thoughts on reverse osmosis make-up water system

TomM

with all the talk about corrosion of newer boilers probably due to salts and chlorides in make-up water, has anybody pondered or used a reverse osmosis filter system for their make-up water?

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 Have any of the pros tested the Total Dissolved Solids in the make-up water for the leaking boilers?  I think i'll pick up a TDS meter and check my water for the heck of it, they're like $50. 

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Looks like a reverse osmosis system would run a couple of hundred bucks for the kit, might be a good idea for some installs.

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TomM

nicholas bonham-carter

Ro makeup water

I have always thought that would be a good idea, but the pros have expressed doubts.

If you do it, keep us informed as to how it works.--NBC

Jamie Hall

The problem...

isn't so much the TDS of the water as it is the overall water chemistry.  Chloride is not good.  Sodium isn't so great either.  A relatively small amount of carbonate or calcium isn't bad; too much gives you scale.



Dissolved oxygen is really bad.



That said, though -- pure, really pure, water, such as would come from a reverse osmosis system -- is astonishingly corrosive.  It will happily dissolve whatever's handy (except for a few plastics) until the balance of chemicals in the water is restorerd...

SWEI

RO water

Acts like a solvent in plumbing systems.  PEX is OK as long as the fittings are non-metallic or stainless, but copper, solder, and iron will erode over time.  Calcium, magnesium, and potassium provide buffering and raise pH.  Commercial RO systems are available with TDS meters and blending valves to deliver a set amount of minerals in the product.  Replacing lost water with RO while monitoring TDS of the system water is the best approach - when water is lost to evaporation or steamed out, its minerals remain in the system, but if it exits in liquid form the minerals go along for the ride.

Hap_Hazzard

As usual our pros are right on the money.

As Jamie says, pure water is highly corrosive, because the pH of pure water is 7.0. The best pH for cast iron is around 9.5. Any lower than this and any oxygen present will cause surface pitting, and at pH 5, it can literally dissolve cast iron. They're also correct in pointing out the importance of moderate alkalinity--just enough to buffer the pH without depositing scale.



If water hardness and/or alkalinity are too high, reverse osmosis would be a good way to reduce them by mixing the filtrate from the RO system with the untreated source water in the right proportions. Another approach would be to add known quantities of desired minerals to the pure filtrate. This would be more expensive, but would give you the best results if there are undesirable minerals and other solutes in your water supply.

TomM

no buffing

Ah, so with RO water, there is no buffer, which would cause large PH swings either way when it enters the system, promoting corrosion. 

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yes, we discussed this before, but i had to refresh my memory.

Toymotorhead

Distilled Water Bad.

Serious automotive mechanics have known that pure distilled water is really bad to use for cooling systems in automobiles, pretty much forever. I see no reason that the electrochemistry of metal would change just because its a steam boiler and not a small block Chevy. You have to add something to it to keep the distilled water from eating all the dissimilar metals and everything else inside your engine.  All the antifreeze products or the magic "water wetter" additives on the market have corrosion inhibitors and or ph buffers in them. Just like the old statement that nature abhors a vacuum. Distilled water really really wants to dissolve anything it can find.



Cheers

Richard.

jumper

distilled water

Pure water doesn't scale. Degassed water with pH above seven won't corrode. What's not to like? It's amazing how long steam and hot water heating systems last when untreated water is continuously added.

Dave in QCA

Welcome Back Jamie!

Jamie, I had just sent an email yesterday to another wallie to see if he knew where you went.  Your presence and thoughtful advice and comments have been missed.

Dave in QCA

Aggressive Water

Many years ago when I was in charge of the operations of a couple of large process type boilers in a health care facility, I theorized that it would be ideal to have deionized water for the boiler feed.  I had a long discussion with our boiler chemical consultant, who was a very educated young fellow with an industrial chemistry education, and explained that while it seems like a good idea, it is in fact a VERY bad idea.  For all the reasons that have already been explained in this thread.  Dionized water or RO water is a very effective solvent and very effectively attacks metals and causes them to corrode and/or erode.

Maine Vent

Welcome Back Jamie

I, too was wondering where you were, hope all is well. I'm not a frequent poster, but as a steam homeowner I have appreciated all your help and advice. Well glad your back, I'll have some news to report once, it gets a little colder here in southern Maine.

TomM

ok

consensus says:  RO water isn't the best idea.

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have we formed an opinion on chloride treated water vs chloramine treated water?  And charcoal filtering?

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jumper

still don't see what's bad

Does aggressive water stay aggressive in a closed system ? Seems to me that as soon as it dissolves a little metal its hunger subsides. An open system like domestic water supply is a different matter.



In industrial steam systems deionization is common. Less need to blowdown.

SWEI

activated carbon

reduces chlorine (CL2) to chloride (2x Cl-) and does a great job adsorbing VOCs.