Introduction
In DesignBuilder „HVAC‟ refers to all elements of the building whose function is to provide heating, cooling and ventilation to provide comfort for the occupants. This includes natural ventilation, but this topic is covered in a separate section.
DesignBuilder offers a range of HVAC system types and also 2 levels of detail:
Simple where the system is modelled by calculating the loads required to meet the heating and cooling setpoints. HVAC system energy consumption is calculated from these loads using seasonal system CoPs.
Compact using the EnergyPlus Compact HVAC. This allows a much more detailed model of some HVAC system types to be developed but using simple parametric input data that can be loaded from templates.
In many cases, especially at early stages of the design process when looking at fabric, glazing, solar shading options, you may not want to model the details of the HVAC system. In these cases the heating and cooling energy consumption can be calculated from loads using assumed seasonal system Coefficient of Performance (CoP) values.
Example
This Example will introduce you to the basics of entering data for Simple HVAC.
Starting from where you left off with the Atrium example model, you will create a simple model with heating and cooling, run a simulation and check daily and hourly results.
1. Ensure Simple HVAC and Schedule natural ventilation model options are selected.
2. Window shading using „High reflectivity slats‟ and 4-Solar control with a 150 W/m2 setpoint, T5 fluorescent lights, no daylight control.
3. At Building level go to the HVAC tab and load the Packaged direct expansion HVAC template.
4. Set the Mechanical ventilation Outside air definition method to 1-By Zone.
This means that the Outside air ac/h rate entered on the slider below will be used directly. Other options will be explored in the next Example.
5. Set the Mechanical ventilation Outside air delivery to be 2 ac/h.
6. Leave the Mechanical ventilation Operation schedule to be Office_OpenOff_Occ.
The HVAC tab should look something like the above screenshot. Note the simple definition of the HVAC system. The Heating and Cooling system CoP values are used to calculate the HVAC energy consumption data from the heating and cooling loads calculated in the simulation.
The Fan and pump energy consumption is modelled using the Auxiliary energy in kWh/m2 (default data for this comes from the UK NCM).
7. Heating and cooling setpoint temperatures are defined on the Activity tab because they relate more to the activity taking place within the space (usage)
The model is now ready for simulation. Click on the Simulation tab and when the Simulation options dialog is displayed select Hourly and Daily results, and Annual simulation and press OK to start the simulation. After a few minutes, when the
simulation is finished and the results have been loaded you should see the daily results below.
Now select hourly results by selecting the Interval as 3-Hourly in the Display options panel in the bottom left of the screen. You should see:
Hourly results are much more useful for understanding the operation of the HVAC system than daily results because you can check that setpoint temperatures are met, see periods of the day when peak demands occur etc.
The above hourly results for the year are interesting for an overview, but to get a clearer view it is best to display less days at a time. Try setting the Days per page to 7 in the Display options panel and you will see results for the first 7 days in the year.
The results show:
1. The heating system maintains the heating setpoint of 20ºC during the daytime.
2. The peak heating load takes place on winter mornings
3. The HVAC system is switched off during weekends and at night and the temperature in the zone is allowed to „float‟ during these times.
4. Infiltration is the only source of fresh air when the HVAC system is switched off – during these time the fresh air rate is 0.5 ac/h.
5. When the HVAC system is switched on, the fresh air rate is the 0.5 ac/h constant infiltration + the mechanical ventilation at 2 ac/h x
Office_OpenOff_Occ schedule value.
6. The thermal effect of the mechanical ventilation is shown in the heat balance graph separately from the cooling loads.
7. The heat generation energy consumption is calculated as heating load (Zone/Sys Sensible Heating on the graph) x CoP.
The thermal effect of the mechanical ventilation (External Vent) is shown separately from the cooling system effect (Zone/Sys Sensible Cooling).
You can use the scrollbars along the bottom of the graph to move through the hourly results for the year. Scroll until you reach the summer period starting with June 18th as
This graph shows the cooling system meeting cooling setpoint temperatures and illustrates how the Chiller energy consumption = Cooling load x CoP. Check for your self that this is the case. Chiller CoP is 2.5. Do the same for heating. Heat generation CoP is 3.5.
Also notice how it is the air temperature that is being controlled and that in the summer the radiant temperature can still rise much higher even while the air is being controlled to 26ºC. This leads to higher operative temperatures of 27ºC and higher (mean of air and radiant temperatures). The operative temperature is often used as a good measure of occupant thermal comfort. It is a better measure than air
temperature because it includes radiant effects.
Warning: There is an option to control to the operative temperature instead of the air temperature but we don’t encourage the use of this option because it requires an advanced understanding of the way the model works to understand limits of applicability and error messages that can arise when these limits are not respected.
For zones with very high radiant temperatures, in order to control the
operative temperature to a particular value the supply air must be very cold to bring the zone air temperature down the value required. Sometimes the
required supply temperature is lower than the supply air temperature
requested by the user and in these cases an error message is generated. If you do use operational control you should look out for error messages from EnergyPlus related to this effect.
More Simple HVAC
Example
In this Example you will modify the model created in Example 1, increasing the night time setback temperature for heating and using minimum fresh air per person mechanical ventilation.
1. At building level on the HVAC tab set the Mechanical ventilation Outside air definition method to be 2-Min fresh air per person. This means that the maximum mechanical ventilation fresh air is calculated from the occupancy of the zone and the minimum fresh air requirement per person. Both these are defined by the activity.
2. So on the Activity tab note that the occupancy density is set as 0.11 people/m2. Also under the Environmental Control > Minimum Fresh Air header. Note the Fresh air requirement per person is set to 10 l/s per person. The Mech vent per area data would also be used if we had selected 4-Min fresh air (Sum per person + per area) option.
3. Still on the Activity tab at building level change the Heating set back under the Heat Setpoint Temperatures header to 16ºC. This will be the setpoint whenever the Heating Operation Schedule has a value of 0.5.
4. Go back to the HVAC tab and single-click on the Heating Operation Schedule.
The schedule Office_OpenOff_Heat should be selected. The data for the schedule is displayed in the Data report panel in the bottom right of the screen.
You will see the schedule has values of 0.5 between 7pm and 6am and a value of 1 between 6am and 7pm. Weekends and holidays have 0.5. This means that the heating system will use setback temperature (16ºC) during the night and at weekends and holidays.
5. Likewise for cooling, the main cooling setpoint of 26ºC applies during the occupied period. For AllOtherDays (i.e. Cooling design days) a value of zero is used, that is heating is not required for cooling design.
Run the simulation for the Winter design week and check the heating operation at nighttime. The heating system should operate to heat the building to 16ºC during the night.
Now save the file as a template (File > Save > Save as template) calling it something like Simple HVAC Example 2.dsb.
Radiant Heating using Simple HVAC
Radiant heating systems supply radiant heat to surrounding surfaces and to building occupant. Convective heating systems heat the air only. Radiant systems are often considered to be more comfortable than convective systems and can be more efficient if the occupants directly receive the radiation. Also convective systems can dry the zone air.
1. Following on from the last example, at the building level on the HVAC load the Underfloor heating system, nat vent HVAC template.
2. Open the Heating >Type header and note that the Heating type is
2-Radiative/convective. The Heating radiant fraction is 1 because all of the heat from the radiant system is to be applied in radiant form to the floor. The Radiant distribution is set to 2-Floor because all the heat is to be radiated to the floor surfaces in the zone. The net effect is that all heat from the
radiant/convective heating system is applied via the surface of the floor giving a good approximation to a floor heating system.
3. Set the Natural ventilation > Outside air definition method to 2-Min fresh air (Per person).
4. On the Activity tab set the natural ventilation cooling setpoint to 10ºC to ensure natural ventilation continues to provide fresh air when the building is occupied irrespective of the temperature in the zone.
5. Check the heating setpoint is 20ºC and the heating setback temperature is 16ºC.
Run a simulation for the design week requesting hourly results. Check that the building is being heated to at least 20ºC during the occupied period and to 16ºC during the night and weekend.
If you experiment further with this system type, you may notice that, as with real underfloor heating systems, the room temperature response is slower than with convective systems. This is because the heating takes place through surface of the relatively massive floor construction and not directly to the air as with the Convective option.
Note that simulations with Radiant/convective heating systems run noticeably more slowly than the standard convective only Simple HVAC option. This can be a
problem when these system types are used in buildings with hundreds of zones.
Version 2 of DesignBuilder will have other radiant heating options including radiant baseboards in Compact HVAC.
Night Cooling using Mechanical Ventilation
You can use DesignBuilder Simple HVAC to model thermostatic night cooling. This example takes you through the steps involved in setting up a basic early stage design study of a building using daytime natural ventilation to provide fresh air and night mechanical ventilation to pre-cool the building fabric before the start of occupancy.
Example
1. Starting from the Atrium example base file, at the building level on the HVAC tab, check that the Mechanical ventilation, Heating and Cooling are all
switched on.
2. To ensure the mechanical ventilation occurs during summer nights and weekends only, create a new schedule. A quick way to do this is to copy the Summer cooling workdays (Northern Hemisphere) schedule and modify the copy (you can‟t modify DesignBuilder library components directly).
a. Click on Schedule under Mechanical Ventilation > Operation to open the list in the Info Data panel on the right.
b. Select Summer cooling workdays (Northern Hemisphere) under the general category.
c. Click on the Create copy icon above and immediately on the Edit icon to edit the copy.
Make these changes to the schedule:
d. Change its name to something like Summer night vent.
e. Then change the data to. The items marked in bold have been changed.
SCHEDULE:COMPACT,
This Compact schedule has values of 1 between 24:00 and 6:00 during weekdays, weekends and summer design days and zero at all other times. The timing will allow the fans to operate between midnight and 6am making use of cheap nighttime electricity tariffs.
3. Select the Summer night vent schedule you have just created as the Mechanical ventilation operation schedule.
4. Set the Mechanical ventilation > Outside air flow rate to 5 ac/h.
5. Still at building level on the HVAC tab switch on Natural ventilation and select the Min fresh air per person option to provide fresh air for the occupants during the day. Leave the schedule as the default
Office_OpenOff_Occ.
6. Now on the Activity tab at building level under the Environmental Control header make these changes:
a. Cooling setpoint = 26°C.
b. Natural ventilation cooling setpoint = 18°C.
c. Mechanical ventilation setpoint = 18°C. Night ventilation will switch off if the zone air temperature gets below 18°C (to prevent
overcooling).
d. Mechanical ventilation Max in-out delta T = 2K. This ensures
mechanical ventilation is only activated when the outside air is at least 2K colder than inside air temperature.
The model is now ready to start simulations to click on the Simulation tab and on the Simulation options dialog select Hourly data and Summer typical week. Press OK to start the simulation.
Select hourly results using the Interval drop-down list and you should see something like the screenshot below.
Note the following:
1. The night-time ventilation cools zone air temperature down to 18°C when possible.
2. The heating system operates first thing in the morning to raise the temperature of the zone from 18°C to the heating setpoint temperature of 20°C. An
improved mode of operation for this building would be for the heating system to be switched off in the summer.
As an extra exercise you may like to investigate the effect of mechanical ventilation rate on the load reduction benefit due to night cooling. Try using this model as a basis for a parametric study. You could carry out annual simulations and use the annual CO2 emissions as the criteria.