Builder wants to install an automatic filling valve

I'm coming towards the end of an extension build and the builder has just put in a 2000m x 600mm rad which doesn't seem to be heating up properly. The builder says it is because the rad is at the end of the loop and the pressure on the system is very low.

He wants to add an automatic filling valve and pressure the system off the mains water supply.

Does this sound like a good or bad idea? Are there any other options or additional measures I should insist on?

Why would he need to install an 'automatic' filling valve to the system unless it is leaking and loosing water?!?!??!

If he wants to pressurize an open vented system, just add the expansion vessel and a mains water 'manual' filling loop & gauge and do it, if it is leaking then you will see the pressure drop

With the rad not heating properly, either piped in a too small bore pipe, partially blocked in pipework, rads need a proper balance, or boiler is under sized (have seen them all happen on new builds and retrofits and especially extensions where the 'plumber' taps off an existing rad to feed the new rad and the pipework is unable to supply both)

Turn off all other rads in the house, bet you the bad one heats great then!, then ask him to explain why!

Competely agree with DGOBS on all points,

very likely a half ich supply was tapped into, rising to bad circulation to the last rad on line. Try balancing the other rads to encourage flow to probelm rad. This may treat the symtom.

Thanks for the replies folks. I suspect he may have tapped into a small pipe alright. He's not back until next week so I am going to see how I get on over the next week or so. (He'd already fitted the auto filling valve by the time I got home today at lunchtime but there is a valve where I can disconnect it so I'll effectively use it only when I see the pressure dropping). I'll give balancing a go and see if this helps also.