New Boiler Size

[thomasar]

Need a new boiler and have had a quotation from a local company who have recommended a 24Kw boiler. From the choices we have taken a Worcester Greenstar 24RI. However, I have a slight concern that this boiler may not be powerful enough as most articles on the web say it for use in a small or medium house and I would describe our house as large.
Our house is detached and is over 4 floors (half floors to be exact) and we have a regular system with a hot water cylinder and a tank in the loft.
The entrance is via steps which is the main floor containing a passage with 1 radiator, a main lounge/diner with 2 radiators and a kitchen with 1 radiator (always turned off). The boiler is in a cupboard in the kitchen.
Above the main floor, we have 2 bedrooms and a shower room containing a toilet and shower but no bath. Each of these rooms has a radiator and the shower room contains the hot water cylinder.
Below the main floor are a bedroom, a bathroom and a room we use as a second lounge. All have radiators with the second lounge having 2.
Below that floor on ground level we have a spare bedroom, a small study and a passage. The passage and spare bedroom have radiators but the one in the second bedroom is always turned off.
In total we have 13 radiators but we only have 11 turned on.
Our current boiler is a Vaillant ecoTEC plus 428 which is being replaced is 28.2KW which is why I am slightly concerned about the replacement being 24KW but the chap who came to give us a quote is obviously knows an awful lot more about boilers than myself.
I'm really looking for re-assurance from boiler installers than 24KW seems reasonable for our house.

 

[SimonG]

24kw sounds on the large size, but without viewing, measuring and calculating it's hard to give an exact answer.

 

[SJB060685]

It is impossible to say what size boiler is required without doing a heat loss calculation. There are several quick guide methods out there but they are only rough guides and completely meaningless if changes to insulation and windows replaced over the years have occurred.
No one on this planet can arrive at the exact output required without running the maths, beit through online calculators, or doing it themselves. I advise you to have a survey carried out, it may very well be the proposed boiler is enough, alternatively it could not be enough, or oversized which can have efficiency problems as a result.

 

[IND_Nick]

24kw would be on the low side for a combination boiler that has to deliver instantaneous hot water (probablywhat you have read online), but for a system boiler or heat only that just provides primary water it is quite substantial. Without doing calculations it is impossible to be accurate but I would say 24Kw for 13 rads and a cylinder would be more than adequate, possibly too big.

 

[ShaunCorbs]

24kw would be on the low side for a combination boiler that has to deliver instantaneous hot water (probablywhat you have read online), but for a system boiler or heat only that just provides primary water it is quite substantial. Without doing calculations it is impossible to be accurate but I would say 24Kw for 13 rads and a cylinder would be more than adequate, possibly too big.

heat only boiler

 

[Timmy D]

I swapped out a 30kw boiler with severely under sized gas pipework (couldn’t economically upgrade it), feeding 5 rads and a cylinder with a 12kw boiler. All works fine.

24kw will be plenty, most boilers installed nowadays are oversized by 50%.

 

[IND_Nick]

heat only boiler

I know

 

[thomasar]

I know

Thanks to all for useful replies.