Water was leaking from the pressure relief valve. Had the customer turn all the thermostats on and checked the system. While the burner and circulators were on, the pressure was maintaining between 10-15 psi. The extrol tank is empty and the pressure relief was dripping. There is also some corrosion on the hi vent. Recommend replacing the pressure relief and hi vent. Drained the pressure from the system. Replaced the pressure relief valve and the hi vent. Filled and purged the boiler. Started and tested. The pressure is staying between 15-20 psi. The system is operating properly