Cold Water Cistern Design – The Good, the Best and the Ugly

by Charlie Brain, on 15-09-2020
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This week’s blog is a guide to cold water cisterns, their design, configuration and varying standards. Guidance for cold water cistern design primarily is given in the Water Supply (Water Fittings) Regulations 1999, but further guidance is given in the HSE’s HSG 274 Part 2 and the Department of Healths’ HTM04-01.

Below is a guide to help understand the differing standards of cold water cisterns and associated equipment.

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Each ‘Feature’ of a cold water cistern will represent varying ‘standards’ or ‘practices’ and therefore risks to water quality. This table shows these differing standards for each feature, from ‘Outdated Practice’ highlighted in red to ‘Best Practice’ in blue. Outdated practices should not be targeted or found in any building and on any system, and in some instances will not meet current water regulations, for example, no lid fitted. ‘Conventional Practice’ features are examples expected on a system in a non-healthcare building such as a school or office. ‘Good Practice’ is a standard up from ‘Conventional’ and expected in most healthcare settings, however, if ‘Best Practice’ is targeted, features in the blue column should be considered.

Feature Outdated Practice 
(Fig. 1)
Conventional Practice (Fig. 2) Good Practice (Fig.3) Best Practice (Fig. 4)
Number of Cisterns More than 2 linked More than 2 linked Two linked Single cistern
Inspection Hatch (>1000 I) Not Fitted Fitted Fitted Fitted
Heat Gain Prone to heat gain from ambient temp Plant room with ventilation Plant room with ventilation Purpose built cool cistern room
Flooding Risk Location with history of flooding Potential risk with controls in place Potential risk with controls in place Not prone to flooding
Cistern Sizing >12 hours supply Minimised and <12 hours supply Minimised and <12 hours supply Minimised and <12 hours supply
Multiple Cistern Configuration Linked in series  Linked in parallel Linked in parallel n/a
Inlet Type Inlet below water  level Conventional ball valve Delayed action valve / level controls Delayed action valve / level controls
Cistern Construction Material Non-WRAS material or lining Galvanised Steel Plastic or GRP Plastic or GRP
Cistern Insulation None fitted Supplementary insulation Pre-insulated Pre-insulated
Pipework Insulation None fitted Fitted Fitted Fitted
Lid Vent None fitted Fitted with insect screen Fitted with insect screen Fitted with insect screen
Cistern Integrity Holes in lid or cistern No holes No holes No holes
Cistern Lid None or ill-fitting Lid fits Lid fits Lid fits
Overflow Pipe None fitted /without insect screen Fitted with insect screen Fitted with insect screen Fitted with insect screen
Warning Pipe (>1000L) Non fitted / without insect screen Fitted with insect screen Fitted with insect screen Fitted with insect screen
Flow Across Cistern Inlet and outlet on same end Inlet and outlet on adjacent sides Inlet and outlet on opposite sides Inlet and outlet on opposite sides
Hot Water Vent Discharges in cistern Discharges in to tundish Discharges in to tundish Discharges in to tundish
Internal Supports Hollow supports or baffle plates fitted Solid supports Solid supports Solid supports
Cistern Labelling None fitted Cistern identification fitted Cistern I.D and volume fitted Cistern I.D, volume and purpose fitted
Cistern Pipework Labelling None fitted Pipework labels fitted Pipework and valve labels fitted Pipework and valve labels fitted
Deadlegs to drained cistern Valve positions create deadleg >2 pipe diameters Valve positions create deadleg >2 pipe diameters Valve positions create deadleg >2 pipe diameters Valve positions create deadleg >2 pipe diameters with vents & drains fitted
BMS Sensors Fitted None Storage temperature Storage, inlet and outlet temperatures Storage, inlet, outlet & sentinel temperatures
Booster Set Pump Switchover Manual Automatic weekly Automatic daily Automatic on-demand

 

Although this guide has been inspired from guidance published in the documents stated at the start of this blog, some are professional opinions only, including the interpretation of terms ‘Outdated’, ‘Conventional’, ‘Good’ and ‘Best’ practices.

 

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The drawings below illustrate these different standards pictorially.

Fig 1

Figure 1. ‘Outdated Practice’ shows multiple cisterns linked in series, no insect screen, holes in the cistern lid, open vent discharges into cistern.

Fig 2

Figure 2. ‘Conventional Practice’ shows multiple cisterns linked in parallel, conventional float valves, insects screens fitted, inlets and outlets adjacent, isolation valves fitted to eliminate deadleg risk.

 

Fig 3-1

Figure 3. ‘Good Practice’ shows 2 cisterns linked in parallel, delayed action float valves, insects screens fitted, inlets and outlets opposite, open vent redirected to tundish and drain, vents and drains fitted to interconnecting pipework.

Fig 4

Figure 4. ‘Best Practice’ shows a single cistern, delayed action float valves, insects screens fitted, inlets and outlets opposite, optimised capacity.

This guide could be used to check if your current cold water cistern meets expectations and then suggest where improvements can be made.

When installing new systems, cold water cistern manufacturers may have their own solutions, but essentially a cold water cistern should be designed so that cold water is kept cold, kept clean and kept moving.

If a new system is being installed, it would be advisable to aim for ‘Best Practice’ in all instances. Where cold water systems are concerned, this may be eliminating cold water storage all together, particularly for risk mitigation. This guide may act as a checklist when designing a new cold water system, to help get it ‘right first time’. 

If you have questions regarding the issues raised above or you would like to speak with one of our consultants please click here to get in touch.

Editor’s Note: The information provided in this blog is correct at date of original publication – September 2020.

© Water Hygiene Centre 2020

 

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About the author

Charlie Brain

Charlie started the Water Hygiene Centre as a trainee risk assessor back in 2010, since then he has developed professionally from risk assessor, project manager and is now a Senior Consultant. During this time he has taken ownership of our risk assessment method and development of our bespoke reporting platform and has been key in our UKAS accreditation to 17020.

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