I've just installed new hot water pipes and can't get any pressure into them. It starts off with good pressure and then goes to a trickle after a few seconds. The pipes all go up towards the vent pipe with nowhere I can see for an air lock. The cold water feed from the tank is fine with good pressure just the hot being the problem.
Sounds to me like a valve almost closed somewhere. With no flow the pressure builds up but as soon as the flow starts the pressure drop across the restriction limits the flow. Just shows that water in a system acts as if it is cmpressible.
You may have inadvertently allowed some debris into the pipework, try back flushing, ( connecting the hot & cold washing machine oulets together is easiest ) flush away 'till water comes out the loft tank overflow just to be sure, if it works just hope the c*#p doesn't find it's way back in Good luck
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