Very slight pressure loss

[dan_the_plumber]

Micro bubbles my friend 🙂

 

[DAVE01]

Gas safe registered (stupid spell check)

All system Boilers take a while to settle because of air so on and so forth what can happen is on a old boiler there could be a leek on heat exchanger and this would not be visible to you because when boiler fires it is then comeing of as a mist and this dryies up with heat this happens a lot .On some HE boilers because most of the parts that you use to drain down boiler to replace a part are Plastic so that's why most of us enginears drain down through PRV only problem is dirt gets under seating of PRV this can cause a leek so easey fix is replace PRV so I think you are worrying over nothing worse case is down on pressure boiler will not work fill it back up to 1.5 bar jobs a good one