Lift your bonnet and check the coolant reservoir. If it has "Mit Silikat" printed on it then this is likely the reason. Certain engines of a certain vintage, mainly the tdi diesels and 2.0 tsi had a permeable bag of silicate granules suspended in the coolant tank, the purpose of this being it supplies a slow release of additional silicate for corrosion protection as the silicate in the G13 coolant get used up with age.

Problem is the bag often disintegrates or splits and empties its entire contents into the cooling system. This forms a fine sand like sludge that blocks primarily the heater matrix and reduces the flow/heat output, can also block the radiator.

Dealers know all about this common problem, there are TPIs issued, but I don't know if VW will honour these outside of warranty.

Visible symptoms are the coolant taking on a murky brown colour and/or the reservoir being coated inside with a residue. The coolant should be a pink or purple colour and rhe fluid clear.

Unfortunately its very difficult if not impossible to flush out taking several flushes, and often not successful as the sludge sets hard in the fine passages in the heater matrix. Needless to say its an expensive fix for a design/quality issue, costing several hundred pounds at a dealer if a new matrix is required.

Later engines the silicate bag was removed, and the coolant changed to G12EVO and a recommendation to change the coolant every x years.

If your car doesn't have "Mit Silikat" printed on the coolant reservoir, then this isn't the problem.