What Size Boiler?

22 Nov 2004
Reaction score
United Kingdom
Hi - We have a flat in Fife where the boiler has finally decided it is time to go. We have been advised to replace it with a Worcester Bosch boiler. Greenstar 30 CDI.

Where my puzzle starts is that this is a smallish 2 bedroom groundfloor flat with only two external walls. We recently had a new boiler fitted at home (a 3 Bedroomed Semi) with the Central Heating Engr installing a Worcester Greenstar Junior 28i. The previous boiler at home was a Hasltead Ace 24 which seemed adequate in terms of performance but behaving like a stroppy teenager hence its replacement.

My question is this: Is the CH Engineer in Fife being over cautious or was the Local CH Engineer over optimistic? Or could they both be correct perhaps?

I have checked the Worcester Bosch web site and that appears to show that the Junior 24 should be adequate at the flat but we are now a bit torn as to whether we stick to our guns?

Any advice gratefully received.


PS - I have used the online calculators suggested and they tend to agree with the Worcester Bosch site calculator.
Sponsored Links
If this is a combination boiler, the size is governed by the hot water flow rate rather than the central heating. Ideally, I suppose the heating output should be able to modulate down below the total output of all the radiators.
I think I understand. So would it be safe to assume that with a simple over bath shower (not electric) [and single bathroom] that the smaller boiler would cope OK? Washing machine is cold fill only if that makes a difference.

At a different time we may simply have goen with the larger one but need to replace the bathroom suite etc so looking to best compromise we can achieve.
Samllest combi will give you adequate shower. more powerful will supply better flowrate (subject to good mains pressure.

I would opt for Vaillant as opposed to WB. I see WB as an installers boiler. Good aftersales but a pig to repair. Vaillant aftersales is shocking
Sponsored Links
The Worcester range start with the 24i junior combi then work there way upto 40kw. Your flat would be approx 10kw for heating only.

The 24 or 28kw combi's are plenty big enough for what you want.
the junior range don't have any controls to adjust the hot water temp at your tap, but that's not normally a problem.

The CDi models are top of the range and cost a lot more.
to determine the size of combi you require use the folloeing method open the kitchen tap and see how long it takes at full fliow to fill a 4 ltr bucket or other known quantity container, calculate into Litres per minuite, this can then be compared to the performance of various size combis i.e 14L/m @35'c rise etc try to select a combi that is capable of heating your max flow rate. probably at least 28Kw! the central heating output is set up using the boilers controls to cap at a maximum out put anything from
5Kw upwards to suit your heating load ;)
need to replace the bathroom suite etc
If it is for you, then consider a thermostatic bath/shower mixer - they help even out boiler temperature variations and get you under the shower quicker.

If it is to rent, then don't bother. ;)

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local