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Water Testing and VDI 2035

What is VDI 2035?
Treatment can be even more simple than adding inhibitors and retesting every year. VDI 2035 is a standard written by the association of German engineers.
Note; VDI is the Association of German Engineers, they create (among many other things) standards to guide industry, for the purposes of this article we will refer to the water quality “VDI 2035” as “VDI”.
VDI isn't a method to achieve lower corrosion levels, it's actually just a few parameters for you to try and achieve.
Now, there are other countries that have different standards, you've got Onom H15951 from Austria and SWKI from Switzerland but they all share the same method of avoiding chemicals, except as a “last resort”.
Onom and SWKI use exactly the same methods as VDI, it's just the parameters which are slightly different and the methods can't be copyrighted because it's just chemistry!
VDI is the best known in the UK and the one we will be talking about most moving forwards as an industry.

Why Treat Heating Water?
There's two main reasons to look at water treatment in heating systems. The first is corrosion, as we all know, and the second, which is much less talked about, is the effect on heat transfer due to scaling.
It's said that one millimetre of scale on the heat exchanger can reduce heat transfer by five to ten percent!

What is Scaling?
Scaling is where salts in the water bake onto heat exchangers. That could be a heat exchanger in a hot water cylinder, the element in your kettle, or a heat exchanger in a boiler.
In the case of a boiler, the flame essentially plays against this surface making the surface hot. When the salts in the water make contact with the surface, they solidify and create a thin layer of scale and that scale creates an insulative layer that insulates the heat from going from the flame into the water.
A simple way to test this in a gas boiler is to compare your flue temperature with the flow temperature from the boiler, the higher the flue temperature above the flow temperature, the more flue gas that's passing through the waterways without being absorbed into the water.
So now we know that the higher the flue temperature above the flow temperature the more scale you potentially have on that heat exchanger. Bear in mind, when doing this you'll need to compare this against the same boiler earlier on in its life.

What are the Sources of Scale?
There are two sources of scale. The first source is calcium/magnesium, which simply comes in from your tap, otherwise known as limescale. When these come into contact with a surface that's 60 degrees or above, they solidify and that creates a scale, exactly the same as you find inside your kettle. This process does happen below 60⁰c, but in a much slower fashion.
If you live in a hard water area and if you think about the sheer temperature of the gas flame, that's going to be around about 900 to 1200 degrees Celsius, the actual flow temperature is kind of the average of all the different flow paths in the boiler, so that really hot surface is way above 60 degrees.
The second source of scale is iron. This either comes in from your mains, particularly if you've got old iron water mains pipe work, or, and more likely for heating but not hot water, it comes off the steel radiators in the form of corrosion. As it comes off the radiators it enters the boiler, finds this hot surface and solidifies, creating this insulative layer.
Notably, VDI also covers hot water production for this very same reason, so where you have fresh mains water coming into your cylinder or combination boiler, and it's heated up in the coil or in the plate heat exchanger, this will also have this same scaling effect, reducing efficiency and meaning that you need higher temperatures in the boiler to have the same recovery rate or flow rates. Calcium carbonate starts to occur at as little as 35°C in hot water systems and gets progressively worse as it gets hotter.

Water Treatment to Prevent Corrosion
Next and most commonly discussed is corrosion of the heating system components. We can never stop corrosion, only ever slow it down and the way we do that in heating systems is to make the system water as non-corrosive as possible.
There are two main reasons for corrosion in heating systems.
The first is that you have the incorrect pH of the water for the specific metals within the system and the second is from electrolytic or galvanic corrosion.

Henry's Law
Henry's law dictates the higher the system pressure, the higher the concentration of dissolved gases in that liquid. If we lower the pressure, the amount of dissolved air within that water reduces and if we increase the temperature, the amount of dissolved air in that liquid reduces.
Dissolved air is a funny thing. When it's dissolved into a liquid it holds no volume, so the liquid doesn't expand at all, it kind of fits in amongst the other molecules, however, when it comes out of the solution it does begin to hold volume.
This is shown in fizzy drinks. When they're carbonated they're held at a high pressure and when they're held at this high pressure and carbon dioxide is introduced they absorb the carbon dioxide.
When you then unscrew the lid, you'll see that all of a sudden it holds volume as the carbon dioxide tries to leave the drink quickly, then after being left you'll see bubbles slowly coming out of solution.
As a liquid returns to an equalised state for that pressure and temperature it will equalise pressure and gas content for its relative temperature and pressure, you'll know when this is because it will be flat.
Likewise you'll notice if you've ever put a pan on the boil, to boil some eggs for example, as the water warms up you'll see some bubbles forming around the imperfections on the edge.
This is where the water is becoming saturated with air and it's trying to eject the gases for that pressure and temperature. If you then take that pan back off the heat, you'll see it'll reabsorb those bubbles because the water is in a lower state, so it'll be actively trying to draw air into the water in a process called diffusion.
Unfortunately testing oxygen within systems is very difficult and expensive. The main reason being, as soon as you take the water out of the system, it's in contact with the surrounding air and your reading will become inaccurate.
VDI suggests that sealed systems will settle at 0.002 milligrams per litre if they're correctly installed and maintained.
That means using pipework that has a sensible barrier to oxygen like press fit or soldered copper, or MLC pipe, and obviously sealing the system up rather than being an 'open vent' system. As such, VDI is fine for good quality sealed systems to be up to 1,500 microsiemens in conductivity.
You'll notice that 0.02 milligrams of oxygen per litre is right at the bottom of this graph, so VDI are happy to have a conductivity up to 1,500 microsiemens, but obviously the lower the better.

Corrosion from an Incorrect Heating System pH
Most of you are probably already aware of what pH is, but for those that aren't, the pH of a liquid is how alkaline or acidic that liquid is.
Traditionally you would have tested this with litmus paper. If the pH of the system water is too much in either direction it will dissolve this passive layer and expose fresh pipe underneath for the oxygen to corrode and simultaneously release this corrosion into the water, raising the conductivity.
A pH of 7 is neutral, below 7 is acidic, and above 7 is alkaline.
We can see here that the ideal pH for copper is around about nine and the iron in our radiators is happy with a pH of around 10.

VDI 2035 in a Nutshell
1 - Make sure your conductivity is below 1500 micro siemens for sealed systems and below 100 microsiemens for open vent systems and systems with air ingress potential.
2 - The pH should be above 8.2 and proper care is taken with dissimilar metals.
3 - In most domestic systems the scale level should be below 300 parts per million. But more likely below 200 if a heat pump system.
4 - Reduce your Oxygen, VDI suggests that most sealed systems will settle around 0.02 milligrams per litre and no additional measures are needed.

What Does VDI Say About Using Inhibitors?
VDI says to fill with potable water and only use inhibitors in extreme circumstances where it's required, and that the engineer putting in the inhibitor should be appropriately trained in that chemical area.
Because of this we're now going to have to start looking at scale levels below 200 parts per million and if you have a look at the map above again you'll see that that's the vast majority of the UK.
By demineralising these systems and stripping out all the minerals, we remove all of the food that bacteria needs to survive.
If you've got 80 to 100 micro siemens floating around your system, there's not much breeding ground for bacteria there.
So if we're going to do low temperature systems, bacteria grows very quickly. It gets cooked off if your system is a high temperature one, so low temperature systems do better with demineralization.