Free trial

Domestic Hot Water

In a home, the consumption of hot water varies considerably according to the habits or the preferences of its inhabitants. For this reason the level of coverage by a solar energy system depends on the consumer’s profile. In principle there is rarely a need for the water temperature to be higher than 50°C, except for the kitchen area, where sometimes the temperature requested is up to 55°C.

If the system is not set up optimally, according to the consumer’s profile, unfortunate consequences may occur: if, for example, a lot of water is consumed in the evening, the tank temperature drops and, if too low levels are reached, auxiliary energy is introduced. In cases in which during the night auxiliary heating works too little, there is a lack of necessary hot water in the morning.

Figure: domestic hot water dialogue

The following parameters relative to domestic hot water can be inserted. The user can define different consumption profiles for daily, weekly, and monthly values.

In many domestic environments the consumption of summertime hot water doesn’t quite equal that of winter, depending on whether one wants to enjoy a nice hot bath in winter or a refreshing shower in summer. Such variations, that are verified over the course of the year, can be taken into consideration in the monthly consumption values.

The temperature of cold water in pipes varies according to region, on the average it is approximately 10°C.

Demand Profile and Absence Periods

The hot water consumption dialogue box allows users to choose between ”Profile“ and consumption-specific “Monthly values”. As a standard six different VDI 6002 profiles are available for hot water.  Each single profile can be individually adjusted to the required yearly demand.

Figure: profile dialogue box

The single files are stored in the “Profiles” folder in Polysun’s installation path. No existing profile file should be changed; Polysun runs a check before each use to ensure no file was manipulated. The software offers the opportunity to tailor profiles to one’s individual needs. The file consists of two columns: the first column defines the time in seconds (from 0 to 31536000) when a change in hot water withdrawal takes place; the second column defines the corresponding hot water requirement (in litres) from this time on.

To best set-up a profile in Polysun the use of spread sheets like, for example, Excel is recommended. 

Below is an extract of a profile:





This extract says that starting from second 18000 a water amount of 4.6 litres will be tapped; the same amount will then be tapped one hour later.


  • The software enables the user to create profiles with precision down to the second.
  • All you need to do is copy a line in the profile catalog, accordingly rename the file and select the correct profile type.

Energy requirements may be further differentiated by defining any absence periods (e.g. holidays) for hot water consumption (if a profile is not in use) and building model. During the defined period:

  • No hot water is tapped.
  • No ventilation is switched on in the building, the temperature is set to the night mode and no internal heat gain is added.

Absence periods (Period catalog) may consist of multiple switching times. As soon as one of these periods sets in results also apply to the combination.

Ex.: the first period defines August as an absence period; the second defines all week-ends as an absence period. As a result all week-ends and August are regarded as absence periods.

A start and end-date are defined for each period. The time running between the start and end-date is regarded as an absence period. Should the start-date be set to a later point than the end-date the absence period will extend over the end of the year. Ex.: the start-date is set to November 1st ; the end-date is set to  the end of February. As a result the period between the beginning of the simulation and the end of February and from November 1st to the end of the simulation is regarded as an absence period.

A weekly repetition can be set for each day of the week. The repetition will be run between the start and end-date. The cut-in time corresponds to the start-time; the cut-off time corresponds to the end-time on the selected week-day.

Ex.: the start-time is set to February 1st 8:00 a.m., the end-time is set to March 31st 8:00 p.m. 20:00. The repetition is set on Wednesday. As a result in February and March each Wednesday the time between 8:00 a.m. and 8:00 p.m. is regarded as an absence period.

Hot Water Demand Profile Types

Table: Hot water demand profile types

Profile typeColumnsRemark
Hot water amount [l]Time [s]; Amount of water [l] 
HotwaterFlowrate [l/h]Flow in l/h; Time stamp in minsHotwaterFlowrate profile in the DHW-format (external format)
Hot water amount, temperature and flow rate („Full hot water profile“)Time [s]; Amount of water [l]; Temperature [°C]; nominal flow rate [l/h]When the amount of water for a time step is used up, the flow rate will be zero. In case the flow rate is too little, the full amount of water will not be drawn.

In all profiles the time steps can be chosen individually and do not have to be regular. For example, a profile may include time steps of twelve minutes each during the day and only two hot water profile intervals for the night.