If we discount the avoidable design, specification, installation, commissioning and maintenance issues, the residual problems are often associated with stagnation i.e. the absence of flow. This applies to both domestic water services and closed heating and cooling systems though the issues are slightly different.
For domestic water services installed in copper tube, if the building can be put into use within a few days after the pipework systems are first filled then any remaining construction or disinfection residues will be rapidly flushed away before they can do any damage and a protective patina of copper carbonate will naturally form on the inner surface of the pipe. This patina reduces the risk of sporadic biofilm development leading to microbially influenced corrosion (MIC) of bare copper tube. Private houses have short construction programmes and tend to be occupied as soon as complete. Unfortunately this is not the case for public and commercial buildings with much longer construction programmes and progressive occupancy after handover. The best corrosion protection for a domestic water system is to use it.
Plastic pipework does not suffer from metallic corrosion but stagnation will encourage the development of biofilms that may adversely affect water quality and increase the risk of colonisation of the system by legionella and other bacteria. Note that chlorination will kill surface bacteria but may not completely penetrate and remove established biofilm so it is preferable to avoid it forming in the first place.
For closed heating and cooling systems there is usually a period of stagnation between filling and commissioning. This allows the development of biofilm that will the influence subsequent corrosion, particularly of steel pipework and steel components, even after the system is cleaned. Application of a suitable biocide and monitoring programme between filling and pre-commission cleaning, as described in BSRIA guide to Pre-commission Cleaning of Pipework Systems (BG 29/2012), should reduce that risk but minimising the stagnation period is better. Once the systems are cleaned and commissioned then careful attention should be given to maintaining an effective water treatment regime, reduction of dissolved oxygen supported by periodic circulation and cycling of control valves to ensure that all parts of the system are continually exposed to corrosion inhibitors irrespective of heating or cooling demand.
The mantra for reducing the risk of failure in pipework systems should be “keep the fluid moving, but not too fast”. If your domestic water, heating or chilled water pipework system has already survived its first three years without problems then, subject to a reasonable standard of maintenance, it probably is good for the next twenty five.