Furnace runs but house does not reach set temperature, long run times with poor heat output

Safety: Gas rate and combustion adjustments must be done by qualified personnel.

• Check controls: Verify that the room thermostat and any Viessmann Vitotrol units are set correctly and not in setback mode.
• Weather compensation: On systems with outdoor sensors, check the heating curve setting on the Vitotronic control. Increase the curve slightly if rooms are consistently cool.
• Bleed and balance: Bleed radiators and ensure all key rooms have adequate flow.
• Professional combustion check: A technician should verify gas rate, clean the burner and heat exchanger, and adjust combustion to Viessmann specifications.