Preparing expansion tank before flushing

I'm about to flush my vented central heating system and I know I have to prepare the expansion tank first, but I'm getting different advice. Some say I should join together the tank's feed and expansion pipes, but others say I should cap off both these pipes. Anyone know which is right?
Any advice much appreciated.

You can link them together temporaily with a valve this way you can flush them out & than turn the valve off to do the rest of the system reinstate after !

Thanks a lot Transam. That's very helpful and a good tip as well.
Cheers
Reg

Commonly, the vent is 22mm and the cold feed to the system is 15mm.

Tell the customer you about to turn the water off, "incase you want to fill the kettle..." Isolate the water at the stopcock, and fit an isolating valve to the cold fill to the tank, then turn the water back on. Tell the customer the water is back on, and you might get a cup of tea.

Using a pipe slice, and not a hacksaw, cut the 15mm cold feed, but catch the water that will run out, even though you have drained the system enough to empty the tank first. As the tank is flexible, and you will move it, some water will spill out. On the tank stub that is left, fit a 15mm speedfit elbow and turn it upwards. When you are ready to clean the tank out, turn it downwards and catch the water.
Connect a 22mm speedfit straight or elbow, to the vent pipe, but check the end first for sharp edges. No need to check the 15mm feed for sharp edges, the cutting wheel will do that.
Reduce the 22mm speedfit down with a 15mm x 22mm speedfit reducer, and connect a 15mm length of either Hep2o or speedfit pipe, using pipe sleaves in the pipe to the reducer and to the 15mm copper feed pipe with a 15mm straight connector. If you have used a hacksaw, take the burrs off the pipe first.
When you fill the system to powerflush it, you will need to vent the pipe in the roof or you will get an airlock and the fill & vent will not be flushed properly.
Have the machine running and check for leaks, so that you can leave the pump running alone, for about 15 minutes. Up in the loft, unscrew the 22mm speedfit fitting, above the tank, and slowly allow the air to escape. Catch the water in the tank and tighten it back up.
Clean the tank out with a sponge and rag.
Reverse the procedure for connecting back up, but you must make sure the vent is left clear and you can simply reconnect the feed with the speedfit fitting. No hot works in the loft.

I used hep in the picture because the feed was hep already, and i could just swap the ends over and exchange the grab rings between fittings.
Speedfit is easy to remove.
Cheers
Mark

That's a fantastic, detailed reply Mark. Thanks very much for taking the trouble to answer so fully (especially the tip about getting a cup of tea!). A great help.
All the best, Reg