Report No DWI0210

Mar 1993

The Water Quality Centre was invited to undertake a review of water quality within vented and unvented hot water systems. The purpose of the investigation was to highlight any potential for deterioration of water quality within the cold water supply, hot water storage and hot water distribution associated with each of these systems.

A total of 99 samples were taken from 10 vented hot water systems and 10 un-vented hot water systems located in commercial and domestic premises.

No Legionellae, coliforms, Escherichia coli or Pseudomonas aeruginosa were detected in any of the samples analysed.

At one site Aeromonas bacteria were detected in very low concentrations from a soft water storage cistern (1 per 100ml) and hot water outlet (2 per 100ml).

No significant increases in bacterial numbers (as measured by bacterial colony counts at 22ºC and 37ºC) were detected from any of the 10 vented systems examined.

Increases in bacterial numbers were detected in 4 of the 10 un-vented systems examined. These increases could be explained by observed design failings of these systems (eg. long runs of unlagged pipework, insufficient hot water temperatures, stratification within calorifiers)

One of the aims of the project was to determine the effect, if any, of the use of expansion vessels within un-vented systems. A slight deterioration in bacterial quality was noted from a sample taken from an expansion vessel fitted to the cold feed to an un-vented calorifier system, although the actual numbers of bacteria detected do not give cause for concern.

The generally satisfactory bacteriological quality of water samples taken may be due to the following reasons:

Water systems within domestic premises generally have a high turnover of water and hot water cylinders are generally set above 50ºC.

The increased awareness of the requirement for legionellosis prevention measures in commercial premises means that conditions which permit deterioration of water quality are less likely to be found within such systems. If a further study of the effect of expansion vessels (or other water system components) on water quality is to be undertaken, consideration should be given to monitoring a small number of sites containing such components over a longer period of time.

Copies of this report may be available as an Acrobat pdf download under the 'Find Completed Research' heading on the DWI website.