# Large Kitchen Exhausts - The Missing Fineprint

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,

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......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.

May 26, 2015
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