If a system is supplied with a charge for a 15-foot line set, which is longer than the set's length, how much refrigerant will need to be added for a 25-foot line set?

To understand the correct response regarding the additional refrigerant needed for the longer line set, it's essential to recognize how refrigerant capacity is typically affected by line set length.

When a heat pump system is charged for a specific line set length—in this case, 15 feet—it is calibrated to accommodate the refrigerant volume required to maintain efficiency and correct operation for that length. If the actual line set length is extended to 25 feet, additional refrigerant is necessary to compensate for the increased volume of the extended piping.

The standard practice often suggests that for every additional foot of line set beyond the length for which the system was charged, a specific volume of refrigerant must be added. This practice can reflect the increased hydraulic resistance and thermal dynamics introduced by the longer refrigerant path.

Thus, when determining how much additional refrigerant should be added for the extended 25-foot line set, that calculation would generally be based on the extra 10 feet beyond the 15-foot charge, leading to a total adjustment that aligns with the system's design requirements.

In this scenario, the estimated refrigerant volume needed for the 25-foot line set aligns with the specific amount indicated in the correct answer, making it both reasonable and grounded in established HVAC calculations.