Worcester Bosch has gas!

[Pyewacket]

I have a W-B system installed in 2015 by a certified trader. It’s a neat job and works really well, but…

if the weather is very hot or sunny (as we have this month) the pump stops when the cylinder gets to 72° and the collector then overheats and gas collects in the system which I have to bleed every few days. 
the fluid was changed two years ago but the problem remains. 
Any suggestions please?

[Nickfromwales]

3 hours ago, Pyewacket said:

I have a W-B system installed in 2015 by a certified trader. It’s a neat job and works really well, but…

if the weather is very hot or sunny (as we have this month) the pump stops when the cylinder gets to 72° and the collector then overheats and gas collects in the system which I have to bleed every few days. 
the fluid was changed two years ago but the problem remains. 
Any suggestions please?

Is it a drain back system?