If the operating vacuum is significantly less than the calculated vacuum, what is likely present?

When the operating vacuum is significantly less than the calculated vacuum, this often indicates that there is an air leak in the suction line. The vacuum in a system is created by the pressure difference between the atmospheric pressure and the pressure within the suction line. If there is a leak, outside air can enter the system, leading to a decrease in the effective vacuum. This condition can compromise the efficiency of the system, affect fuel delivery, and lead to various performance issues.

In contrast, a clogged filter would typically cause an increase in the vacuum reading because it restricts flow, while oil expansion is not a direct cause of reduced vacuum. Excessive radiant heat is more likely to be associated with temperature control issues instead of impacting the vacuum directly. Hence, the presence of an air leak in the suction line is the primary reason for the discrepancy in operating vacuum readings. Understanding this principle is key in diagnosing and maintaining heating oil systems effectively.