Not a gret idea. 1 Z) E' F, _: y- s& b* x 7 z7 [5 U7 T$ fHot water has went through all your home's internal pipes whereas cold water comes "direct" from your local reservoir. Which should be pretty safe. 9 h4 b8 \! i7 h( w# e9 ~' z1 q3 {- J" u& Z- U9 A* A* L
Beside,furnace hot water coil is made of metal. Some of it will come into the hot water.