Google for "Heat Loss Calculation" will find more than a few ways to 
calculate the heat gain/loss. Here's one link (I didn't look beyond the 
front page) http://www.heatload.com/.

The calculations are based on basic Thermodynamics with input of 
temerature difference, area, and "R" value. The R values are from 
handbooks, manufacturer's data, etc. The calc can be done by hand and 
generally for a moderate sized house will fill one page of paper. 
Spreadsheet can make better since can do "what if". Depending on the 
detail (just to size equipment, or individual room heating wire, pipe, 
duct sizes, etc) might be much less.

Rich wrote:
> Basically, you are correct.  The rapid and extreme temperature changes 
> introduced at the windows will cause excessive cycling of the thermostat 
> unless the windows are heated.  Also, the moisture from the warm room air 
> will condense on the cold windows and drip down onto the frame.  This 
> condensation is reduced if the windows are kept warmer.  Some homes have 
> "storm windows" to keep the heat of the inside (room) window from 
> transferring to the outside air.  The airspace between the windows acts as 
> an integrator.  This also helps the heater under the inner (room) window to 
> be more effective.  There are HVAC books that have some rules to work out 
> heat (Q) for a given room size, configuration and number of windows and 
> location of heaters.  I have not seen any hard mathematics regarding this, 
> although there are some formulas for calculating the heat transfer, energy 
> consumption and stability factors.
>
>
> ----- Original Message ----- 
> From: "Vasile Surducan" <piclist9@gmail.com>
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
> Sent: Tuesday, May 22, 2007 11:58 AM
> Subject: Re: [EE]:: Why are radiators usually installed below windows.
>
>
>   
>> On 5/22/07, Russell McMahon <rusl@paradise.net.nz> wrote:
>>     
>>> A friend asks:
>>>
>>> Industry practice is to locate heating radiators (most often fed via a
>>> reticulated hot water system) under windows.
>>>
>>> Nobody can tell me why this is done and to my mind it would perhaps be
>>> the
>>> worst place from an energy efficiency point of view although there may
>>> be
>>> other reasons (like reducing convective draughts within the room  -
>>> which
>>> can reduce the apparent temperature by several degrees) why it is a
>>> good
>>> idea.
>>>
>>> I would hence appreciate your commentary as to what you would expect
>>> to be
>>> the optimum location for a radiator and why.
>>>
>>> Any thoughts?
>>>       
>> Before the heat proof window pan era, locating heating radiators below
>> windows was a must because it generate a courtain against heat lost on
>> widows surface.
>> Such heat air courtains are often used today for very large open doors
>> (no windows, just the open door).
>> Now this perspective has changed as long there are available windows
>> systems with 3 or 5 glasses, with less thermal loss than the wall
>> itself.
>>
>> Vasile
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