Boiler Size ?

[woodchopper]

How do you calculate the size of boiler required for a house?
The current standard oil fired boiler is a 50/70 Supaflame which is due for replacement. The house was originally heated by a back boiler on the fire in the living room but this was changed sometime before we bought the house, and further to this the upstairs was added including another 4 radiators.
I'm not sure of the time sequence whether the boiler size was calculated before the upstairs was added.
Downstairs has 5 rooms in total inc. bathroom all have single convectors fitted except living room. Upstairs has 4 rooms inc. small bathroom all fitted with single convector.
It takes at least 4 hrs to bring the living room up to temperature in this weather. That is why I have my doubts the boiler size is adequate.

[bhg481]

Try here:

http://www.idhee.org.uk/calculator.html