Is inhibitor needed in sealed combi system?

[Lobster]

Surely the answer is yes... but I've just had a plumber adding in and moving radiators etc. Near the end of the job I asked him casually 'so when does the inhibitor go in'? meaning of course, 'don't take me for a numpty, I know you haven't put any in yet, so don't think you can get away without' and he says 'oh no, you don't need that these days; you had to use it with the old vented systems but not with these sealed combi jobs. And anyway there's no header tank to pour it into, to get it in the system'

As so often when I'm faced with an apparently experienced tradesman telling me that black is actually white, I just caved and left it at that (despite having personally injected inhibitor into a sealed system before using the special syringe provided with the stuff, so of course it's 'possible'). This guy has been a qualified plumber all his life (now in his sixties) and I'd feel very uncomfortable telling him he's talking b*ll*cks. But apart from the syringe thing, is he? I suppose if no air can get in to the water, corrosion is less likely isn't it? I'd planned on just injecting some inhibitor myself later without letting on, for a quiet life, but do I need to?

David

[Andy Hall]

Yes you should.

There is a grain of truth in what he says in so far as since the system is not exposed to the air there is a limit on oxygen content for corrosion.

However, some corrosion can still happen, and for the few pounds that a tube of Fernox gel costs, it isn't worth not doing it. I would avoid the discussion and just do it later.

.andy

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[Andrew Gabriel]

That depends how often you have to top it up, i.e. how well he managed to make it a "sealed system" ;-)

ISTR it was a condition of the manufacturer's guarantee for most boilers when I was trawling though their installation instructions.

[Alex]

Taken from a Potterton installation manual I have to hand - sorry if the lines are broken, I just copied it from Acrobat (the last line is the important bit).

"For optimum performance after installation this boiler and its associated central heating system must be flushed in accordance with the guidelines given in BS

7593:1992. "Treatment of water in domestic hot water central heating systems". This must involve the use of a proprietary cleanser, such as GE Betz Sentinel X300 or X400, Fernox Superfloc or Salamander System Cleanser. Full instructions are supplied with the products, but for immediate information please contact GE Betz (0044 (0)151 420 9563), Fernox (0044 (0)1799 550 811) or Salamander (0044 (0)121 378 0952) directly.

For long term protection against corrosion and scale, after flushing dose the system with an inhibitor such as GE Betz Sentinel X100, Fernox MB-1 or Copal or Salamander System Inhibitor in accordance with the guidelines given in BS 7593:1992.

Failure to flush and add inhibitor to the system will invalidate the appliance warranty."

Alex

[Alex]

Oops, forgot to add that's for a sealed combi.

Alex

[Andy Hall]

It doesn't really matter whether it's combi or not, though Alex, it should still have inhibitor.

.andy

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[Owain]

"Lobster" wrote | This guy has been a qualified plumber all his life (now in his | sixties) and I'd feel very uncomfortable telling him he's | talking b*ll*cks.

Good though he may have been in his day, if he hasn't been keeping up to date on installation techniques he may be missing out other things. Is he, for example, dealing with the condensate drain properly (if appliccable) or the pressure relief valve discharge (it's not acceptable to poke it out of a wall where it might spray boiling water over someone)? What about updating controls to Part L?

Owain

[Brett Jackson]

I put my sealed system in a couple of years ago and originially put inhibitor in. I've replaced a couple of raditors since as the original install was 'quick and dirty' and I've decided to re-locate a few raditiators since. I never topped up the inhibitor and the last radiator I pulled out had a tiny amount of black water (I wouldn't call it sludge as it wasn't at all thick) in the bottom.

I'll be adding the inhibitor back in when I get round to it.

[John Carlyle-Clarke]

[e-mail] (Andrew Gabriel) wrote in news:ci1uul$3qq$ [e-mail]:

My so called sealed system has never been very sealed, I must admit, but I can certainly say that after 5 years with no inhibitor in it, the water was brown/black and there was a lot of magnetite in the pump and the radiators (and presumably everywhere else too). I think it's worth it.

[Aidan]

" but do I need to?"

Yes.

The wizened one was talking about corrosion by dissolved oxygen. The water in a sealed system system will typically absorb less oxygen than that in an an open system, but it is likely that there will still be some oxygen getting into the water. An inhibitor would contain an oxygen scavenger, which reacts with the oxygen and renders it harmless before it reacts with the inside surface of the radiators.

ALSO, because the system contains different metals (steel rads, copper pipes, brass fittings, etc.) and an electrolyte, there will be galvanic corrosion of the steel radiators. The inhibitor will contain a pH adjuster which will make the water alkaline and so much less conductive as an electrolyte, stopping the galvanic corrosion. If the system hasn't been thoroughly flushed, it may contain acidic flux residues which will accelerate the galvanic corrosion. The inhibitor would neutralise any flux residues, within reason.

[Brett Jackson]

What is the best inhibitor to use in this sort of system anyway? I was going to get some more Fernox MB1 but it's a bit pricey (£19/4l).

[Aidan]

No idea, I don't know what's in any of them. I use Fernox or Sentinel. Corrosion is cheaper, in the short term..

[IMM]

Sentinel X-100 is far cheaper and just as good.

[adder]

Some of the old combined pressure hot water & heating (i.e. the hot water tank acts as a pressure vessel for the heating) types you're not allowed to put inhibitor in because of possible contamination of the potable water.

My old sealed system used to be like this & when it eventually stopped working I found that a lot of the pipework was almost completely silted up. It didn't help that the original install had the pump pumping down so that air got trapped in. I now have a replumbed sealed system with inhibitor & everything is fine.

[Aidan]

PS to the above.

"there will be galvanic corrosion of the steel radiators".

I should have said that the corroison by dissolved oxygen should be greatly reduced in a sealed system, but that the galvanic/bi-mettallic corrosion will be unaffected by whether it is an open or a sealed system. So you need inhibitor.

[Lobster]

OP back again...

I'm now a bit confused as to which inhibitor I can use in my combi system. I was going to buy one of those injector kits, but found a Fernox one in B&Q for £26 so thought it was worth looking into further, particularly when you can buy a 1L bottle of el cheapo Protex stuff from Screwfix for only £3.59. What's the difference?

I gather Fernox and Sentinel are the brands to go for, for whatever reason, and note that on Screwfix's site

X400 is stipulated as being for indirect systems (combis are direct, right? ie no h/c water storage, all water at mains pressure?) whereas the Protex one isn't. Does this relate to potable water issues as mentioned above? Have I *got* to buy the cheapo stuff, or go to B&Q for a whopping £26 kit?

Thanks David

[Andy Hall]

I've used Fernox products - originally MB-1 but more recently Superconcentrate gel and Alphi-11 in various systems for over 25 years. I have never had sludging problems at all.

Ergo, I know that these products work.

I am certainly not going to fret over trying to save £20 every few years when the risk factor in products that are unknowns and may or may not be good in the long term is many thousands of pounds.

When a chemical is specified as being for indirect systems it refers to those with a cylinder where the indirect aspect is because the heating of it is via a coil. Therefore the primary and secondary water never come into contact.

A combi is a different arrangement where a separate heat exchanger inside the boiler warms the DHW directly. Again the two sets of water are kept apart.

Sentinel have a good reputation as well. I would not attempt to save a few pounds by buying an unknown brand with unknown means to test its effectiveness. The saving is far outweghed by what is at stake.

.andy

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[IMM]

No.

Protex will do. Put two in as it is so cheap.