Domestic Hot Water

HVAC tab in model data Simple HVAC

 

The effect of hot water consumption (hot water taps, showers etc) on building energy use can be included in your thermal model by switching on DHW on the HVAC tab.

 

When using Simple HVAC, Domestic Hot Water (DHW) is modelled in DesignBuilder by using hot water consumption rates based on the activity in each zone.  DHW hot water consumption is defined on the Activity tab and the definition of the hot water generating equipment is described below) is defined on the HVAC tab.

Type

Three types of DHW are available:

 

Delivery hot water temperature

The temperature of the water supplied to the taps.

Mains supply temperature

The average temperature of the water delivered to the building over the year. This can be approximated to the annual average outside air temperature.

Operation

The DHW Operation schedule defines the timing of the hot water draw off.

 

The DHW consumption rate is defined on the Activity tab in l/m2-day (gal/ft2-day in IP units) but this needs processing before being passed into E+ which requires max DHW flow in m3/s. To do this DesignBuilder needs to use the concept of number of operation hours per day (or 'Typical hours per day') based on this operation schedule.

 

When a Compact schedule is used to define DHW operation, DesignBuilder calculates the 'Typical hours per day' based on the day in the year which has the most operation hours. Fractional hours operation are included as fractions. A 30 minute timestep through the year is used to perform this calculation.

 

ClosedExample 'Typical hours per day' calculation for Compact schedules

 

Schedule:Compact,

Domestic Occupancy,

Fraction,

Through: 31 Dec,

For: Weekdays SummerDesignDay,

Until: 08:00, 1,

Until: 16:00, 0,

Until: 18:00, 0.5,

Until: 24:00, 1,

For: Weekends,

Until: 14:00, 1,

Until: 20:00, 0,

Until: 24:00, 1,

For: WinterDesignDay AllOtherDays,

Until: 08:00, 1,

Until: 16:00, 0,

Until: 18:00, 0.5,

Until: 24:00, 1;

 

For this schedule, the Weekdays give the most operation hours:

 

Until: 08:00, 1,

Until: 16:00, 0,

Until: 18:00, 0.5,

Until: 24:00, 1,

 

In this case 'Typical hours per day' would be calculated as:

 

8 * 1 + 8 * 0 + 2 * 0.5 + 6 * 1 = 15 hours.

 

When 7/12 schedules are used the typical occupancy hours per day is determined from January/Monday data.


When typical workday schedules are used, this is simply the number of hours occupied during the working day.

 

The DHW volumetric consumption rate supplied to EnergyPlus is calculated as:

DHW (m3/s) = DHW (l/m2-day) x 0.001 x m2 / (Typical hours per day x 3600)

Note: the zone DHW operation schedule is also used in Detailed HVAC when the default 1-Simple HVAC Detailed HVAC Activity data setting is made.

DHW Type

A range of DHW types can be selected but note that different types of DHW system do not affect results in EnergyPlus simulations, apart from 1- Same as HVAC which causes heating CoP and fuel to be taken from the heating system. Other options all give the same results.

1- Same as HVAC
2- Dedicated DHW boiler
3- Stand alone water heater
4- Instantaneous DHW only
5- Instantaneous Combi
6- Direct (or off-peak) electricity
7- Heat pump