DHW Demand (Sanitary Hot Water)

Domestic Hot Water (DHW) is a term that has become popular in recent years with the new heat pump or water storage heating systems. However, the reality is that we are much more familiar with it than we might initially think.

When installing a heating system or water heating for DHW, it is advisable to calculate the DHW demand of our home, building or premises. Especially if we want to adapt and certify our installation to the new updates of the DB HE regulations.

In this post we will focus on the calculation of DHW demand, its types, application regulations and those parameters and considerations to be taken into account before making the calculations.

But what is DHW?

But before we start, what is DHW? As we have seen above, DHW is the acronym for Domestic Hot Water, which is water that has been previously heated for human consumption.

It is usually used for sanitary purposes, i.e. for bathing, showering, etc., and also for cleaning with hot water, such as dishwashing. This water comes from an installation such as an accumulator, a boiler, or a heater.

How to get DHW?

There are two common methods for obtaining domestic hot water; one is by accumulation, when the hot water is stored in a tank; the other is instantaneous, with the water being heated as it passes through a coil. It is the system of a heater.

By accumulation

By using energy generated by a device (a heat pump, solar panels, an electric boiler, etc.), the water is heated and transferred to an accumulator.

The hot water is thus stored in a tank that maintains a constant temperature (unless its operation is interrupted).

This is a centralised system, as from the tank the water is distributed through the pipes of the house or building. It guarantees a constant supply of DHW.

For its installation, it is necessary to calculate the DHW demand of the property, taking into account the use of the property and the number of people living in it or using it.

Depending on the use and type of installation, DHW by accumulation can be used in different points or homes at the same time. It is a system that we usually find in residents’ associations, sanitary installations, etc.

Accumulation systems are classified, according to their volume of circulation, into

  • Accumulation systems
  • Semi-accumulation systems, focused on peak DHW demand
DHW Demand by accumulation

By instantaneous water heaters

By means of this system, the entire water flow will be constantly heated, and will be interrupted once its function is interrupted, without accumulation.

They have the advantage of being less expensive than accumulation systems, as they do not require a special tank and are much easier to install. They also save space.

The most common instantaneous heaters are:

  • Electric boiler, which uses an electric resistance to heat the water flow.
  • Gas boiler, which needs a source of gas to burn to heat the coil that will, in turn, heat the water.
DHW Demand by instantaneous water heaters

Calculation of daily DHW consumption

To be able to certify the energy performance of a new building or a renovation, the ACS demand calculation is essential. Related to energetic efficiency, estimations must be followed for the calculation in the case of single-family, multi-family or tertiary use homes.

The calculation of DHW demand is made taking into account a DHW outlet temperature of 55ºC, the estimated number of people and their daily consumption taking into account the nature of the building.

In the case of buildings for tertiary use, the reference value for daily DHW consumption per person changes according to the nature of use.

In order to take into account the number of people for whom ACS demand calculations are to be made, fire safety occupancy data will be taken.

The ACS demand data for tertiary buildings takes into account the following demand/person:

DHW Demand for tertiary use buildings