Idle RPM increases when heater knob turned to max heating.

I recently noticed when I turn the heater selector to MAX on my 2010 Jetta TDI, I hear the auxiliary heater's click and shortly after warm air would flow from the vents. However, at the same time the engine rpm slowly increases to ~1000 and stays there. It does not matter whether engine is at normal operating temperature or whatever the ambient temperature is.
It never did this before, has anyone experienced the similar symptoms? Checking with VCDS, there was one fault; 01134 ecu intermittent fault 004. The vehicle runs fine with no issues otherwise. Thank you in advance.

The ecu will increase rpms at idle when in high electrical load situations.
What is your battery voltage before and after turning the heater on?
If your car battery is past its prime it may be requiring longer "top off" charge after the car starts and then you activate the aux heater causing a higher demand the alt cannot supply at regular engine idle speed.

The battery is fairly new---5 months old--but it's a good point, I will check the before/after battery voltage to see if there is noticeable difference.

I forgot to add... if the glowplugs are activated at the time the heater is turned on, the electrical system may be on full capacity demand