Identify the source of the diagram
Request the full model, serial number, installed nameplate, applicable manual revision, and the diagram supplied with the actual heater. Check whether the contractor is comparing the correct product and electrical variant.
Provide existing paperwork and accessible photographs. Do not open electrical compartments, probe wires, or infer the supply from wire colors, cable shape, or the number of visible conductors.
Ask the electrician to explain “single phase” in the context of this heater and to document the measured supply, circuit, grounding, protection, and connection requirements. The finding should identify any mismatch clearly, rather than simply saying the wiring is unusual.
Determine what work is needed
If the circuit is suitable, request the evidence and completed-installation verification. If it is unsuitable, ask whether the correction involves the circuit, supply, selected heater, or another specific item.
Do not accept an improvised conversion, bypass, or altered internal wiring as the explanation. Any proposed equipment configuration must have manufacturer support and satisfy applicable electrical requirements.
If there is apparent damage, overheating, or a repeated trip, keep the equipment out of use and report it when arranging help. Ask the qualified provider how to keep it safely isolated without approaching hazards.
Close the handoff
Have the electrician and heater installer identify who corrects the issue and who verifies the final result. Save the approved model information, inspection record where required, and startup findings with the service history.
Your next step is to request a written electrical compatibility finding. Use that finding, rather than the apparent difference in a photograph, to decide the correction.