Some old buildings have old floor drains with p-traps and if there is no water in it the sewer gas could come in and make it's way to the vents.

I put a new well in back lot to feed water to all new buildings but since I have recently sold that property along with two other duplexes and retained rights to use water lines I put in for a monthly fee.  

I know to check shutoff valves the water should be turned off.  

“Do not let wet clothing or towels sit for long periods — they can cause black stains on walls or ceilings.”Have tenants sign a Mold Addendum acknowledging that they understand their role in preventing moisture issues and will notify management promptly if they see any black stains or experience leaks.Landlord ResponsibilitiesAddress leaks, water intrusion, or condensation issues immediately — prevention is key.Hire licensed contractors for roof, HVAC, or plumbing repairs.Keep documentation and photos of all tenant complaints and maintenance actions.Maintain proper ventilation and dehumidifiers in basements or areas prone to moisture.If mold is confirmed, notify tenants in writing about the remediation process and timeline.Tenant ResponsibilitiesReport leaks or water damage as soon as possible.Use ventilation fans during and after showers or cooking.Maintain reasonable cleanliness and avoid over-humidifying the space.Follow the instructions in the Mold Addendum and cooperate with access for repairs.Final NoteMold prevention is a shared responsibility between the landlord and tenant.

In the 1930s lead pipes were supplanted by copper as people realized lead was unsafe.The use of lead wasn't officially stopped until 1986 by the Safe Drinking Water Act Amendments.Also, up until 1986, many homes were still using lead-based solder on copper pipes. 

Virgin steel to stand up to Salt water environments.

.  in our area, it is common to pre-lease 9 to 10 months in advance before the academic year begins and for tenants to pay every utility including water .

However, let’s go with my premise that when you first purchase it will be significant cash negative when including all expenses/revenue impacts (piti, cap ex/maintenance, PM (always include pm in underwriting even if self managing because your time has value and circumstances change), vacancy, bookkeeping, portion of asset protection (umbrella, LLC, etc, and miscellaneous (this includes things like unexpected utilities for example from a slab leak, not fair to charge the tenant (I recently had a $2k unexpected water fee for a water leak), legal fees for various consultations, eviction fee, or the recently mandated stair and balcony inspections, etc.  

Do gutters/downspouts move water away effectively?

Also added mailboxes for box units and new water heaters for both units (plus a new electric meter for the other unit and internet hookup).Apt A (two beroom, one 3/4 bath) rents for $2200 and is occupied.Apt B (one bedroom, one full bath) rents for $1450 and is occupied.The problem I'm facing is with the refinance.