Not a gret idea. , M% }3 M, R3 D: E2 k. h q % v. F% C" G4 j& zHot water has went through all your home's internal pipes whereas cold water comes "direct" from your local reservoir. Which should be pretty safe. - J4 V. S% k6 ~ _. w+ k5 d( \ 9 c& H# _3 z$ e' KBeside,furnace hot water coil is made of metal. Some of it will come into the hot water.