Cast iron boiler descale - is this overkill?

[johnkirk]

Thanks, Onetap. I'm about to start the descale later this week, and I'll report back on what I've done, and the outcome, soon after.

 

[Agile]

If you really do have lime scale then HCl is far better at removing it than citric acid ( DS40 ).

But HCl can produce the very agressive ferric chlorides with iron which is bad for the copper components.

 

[johnkirk]

But HCl can produce the very agressive ferric chlorides with iron which is bad for the copper components.

I'm aware that the copper flow pipe protrudes vertically downwards into the heat exchanger. So to minimise copper attack, to the point that is negligible, only 10% strength HCl would be used, cold, in the heat exchanger alone, and for only 45 mins, then the HE flushed thoroughly and neutralised.

I can't tell whether 45 mins of HCl contact is long enough for the HCl to do its full job, so to err on the side of caution I'm inclined to then resort to the citric acid treatment to finish the task.

 

[zstew]

If HCl then you dont need sulphamic acid.

If citric acid then you dont need an expensive DS40 but could use a Ph meter or strips of indicator paper.

But most would just overdose with the citric acid and not be concerned about titrating the exact amount to exactly neutralise the oxides.

Tony

Agile,

It sounds like there's no danger in overdosing the system on citric acid.
I've got about 2800sq-ft (260sq-meters) of space. 15 radiators, and a cast iron boiler that's making kettling/pinging noises. It did have significant water loss for a good while (many months, maybe years) so I suspect hard water deposits.
I'm thinking of adding 1 kg of citric acid and letting that circulate for what? A day or two?
Then Drain and refill, lTry again until I still see an acidic test strip? Try again until noise stops. Do I need to be careful not to overdo it.

All, thanks for your discussion. It's an old thread but one of the most informative I found on the subject. Thanks for being so through everyone.

 

[Mike13]

You are better off with citric or sulphamic acid, both are less likely to eat away the cast iron. they are a bit slower than hcl, but attack the metal itself much much slower.
DS40 (mostly citric acid) run round for a week, measure the ph daily to see how it is going, initial ph is about 3, then part way through it drifts up to a ph of 6.3, after that it is spent. It is a three acid tails on the molecule, so ph varies through its use.
Fernox recommend up to a week each time, so some DS40, volume up to you, when spent or up to a week, then drain and flush to remove loose crud, and repeat. If the ph stays at 3, then it is clean and you can stop.

 

[zstew]

Thanks Mike13. I'm thinking I'll start with citric acid, I figure half or a full kg. Then monitor with test strips.
Is there a convincing reason to use DS40 over citric acid?

 

[Mike13]

No convincing reason, but the nuclear industry usually also add a reducing agent to make the citric acid a bit more active (google told me that), I suspect but dont know that fernox has done the same with the ds40, its cheap enough if you get the citric acid by the kilo I guess.