Not a gret idea. / }2 [& [& L& m 5 K% V: U2 p; ~6 `Hot water has went through all your home's internal pipes whereas cold water comes "direct" from your local reservoir. Which should be pretty safe. 1 C5 ]+ `" I. ?$ X " t- O/ o6 A3 t) M7 z9 OBeside,furnace hot water coil is made of metal. Some of it will come into the hot water.