There should be a blow off valve, but best bleed the rads to let a bit out. Remember if there's air in the system it is likely to expand and contract rather more than liquid so the pressure will vary a lot. Bleeding should reduce that and hence reduce the swing. Mine (different make) conks out if the pressure is too high so I needed to bleed it to fire it up. I guess there should be a blow off valve too but for some reason it doesn't seem to be working