well i used it great sucess, but the label said don't use unless water bed in full. So I lined a box with rubber roofing that I had with the heater under the rubber, I put water in it then bricks to set my seed boxes on, it worked great. The idea is that if the heater can't dissapate enough heat it will burn up (electrically) or shut off. I was thinking of putting under just bricks this year. will be setting it up this weekend. So to answer you question is its hard to say whether that would work or that it would be safe, If you get advice and you have a problem with it and you are not following the directions then there could be a problem. I am not saying that it won't work and I am struggling with it myself. so if anyone has used them dry please let me know. I am pretty sure it has to do with heat transfer and overtemp. but have not seen any data on that.