Calculating Size Of Boiler

[FredPuli43]

How is the calculation of boiler size done ? I need a rough idea before calling in any heating contractors. Is it by cubic capacity of rooms? This house is a four bedroom, two being large (about 20 feet by 18) and two ordinary double bedrooms, with a dining room about 16 feet by 12, drawing room 25 feet by 20, office 15 by 15 feet, kitchen 20 by 18 feet and hall. The ceilings are at 8 feet 6. There's a downstairs cloakroom and lavatory but nobody's ever thought to heat that so I'm inclined to exclude it.

[buildersmate]

It's far more important to know something about the level of insulation in all aspects of the property, Fred, as you will know from your physics O-level that heat in a well-insulated device like a thermos doesn't escape very quickly.
I'm talking walls (cavity insulated?), roofs (lots of Celotex?), windows and doors (DG installed?), and to a lesser-extent floors (assume zilch unless built in the last 30 years).
Boilers come in big jumps between size models. If it is detached but reasonably-well insulated, I would guess around a 30kW output model. Better to oversize a bit in case you extend the house later. You heating technician will be able to assess this on site.

[malagabob]

The lengthy method is measure volume of rooms,Allowance for heatloss through windows. subtract windows whether they are single/double/triple glazed. Plus a lot of other factors.What the outside walls are made of. North/South facing
Cavity insulation. Solid/suspended wood floors Thickness of.loft insulation. Loads of things. This will give you output of radiator required for each room. Add in hot water requirement.
A very lot of calculations, but most dont bother these days, If a rad fits under a window that will do attitude.
Any good heating contractor will or should come to the right size of package.If he tells you within 5 minutes of surveying your house call someone else.

[FredPuli43]

BMate, I either failed O level physics or wasn't allowed to take it (I forget which) but I do watch Brian Cox. Is that any good?

The house has cavity wall insulation and layers of loft insulation but the windows don't permit of double glazing; have to make do with secondary glazing because the windows are all leaded lights. The floors are nearly all solid oak strip downstairs and ordinary deal upstairs.

[CoffeeLounge]

https://www.worcester-Bosch.co.uk/specifier/boilers/find-a-boiler

This little q & a helps give you an idea