It depends on the type of wood and construction of your wood floor. Go educate yourself at these sites:
http://www.radiantdesigninstitute.com/page2.html
http://www.radiantpanelassociation.org/
http://www.hydrothermcan.com/products/design.html
http://www.runtal.com/
http://www.radheat.net/
http://www.radiantheat.net/leaks.htm
Luck, Kizmet