What's the status of your hydronic heating and cooling system?
28 September, 2017
Unfortunately, the answer to that question is not as straightforward as one might wish, but there are ways and means of measurement and analysis that can help. The first question is: what are the consequences if the system is in poor condition? In a recirculated water system with suboptimal water chemistry there is a risk that sludge build-up, excessive pump wear, stuck valves, impaired heat transfer, noise, corrosion and leakage can lead to disruptions and shorten the service life of the system.
The single most important parameter is the amount of oxygen in the system. The presence of oxygen is a prerequisite for corrosion to occur. When a system has been filled, the initial oxygen level is high (approx. 8 mg dissolved per litre water). The oxygen is consumed over time, among other things, through oxidation of iron and steel, until the system becomes virtually oxygen-free. In the latter case, corrosion ceases and will not continue to progress, as long as no new oxygen is introduced into the system. If oxygen leaks into the system, the components that are sensitive to the given water chemistry will begin to corrode. New oxygen can enter the system, for example, during cold water fill, diffusion through permeable components or due to negative pressure in the system.
According to the Swedish standard for "Heating systems in buildings – Design for water-based heating systems, SS-EN 12828:2012+A1:2014", which refers to ”Verein Deutscher Ingenieure, Vermeidung von Schäden in Warmwasser – Heizungsanalgen, VDI 2035 Blatt 2 (Prevention of damage in water heating installations. Water-side corrosion.)", there are guidelines with respect to electrical conductivity, appearance, pH and oxygen content. Oxygen content should be <0.1 mg/l for freshwater and <0.02 mg/l for water containing salt.
"If there is a suspicion that a system is in poor condition, on-site measurements can be taken to determine the oxygen content in the system," explains Carolina Schneiker, senior research engineer? at Swerea KIMAB. Low oxygen content may mean that the system is in good condition and the risk of significant corrosion is low. But low oxygen content may also imply the opposite; that the corrosion that is occurring is using up the oxygen that is leaking into the system. It is therefore very important that water samples for analysis are taken at the same measurement points. Parameters such as pH, alkalinity, conductivity, dissolved salts, metal ions and chlorides are analyzed and the values are used for an overall assessment of the system's status.
A hydronic heating and cooling system comprises many different components, such as pipes, sleeves, pumps, heat exchangers, cooling baffles, etc. These are made of different materials, complicating matters where corrosion concerned. The various materials are sensitive to water chemistry parameters. For certain materials, alkalinity or pH have the greatest impact and, for others, the anions, mainly chlorides, pose the greatest risk. Junctions or joints between different materials also pose a risk for galvanic corrosion.
"To be able to assess the status of hydronic heating and cooling systems, Swerea KIMAB can perform on-site measurement of oxygen content using a portable optical oxygen sensor," explains Carolina Schneiker. We also take water samples for analysis by an accredited laboratory.
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