Upon arrival noticed the pressure relief was dripping even though the boiler pressure was only 20 indicating the pressure relief had gone bad. Drained the pressure from the boiler and removed the old pressure relief. Replaced with new pressure relief and filled the system back up with water. Purged all air from the system. System is working properly at this time. Also found that the hi-vent had been leaking water in the past and is no longer functioning properly. Looked in company notes and found the hi-vent and extrol tank had been recommended to replace at the last visit. Recommend replacing the hi-vent and extrol tank. The unit may not run properly in the future if these are not replaced