Heater blowing cold

[GBSUPERB]

Hi guy, my heating has started blowing cold air it is slightly warm on passenger side but cold on drivers . I have put a bottle of forte coolant cleaner through the system in case matrix was blocked but still no change carpets seem dry in front so don’t think it’s leaking I have reset the flap motors on the duel climate too . No coolant loss either I am going to back flush the matrix with a garden hose this weekend just incase that helps . Temperature gauge seems ok so don’t think it’s thermostat either.Anyone had similar issues or have any constructive input? Cheers guys 

[Smart Pro]

Check the condition / voltage of your battery and if this is within limits, check the resistor on the HVAC blower motor.