On Sunday, October 30, 2016 at 9:44:05 AM UTC-5, wendy vonlienen wrote:
am putting new floor under gas hot water heater,there is a hole in old floor
under water heater for vent i think?can i cover it up

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To summarize: if it is a vent to draw in combustion air, it is needed if the water heater space is small or airtight or if you don't want to use heated/cooled house air for comubustion. If it is a drain in case of a leak, you may someday wish you still had it. I assume there is some protection from insects or rodents entering the house through this hole.