What is a Domestic Hot Water System?
A Domestic Hot Water (DHW) system provides potable hot water to our sinks, showers, and tubs and other places where hot water may be ingested by, or come in direct contact with humans. This is as opposed to non-potable water which is not suitable for drinking and other domestic purposes but may be used for hydronic heating, industrial cleaning and other applications. Most plumbing codes specify that all piping, components, and heat-transfer devices in contact with the potable water shall be intended for use in potable water systems.
Typical commercial DHW systems consist of one or more boilers that provide a reserve of hot water for use. Since demand for hot water is not uniform across the day, DHW control systems are sized to try to balance the costs of heating large tanks of hot water, while still being able to meet the anticipated demand during a peak period (usually when people are getting ready for work in the morning)
Key Parts of a Typical Commercial Domestic Hot Water System
The diagram below shows a typical configuration for a domestic hot water system with a single boiler/storage tank. Many configurations for a DHW system are possible depending on the size, age, demand profile and layout of the buildings involved but they all share a few basic components.
- Hot Water Tank/Boiler – In every system there needs to be a source for the hot water. In large apartment buildings this can be a series of boilers and storage tanks, or in a smaller office building a single combined boiler/storage tank.
- Recirculating Pump – As shown in the diagram, this pump recirculates return water so that water in the hot water piping remains hot during periods of no usage, avoiding the “cold shower” syndrome.
- Mixing Valve – Central to the safe performance of any commercial hot water solution is the use of a 3-way mixing valve. The mixing valve allows for the hot water in the tank to be cooled before delivery for domestic use. In this way Legionella and other bacteria are killed while insuring that superheated water does not scald people at the tap. Valves like the Heat-Timer ETV run under a computer control which uses temperature sensors to achieve this and provides alarms in case of system thresholds are exceeded for any reason.
- Actuator – The actuator is part of the Heat-Timer ETV solution, and represents the motor that turns the valve under direction of the control unit.
- Safety Valve – While optional, this use of this valve is highly recommended. Essentially the valve makes sure the system “fails cold”, so that in the event of any distribution valve or boiler failure, the Heat-Timer ETV control will sense it, and direct the safety valve to trip.
A typical Domestic Hot Water heating configuration showing these components is below:
In this system, the hot water tank is providing superheated water at 160F to a 3-way mixing valve that cools it for delivery to the tap at ~110F.
Other Key Design Considerations for a Domestic Hot Water System
The bacterium legionella pneumophila is found in both potable and non-potable water systems. Studies have shown that Legionella does not grow in cold water below 89.6F1 but in water temperatures above 90F especially in stagnant water between 95F (35C) and 122F (50C), legionella bacteria begin to multiply. Legionella can cause legionnaires’ disease or legionellosis, a severe, often lethal, form of pneumonia that occurs primarily when steam or water vapor containing legionella is inhaled. The disease’s name was coined in 1976, when American Legion members who attended a convention in Philadelphia, suffered from an unusual lung infection (pneumonia). Legionella cases have been on the rise for over a decade, with several outbreaks identified in large cities, that have caused severe illness and death. Our society is vulnerable to these waterborne bacteria, and the HVAC/plumbing industry is responsible to do what it can to protect the public.
Studies show that flushing a water system to effectively kill legionella requires water temperature over 160F (71C) throughout the entire piping network for at least 30 minutes; at lower temperatures, some bacteria can remain protected inside the biofilm lining the pipes.
Yet if water at this temperature were to come in contact with your hands, severe scalding would occur in only 2-3 seconds. Key to satisfying both objectives is the use of an ASREE mixing valve which allows systems to maintain high temperatures in the DHW storage tanks, while mixing that water with cold supply to derive just the right temperature.
Heat-Timer® Motorized Tempering Valves Get The Job Done
A valve like the Heat-Timer Electronic Tempering Valve (ETV) is often used as the mixing valve in domestic hot water applications. Whether the system uses boilers and tanks, or generates hot water instantaneously, the function of this valve is absolutely critical in preserving the safety equation. Water must be heated to 160F kill legionella, and a 3-way mixing valve must be used to scalding at the tap. Under control of a programmable console, the motorized valve is automatically adjusted to drive the perfect temperature of water through the system using temperature sensors placed on piping to record the exact temperatures (1 degree accuracy).
Visit our website pages for more information on how domestic how water systems work, or contact a Heat-Timer Representative today
- Wadowsky, R. M., R. Wolford, A. M. McNamara, and R. B. Yee. 1986.Effect of temperature, pH, and oxygen level on the multiplication of naturally occurring Legionella pneumophila in potable water. Environ. Microbiol. 49:1197-1205.