A wall-hung combi looks like the easy part of a boiler upgrade. The box is smaller than a floor-standing unit, it keeps the basement floor clear, and the crew often has it hung in an afternoon. The trouble starts when the wall behind the unit was never built to carry it, or when the service side has quietly been lost to a water heater, a shelf, or a stack of paint cans. Before the new unit arrives, walk the mounting wall with the same care you give the electrical panel.
Start with what the manufacturer expects that wall to hold. The Navien NCB700 story at ACHR News describes the unit as "providing heating and hot water performance in a single wall-hung unit." The listed figures include space-heating inputs of up to 150,000 Btuh and domestic hot water inputs of up to 210,000 Btuh, with hot water flow of up to 5.4 gallons per minute at a 70°F temperature rise. Those numbers drive the gas line, the venting, and the condensate plan. A 96% AFUE figure in a headline tells you how efficiently the burner converts fuel, though, not whether the wall can carry the cabinet.
Check the Wall Before the Box Arrives
Hanging weight is the first question, and the answer depends on the wall, not the catalog. Find the studs with a stud finder, then confirm them with a small pilot hole. Plaster over lath, hollow block, and thin paneling each behave differently under a load that will sit there for the life of the unit. If the wall is a single layer of drywall over an open cavity, plan for backing or blocking between studs. Ask the installer to confirm that the bracket spans two or more studs, and ask whether the bracket is the manufacturer's part or an aftermarket substitute. The install manual sets the bracket and fastener specification, and that document, not a rule of thumb, decides what the wall needs.
Clearances Are the Service Route
Manuals list clearances to combustibles and minimum service space because someone has to reach the valves, the gas connection, the vent connection, and the condensate trap after the install is finished. Measure from the unit footprint to the water heater, the washer, the shelf, and any return air path. Then stand where a technician would kneel and check whether the drain line and the isolation valves can be reached without moving furniture. A unit that goes on the wall but cannot be serviced without a full room reset is a unit that will get serviced late.
Count the Electrical and Venting Path
A wall-hung combi still needs a properly wired circuit, a disconnect where code calls for one, and a vent route the installer can justify on paper. Use a multimeter to confirm the existing receptacle is hot, correctly polarized, and grounded before anyone runs the new feed. Measure voltage at the receptacle with the unit off, then check continuity on the ground path with the breaker off. Trace the vent route through the attic or chase and note every elbow, since each one changes what the vent run can handle. Ask the installer to write the vent length and elbow count into the job paperwork. These checks are not glamorous, but they are where an install stops being a bracket on a wall and becomes a working system.
Keep the Paperwork With the Wall
After the install, keep the mounting specification, the clearance sheet, and the vent layout in the same folder as the equipment record. If a repair later means taking the unit off the wall, the next technician needs to know how it was hung and what it sits on. Some homeowners look into coverage for home systems and appliances to plan for unexpected repair costs, and the policy terms matter more than any sales language attached to them. Read the exclusions, confirm what counts as a covered repair, and file the policy with the rest of the equipment record.