Problems with Multiple Conditions

So I have been able to create what I need but it does not work the way it should. So Im missing something. In the Attached form Im trying to provide a outcome with two to three different conditions. Based on Construction and Square footage I get a Heat Factor. (See table in pdf) But For some reason I can only get it to work half the time and usually it will only display Tight construction from the drop down even though another type of construction is selected.

Construction drop down values:

Tight =1

Average=2

Loose=3

Heat Factor field Calc script:

event.value = 0;
var nSquareF = this.getField("F72").value;
var nCon = this.getField("D3").value;
if ( (nCon=1) && (nSquareF < 900) ) event.value = 0.41;
else if ( (nCon=1) && (nSquareF >= 901) && (nSquareF <= 1500) ) event.value = 0.31;
else if ( (nCon=1) && (nSquareF >= 1501) && (nSquareF <= 2000) ) event.value = 0.26;
else if ( (nCon=1) && (nSquareF >= 2001) && (nSquareF <= 3000) ) event.value = 0.22;
else if ( (nCon=1) && (nSquareF > 3001) ) event.value = 0.19;

else if ( (nCon=2) && (nSquareF < 900) ) event.value = 0.61;
else if ( (nCon=2) && (nSquareF >= 901) && (nSquareF <= 1500) ) event.value = 0.45;
else if ( (nCon=2) && (nSquareF >= 1501) && (nSquareF <= 2000) ) event.value = 0.38;
else if ( (nCon=2) && (nSquareF >= 2001) && (nSquareF <= 3000) ) event.value = 0.32;
else if ( (nCon=2) && (nSquareF > 3001) ) event.value = 0.28;

else if ( (nCon=3) && (nSquareF < 900) ) event.value = 1.29;
else if ( (nCon=3) && (nSquareF >= 901) && (nSquareF <= 1500) ) event.value = 0.94;
else if ( (nCon=3) && (nSquareF >= 1501) && (nSquareF <= 2000) ) event.value = 0.8;
else if ( (nCon=3) && (nSquareF >= 2001) && (nSquareF <= 3000) ) event.value = 0.66;
else if ( (nCon=3) && (nSquareF > 3001) ) event.value = 0.58;

https://dl.dropboxusercontent.com/u/2944617/00%20work/Load%20Cal.pdf