I add inhibitor via a rad: take out the top bung (like the air bleed screw but just a blank on other top corner). Insert one end of a 15mm iso valve and gently tighten. On other end of iso valve is small length of copper pipe connected to small length of garden hose and then funnel. Pour inhibitor into funnel which then flows thru hose and past iso valve into rad (make sure iso valve open lol). You may need to open opposite air bleed valve to allow air to escape as inhibitor enters. One can add inhibitor like this at any time - just drain down some water, add inhibitor to a upstairs rad, then refill.
You will have to use a rad that has been part drained
Whole article below if you are bored... copied from another post I wrote...
Sounds like you have a combi boiler and assuming it is... if post a little long I copied and pasted from old post I wrote for somebody. I'm not a CH expert but have fitted a few rads.
Turn boiler off (ideally at fused spur). Do not use boiler at all if no water in CH circuit.
If you can find a drain point: Fix hosepipe to a drain pipe on one of the downstairs rads (or might be a drain off point near boiler) and drain off an amount of water to outside drain or garden (you'll see combi boiler pressure drop from whatever it is, likely between 1 bar and 2 bar... eventually to zero). Keep draining until you have emptied the rads (you will need to open the rad air bleed screws - starting at the top of the house and gradually working down - to allow air in, which in turn allows water out the hosepipe - essentially you are getting rid of a vaccum that would otherwise prevent draining). Take care that you are not just draining a "drop down leg of pipework" - this is okay but don't then go and work somewhere else thinking everywhere is empty.
If no visible drain point then you'll just have to choose a low radiator and close both the TRV (switch it to off or lowest setting) and lock shield valve (this then means that the rad is cut off from the circuit). Then carefully drain the ONE rad only by loosening the connection between the rad and the lockshield valve and letting the water flow into a paint tray or something suitable (open bleed valve on rad to allow air in). Be ready to tighten connection as water fills paint tray as rads hold a lot. When rad is empty you can then think about rest of water in circuit (under pressure still!). Obviously if you now open the lock shield valve or the TRV then water will spurt out... what can you insert or attach to this? For example on the TRV next to my desk now I could insert 15mm pipe (inc. olive and nut from a 15mm iso valve or compression fitting) into the TRV, tighten it up and then attach hose to that with a jubilee clip... then switch on TRV and water will be off and away. If it’s a big thread then a wash machine hose will likely fit with other end connected to some plastic 15mm pipe (or tucked into some waste pipe and fed outside). It’s just about getting the water out and diverting to a drain or garden.
TOP TIP: If a rad is taken out but the flow and return still pressurised then be aware a TRV can open if the temperature in the room drops enough due to frost setting sort of thing..
When re-filling you do this using the combi boiler filling loop (boiler still off). Google some pics to see what one looks like but sounds like you found it already. Stop at 1 bar pressure and then bleed downstairs rads (so water filling from bottom of house upwards)... pressure will drop so top up to 1 bar and bleed again and continue this bleeding all rads inc. upstairs. When no air in all rads pressure up the system to what it should be (likely 1 bar to 2 bar - check your boiler manual). You should add some inhibitor (early in this process - add inhibitor whilst drained down, either via pipework at TRV or by taking bleed valve out and putting in top of rad)..
I add inhibitor via a rad: take out the top bung (like the air bleed screw but just a blank). Insert one end of a 15mm iso valve and gently tighten. On other end of iso valve is small length of copper pipe connected to small length of hose and then funnel. Pour inhibitor into funnel which then flows thru hose and past iso valve into rad (make sure iso valve open lol). You may need to open opposite air bleed valve to allow air to escape as inhibitor enters. One can add inhibitor like this at any time - just drain down some water, add inhibitor to a upstairs rad, then refill.
I am not a CH expert ... I'm sure somebody will chip in if I am wrong or have missed anything.
