boiler: for homes with three bathrooms, read our best system boilers guide.
Is The Worcester Bosch 42CDi Classic Worth It?
Not as a long-term option, because most 42CDis are now over 8 years old and a new boiler is usually the better choice at that age.
In its day, it was one of the strongest wall-hung combis you could buy. Today, a Greenstar 8000+ Life in the 45kW or 50kW size will match or beat it on hot water. On an older 42CDi, that’s usually better value than paying for repairs.
Choose A 42CDi-Sized Combi If:
- You have a large home with two bathrooms and good mains pressure
- You want as much hot water as possible without a cylinder
- You’ve no room for a hot water cylinder
Look Elsewhere If:
- You have three or more bathrooms in regular use
- Your mains water flow is low
- Your current 42CDi is over 8 years old, unreliable or needs expensive parts
Get A Free Boiler Quote From Clever Energy Boilers
We carry out new Worcester Bosch boiler installations for homeowners and landlords across the North West, Yorkshire and the Midlands. As Worcester Bosch accredited installers, we’ll size the replacement properly, check your gas and water supply, and fit the right controls.
Long, market-leading warranties are available through Clever Energy on the boilers we fit.
Get your fixed-price boiler quote online today.
Frequently Asked Questions
Is The Worcester Greenstar 42CDi Classic Discontinued?
Yes, the 42CDi Classic is no longer made. Worcester replaced the CDi Classic combis with the Greenstar 8000 range, now sold as the Greenstar 8000+.
What Is The Flow Rate Of The 42CDi Classic?
It’s 17.2 litres per minute at a 35°C rise and 15 litres per minute at a 40°C rise. Your actual flow depends on incoming mains pressure.
What Is The Difference Between The 38CDi And 42CDi?
The 42CDi has more hot water power, 42kW against 40kW, giving 17.2 l/min instead of 16.4 l/min. Both have the same 30kW heating output and the same casing size.
What Should Replace A Worcester 42CDi Classic?
A large Greenstar 8000+ Life combi, usually the 45kW or 50kW model, is the most common replacement. If the home has three bathrooms, a system boiler with a cylinder may suit it better.
Can A 42CDi Classic Run On LPG?
Yes, an LPG version of the 42CDi Classic was made. Any conversion or replacement on LPG must be done by a Gas Safe registered engineer qualified for LPG work.