Examined Booster Heater unit, blowing pressure relief when heating. Found lower thermostat overshooting temperature. Recommend replacing both thermostats. Turned lower thermostat down to prevent overheating. Pressure relief should also be replaced. 10/23/20 Replaced upper and lower thermostat, and pressure relief valve. While removing another thermostat, capillary for high limit broke and is not repairable. Unit is working properly otherwise but limit is a safety device and needs to be operational. Created another work order for replacement.