Audi Repair: Audi Climate Control, climate control, what this means

Question
Hi - I have a 98 Avant A4 (1800). The heating has just stopped working (at the coldest time of year in the UK). What this means, in effect is that I can get air to blow through any of the vents, but the air is cold, and doesn't warm up at all, even on the HI setting
I suspect I'm going to have to replace the unit, but was wondering if anyone out there has had similar problems, and if there's any way I can check first, before splashing the cash!

With many thanks

Dick

Answer
Hi, typically the valve in the heater pipe gets stuck.  What I would do is have another person operate the heater control from lo to hi, and then watch if the valve in the heater pipe, at the back of the engine before the pipe goes into the firewall.  there are two 3/4-1" pipes next to each other, one has a mechanism in it which is operated by vacuum.  If it does not move, then a little persuasion sometimes helps but be careful, there are a lot of plastic expensive parts...  Also check the temperature of the pipes, they should both be hot when the engine is hot.  Let me know,  Jan