Combi Boiler Draining to re-pressurize the expansion tank/PV

[dignity473]

I inherited in my house a 10-year old Worcester 28CDi combi boiler - it's a sealed system - which had been standing idle for about 6 months. When it's operating all the right lights and sounds appear. However, the pressure in the CH system when running, as per the gauge, raises very slowly and to about double the starting pressure and with the system 'cold' and at 1.5bars it starts to vent (through the PRV I assume) at between 2.5 and 3.0 bars.

I have bled some water from an upstairs radiator to lower the cold start pressure to about 0.5 bar; max pressure when running the CH is then about 1.5 bars. Pressure increase is again very slow. To begin with, when topping up the water in the system it filled very quickly.

Elsewhere on this excellent site I have read that the symptoms above point to the PV requiring re-pressurising and that the CH system must be drained so that system pressure is 0 before starting this task. Then plug in an air pump to the PV and rack it up to about 0.7bars/10psi

Questions are:

1. how do I drain the CH system to reduce the pressure to zero and if I must open a stop-cock/drain valve is it on the bolier or elsewhere?
2. Is this task achievable by a handy but un-knowledgeable-in-plumbing matters novice?
3. Is this task achievable DIY on this Worcester combi-boiler?
4. Any spare parts/washers etc required?
5. Is the diagnosis correct?

Many thanks in anticipation ...............

 

[dcawkwell]

Open a drain cock on the central heating piping and drain down.
Pressurise the expansion vessel to about 1bar.
Refill system.
Bleed radiators.
(I always switch the gas off and start the boiler up on central heating with the gas off to give the boiler chance to bleed any air out, boiler will lock out but you can reset this)
Sorted.

 

[dignity473]

Many thanks for this advice - I will follow it!

Will all the rads need bleeding btw?

 

[Robiow70]

Many thanks for this advice - I will follow it!

Will all the rads need bleeding btw?

Yes you will need to bleed all the rads and don't forget to replace the inhibator.

 

[dignity473]

Thanks again - I assume that the inhibitor is the gunk that stops the system rusting up or some such?