The basic principal is that for every cfm that is exhausted outside.......the house will naturally re-pressurize bringing in outside air. If that air is cold then the furnace has to re-heat 100% of that exhausted air. I find the below method the easiest to understand......calculate it in KW's first then transfer to Btu/h which is how furnaces are sized. The other reason its in KW is that Makeup Air heaters are usually small electric heaters thus sized in KW.
Temperature Rise Calculation
Equipment Load = 1.08 * TempRise F * CFM
(TempRise = the difference between the inside temperature and the outside temp)
If it's -13F (-25C) outside and it's 72F (22C) inside then the Temperature Rise is
72 - (-13) = 85 degrees F,
and Equipment Load equals to....
1.08 * 85F * 1000cfm =
In kilowatts, that converts to...
91,800 BTU/h ÷ 3412 =
So, when its -25C outside and the inside t-stat is 22C......the 1000cfm of exhaust requires 91,800 BTU/h to heat up. This doesn't include the actual heat loss of the house when it's that cold. To put 91,800 BTU/h of heat that is enough heat to heat about a 4000sqft house when its -25C. The formulas can be sliced and diced into full metric or full imperial.....but I find the above the easiest to understand.