What should be replaced along with a failed water heater in a Bushwick pre-war multi-family building?

The problem

A property owner managing a four-unit pre-war tenement building in Bushwick called us after the building's water heater failed overnight in January. The existing unit was an original direct-vent gas water heater that had been in service far beyond its expected lifespan. With four units relying on a single heater and temperatures dropping, restoration of hot water was urgent. Beyond the failed unit itself, the original installation had no expansion tank and no circulator, just a direct connection to the building supply, which had been creating hammering in the pipes for years. The original cold supply connected directly to the water heater inlet with no expansion tank. In a closed system with a pressure-reducing valve on the building main, thermal expansion had nowhere to go. That explained the pipe hammer the tenants had reported for years.

What we did

We removed the failed water heater and assessed the existing piping configuration. Rather than a straight swap. We fabricated a new manifold assembly on the bench using copper and brass fittings, incorporating isolation valves on inlet and outlet, a properly sized diaphragm expansion tank on the cold supply side, and a bronze circulator pump for hot water distribution to the upper floors. The new water heater was set, vented, and connected to the manifold. We flushed the system, verified gas pressure and combustion, set the thermostat, and confirmed hot water delivery at all four units before leaving the job. The owner reported no pipe hammering after the new system was commissioned. Expansion tank was a Watts PLT-5, sized for the system volume and the 65 PSI supply pressure we measured at the cold inlet. Circulator was a Grundfos UP15-42F bronze body pump, appropriate for a domestic potable water system.