LARGE KITCHEN EXHAUSTS

The basic principle is that for every cfm that is exhausted to outside, the house will naturally re-pressurize bringing in outside air. If the incoming air is cold, the furnace has to re-heat 100% of it. The example below will explain how it works:

The calculation is done in BTU/h first (that is how furnaces are sized) and then converted into kW (since MUA heaters are usually electric).

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 =

91,800 BTU/h

In kilowatts, that converts to...

91,800 BTU/h ÷ 3412 =

27KW

So, when its -25C outside and the inside t-stat is 22C then, 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. As an example, 91,800 BTU/h is enough to heat approximately 4000 sqft house when it is -25C outside.