Addition of Fernox boiler noise reducer to CH header tank - do I need a partial drain down?

Hi experts,
I have what is probably a very simple question - I'm a newbie with looking after central heating systems, being a new homeowner and having grown up in a house with warm air vent heating (these radiator things really are a new experience...).
To set the context, my boiler is on its last legs and received a "Not to Current Standards" (NCS) warning from the gas engineer who recently serviced it, on account of the flue position under an open porch - so I'm the process of getting estimates for a replacement. In the meantime, however, I'd like to do something about its kettling, which has become very bad (the sound is actually more like knocking and banging than kettling). As this is only a short term measure, I don't want to descale and flush the system, because that would get done as part of the new boiler installation. Reducing the temperature on the boiler thermostat (it's now set at about 66 C) has improved but by no means eradicated the problem. Also, I am absolutely sure there is no air in the system (I've bled all the radiators, the radiators themselves are not noisy, and they all heat up pretty evenly all over).
So, instead, what I've done is get hold of some Fernox noise reducer and I've poured that into the expansion tank in my loft, as indicated in the instructions on the bottle (which are really pretty brief). I'm left wondering if this is all I need to do? Having been up in the loft while the CH is running, it appears that nothing is moving up into the expansion tank at all - the water in it is cold (and a little manky, with one or two dead insects in it and cobwebs over the top), and the pipes connecting it to the hot water tank are also cold - stone cold. I checked the valve on the relevant pipework, and it's definitely open all the way.
Now, perhaps this is normal if the system is running nicely and not getting too hot? However, am I to expect that the Fernox additive will get into the main part of the system simply via diffusion along the pipework (as per standard chemistry/thermodynamics) - or should I drain a little from the bottom of the system to draw what's in the expansion tank down into the main part of the system? The instructions on the Fernox didn't say anything about this, but it seems to me that it might be necessary?
I also have two related questions:
1. Should the expansion tank and relevant associated pipework really remain stone cold all the time, even when the CH has been running for a while? Could there be some kind of blockage between the expansion tank and the hot water tank? If so, what would be a good way of investigating and correcting?
2. Should the expansion tank really be uncovered, so that the afore-mentioned gunk and dead insects can end up in it? I appreciate that the purpose of the tank is to allow the sytem to "breathe" in and out as the CH water volume adjusts with its temperature - however, presumably it would be OK to place some kind of non-airtight cover over the tank?
Thanks very much, and apologies if these are very simple questions for which the answers are obvious to most others!
Andy
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Axeman wrote:

No expert.... just plenty of practise recently :/

I'd tie up the ball-cock in the F&E tank, or turn the water mains off, then drain 8-10 litres from the lowest drain point to draw the fernox into the system, release the ball-cock (or restore mains pressure), to refill as needed. Check the rads (bleed them)... but chances are they'll be okay.

Ideally, yes, the F&E should be cold, although not unusual to get some heat in there e.g. if you introduce air into the system. In my experience (this morning) my F&E was cold.

No. The F&E should be covered (and insulated). If you get any CH water drawn up the expansion pipe expect that to bring some of the sludge from the system with it

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Thanks for the advice Mike, I'll drain a few litres from the expansion tank as suggested (I'll scoop out some of the gunk first!). I'll also get the expansion tank covered and lagged.
Thanks,
Andy
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Axeman wrote:

Just to clarify... drain from the lowest drain point (downstairs radiator, or boiler)... NOT from the expansion tank.
Yeah, take some paper towels or J-cloths with you into the attic... its a pretty grim job, but better have the crap out of the system.
Mike
<wonders how long he can last before unscrewing the newly installed magnaclean to see how much crud that has picked up>
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