Central Heating Boiler help

[RandomMess]

How do I work out how many radiators/what house size an existing boiling would comfortably run?

We have a Baxi boiler that is 11 years old installed by the previous owners

www.baximarketing.co.uk/discontinued-literature/heat-only-boilers/5106725-06-User-Solo3-PFL.pdf

We are going to extend and add on 2 extra small rooms, however I'm concerned that previous owner didn't actually use the whole house previously and it may not be suitable. So I don't get ripped off by a heating engineer how do work out what size house it would suitable for?

I have read the instructions aren't am none the wiser!

It's an 18kw output but nowhere on their website does that tell you what house size that would work for...

Am on mobile

Google "Boiler Sizing Baxi" and look for an online calculator.

As a rule of thumb, a single panel radiator 600mm high can use about 1kW per metre length. Measure all your rads and add them together. You may want bigger rads than you already have if your house is not warm.

If you have a combi, you will need 30kW or more to supply a decent hot shower. This will almost invariably be more than you need to heat the house.

No it's not a combi.

I did some online house calculator and it came up with us only needing 14kW

Problem is we've just bought the house and not living in it so we don't know if the current heating is adequate and need to get on with the building work asap

well if the calculator say 14kW needed, and the boiler is 18kW, I see no need to change it. 14kW is quite normal for a modern house that is a bit bigger than average. Provided the boiler is big enough for the house, it is OK to have radiator sizes which added together exceed the capacity of the boiler, because it is seldom that you will be heating all rooms to the max at the same time, unless you have just come home from hols in winter and the whole house has got cold. If you have TRVs they will throttle themselves down as the house warms up, and you will have a more even, gradual heat.

If you improve e.g. loft insulation or add CWI, the amount of heat needed will drop noticeably.

You can carry out the calculation twice; once as it currently is, and again as if the extension has been done and the insulation improved. You can tinker with the input settings to see what gives the best results.

Have a look at the size of the existing radiators and see if they seem big enough. If they are single-panel and reasonably modern, it is fairly easy to swap them for double-panel if you find the rooms are not warm enough.

p.s.

I can see several models but I can't see an 18kW one.