No heat!
 

So the weather is changing finally, and I was tempted to up the temperature in my car a few days ago. I was shocked to discover that regardless of how high I cranked the temperature, no heat!

The A/C works fabulously and has no issue, whatsoever. Any ideas as to what could be causing the heating issue?

No heat = check thermostat and then heater valve block to start
 

Of course also make sure motor thermostat isn't stuck open. If it is, it would take a long time until heater warms, but it finally should. If it does eventually warm--best NOT to run A/C while checking so fan only kicks on with high water temp--its probably the thermostat, which is buried under the timing belt and tends to force that job, assuming port injection (pre FSI) 4.2.

Assuming it isn't the thermostat, pull the plenum cover to the passenger side of the HVAC air cleaner. In there fairly close to the heater housing is the heater valve block that switches water flow to the heater cores. It's electrically driven, not the old cable stuff of yore. It has a number of wire connectors and several hoses attached to it, plus two fairly prominent round solenoids on the upper side of it. Once car is warmed up and you have heater switched to high on both sides, feel those hoses--one set is "in" and one is "out," but they aren't particularly marked. Doesn't really matter since with heater switched on and motor warmed up, water should be passing through there and all hoses very warm/hot to touch. If not, there is your issue. A very expensive part new; possible to disassemble (really carefully) if something is gummed up or solenoids just stuck). Do not hook 12V power to it or you will fry the solenoids--they are pulse width type.

Thanks, MP, for the feedback. I have a 2007, so no timing belt, thank heavens.

"its probably the thermostat, which is buried under the timing belt and tends to force that job, assuming port injection (pre FSI) 4.2."

So being that my engine is FSI (no timing belt), is the thermostat in the same location?

If it is thermostat your engine is running cold. Gauge would indicate that.

Thermostat is on the water pump, easy to access.

I was waiting for your sagacious input, Mishar! Thanks for chiming in.

Engine is definitely not running cold. What else could it be?

As MP said, next would be the valve block.

I suspect the valve block too, around 1500€ here as a new part, ridiculously expensive. Can be diagnosed as said by the means MP described.

I had a similar issue on my W12 2006 model. Tough, the problem was, that there wasn't sufficent heat at under 10ºc, especially on the passenger side, AND the A/C wasn't working at full power, so hot in the summer after the temp reached 25-30ºc.

The A/C issue was solved by blocking the hose to the heater core, then there was enough cold, but the problem with winter heating i never solved.

I think i had a problem with the temperature control flap inside the HVAC unit under dash, tough the measured values from the driver motor were OK. I sold the car before i solved the problem, i put countless hours dissassembling almost everything connected to the HVAC.