best place to go for this advice is the following link to a reputable manufacturer. You will find their info pages helpful and informative. I have installed a significant number of these products without failure.
http://www.floorheating.com/
billybeltemout
October 2009
Automatic Answer
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howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix baths and showers.