Protecting health and performance in DHW

JOHN COOPER explains why thermal balancing is much better at responding to safety and performance issues in domestic-hot-water systems than static balancing.What are the key factors affecting the design and installation of domestic hot water systems in commercial, public and multi-occupancy residential buildings? Issue As always, cost is arguably an issue, especially with the increasing expense of hot-water production and the rising price of water itself. Reliability of the supply is also important — as, increasingly, is the quality and hygiene of the water. Media interest in water quality remains easily sparked by public-health scare stories — especially where there is bacterial contamination, and the presence of legionella pneumophila bacteria, which causes Legionnaires’ Disease. These are key reasons for the building-services industry to give the subject of domestic hot water systems proper respect and care. This legionella bacteria finds a fertile breeding ground in domestic hot water systems. In fact, it grows best at temperatures between 22 and 43°C. The man-made aquatic environment of hot-water tanks and pipework installations in multi-dwelling residential, commercial and public buildings, like hotels, hospitals and offices are its natural home. We also provide the perfect route for bacteria transmission from these systems into our lungs through the water spray typical of, for example, showers and spa pools. Comprehensive approach Taking a fully comprehensive approach to domestic hot water systems can successfully tackle all the issues of cost, reliability and hygiene — generating major end-user benefits into the bargain Such a solution has two key components — a combination of thermostatic mixing valves, and the thermal balancing and disinfection of the hot-water system. Thermostatic mixing valves are capable of tempering the water temperature so that it will not be hotter than the valve is set. This is ideal for applications where scalding is a risk. Thermal balancing can be provided using thermostatic circulation valves which control the minimum hot-water temperature in the circuits. These self-acting valves can also be electronically controlled to facilitate a high-temperature disinfection program. Thermal balancing is different in approach to static balancing. In static balancing, excess system pressure is reduced, typically by using variable-orifice double-regulating valves (DRVs) in the return pipe of a circuit and is ‘set up’ under a fixed condition. However, this method of balancing does not secure an equal temperature in all points of the system, for several reasons. • Changes from the design consumption of hot water affect both water flow and temperature. • Changing system pressure losses due to ongoing scaling in the pipes, filters filling etc. • Changes in ambient temperature resulting in varying heat losses. • Conditions being different from the original design — especially in larger systems. • The dynamic nature of hot water consumption, where design conditions are rare. Because the dynamic or thermal balancing method is based on the use of thermostatic balancing valves (circulation valves), it secures a constant temperature in all circulation risers in the system, irrespective of changing operating conditions. Regulation is made simply by setting the required temperature on the valve. Complicated system calculations to balance pressures in all parts of the system are no longer needed. The thermostatic balancing valve automatically provides a thermal balance in hot-water installations at the valve’s set point. By keeping a constant temperature in the system, the flow in the circulation pipes is minimised, and scaling is limited because excessive water temperatures are avoided. At the same time, such a valve (like the Danfoss MTCV from Taconova) can be enhanced to realise a high-temperature disinfection process by means of two optional features. An automatic (self-acting) thermal disinfection module, or thermo-element, can maintain temperatures above 68°C with integral safety protection of the installation to prevent the temperature rising above 75°C. Alternatively, an electronic controller, complete with thermal actuator and temperature sensors can be connected to the circulation valves. This controller also programs the disinfection temperature and duration. Lower risk To sum up, a comprehensive thermal-balancing approach incorporating automatic thermal disinfection will significantly lower the risk of Legionnaires’ Disease. At the same time, it will help protect the installation against excessive scale and the user from scalding. In such an approach the cost of hot-water production and water consumption is also minimised, because there is no need to draw off water unnecessarily to achieve the right temperature at the tap. Instead, thermal balancing will maintain an equal temperature at all outlets, and the water temperature can also be monitored and electronically controlled during the disinfection process. John Cooper is managing director of Taconova UK Ltd, 20 New Forest Enterprise Centre, Rushington Business Park, Totton, Hants SO40 9LA.