The best practice industry standard way to come up with a number is to use the Manual J calculation or use a piece of software to do it for you. If the software doesn't ask about orientation, building materials, sqft of doors and windows, etc it's just guessing for you. When I had to install a new furnace I paid for a homeowner version of HVAC-Calc software from www.hvaccomputer.com
. It's $50 but I think it's worth it to get the right size heating system.
Any of the rules of thumb mentioned in this thread are just that, rules of thumb that might get in you in the right ballpark for a given set of conditions. Where I live, trying to heat a 3000 sqft home with two 1500w heaters would be rediculous. And using Nevada's rule of thumb would oversize my furnace by a considerable margin.
As a point of reference, using the software I mentioned, my 1650 sqft house with 2x6 walls, insulating sheathing, about 12" of attic insulation, on an unheated crawl with average ammount of double pane windows and doors has a heat loss of about 70k btu at the design temperature. Before I did the calculation, some online calculators got me close to this number, some were off enough that I would have bought the wrong size furnace.
Installing an undersized furnace will mean you may be cold during extreme weather. Installing an oversized furnace will mean you will get short cycles and probably won't see anything near the advertised efficency. It pays to get it right on this one.
The calculator that I mentioned here:
uses the calculation methods listed in the ASHRAE Fundamentals Volume -- essential the same as the manual J calcs. It does ceilings, walls, windows, ... all with the R values you plug in. IMO there is really no need to pay for a calculation unless you have a very complex situation.
The calcs are not even close to rocket science -- three simple equations covers it all:
The first is for heat loss through anything that can be characterized with an R value (walls, ceilings, most floors, windows) -- this is
Loss = (Area)(Temperature difference)/(Rvalue)
The 2nd is air infiltration:
Loss = (Temperature Difference)(Volume of house)(Air Changes/hour)(Density of air)(Specific heat of air)
There is another simple equation for losses out of concrete slabs.
The calculator takes your inputs for areas and R values and gives you a heat loss. If you put in a value for the number of heating degree days for your area, it will also give you a rough estimate of total yearly heat heat loss, what your fuel bill will be based on the fuel you use, and green house gas emissions.
I've had the calculator up for more than a year, and have asked people to report anything that looks like an error. I've made a couple small changes to reflect comments, but nothing significant -- it gets a couple hundred uses a day, so I think I would have heard about any serious errors.
All of these methods, including the Manual J and ASHRAE calcs are just rough engineering approximations of real world heat transfer, and can easily be wrong by a fair margin for reasons listed with the calculator, but luckily you don't have to be right on.
I do certainly agree that a calculator that does not know how much insulation you have, or how good your windows are could be subject to huge errors.
There is a program called HEED that does an actual hour by hour simulation for the whole year using hour by hour weather from a file for your area. I think this will give a better picture of what goes on, and will probably be more accurate. It includes some additional things, like solar gain, but is still subject some errors. Its a free download, and is easy to use (it was written for use by CA utility customers).
Its listed here with some other helpful software:
The calculator and the HEED program are are great ways to see which kinds of insulation, window, or sealing programs it might pay you to do, and how much.