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Central Heating

Thinking of changing our central heating boiler to either worcester 42cdi or 440cdi ?

we currently have a worcester floor-standing highflow with its heatbank. So the 440CDI would be similar to what we have at present. But the wife prefers to change to wall-mounted so best is 42CDI. I am worried that we will have problems with the 42CDi not producing the amount of water we are used to. rough test while running downstairs hot tap, bath gave 11 litres in a minute, so are we wasting our money buying a 440CDI to get a flow of 20 litres per minute. normally only maximum of 3 people in this 4-bed semi house. One bathroom, but there is space for a futue en-suite in bedroom 1 and also to extend into loft for a further 2 bedrooms and a second bathroom. if that was ever done surely we would need a bigger boiler, we currently have about 16 rads. we want to stick to worcester but worried about what is best for the long term.
any help would be appreciated.

8 Answers from MyBuilder Heating Engineers

Best Answer


My name is Stuart Johnson I'm a registered gas safe engineer after reading your question i'd recommend a Remeha Avanta 39c or 35c combi boiler this will give you 16l per minuter for hot water, i've installed this and the 35c into properties that have 5 bedrooms with a bathroom and 2 en-suite's plus more radiators than you currently have at your property and it works fantastically well I can't rate this boilers enough I probably install over 30 Of these a year and haven't had a problem with any of them it comes with a 5 year warranty which most manufactures only provide 12 - 24 months. I'd be more than happy to quote you for supply and installation or if you would like to ask any further question please don't hesitate to contact me, Kind Regards Stuart, S Johnson Heating


Answered 18th Oct 2011

Both these boilers would suite your needs.The 42cdi will deliver 17.2liters per min the 440 will deliver 20lpm dependant on mains incomming pressure .heating wise the 42 is 30kw capable of 102,300btus the 440 is 29.2kw the same btus price 42cdi a lot cheaper to both buy and install


Answered 18th Oct 2011

hi there
combi boilers will only provide water for one hot tap outlet at a time
if multiple outlets are to be needed then you will need water storage ie a pressurised cylinder providing mains pressure hot water.
on the other hand how many times are people going to be bathing at the same time so your cheapest option will always be the combi.
am heating


Answered 18th Oct 2011

It would seem u would be wasting ur money
On anything more than 20/35
Kw boiler

Worc ok but pricey and not user friendly
Ideal good alternative

Regards ian
Combi boiler services


Answered 18th Oct 2011

The test you carried out is not correct for finding out the flow rate
There are two things that need to be known to determine flow rate and the best way to do this is via an outside tap that does not have a ballafix valve fitted to it so that you're getting the full flow
What you need to find out are your dynamic flow and static pressure then you can decide what combi boiler will best suit your needs
From what you say I feel that the best route to go down would be with a system boiler and an unvented cylinder but this will depend upon the water flow rate and static pressure
Hope this helps


Answered 18th Oct 2011

A very good Alternative is Vaillant and Particularly the;

Vaillant 937kw Storage Combi
This is a High output 37kw Storage Combi more than enough Heating and Hot Water Output than you need.

The good thing with this Vaillant is in the back it has 2 small Hot Water Cylinders so output is very good and powerful cant remember the LPM on DHW but its the only Combi weve ever Installed that gives Hot Water when 2 or more Hot Taps are opened.

Worcesters are good boilers as someone above said, but not he most user friendly with their controls, vaillants keep everything simple with their Wireless Roomststats on, off etc..

As with all Vaillants build Quality is Excellent, 5 years Part and Labour Warranty if installed by Vaillant Accredited Engineers, which we are if your in Northwest we can give you a quote install this for you.

Good luck with what Boiler ever you choose.


Answered 19th Oct 2011

You would be better with a system boiler and a unvented cylinder to get the high flow rates


Answered 18th Oct 2011

You have quite a few variables in this question.

The boiler and system should be sized to the property and potential demand not the amount of people that may or may not be staying their.

The flow rate and pressure would need to be looked at (you can only get out what you've got going in)

Definitely get some quotes and see what the engineers have to say.

You would want them to do a heat loss calculation for your property taking in to consideration future extensions which will then tell you if your proposed boiler will be able to handle the extra heating demand

An over sized boiler will modulate (fire to the required demand) but would not be as economical as if it was correctly sized as this would condense for longer periods getting you more for your gas

I would say that your current set up could do with a different solution to be satisfactory with your proposed extensions, ideally a boiler that you can range rate and maybe an unvented cylinder.

Kind regards



Answered 18th Oct 2011

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