Before buying a furnace, take some time to evaluate the insulation and air-sealing of your home. Insulating and sealing improvements should be done before installing any heating system. Otherwise you will end up with a grossly oversized heating system, that will not perform well and will cost you more.
It’s true that oversized multi-stage condensing furnaces are not a big issue. Their design allows them to be efficient even when oversized. But you haven’t any advantage in oversizing your heating system, and it will cost you more.
A properly sized furnace will run continually and smoothly during the colder winter days. Unless you have a multi-stage furnace, a grossly oversized unit will run only for a short period, without coming up to peak efficiency, and causing higher energy bills and temperature swings and discomfort. Ideally you should seal and insulate your home to very high levels, to ensure that heat loss is kept to a minimum, and to allow you to have a smaller furnace system (or no central furnace at all).
And consider also, with care, possible leaks and lack of insulation in your ductwork, as well as their size and the venting system.
Furnace ducts are often undersized, which is a cause of airflow problems, and one of the major reasons for low energy-performance.
That should be fixed, and you may have to replace your existing duct system. Note however that installing a smaller furnace, after sealing and insulation improvements, can solve the problem of undersized ducts...
Upgrading involves often a smaller furnace, with a smaller volume of combustion gases - that are also cooler. And that requires adaptations in the venting system (when upgrading to a 90%+ AFUE furnace) or the re-lining of the existing chimney, to avoid malfunction of the furnace and acidic condensation and deterioration of the chimney (when upgrading to a 80%+ AFUE furnace).
New and high efficient furnaces have their own venting system: they use plastic pipes for both venting the combustion products to the outdoors and to draw combustion air from the outside. They don’t use the traditional vertical chimney system. But that can also raise a problem: the existing chimney becomes too large for the gas water heater; and in most cases you will also have to install a smaller chimney liner, or to redesign the chimney.
Unfortunately, homeowners and contractors often ignore and under-estimate the importance of these modifications, in part due to their costs. But that has huge implications: heat loss, malfunction of the furnace and other problems. Furnace capacity
The heating capacity of a furnace is measured in thousands of BTU (British Thermal Units), and is in close relationship with the AFUE of the equipment, that is, its efficiency: a furnace with an input BTU of 100,000 BTU and a 80% efficiency/AFUE will produce 80,000 BTU of heat (the BTU output), while a furnace with the same 100,000 input BTU and a 95% AFUE will produce a higher heat output: 95,000 BTU.
Good and bad Sizing methods and Tips
Sizing a furnace is a job for a professional contractor (some utilities may provide that service free of cost) and should consider:
1) the local climate,
2) the sun-orientation of the building;
3) its shape and size;
4) its insulation and sealing levels (walls, attic, ceiling);
5) the quality and type of windows and their glass;
6) home appliances...
Typically, contractors use computer software to get the right size, and not just general tips. In North America it's common to use the Manual J, "Residential Load Calculation" from the ACCA (Air Conditioning Contractors of America).
Simple rule of thumbs, based on the home’s size or the area of the windows can provide first estimates, but just that.