Definition: When the discussion turns to oxygen diffusion or permeation and the use of non-barrier tube, pipe or hose in a hydronic radiant heating system, one alternative that is often mentioned is chemical water treatment. Water treatment, corrosion inhibitor, oxygen scavenger and other terms are used to describe chemical products which are added to water in closed loop systems to prevent system failure due to corrosion. The question is, are they effective?
Relevance: The answer to this question is far from simple and oxygen corrosion is only one of the many types of water problems that can foul a hydronic system. It is often heard, "I've never used water treatment in closed systems and have never had problems." Or: "After initial corrosion takes place, the process stops and therefore treatment is not necessary." On the other hand this is often heard, "I have two identical systems, one with no problems and the other with serious corrosion, ... why?"
Lets take two samples of actual supply water, one raw hard water (a) and the other softened water (b). (pH= a. 7.7, b. 7.8;alkalinity ppm= a. 215, b. 225; hardness ppm= a. 700, b. 14; solids ppm= a. 750, b. 820; Is at 40°F= a. +0.39 b. -0.15; Is at 180°F= a. 1.77, b. -1.29) In sample (a.), the raw water becomes less corrosive as the water temperature increases and would probably not cause a problem except for its hardness. As hard water is heated, hardness will come out as scale on heat exchangers. If enough hardness comes out, the water will become soft and a problem could develop. The hotter the heat exchanger, the more likely this will occur. Thus the same water in two separate systems running at different temperatures will exhibit different corrosive tendencies.
The soft water in sample (b.) will very likely cause problems. Under certain conditions - positive Is, low temperatures, and a leak-free system - without treatment a soft water system could operate for several years, but a problem will occur. Soft water should be avoided or, at least, treated in closed loop heating systems.
Untreated closed loops can be loaded with black debris, rust, which may have a slight sulfuric odor. They can also be loaded with living masses of slime. This possibility increases for temperatures below 120°F. All this without the complication of a continuous source of new oxygen. This new oxygen can come from fresh make-up water that replaces water lost through seepage from fittings, valves, or leaky air vents. Oxygen can also penetrate the permeable walls of a non-metallic, non-barrier tube.
The selection of a water treatment is tricky at best. There is no "one size fits all" solution. To properly apply water treatment, a water sample should be taken and a treatment prescribed for that particular system. The system should then be checked periodically, i.e. every 1-3 years, and the treatment adjusted if needed. Treatments for closed systems do not wear out, so a constant and/or sudden drop in treatment level should be investigated. It is usually the result of seepage and make-up water.
Chromates have long been the mainstay of water treatment and provide excellent corrosion protection when used in the proper concentrations. However, higher concentrations needed to protect hot water heating systems can cause mechanical seal failure. Compatibility with system components should be investigated before using chromates. They are also considered a hazardous material and prohibited in some localities.
Nitrate/Borate formulas are equivalent to chrome but will support bacterial growth at temperatures below 120°F (some say as high as 180°F). They provide excellent protection for steel components but offer little protection for the nonferrous parts of the system.
Sulfites and Hydrazine react with dissolved oxygen, thereby eliminating only one contributor to the corrosion process; oxygen corrosion of ferrous metals. They are sometimes referred to as oxygen scavengers.
Molybdates are on the high end of the cost scale. They protect system components by laying down a very thin film barrier between the component and the water. Some say that a spot missed by the film will receive a concentrated attack and result in accelerated corrosion. Proper mixing and circulation is imperative with these treatments.
Organic Materials come in a wide variety of formulations. They should be satisfactory if they are safe to use, non toxic, not a bacterial nutrient, environmentally safe, easily tested by the user and control corrosion at an acceptable rate.
Technically, all water used in closed loop hydronic heating systems should be tested and treated. Unfortunately, this is not practical nor is it likely to be practiced in the field. Use of non-oxygen barrier pipe, tube or hose does not, in its self, guarantee that there will be failures of ferrous metal parts within the system. It does increase the possibility and therefore warrants the use of water treatment or the elimination of ferrous metal from the system altogether.
The keys to successful application of water treatment are: 1.) sample the water, 2.) thoroughly clean the system, 3.) use correct dosage and formulation, and 4.) perform periodic checks. Use of a water treatment information and test log posted near the boiler is highly recommended. The owner will then have a record of what water treatment was used and when it was last tested.
It must be stated that there are thousands of hydronic radiant systems utilizing non-barrier tube without water treatment and have operated for years without failure due to corrosion. There are also those that have experienced varying degrees of problems. Use of water treatment is an insurance policy. It insures protection for those times when water conditions are ripe for corrosion to take place.
Water treatment is a viable solution for non-oxygen barrier pipe. The dosage of additive is specified by each individual manufacturer. I cannot include a list of water treatment companies for the following reasons:
1. There are over 100 water treatment companies.
2. Water treatment chemistry is not an exotic art, any firm with a desire can get into water treatment.
3. One has no way of knowing the quality of the backup or laboratory service being provided.
4. The field representative is key to properly applying any given water treatment product. His or her qualifications are extremely important. A strong background in chemistry and biology is of great value to the client.
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