Are you looking for a high-output boiler with more than enough power to heat your large Edinburgh home?

If so, you may already be aware of two main contenders; the Worcester Bosch 42CDi and the Worcester Bosch 440CDi Highflow floorstanding combi. Both of these boilers have a lot of oomph to them, and because they come from premium brand Worcester Bosch, they have a great reputation.

But which one is best for your home? Well, there are a few tell-tale points to consider.

As an experienced engineer, I’m going to talk you through both of these two boilers, so you can help decide which is best for your home. Hopefully you’ll find it helpful!


Premium brand boiler manufacturer Worcester Bosch are the market leader with an unbeatable reputation. They’ve been Which? ‘Best Buy’ for at least eight years now – which is impressive in itself. Both of these boilers will give you plenty of bang for your buck – with fantastic flow rates some other brands would find difficult to match.

I’ve fitted many of these boilers in my time, and I never have any issues – you can just fit them and let them happily run in the background.

But to find out which boiler is right for your home, the first thing you should look for is the heating and hot water outputs, along with the flow rates you can expect to your taps…

Output and flow rates

The 42CDi provides 42kW to your hot water, and 30kW to your heating. This results in a flow rate of 17.1 litres per minute – which is pretty incredible. If you wanted to, you could run two showers at once – assuming you had enough incoming water pressure to your house, that is.

The 30kW it provides to your heating circuit will happily heat a five-bedroom house with two public rooms and around 10 – 16 radiators.

On the other hand, the Worcester Bosch 440CDi Highflow has a heating and hot water output of around 29kw, which is great for large homes with around 10-16 radiators and multiple showers and baths. With a flow rate of 20 litres per minute It’ll handle a high pressure shower or deep cast iron bath no problem.

TIP: Don’t forget, always get an engineer to check your incoming water pressure to ensure you’ll definitely benefit from fantastic flow rates these boilers can offer you.

Size and special features

With dimensions of 760 mm x 440 mm x 360 mm, the 42CDi combi isn’t a compact boiler, so unfortunately it won’t fit in a standard kitchen cupboard or Edinburgh press. It is a wall-hung boiler though, so it can look at home on your kitchen wall.

Measuring in at 850mm x 600mm x 600mm, the 440 CDi is what’s known as a floor-standing boiler, which is around the size of your washing machine – in fact, it’ll slot quite nicely under a kitchen worktop or in a basement or garage. However, it’s not ideal if you don’t have the extra space in your home.


Good news! Both of these boilers offer this premium brand’s 10-year no-quibble guarantee. The only thing is, you’re also paying a premium price for those 10 years, and you also have to get the brand’s own filter and controls installed.

At a glance

To begin with, let’s take a look at the Worcester Bosch Greenstar 42cdi classic:

  • Heating output: 30 kW
  • Hot water output: 42 kW, with a flow rate of 17.1 litres per minute
  • Ideal for: A large five-bedroom home with 2 bathrooms, and 10-16 radiators
  • Size: 445mm x 740mm x 330mm
  • Guarantee: 10 years (when fitted with own-brand controls & filter and installed by an accredited installer)
  • ERP rating: A

Now, let’s see how the Worcester Bosch 440CDi Highflow compares:

  • Heating output: 29.2kw
  • Hot water output: 29.5kw, 20 litres per minute at a flow rate of 35 degrees Celsius
  • Ideal for: Large homes with 10– 16 radiators with multiple showers and baths
  • ERP rating: A, 93% efficiency
  • Guarantee: 10 years (when installed with the brand’s own filter and controls from an accredited installer)

Advice from an experienced heating engineer

I had a client recently who wanted a combi boiler – a large one. What they had was a Potterton ProMax – a boiler with a hot water cylinder in it. It gave wonderful flow rates – around 17 litres per minute. However, it was old and had become very unreliable. My client wanted a combi boiler, but it was going to be tricky to achieve the same flow rate.

The only boiler that could do the job without having a separate cylinder was the Worcester Bosch 42CDi. He spent a lot of time looking at reviews and quotes, and realised the flow rate was fantastic. It would allow him to run two showers at once. The 10-year guarantee was another huge selling point.

The 42CDi may be more expensive than other options, but there aren’t really any competitors out there that are also wall-hung, which is what my client wanted. So, if this is a deciding factor, the 42CDi may be your best bet. However, if you have a large utility room, the 440CDi Highflow may be a viable option.

Because a 42kW boiler burns a lot of fuel for a short time, it is highly likely that your gas pipe will need to be upgraded. It should ideally be 28mm from the meter to the boiler. The reason being, if you don’t have the right gas pipe, you won’t get the right output. This will mean you won’t enjoy the same great flowrates.


Well, there we have it! I hope this blog has helped you narrow down your decision when comparing the Worcester Bosch 42CDi Classic combi and the Worcester Bosch 440CDi Highflow combi.

If you have a large family home, both of these boilers may be on your radar, the deciding factor may be wall hung boiler or floor standing boiler.

Do you have any questions about either of these two boilers? Let me know in the comments below!