System cleaner - worth it 2 days before drain down? Dose through magnetic filter?

[Oxbow16]

Hi

The onslaught of plumbing questions continues!  Sorry folks...  The plumber is coming on Monday and will be draining down to do the bathroom etc.  I was considering whacking in some system cleaner before he comes, but would need to get some tomorrow.  

 

1. Is it worth using for such a short time (less than 48 hours)?
2. If so, which one would people recommend for that timescale? 
3. I think it's Fernox who say the system should be drained and refilled BEFORE using the cleaner.  Is that generally true for all cleaners?  I won't be able to do that and can only add it to the system as it is now.  Is that ok?  
4. I'd prefer to add it via the magnetic cleaner.  I've got one of these:

 

https://www.wolseley.co.uk/product/center-(cb)-system-filter-22mm-black/

 

Are the quick dose etc versions from different manufacturers all compatible, or would I have to buy a specific one for that filter?  

 

5. People sometimes say, half jokingly, that if you have a *really* old banger of a car you shouldn't clean it because the dirt is holding it together  A plumber once told me that the same is true of old heating systems, and that system cleaners can potentially cause/expose problems if there's rust inside radiators, dodgy piping behind the scenes, etc.  I've googled and not found others saying the same, so wondered what people here think about that?   

 

We've on oil, normal boiler (vented system), most of house heating around 20 years old, but some radiators probably much older than that.  

 

Many thanks for putting up with me!

Edited November 19, 2021 by Oxbow16

[gaz_moose]

i recently had a whole new heating system installed and the plumber left it for a month with system cleaner in before draining it down to ensure it was flushed out. 

if he is just doing 1 room in your house then he will probably just isolate that radiator before taking it off so you still have heating in the rest of the house.

i supose you could drain it down and just fill it up with water to flush it out.

[Temp]

In my opinion most of the cleaning is done by flushing and refilling.

 

Some cleaning products are mildly acidic and there might be some truth in old systems being held together by scale.

 

I would flush it two or three times, refill, test everything works (so no trapped air) then put in corrosion inhibitor. If you measure how much water comes out when you drain the system you can calculate the amount of corrosion inhibitor needed correctly. See instruction on bottle. In general a bit too much inhibitor doesn't harm.