Responded to no-heat call for two 11-year-old Reznor unit heaters in wash bays. Diagnosed and replaced failed thermostat on larger unit and discovered concentric vent intake issue causing exhaust recirculation; temporarily disconnected intake to allow unit to draw room air. For smaller unit, cleaned flame sensor which restored operation. Noted larger unit's fan motor and control board showing significant deterioration. Scheduled follow-up with service advisor to evaluate complete replacement options for both aging units.
