Risk Assessment and Control Measures for Household Hot Water Systems Introduction and Scope The impending changes to the Part G Building regulations have focussed attention on some of the potential hazards involved in the provision of hot water services for domestic premises. This publication concentrates on a common sense approach for the normal domestic household and is aimed at providing practical advice and knowledge to heating engineers and other interested parties. For other circumstances such as public buildings or where there are persons at high risk then other factors and different legislation will apply. When planning to install or make modifications to a hot water storage system, it is recommended that a simple risk assessment should be carried out. This approach is particularly important when considering the use of alternative heat sources such as solar collectors and heat pumps. The main risks are broadly associated with temperature; if delivery temperature is too high then scalding can occur. There is of course a theoretical risk of explosion but this can be regarded as virtually zero provided that existing good practice and the G3 requirements for vent pipes on vented cylinders and temperature & pressure relief valves (TPRV) on unvented cylinders are adhered to. At the other end of the temperature scale then the use of heat pumps and solar can lead to some of the water being stored at lower temperatures than usual which could, if good practice is not followed, encourage microbiological growth. Scalding Risk Hot water is generally stored at 60 o C but we should remember that cylinder thermostats are not precision devices and can occasionally drift or fail. The area of greatest risk is at the bath tap and the revision of the Part G Building Regulations 1 advises that thermostatic control should be fitted to prevent temperatures at the bath taps from exceeding of 48 o C. Hot water at any outlet can of course give rise to scalding and the young and elderly are most at risk. In order to maximise the benefits of solar input many control systems allow the cylinder contents to go to much higher temperatures than the normal 60 o C and we would strongly recommend the use of a thermostatic blender valve or equivalent device to prevent high temperatures reaching any hot water outlet. It should be noted 1 G3 Applies to England and Wales, equivalent legislation is already in place in Scotland Page 1 of 5 First Edition Feb 2010
that in order to reduce scaling problems the use of temperatures above 60 o C should be limited to areas with soft water or to installations fitted with appropriate water treatment devices. Where a blender valve or equivalent device is fitted at the outlet of the cylinder this may typically be set at between 55 o C and 60 o C and does not negate the requirement for the temperature to be further blended to 48 o C at the bath taps. Vented cylinders will normally be fed from a cold water tank situated in the roofspace. It is important that this tank complies with BS4213 and is fully supported over its base area with a 150mm overlap as stipulated in the guidance to the G3 building regulations. This precaution means that even if the tank should seriously overheat, it will not collapse and discharge its contents through the ceiling. Please note that in addition to the normal control thermostats : Electrically heated vented cylinders must incorporate a non-auto-resetting energy cut out in the immersion heater. Indirectly heated vented cylinders also require an additional level of safety control which in practice can be the boiler thermostat controlling the primary temperature All unvented cylinders must have the two levels of over temperature control for all heat sources as specified in Part G3 of the building regs. In practice, there is typically a cylinder thermostat or equivalent, a non-auto-resetting energy cut out and a temperature pressure relief valve. Microbiological Growth It should be understood that Legionella and other bacteria are always present in the water supply. If these bacteria are given favourable breeding conditions then their levels can become unacceptable. The highest risk areas are large scale hot water systems such as hotels and hospitals where the Health & Safety Executive L8 document gives appropriate guidance for both system design and on-going management regimes. In this document we will focus on the less onerous guidance more appropriate for the potential risks associated with typical domestic properties where cylinder capacities are generally below 400 litres. The preferred breeding temperature range of Legionella is from 20 C to 50 C and it generally takes several days at this temperature to promote colonisation. Once temperatures exceed 50 C the bacteria start to die off. This is almost instantaneous at 70 C or above whilst over 90% of legionella are killed in less than 10 minutes at 60 C and in around one hour at 55 C. However, due to back up heat source temperature lag, and other factors such as the different times it takes to kill off different bacterial growths, we recommend that where temperature is the main method of control that before entering the hot water distribution system the water is heated to either: Page 2 of 5 First Edition Feb 2010
60 C for 1 hour once a day or 55 C for 6 hours once a day or 70 C once a day Evidence from elsewhere in the world where solar, heat pumps and similar heat sources are well established suggests that any additional risk from alternative domestic heating systems is minimal and can be controlled by good design practice. Control strategies for dedicated solar volumes related to minimising bacterial growth Many solar systems are designed with either a two zone cylinder or with two cylinders. In these systems, the top of the twin-coil cylinder or the cylinder closest to the points of use is connected to the back-up heating system (e.g. boiler or immersion heater). This is called Vb, the back-up volume. The back-up volume should be sized to the peak demand of the property. Where the cylinder is used to preheat water for a combi boiler then there is generally no requirement for a back-up volume and the boiler manufacturer will recommend appropriate Legionella bacteria risk management strategies. The bottom of the twin-coil or cylinder furthest from the points of use is connected to the solar heating circuit and the volume that can normally only be heated by the solar collector is called Vs, the solar volume. A similar logic applies to heat pump systems where Vs is the volume that can normally only be heated by the heat pump. In these systems, the back-up volume Vb should be heated to 60 C for an hour once a day. Vb Backup input Solar input Vs Vs Vb Single two zone cylinder Twin cylinders Page 3 of 5 First Edition Feb 2010
In systems where it is not possible or desirable to adopt this strategy then another option is to pasteurise the cylinder contents by heating to 60 o C or above on a regular basis (at least once a week). This strategy may be preferred for heat pump systems where larger volume and lower temperature storage systems may be specified. Another effective means of controlling the growth of legionella is by reducing the dwell time within a system and avoiding stagnation. Typical of this approach is the use of thermal (primary) stores where the minimal amount of stored domestic hot water significantly reduces the risk. However, please note that it does not completely eliminate the risk and so it is important to implement good practice as documented below. Good practice bacteria management strategies Please note that unfavourable conditions for bacterial growth are: Regular exchange of water No pipe work dead legs Regular cleaning (especially of shower heads) The use of appropriate time/temperature regimes to ensure pasteurisation And favourable conditions for bacterial growth are: Temperatures of 20 to 50 C with an optimum temperature around 38 to 40 C Available nutrients and factors that support growth such as, foreign objects (insects, bird droppings etc) Stagnant and/or dirty water Scale and corrosion deposits including limescale Strategies before the hot water store The best form of control is of course not to let the bacteria multiply in the first place so if dealing with an existing vented system fed by a cold feed tank then the first area to look at is the feed tank itself. The tank should have a close fitting lid with any breather or discharge pipe incorporating a protective mesh. If there are any doubts about the integrity and cleanliness of the tank then it should be replaced. A check should also be made to ensure that the tank is adequately insulated. Whilst such insulation may be primarily for frost protection it also helps avoid excessive water temperatures during the summer period which might lead to Legionella growth in the feed tank. Page 4 of 5 First Edition Feb 2010
If it looks as if the cold water tank needs replacement then it may be more cost effective to install a mains pressure unvented cylinder or thermal store. Strategies after the hot water store. The second area to consider is the water pipe-work and its insulation with particular reference to undesirable features such as dead legs or instances where hot and cold pipes run in close proximity. Again if in doubt then replace the offending sections and consider disinfection of the entire system as part of the commissioning procedure. In most homes the areas at most risk are shower outlets as Legionella is transmitted as an aerosol. Particular attention should be given to the pipes feeding the shower including the condition and positioning of any flexible hoses so that they are as far as possible self draining and not likely to be immersed in warm bath water. Customer Advice Finally, the user instructions should stress the importance of proper setting of the controls to ensure that the system integrity is not compromised by the householder reducing storage temperatures and altering control regimes in a sometimes mistaken effort to save energy. These user instructions should include: Leaving the hot water programmer on for at least an hour a day cycle and with the cylinder stat set to 60 C (for solar systems, solar gain is improved if this hour a day cycle is set for the evening rather the morning) Weekly cleaning of shower heads and emptying shower hoses after use Before any use, to make sure that the cylinder is heated to 60 C for an hour on return from a holiday or a period of several days when no hot water was drawn off and the hot water heating circuit was switched off. To avoid scalding; if thermostatic blending valves aren t fitted, take due care and attention, especially with children and older people, when running a bath If all of the advice provided here is followed then the building occupants should be protected from bacterial infection and scalding. Page 5 of 5 First Edition Feb 2010