Gas central heating

[IDontKnowWhereMyMedalsAre]

Does anyone know how I work out what size boiler I would need for 7 radiators over 6 rooms. The central heating chap quoted a 30kw but for some reason I'm not sure I beleive him, possibly because he doesnt think tanking the cellars would be worth it. Although its 2 12x12 ft rooms already with plus 7 foot head height so no digging out needed. Ideally I would like a large enough boiler incase I do choose to tank the cellars and then add two more radiators.

The size of boiler you need is determined by the heat requirements of the size of the rooms you need to heat and the number of bathrooms/showers you have. Combi boilers are good for heating smaller properties - eg 2/3 bed properties with one bathroom and maybe an additional shower. Any more than that and a condensing boiler may be needed.
To calculate the heating requirements of a room you need to calculate its size in cubic metres ie heightxwidthxlength. Then take into consideration the wall construction, size of windows, position of windows ie north/south and whether or not windows are double glazed, or pvc/wood.
There are lots of tools online to help with this calculation.
Once calculated, you will be given the Kw heat output that is needed to heat each room - this will allow you to determine what type of radiator that is needed to produce that heat output. Depending on what size of radiators are needed - ie the BTU/kw! Combined with how many bathrooms etc will determine how large your boiler needs to be!
Disclaimer - I'm not a plumber/ gas engineer but I'm about to put gas CH/ boiler into my property so I have been reading up on it!
Hope that helps you initially.

a 30kW boiler is almost certainly more than is needed to heat the rads - it is enough for a 5-bedroom detatched Georgian house.

however

if it is a combi boiler, it will be sized to give adequate hor flow to run a good instant shower and a tolerable instant flow to fill a bath, and 30kW is very common for that purpose. The fact that it needs less power to run the rads just means that it will modulate itself down to provide no more heat than the radiators can dissipate. As a very very rough rule of thumb, 1 square metre of radiator surface can use 1kW of heat.

If you have a hot water cylinder or a Megaflow you don't need 30kW. However maybe the brand of boiler he wants to sell you has that as the smallest boiler he can get with a special offer to maximise his profits.

Worcester Bosch and Vaillant are two good-quality mass-market brands and a good bet for most buyers.

Atmos, Broag and Viessman are less common but very good. There are various others.

there is a calculator here though on my quick sample it gives an undersized rad to my mind, thinking of rooms in my house. I would cheerfully use 50% more than it says to allow for bitter cold and/or fast heating of a house that has had a few days to get very cold. The radiator output tables usually show the maximum output you could possibly get in ideal conditions, with a very cold room and a very hot radiator.