mcurtiz

Problem is now solved FWiW! I decided to flush the heater core (only) to see if there was any blockage - theory being that as some heat could be felt on the passenger side but none on the drivers side, suggested to me a partially blocked heater core. It's not a single capillary path in the core but a classic radiator style matrix. Method: I disconnected the hoses to the heater core in the scuttle area between the fire wall and bulkhead. It was a PITA but I got the hoses off. I connected some garden hose (softened to get it to expand from the 1/2" bore to the c.25mm pipes of the heater core). I hooked it up to mains water and ran water through the core in both directions (i.e. reversed the flow). There was what can best be described as sand-like sediment but not a lot. Reconnected the hoses, topped up the coolant, and ran the engine for c.15 mins, with the heat and blower at maximum. Heat was now coming through all vents. When the weather improves (or the fault returns!) I will do a full additive-based system flush then refill with the recommended coolant.

@cliffcoggin - not sure who mentioned silica gel but no confusion on my part. If you are referring to my mention of silicate bags and reservoirs, they are used in various VAG models - small bags or reservoirs of silicate (not silica gel) in the expansion tank. There are reports that describe the bags decomposing or rupturing and the silicate distributing around the cooling system and so the theory is that this blocks the heater core due to its inherently narrow channels.

I've been reading up... Silicate has always been needed since aluminium-based parts were introduced c.1996, to prevent aluminium surface corrosion (such in as the rads / heater matrix!). It was introduced post-cast iron blocks and brass rads etc. I also have an A3, a 2011, that also does not appear to have the bag -at least it does not have 'Mit Silicat' on the expansion tank and uses G12 (pink or red) coolant. I do wonder if the bag or reservoir is there, just the marking is missing. I say this because G12 does not contain silicates apparently hence supplemented silicate would logically be needed. FWIW - G12 is made of 5% distilled water, ethylene glycol alcohol, 2+ carboxylate compounds and a colourant. G12 life is 4-5 years / 80k miles. G12 freezes at -50c, boils at 118c. G12+ (red) and G12++ (purple, c.2008) are also available and are long-life organic coolants and do contain silicates combined with carboxylate to overcome the weakness of G12 that only works when corrosion has appeared. You shouldn't mix G12 with G12+ (and def not with G11) or even G13, etc., so full flush out is needed if using different to what is in the system. Everything I read recommends sticking with the recommended coolant and not to mix - although moving from the recommended G12 to G13 on a full system flush basis is generally acceptable but usual caveats apply (check first etc). I also have 2015 Golf that does have 'Mit Silicat' (so will be swapping its expansion tank as a precaution!) using G13 coolant. The main difference in G13 is Propylene Glycol (less harmful to the environment when disposed but also in its production) and like G12+/++, contains silicate. G13 came into force c.2012, freezes at -69c, boils at 175c and has a similar life span to G12. A change of the coolant colour from its original (bright orange, purple, red, pink) indicates it is losing its protective properties. With all that said, having gone down a rabbit hole on coolant and silicate bags and what-not, I am still no closer to knowing why my A4 does not have heat on the driver's side!

My tank does not have the "Mit Silicat" (German for "With SIlicate"). Apparently, the round spherical tanks have this marking if they have the silicate bag. But, different shape tanks 'might' have a silicate reservoir (so no bag) with a labyrinth to prevent premature Silicate dispersal but there does not appear (yet) to be a list of which models have a bag or which have a reservoir -but it seems ALL models have silicate as this prevents premature corrosion of aluminium surfaces/parts. This suggests its not the silicate that is the problem as that is needed, but the bag. Once burst, logically it goes with the flow and chokes the parts with the narrowest pathways - the heater matrix!

I would never have believed that VAG would do such a crazy thing - just viewed a really useful YouTube vid on this. I suspect this maybe too late for me though...

I think have this same fault or similar, so interested to see suggestions/fixes. What I've done so far: I replaced the A/C pump and A/C rad as I suspected them because last summer I had no cold air -only hot(!), and had a really noisy AC pump! This fixed the noise and A/C cold but no heat! Now, fault code said RH side flap motor had unfeasible response so replaced with a new part, recalibrated the flaps, but still didn't work so put the old motor back in to rebase my investigation and damn it, it now worked (=dodgy connection?) and I could see the flap opening/closing - but it didn't fix the symptom of zero heat on the drivers side -and low heat on passenger side. Fault must be deeper inside as there is no heat coming from the central duct i.e. before it even gets to the driver's side footwell.