Hi Jerry I assume that's in case a leak mixes the coolant with the water. Deadly. A few local pets, especially cats, die each year from lapping up puddles under cars. They like the sweet taste, apparently. I remember some years ago there was a great scandal about Austrian vintners(?) having added trace amounts of glycol to their wines which hadn't sweetened sufficiently on the vine. I don't know if my hot water heater is set up that way (H20 squared?), but it's just another particular I never thought to check. I'll certainly do so when I go to the boat tomorrow. Thanks, Garrett