Heater problem

[barnaclebill15]

For the past few days the heater is only warm, if i turn both control knobs to max you can hear the fan speed up but only low pressure at the outlets, if I do the flap reset air blows out full.
The system has always been strange, with both on 22C it would be ok, if I wanted more and turned them one click higher the fan would go to high speed, get very hot then slow down and even go to AC cooling and settle down so I did not touch them often but a few days ago it was only slightly blowing tepid air but the fan sounded as if it was on high, in the end I tried the control knobs on full but it is not doing as before.
I feel the heater matrix is not getting hot but I have done no work on the cooling system and the engine temperature is ok 88c.
The only other thing would be if the system is letting too much outside air in to mix with the hot air.
What to try first?

 

[LostKiwi]

Start with what costs least. Flush the heater matrix both ways.

 

[barnaclebill15]

One other thing, the heater does not work at all if auto is off, according to the instruction book it should work in manual but with auto off there is no fan operation

 

[barnaclebill15]

Operator error, the heater does work in manual, I did not have the fan turned up high enough to hear or feel the flow.
Now If I leave in manual turn the fan up to 5 or 6 and turn both temperature controls to max I will get warm air enough to be comfortable, it is not hot but ok for now if I turn to auto the temperature falls so I will run like this until better weather when I can check the system.