Heater not working -2000 s6

[Mike2d]

I have a year 2000 Audi s6 in which there is almost no heat from the heating system. Once the engine is up to temperature if you switch on the heater then there is probably about 10 seconds of warm air cooling pretty quickly after that. 
 

One  mechanic looked at it and decided it was a solenoid valve which presumably directs hot water into the heater matrix. Another thought it was a viscous fan issue. 
I suspect it is the valve which has seized up but can’t find a diagram to determine the part to look for  

any suggestions would be very welcome

 

Mike