Pre dimensioning the DHW system

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Pre dimensioning the DHW systemDomestic hot water (DHW) is a term used to describe new heat pumps or water heater systems in recent years. However, the truth is that we know him a lot more than we thought.

It is recommended that we calculate the hot water demand in our house, building, or room before installing a heating system or producing hot water. Especially if we want to adapt and certify our installation to new DB HE rule updates.

Pre dimension the (domestic Hot Water) DHW system

This article will concentrate on calculating hot water demand, its types, application rules, and the parameters and considerations that must be considered before performing the calculations.


Predimension the DHW system

But what exactly is DHW?

But first, what exactly is DHW? As previously stated, DHW is an abbreviation for Domestic Hot Water, which is water that has been previously heated for human consumption.

It is typically used for sanitary purposes, such as bathing, showering, and so on, as well as cleaning with hot water, such as dishwashing. This water is derived from a system such as an accumulator, a boiler, or a heater.

What is the best way to obtain DHW?

Domestic hot water is usually obtained in one of two ways: by accumulation, in which the hot water is kept in a tank, or by instantaneous, in which the water is heated as it passes through a coil. It is the heating system.

By means of accumulation

Pre dimensioning the DHW system

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

The hot water is thus held in a tank that keeps the temperature constant (unless its operation is interrupted).

This is a centralized system, as the water is delivered from the tank through the house or building’s pipes. It ensures a steady supply of DHW.

DHW by accumulation can be employed in multiple points or dwellings at the same time, depending on the use and kind of installation. It’s a system that’s commonly seen in homeowners’ associations, sanitary installations, and other places.

Accumulation systems are divided into three categories based on the volume of circulation.

  1. System of accumulation
  2. Semi-accumulation systems are designed to meet peak DHW demand.

Instantaneous water heaters are used.

Pre dimensioning the DHW system

The entire water flow will be constantly heated by this system, and it will be terminated whenever its function is disrupted, without accumulation.

They are less expensive than accumulation systems because they do not require a particular tank and are considerably easier to set up. They also take up less room.

The following are the most typical instantaneous heaters:

  • 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.

Calculation of DHW consumption on a daily basis

The ACS demand calculation is required to validate the energy performance of a new building or refurbishment. In the event of single-family, multi-family, or tertiary use residences, estimations must be followed for the calculation of energetic efficiency.

The DHW demand is calculated using a 55°C DHW outlet temperature, the projected number of people, and their daily consumption, all while considering the building’s nature.

The reference value for daily DHW consumption per person in buildings for tertiary use varies depending on the nature of use.

Predimensioning the DHW system


The key is to find the right compromise between an attractive solar fraction and sufficient production per m² for the solar installation to remain economically justified.

For low solar fractions that guarantee greater surface production (better efficiency and therefore shorter return times), the system will be sized in the summer (solar needs and supplies).
To obtain greater solar coverage, the system will be sized in relation to the solar energy available in mid-season.

However, in operation, an appropriately sized plant should produce between 300 and 450 kWh / m².
The following table presents the sizing values ​​commonly found in practice (Source ATIC):

Solar fraction% Type of installation Sensor surface Buffer volume
20 to 40% Great 1m² for 50 to 70 l / day of DHW at 60 ° C 50 l / m²
40 to 50% medium 1m² for 50 to 60 l / day of DHW at 60 ° C from 50 to 60 l / m²
50 to 60% Little 1m² for 30 to 40 l / day of DHW at 60 ° C 60 l / m²

As a general rule, for very small installations (domestic type) a minimum of 4 m² of collectors are considered to make a solar system profitable, while for collective dwellings 1 to 2.5 m² of solar panels per house are sufficient