Well we used to have orphanages for such situations. Now they just go to foster care where the government pays through the nose for adults to mistreat children.
Here is a Standard comment we’ve used for 25+ yrs …
FYI - The water heater was the only appliance venting to a chimney or flue originally designed for more than one appliance. This condition is referred to as an “Orphaned Water Heater” and has been known to cause damage to the inside of a chimney. Sometimes lining or resizing a flue is needed when one of the appliances is removed. Verify with the seller that the professional contractor removing the other appliance performed the necessary calculations to determine if this was needed. If the seller does not have this information a competent HVAC contractor or plumbing specialist can evaluate the flue and determine if an upgrade or modification is necessary.