Heat-Timer® Corporation
Domestic Hot Water Systems

How Do Domestic Hot Water Systems Work?

A Domestic Hot Water (DHW) System delivers hot water to fixtures used by people at the sink, shower, tub and any other appliance where water may contact humans. They typically work by delivering  hot water through a centralized storage tank separate from water that may be used for steam or hydronic heating.

Residential DHW vs. Commercial DHW

In both Commercial and Residential systems, the definition of DHW is the same. Domestic Hot Water refers to any hot water delivered for use by people, well…., domestically. For residential DHW systems, this includes the cylindrical, floor standing 40 or 50 gallon hot water heater found in many homes across the country, used to deliver hot water to appliances throughout the house or apartment. These are simple systems with cold water in, and hot water out, sized to meet the needs of a family.

A commercial domestic hot water system, however, must consider issues of water demand across many users, many floors, and many fixtures. Commercial DHW must comprehend how much water is needed during peak usage periods, and the maintenance of constant water pressure under all loads. DHW systems must be controlled and plumbed so as to prevent the growth of deadly legionella bacteria, while reducing any potential scalding hazard at the sink or shower. In large hotels, office buildings, and resorts, calls for hot water peak during the morning rush to get ready for work. In these businesses, having no hot water or alternately burning people’s hands and face in the shower is simply not an option. Just one legionella outbreak at your hotel has the potential to poison and even kill your guests, paving the way to multi-million dollar legionella lawsuits. No scare tactics here, it has happened.

How Do Domestic Hot Water Systems Operate

In commercial domestic hot water systems, solutions are most often tank based. A central reservoir of hot water is heated to ~140F – 160F (60C-70C) and then cooled by a mixing valve to between 110 and 120F before it hits sinks and showers so that no scalding occurs. The water is centrally stored in tanks at a higher temperature to prevent the growth of Legionella bacteria and to insure that demand for hot water can be met during peak periods.

A typical system installed using Heat-Timer’s electronic 3-way mixing valve is shown below:

The above system is typical of larger installations at apartment buildings, office buildings, hotels, and other multi-unit dwellings.  Multiple boilers supply multiple storage tanks with hot water near 160F. When there is a call for hot water, sensors used with the Heat-Timer ETV 3-way mixing valve, feed temperature information back to the ETV computer control. The control uses advanced PID logic to mix in just enough cold water to deliver the desired temperature of domestic water to the fixtures in the building.  The diagram above can be extended to as many boilers as are used in a particular mechanical room.

Mixed DHW and Heating Systems (Domestic Hot Water Priority)

Sometimes a boiler system can be designed to provide both DHW and the hot water used for hydronic (radiator based) heating. After all, both these systems require hot water so it can be economical to use the same boilers to provide both. In such a system additional logic is required to allow the boiler controls to operate the system efficiently and correctly.

When a single boiler is used to provide both building heat and domestic hot water, the boiler may sometimes provide only heat (during the winter months). Other times it may need to provide only domestic hot water (during summer months). At other times, the boiler may be called upon to provide both heat and domestic hot water at the same time. In these scenarios’ something called “Domestic Hot Water Priority” comes into play. Since calls for hot water are generally satisfied quickly the boiler is switched to hot water production and the loss of building heat during that time is generally negligible. Visit our page on domestic hot water priority to see a detailed discussion of how these systems function.

How Do You Control Large Scale Domestic Hot Water Systems?

Whether your boilers are modulating or staging, or your system requires DHW priority, or is a completely standalone domestic hot water system, Heat-Timer® provides a Platinum Series control that is the most sophisticated, energy management control in the world. Coupled with our 3-way, Electronic Tempering Valve (or Electronic Tempering Station) we provide a complete solution to domestic hot water system control for apartment complexes, hotels, office buildings, schools, campus settings and more.

You can choose the right Platinum Control for your Domestic Hot Water System depending on a variety of different factors as shown below:

Platinum HWR Control – This control is for a single hot water boiler application that is being used for building heating and domestic hot water. It manages domestic hot water priority, and completely controls the domestic hot water pump for maximum efficiency.

Platinum HWRQ Control – This control can operate multiple boilers, and multiple boiler stages. It can operate as a control for a standalone domestic hot water system, or as a control for domestic hot water priority options in a shared boiler system. When used as a heating control it also features outdoor reset capability for maximum thermal efficiency.

Platinum Multi-MOD – For standalone domestic hot water systems that require multiple boiler modulating stage control. The Multi-Mod controls set point precisely, while auto-rotating stages for load balancing and even wear. It also includes outdoor reset capability, wireless sensors, and comprehensive mobile device and internet remote management capabilities.

Electronic Tempering Station – Rounding out the solution for domestic hot water is our electronic tempering station which incorporates our motorized electronic tempering valve (ETV) into a pre-plumbed solution. This motorized 3-way mixing valve has its own control computer which can be located up to 500 ft. from the valve.

For further information on the use of Heat-Timer Products in commercial domestic hot water applications, contact a Heat-Timer® representative in your area.