Draining / re-filling CH system need to replace pump

Hi
I have a oil fired boiler system where the Wilo gold 50 pump is knocking but still working, property is a bungalow with 10 rads and a hot water tank.
The two shut of valves at the pump are screw type and are solid tried to move them but wont budge been there 20 years as has the pump.
Looks like I will have to drain some water out to save a flood from above when I loosen the isolating valves, I can see two drain off valves in the airing cupboard just not sure which is the correct one to use to drop the level of water, one maybe for the hot water cylinder as looks connected to pipe going into the cylinder at the base.
Is there any way of loosening these isolation valves ? only wish they installed better quality valves years back but I am stuck with the problem now, any advice on either freeing the isolating valves or draining the system.
Thanks.

DELTA16 said:

I can see two drain off valves in the airing cupboard just not sure which is the correct one

Click to expand...

Photos would be good.

Hi
These are the two drain valves the one to the right side of the cylinder is always cold regardless of heating CH or water.

The one on the left of the cylinder gets hot.

Hope these photo's help. Thanks.

Assuming it's a gravity fed system (header tank in loft), bung the tank outlet & the vent pipe open the drain cock to release a few cupfuls of water until a vacuum forms in the system, then change pump and valves without draining.

Hi
There's no header tank only a large water tank about a metre square with a ball cock, I think a header tank are small if I recall and there's nothing like that in the loft.