Adding corrosion inhibitor to a heating system

Hi,

I'm planning on replacing a radiator in a flat - which has a sealed combi-boiler system. Also, moving the rad along the wall at the same time.

My thought is to put the new rad in place and pipe up to the old pipes - then freeze them and attach the new pipes.

However, being a sealed combi-boiler system, and one with only 4 rads, I'm thinking I'm going to have to add extra inhibitor as the current amount will become more dilute when the system is topped up to fill the new rad and pipes.

The questions I have are:

1. Are the inhibitors generic enough so that the chemical I add should match that already in there? Or could they be incompatable which would require draining the whole lot?

2. What would be the best way to tell if I've added enough?

Thanks

Your best bet is to drain down, fit new rad and pipes, then before you refill take the rad off the wall and turn it upside down pour the inhibitor into the valve (use a small funnel). Close the valves and refit rad. Or you could buy the tubed inhibitor and fill through the bleed nipple. You can buy a testing kit but 1L should easily be enough for the size of your system.

Thanks. As it's a sealed system, there's no obvious drain point in the circuit (that I can find anyway). What would you recommend would be the best way to drain the system? It's all on one floor.

Cheers