I bought one as a spare to keep in the cupboard for my Thermo-top setup which I hadn't used and then bought another to fix a job on a pals unit which had a faulty pump.
Upon trying the 1st Chinese one on his kit, it fired pretty much straight away but put no heat out, as if it was only running on 'tickover', tried my spare which gave same symptoms.
Tried them both on my 'known' working setup, same again, running but not enough to give out any heat.!
Ended up buying a used genuine Webasto one off ebay to fix his fault, worked as should straight away
I presume they don't deliver enough fuel per pump to get the unit firing on full pelt but haven't / cant be arsed getting all technical and measuring their output against a genuine one.
Good luck!