The average new construction heat loss average is 18-20 Btu/hr at -10*F. That is even for a "green" tight house. Old houses average 45-50 Btu/hr heat loss. Those numbers are obtained using heat loss/heat gain software.
I understand OLD house = built prior to ~ 1963
It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE
Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities