Combatting scale in a thermal store

[Indecisive]

After depending for three years on a polyphosphate water treatment on the rising main to protect the coil heat exchanger in my thermal store..... I now have doubts. Does anyone else has experience?

I use the Permutit from B&Q, but perhaps the Cistermiser CombiMate is better known. There are many others eg from Screwfix. Polyphosphates are widely used for treatment of potable water in the USA, not solely as an antiscaling agent. Southern Water in UK uses it to prevent lead leaching into water from old pipework. But in terms of protecting against scale at domestic hot water temperatures, I have just read a scientific paper published in Desalination journal in 2009 which lab-researched five commercial phosphate anti-scaling additives and concludes: 'The experimental tests showed that the inhibitors added at the optimal concentrations previously determined at 20 °C are not effective at 50 °C. At 50 °C, the phosphate inhibitors were shown to inhibit well scaling at the beginning but end up losing their efficiency.' This drop-off occurs in a matter of minutes - and in a domestic setting, water will often remain in the storage tank coil for hours at a temperature of 60 degrees or more. The break-down of polyphosphate by hydrolysis at higher temperatures has long been known, but this study appears to be one of the first to measure it in this way. The conclusion was that of the five treatments tested, only the phosphonate worked at 50 degrees. Phosphonates are used in cooling processes, geothermal extraction and wastewater treatment, but cannot be left in potable water.

I am writing to manufacturers, and in the case of PERMUTIT to B&Q - since they advertise that: 'It protects all pipes, hot water boilers, heating systems and domestic appliances.' CombiMate makes comparable claims. I would like to find out that I am wrong, or I have three years of scale in my hot water tank! Otherwise, at least this post can serve as a caveat for anyone attracted by this apparently inexpensive solution to scaling damage. I am considering turning to a standard water softener, salt and all..... which I had hoped to do without.

[djh]

I have just read a scientific paper published in Desalination journal in 2009 which lab-researched five commercial phosphate anti-scaling additives

Do you have a reference for that paper please; either a URL or citation?

You could also consider not trying to protect the coil and then descale it every few years.

[MarkB]

I have just read a scientific paper published in Desalination journal in 2009 which lab-researched five commercial phosphate anti-scaling additives

Do you have a reference for that paper please; either a URL or citation?

You could also consider not trying to protect the coil and then descale it every few years.

I was planning to fit a similar water softener (maybe kinetico) because our water is very hard. As an insurance, I plan to fit the coil in the thermal store with some isolation valves and an alternate means of connecting pipes/pump for descaling if/when it is needed. (Perhaps even using flexible pipes to eliminate/reduce any risk of mains contamination.)

[Indecisive]

Yes DJH, the paper can be read or downloaded from scribd: title is 'Efficiency of five scale inhibitors on calcium carbonate precipitation from hard water: Effect of temperature and concentration', by R Ketrane, B Saidani, O Gil, F Baraud, in Desalination 249 (2009) 1397-1401.

I had consoled myself at install with the plan that I could treat the coil with acid..... but in my case the tank was squeezed in to maximise bathroom size, and I would need to take off the side of the cubboard that I built around the tank to get good access. (If I lived alone I would have left it functional and accessible!) As the years pass, I feel less inclined for such plumbing antics. But it is an option, as long as I do it cheaply with my own labour.

I think my concern is the unsubstantiated advertising of products - as with the much more infamous magnetic and electronic treatments. They get away with it because we can't easily peer in to monitor our pipework, or even then attribute blame easily. I doubt I will get much response by writing to the manufacturers - I doubt they know the answer, or would carry out the rigorous testing required. Some are just copycat products. But long-established manufacturers eg like Gledhill also endorse the use of a polyphosphate doser ahead of the storage tank. One thing that is apparent from the few studies I have read is that I don't have a clue what is actually going on in the water chemistry. For example polyphosphates can, apparently, readily break down into orthophosphates which in turn can combine with calcium to form an almost insoluble calcium phosphate. The chemistry would depend on temperature, Ph of the water, concentrations of Ca and other ions, and the amount of phosphate.  

By the way, I see a couple of objective studies that show that magnetic fields can and DO work, at least to an extent - though it seems that it is senstive to system layout.

[djh]

One thing that is apparent from the few studies I have read is that I don't have a clue what is actually going on in the water chemistry. For example polyphosphates can, apparently, readily break down into orthophosphates which in turn can combine with calcium to form an almost insoluble calcium phosphate. The chemistry would depend on temperature, Ph of the water, concentrations of Ca and other ions, and the amount of phosphate.

Many thanks for the reference. It seems to be a sensible paper at first glance. All those dependencies were what struck me too. IIRC, they were increasing the pH to cause precipitation but I don't know enough to know whether that is a typical condition to be found in a tank. I also wasn't clear whether the effect reduced because they perhaps put a fixed amount of polyphosphate in, rather than continually dosing. But it would be interesting to know what happens in a real tank. Presumably there's actually a wealth of knowledge out there in the heads of all the plumbers that have ever lived with a dosing system for long periods, if only the knowledge could be collated.

By the way, I see a couple of objective studies that show that magnetic fields can and DO work, at least to an extent - though it seems that it is senstive to system layout.

I'd be interested to read those too.

[Indecisive]

djh, I know this is the best place to have any serious discussion!  I agree - you can approach this from a scientific viewpoint, but what happens in practice is probably the best guide. And I guess diynot forum or Screwfix forum is the best place to sound out plumbers.

The accelerartion of hydrolysis of polyphosphate at higher temperatures is basic chemistry and widely recognised in the industrial literature. However the tests carried out for the paper in Deslination journal did not treat the water and then heat it. The treatment was added to water at different temperatures from 20 to 50 degress C. In a domestic situation the incoming water is treated at mains temperature, when the effect of the polyphosphate is thought to be the coating of smaller particles to prevent them forming and depositing scale. The deflocculated particles are suspended and show little tendency to settle. Whether this then 'proofs' them against causing scale when subsequently heated, I do not know - not having studied chemistry since opting for Arts and Languages at 14 (terrible, I know.) I understand that they are used eg for commercial coffee machines.

However the industrial literature eg GC3 indicates that when polyphosphates hydrolyse it is most commonly into the next chain down, orthophosphates, and these do not inhibit scaling and indeed can form a relatively insoluble deposit of calcium phosphate - assuming the heated water has Ca ions present. I have written to the sellers of domestic polyphosphate devices but I don't expect a scientific commentary in response. I think most are just on the bandwagon, as it is a simple product to make. All this seems to confirm the comment of the authors in the Desalination article that: “The fundamentals of inhibition mechanisms, particularly from their quantitative aspects, are poorly understood so that the effects on inhibition effectiveness are largely unpredictable.”  Which would be a bit scary - except I am only talking about my domestic tank!

For the sake of completeness, I will post the essentials of any replies that I receive.