Comfort Analysis

DesignBuilder EnergyPlus simulations can generate extensive data on environmental conditions within the building and resultant occupant comfort levels.  The following comfort-related output is available:

 

 

The above data can be displayed at all time increments. In addition, air, radiant and operative temperatures can be displayed as distribution curves (below).

 

You can find more detailed information about Fanger, Pierce and Kansas State University Thermal comfort prediction algorithms in the EnergyPlus Background Section.

 

The Mech Vent + Nat Vent + Infiltration data is the sum of fresh air delivered through the HVAC system, infiltration and natural ventilation outside airflow in air changes per hour (ac/h). When used together with the other environmental output it can be very useful in checking for occupant discomfort.

Note 1: It is possible to display comfort data for occupied periods only by selecting 2-Just occupied periods as Zone environmental and comfort reports on the Output tab of the simulation options dialog. This option applies to all data described in this Comfort Analysis help section.

Note 2: Comfort data are not generated when the Gains data model option is set to Lumped. This is because in EnergyPlus the request for comfort calculations is made in the 'PEOPLE' statement and this is not used with Lumped gains.

Block and Building comfort level data

The comfort data listed above is calculated for blocks using floor area weighted averages of the zones in the block. For example:

 

Block average Air temperature (Ta) = (A1 x Taz1 + A1 x Taz2 + A3 x Taz3 + ...)  /  (A1 + A2 + A3 + ...)

 

Where:

 

An is floor area of zone n,

Tazn is the air temperature of zone n

 

There is one exception, Mech Vent + Nat Vent + Infiltration, which is calculated in a similar way but using volume-weighted averages, i.e. using zone volumes instead of floor areas in the above equation.

 

Building data is calculated in the same way but including all blocks.

 

if the Include unoccupied zones in block and building totals and averages option is not set then unoccupied zones are not included in the above equation.

Note: if all zones in the block/building are unoccupied and the option is not set the denominator is zero and an error message is shown and results are deleted.

Temperature distribution

To view temperature distribution graphs follow this procedure:

 

  1. Select the Temperature distribution option on the Output tab of the Simulation options dialog before running the simulation.
  2. If, as is common, you need temperature distribution data for occupied periods only, then you should select the 2-Just occupied periods option for Zone environmental and comfort reports on the Output tab of the Simulation options dialog, again before the simulation. Note that if you do this and the zone is always unoccupied then you won't see any output.
  3. Run the simulation for the period of interest. There is no need to select sub-hourly, hourly, daily or monthly output data if temperature distribution data is all you require.
  4. Navigate to the zone of interest (you must be at zone level to view temperature distribution data).
  5. Set the Interval option in the Display Options panel to Distribution and the Data option to Comfort
  6. View the temperature distribution chart.

If you requested Temperature distribution data prior to the simulation, you can generate temperature distribution curves showing 'hours at', 'hours below' and 'hours above'  temperature levels in 1°C temperature intervals during the occupied period.  Bins include data for the labelled temperature through to that temperature + 1°C, so for example, the bin labelled '12°C' holds data for all temperatures in the range 12°C <= T < 13°C.

 

The temperature distribution result set is calculated using sub-hourly data generated during the simulation regardless of whether sub-hourly or hourly results were requested for output. You should therefore expect to see slightly different distribution results between runs if you change the timesteps per hour setting.

 

To view temperature distribution data set the Interval option in the Display Options panel to Distribution and the Data option to Comfort. This data can be useful for getting an overview of the levels of comfort (or otherwise) provided by the building over time. The results can be compared with benchmark comfort criteria.

 

For example, CIBSE provides recommended benchmark summertime peak temperatures and overheating criteria as follows:

 

Offices, schools, dwellings (living areas) should have no more than 1% of the annual occupied hours over operative temperature of 28°C.  Bedrooms in dwellings should have no more than 1% of the annual occupied hours over operative temperature of 26°C.

 

UK DfES Building Bulletin BB87 recommends an allowable overheating criteria of 80 occupied hours in a year over and air temperature of 28°C.

Note 1: Temperature distribution data is only available at zone level.

Note 2: This data is not available when using Lumped gains model option as the data is tied in with the EnergyPlus 'People' data set and this is not generated for Lumped gains.

Note 3: This data is not available if the 1 time step per hour option is selected

 

Example Temperature Distribution Output

Simplified ASHRAE 55-2004 Graph Related Outputs

The Discomfort hours data is based on whether the humidity ratio and the operative temperature is within the region shown in ASHRAE Standard 55-2004 in Figure 5.2.1.1. For these outputs the operative temperature is simplified to be the average of the air temperature and the mean radiant temperature. For summer, the 0.5 Clo level is used and, for winter, the 1.0 Clo level is used. The graphs below are based on the following tables which extend the ASHRAE values to zero humidity ratio.

 

In some cases you may find a large number of discomfort hours for a particular zone even though the air temperature in the space is within normal ranges. If this happens you should check:

 

  1. Operative temperatures which can be very different in highly glazed spaces in the summer or poorly insulated spaces in the winter.
  2. Humidity levels are within range - you may find very high or very low humidity caused by inadequate ventilation. Note that the minimum comfort temperature with winter clothes when the air is very dry is 21.7°C / 71°F which may be lower than some heating temperature setpoints. Likewise in the summer when the air is very humid operative temperatures above 26.8°C / 80.2°F are considered too hot with summer clothes.

Winter Clothes

Summer Clothes

Note: in ASHRAE 55 comfort assessments the criteria can be exceeded, regardless of the temperature, if the relative humidity is too high.

You can also determine distributed comfort levels using CFD Comfort Calculations.

 

 

The Discomfort hrs data displayed by DesignBuilder is similar to the "Time Not Comfortable Based on Simple ASHRAE 55-2004" data displayed in the Summary report, but EnergyPlus uses a different method to DesignBuilder for calculating building level discomfort hours. "Time not comfortable Based on Simple ASHRAE 55-2004" is the summation of the number of hours over the year when one or more zones in the building fail the ASHRAE 55 comfort criteria. DesignBuilder on the other hand calculates discomfort hours at building level in the same way as other building average values as the floor area-weighted average of all zones.