Check the complete installation
Send the exact model and photographs of the proposed space. Include the ceiling, nearby wall, shelves, pipes, doors, and anything that would remain in front of the heater. A dimension measured before piping or trim is added may not represent the finished access.
Ask which instructions govern the filter, fan, motor, electrical compartments, and other service components. Request an explanation of how a technician would remove the relevant covers and parts. Do not remove service panels yourself to demonstrate that they fit.
The useful question is not simply whether a hand can reach the filter. It is whether the installed arrangement permits the manufacturer's required work without damaging the heater or surrounding home.
Make restrictions visible in the quote
If a removable wall panel or access door is proposed, ask who supplies it and how it would be removed. Have the installer identify any piping, duct connection, or building work that could obstruct service. Ask which qualified provider would handle that work and whether its cost is included.
Request written confirmation that the final arrangement meets the selected model's instructions. If service would require moving the heater, ask why that layout is recommended and what a reasonable alternative would involve. Do not accept an undocumented assurance that a future technician will figure it out.
For an existing installation, send the site photos to a provider who services that model before building permanent cabinets around it. Your next step is a model-specific access review, followed by a layout that supports both everyday care and future repairs.