Tested the DEFA aux heater installation on my BHC car by running it for half an hour.
This was fitted over a year ago but I never got round to working it out last year. Decided to plug it in and get a feel for how it works.
The background : external temperature around 14 degrees. Coolant temperature on 15 when I switched it on (ODB reader plugged in with the ignition key turned to just before starting).
When plugged in, you hear a gentle noise of moving liquid rather like when someone runs a tap in a different part of the house emanating from around the coolant / oil heat exchanger and somewhere near the alternator. The return pipe from the oil heat exchanger is where the heater is plumbed in, and the first signs of something happening is that the pipe above the heater immediately warms up. After about 15 minutes you can feel heat in the pipe returning from the oil heat exchanger to the heater, and by 30 minutes the pipe leading to the base of the coolant expansion tank is physically warm. The pipe immediately above the heater by this stage feels like warm bath water - my guess would have been 40˚C or so, backed up by the reader suggesting it was mid 30s ˚C.
I'm now convinced enough by how it works (not that I shouldn't be given how prevalent these are in colder climes than ours), and am going to get a digital timer so that it can be switched on from the garage power point around an hour before the car is needed as it gets colder over the next few weeks. I can also swap on a spare slightly damaged ridged service panel from the garage that I will modify enabling the socket to be accessed through the fourth Audi ring without leaving the whole service hatch open to the elements - probably insert a grommet or something similar.
Postscript - Monday morning 0550am .. borderline freezing fog conditions, Colour DIS says air temperature 2 degrees, coolant temperature 3 degrees. Remove DEFA from timer as it hasn't switched it on, and plug into normal socket. 0625, air temperature 2 degrees, coolant temperature 22 degrees. Just need to work this clockwork timer socket thing out so that it turns the power on at 0530 as planned instead of having to do it manually.
Edit .. or buy a digital one with a clear user interface and use the clockwork one for something else!