I have a natural gas water heater on the first floor that is vented up through the roof. I want to add a vented gas fireplace to the first floor. Is it safe to vent it to the water heaters vent?
Some jurisdictions permit sharing of a single chimney flue with multiple appliances or fireplaces, so I would start asking at your local building department. If approved, you have to compare the fireplace manufacturer venting requirements (type of a pipe, min height, size, combustion air requirements) with your current WH vent flue pipe.
Every time you connect more than one appliance into the same chimney flue, you’re increasing your chances for back-drafting, so going with properly installed separate flues (of course more expensive) would be safer and easier to control. Another option is to use a direct vent fireplace (venting through the side wall of your property), or even ventless, but all of the options should be checked first with your local jurisdiction.