DHW heat exchanger problem.

[A.Lee]

I've been having problems with my hot water for nearly a year now, where the boiler burners would turn off for 30 seconds or so when the hot water was on. I had a CORGI man round to have a look, he seemed to think it was the DHW heat exchanger blocked up, he said I was quite capable of doing it myself, so I took it apart, flushed it through, and it was fine for 6 months or so. It was cleaned again a month or so ago, but it didnt cure it this time,just made it a little bit better, in that the hot water would run for 5 minutes before cutting out, rather than the previous 2 minutes.

I have since put in cleaning fluid to the primary side via one of the radiators, and ran them on and off for the last 3 weeks with this cleaning fluid in. I flushed them through yesterday, though there didnt appear to be any sludge in the system at all.

This made no difference to the DHW.

What is the next step? A new DHW exchanger is £70ish - maybe get one, or could the old one be brought back to life by soaking it in cleaning fluid.

If a new exchanger is the best solution, then I would rather do that than spend more time trying to fix the old one. If not the exchanger, what else should be checked? BTW, the CH works fine.

Ta Alan.

[A.Lee]

And of course, I left out the important bit that this is on an Alpha Combi boiler that is around 10 years old. Alan.

[Ed Sirett]

What are you cleaning it out with? If dilute HCl then you shoudl get a result or a leak 8-(.

Buy replacement unit. Put existing unit in Dilute HCl (e.g. Screwfix Patio Cleaner) and leave for a couple of days.

You then have a spare.

[Andrew Gabriel]

It's easy enough to clean with Furnox DS3 (sulphamic acid with an indicator dye so you know when it's exhausted). I stood the plate exchanger in a washing up bowl and poured a jug of hot dissolved DS3 in one end. Fizzes like mad. Bowl allows you to collect it and pour it through again. When it changes from yellow to green, you need to chuck it and mix up a fresh lot. Keep doing this until it's stopped fizzing and won't go green (i.e. no scale left). Probably took me about 20 minutes for a Baxi one. Watch out for the fine acid spray from the fizz going up your nose - not very pleasent. Also the indicator dye stains some things (washing up brush had green or yellow bristles from then on, depending if you dunked it in something acid or alkali:-).

If it's the primary side which is blocked, then sulphamic acid probably won't clear that. That tends to block with rust particles, not scale. If that's the case, some tell-tale ones would like be dislodged by running water through it and tapping it.

[geoff]

In message , A.Lee writes

Yes, but did you descale it properly ?

How did you flush it

well, it wouldn't, would it ?

descale it properly?

[Martin Pentreath]

When I had similar problems I found that the primary side was clogged up with black flakes which I assumed were magnetite - iron I oxide I seem to remember from school, as opposed to red rust which is iron III oxide. Magnetite is much more or a pain in the arse than limescale on the secondary side, which as Andrew says is easily cleared by a bit of DS3.

I've asked in here before whether there is any straightforward chemical process for dissolving iron I oxide, but the general opinion seems to be "no". (Further vague schoolboy memories suggest some sort of reduction reaction would be necessary?).

Anyway, in my case I did manage to restore the heat exchanger to working order by blasting a lot of mains-pressure water through the primary side and bashing it about quite a bit to loosen the crud. It's got a few dents now as the scars of battle, but it works like a good'un. If only that worked for all of life's little problems.

Cheers!

Martin

[A.Lee]

Obviously not.

Hosepipe in one hole, water comes out other.Repeat for other holes.

But the primary water does go round the DHW exchanger doesnt it, so the cleaning fluid, I thought, would help to clean it out.

Easier said than done.You cannot see in there apart from the first inch.If there is a rust blockage, it is not visible.When I cleared it a few weeks ago, nothing at all came out, apart from clear water - I was spraying into a large bucket so would see any flakes. I'll be ordering a new one now anyway, as I'm getting annoyed with it. Ta Alan.

[geoff]

you need some way of keeping a flow going or else you get dead spots of "neutral" with no acid left to remove it

through the other side of the exchanger.

Yeah, without some means of circulation, it's difficult to do it properly