Cleaning secondary heat exchanger for domestic hot water

The local heating guy told me to pour sulfuric acid (the drain cleaning stuff) down the CH flow or return connection until it come up the other side and then leave it overnight before rinsing all the crap out with lots of water, I've used citric acid in the hot water side. Make sure its stainless steel though!

Furnox DS-3 is the stuff to use. It's Sulphamic (not Sulphuric) Acid, plus an indicator dye to tell you when it's used up, plus a scent so you don't drink it. You used to be able to buy it in B&Q, and builders merchants used to stock it, but the larger tubs have got harder to find, so I order it from BES.

Interesting, will there be any harm in using the sulphuric acid though? I have already got a bottle and just about to pour it in the heat exchanger.

In message , dog-man writes

Phosphoric acid

For descaling the DHW side I use Kilrock-K ("inhibited formic acid" IIRC)

- which is also sold in diddy little bottles as kettle descaler. Then fit a proper scale inhibitor

If it's blocked on the primary side then a powerflushing machine with a PHE adaptor seems to do the trick temporarily but you'll need to address the cause of the problem which probably means powerflushing the whole primary system and/or fitting a Magnaclean/Boiler Buddy/equivalent filter to the primary circuit to remove/catch crap circulating around it.

That's really a question for the manufacturer. Depends what the heat exchanger is made from. It's probably ok with stainless steel. Heated sulphuric acid can corrode some stainless steels and the walls tend to be very thin, but you aren't going to have it in contact for long. If it's not stainless steel, then you'll have to say what it is.

It's a stainless steel from a Vaillant turboMAX