Heat-Timer® Corporation

Domestic Hot Water Priority

What is Domestic Hot Water?

Many residential homes and small commercial buildings have two hot water systems; a heating system and a domestic hot water system (DHW). As the words entail; the heating system is used to provide heat to the building during cold weather. The domestic hot water system (DHW) is used to provide hot water for the kitchen and bathroom sinks, tubs and other appliances. The same boiler can provide both water systems as long as both water systems do not mix. That is the water that circulate through the heating system cannot be circulated through the domestic hot water system.

Types of Domestic Hot Water Systems:

Domestic hot water can be supplied through a stand-alone water heater or an indirect water heater (a tank with a coil) that is heated using a boiler. A stand-alone water heater can be instantaneous or a hot water storage tank. Instantaneous water heaters have no storage capacity. The heater fires up and heats the water when someone opens any of the hot water faucets. On the other hand, the hot water storage tank will heat the water and store it for later use. The indirect water heater is a storage tank with a coil in the middle. The hot boiler water is ran through the tank coil to heat the tank water surrounding the coil. That tank water is used as domestic hot water.

What is Hot Water Priority and How Does it Work?

When a single boiler is used to provide both building heat and domestic hot water, the boiler may sometimes need to provide only heat (during the winter months). Other times may need to provide only domestic hot water (during summer months). However, there are times where the boiler is required to provide both heat and domestic hot water. To satisfy this scenario, the boiler size must be large enough to satisfy both loads.

Heating a building is a very slow process as it takes hours to raise the building space temperature just a few degrees. In addition, the water temperature used is mostly at around 160°F. On the other hand, domestic hot water is a very fast process, as it requires a large amount of energy to be put into the water in a very short time. The boiler water temperature may reach 180°F during a domestic hot water call. This helps in fast domestic hot water recovery. In most residential installations, the domestic hot water load is greater than the heating system load.

What “Domestic Hot Water Priority” does is allow the use of a smaller size boiler to do both jobs. It does that by turning off the boiler output to the heating system when there is a domestic hot water call, which normally does not take a long time. And since, heating is a slow process, the building space temperature drop in that period is so small. To organize this operation, an outdoor reset heating boiler control with domestic hot water priority is required. During heating, the control will regulate the boiler water temperature to satisfy the heating load. However, during domestic hot water calls, the control will turn off the heating system circulating pump and will raise the boiler water temperature to a minimum of 180°F to satisfy the domestic hot water load.

Other Benefits to a Smaller Boiler

Using a smaller boiler reduce short cycling of the boiler during mild weather conditions. Reduction in boiler efficiency and longevity has been attributed to boiler short cycling.