Introduction to Thermal Comfort in Sefaira

This section provides an overview and links to how Thermal Comfort Calculations in the Sefaira Web-application work (Systems or the New Architecture view) including: 

  • What is Thermal Comfort
  • When Thermal Comfort is important
  • How we process raw results from EnergyPlus
  • How Thermal Comfort works in EnergyPlus
  • Why runs take longer and results may change
  • Why some air-conditioned models will still fail comfort

What is Thermal Comfort?

Thermal Comfort is a generic term that describes how satisfied an occupant is likely to be in any given space.

There are many metrics used to measure thermal comfort. The most common is air temperature (which is how most HVAC systems are controlled) but there are others that account for radiant temperatures, humidity, air movement and even what clothes occupants are wearing.

This link takes you to an extensive article describing what all the different comfort metrics are.

Why is Thermal Comfort important?

There are many use where being able to calculate Thermal Comfort is important. Here are some typical reasons for considering Thermal Comfort as part of a design project:

- The project has a natural ventilation component and the envelope needs to be designed to maximise the frequency of comfortable conditions

- The project includes zones with large amounts of glazing and it's important to see how comfortable occupants near that glazing are likely to be

- The project is working towards a green rating or compliance outcome relating to comfort and early tests of designs need to be carried out to see whether it's on track to achieve the required performance.

This link takes you to an article that describes these use cases in more detail.

How Thermal Comfort works in Sefaira

Comfort values are calculated hourly and represent a lot of data that needs to be understood to determine the impact on a design. Sefaira simplifies this by providing a simple pass/fail criteria that is calculated based on hourly comfort data from EnergyPlus. Here's how it works for each of Dry Bulb Temperature, Operative Temperature and PMV.

Dry Bulb Temperature

  1. Sefaira takes all the hourly data from the annual simulation in EnergyPlus and filters out results that are not during operating hours (Operating Hours are defined in the Space Use settings are are when the HVAC system is on).
  2. For each hourly zone temperature results in Operating Hours, we then sort it into one of 3 groups:
    1. Comfortable - the temperature recorded is between the heating and cooling setpoints for the zone (defined in the Space Use settings)
    2. Too Hot - the temperature recorded is above the cooling setpoint
    3. Too Cold - the temperature recorded is below the heating setpoint
  3. For each zone we then convert these values into percentages and compare the Percentage Comfortable with the criteria entered in the Criteria Card and colour the zone as follows:
    1. Red: Zones where the % of hours comfortable is more than 1% less than the target.
    2. Orange: Zones where the % of hours comfortable is below the target but not less than 1% below.
    3. Light Green: Zones where the % of hours comfortable is above the target but less than 1% above the target.
    4. Dark Green: Zones where the % of hours comfortable is above the target by more than 1%.
  4. The summary of the whole building's performance is presented in the Card at the top of the tab. 
  5. By clicking on the zone, you can see the % of hours comfortable, too hot and too cold for that zone.

There is a Tolerance range that is applied to results. The default tolerance is 0.2°C / 0.36°F. This allows for some slack in accepting EnergyPlus outputs as a pass - for example if the setpoint range is 21°C to 24°C then an hourly result is still considered comfortable if the zone temperature is 20.8°C to 24.2°C. This setting is adjustable - more about how it works here.

Operative Temperature

  1. Sefaira takes all the hourly data from the annual simulation in EnergyPlus and filters out results that are not during operating hours (Operating Hours are defined in the Space Use settings are are when the HVAC system is on).
  2. For each hourly zone Operative Temperature results in Operating Hours, we then sort it into one of 3 groups:
    1. Comfortable - the temperature recorded is within the range defined in the "Operative Temperature Criteria" card
    2. Too Hot - the operative temperature recorded is above the range defined in the "Operative Temperature Criteria" card.
    3. Too Cold -he operative temperature recorded is below the range defined in the "Operative Temperature Criteria" card.
  3. For each zone we then convert these values into percentages and compare the Percentage Comfortable with the criteria entered in the Criteria Card and colour the zone as follows:
    1. Red: Zones where the % of hours comfortable is more than 1% less than the target.
    2. Orange: Zones where the % of hours comfortable is below the target but not less than 1% below.
    3. Light Green: Zones where the % of hours comfortable is above the target but less than 1% above the target.
    4. Dark Green: Zones where the % of hours comfortable is above the target by more than 1%.
  4. The summary of the whole building's performance is presented in the Card at the top of the tab. 
    Screen_Shot_2017-07-07_at_13.46.14.png
  5. By clicking on the zone, you can see the % of hours comfortable, too hot and too cold for that zone.

Screen_Shot_2017-07-07_at_13.47.09.png

There is no tolerance applied to Operative Temperature results. This is because the HVAC system is not controlling to that setpoint (so there is no need for a tolerance control).

Predicted Mean Vote (PMV) used to calculate ASHRAE 55

  1. Sefaira takes all the hourly data from the annual simulation in EnergyPlus and filters out results that are not during operating hours (Operating Hours are defined in the Space Use settings are are when the HVAC system is on).
  2. For each hourly zone PMV results in Operating Hours, we then sort it into one of 3 groups:
    1. Comfortable - the PMV value is within the range defined in the "PMV Criteria" card
    2. Too Hot - the PMV value recorded is above the range defined in the "PMV Criteria" card.
    3. Too Cold -the PMV recorded is below the range defined in the "PMV Criteria" card.
  3. For each zone we then convert these values into percentages and compare the Percentage Comfortable with the criteria entered in the Criteria Card and colour the zone as follows:
    1. Red: Zones where the % of hours comfortable is more than 1% less than the target.
    2. Orange: Zones where the % of hours comfortable is below the target but not less than 1% below.
    3. Light Green: Zones where the % of hours comfortable is above the target but less than 1% above the target.
    4. Dark Green: Zones where the % of hours comfortable is above the target by more than 1%.
  4. The summary of the whole building's performance is presented in the Card at the top of the tab. 
    Screen_Shot_2017-07-07_at_13.51.01.png
  5. By clicking on the zone, you can see the % of hours comfortable, too hot and too cold for that zone.

Screen_Shot_2017-07-07_at_13.52.13.png

There is no tolerance applied to Operative Temperature results. This is because the HVAC system is not controlling to that setpoint (so there is no need for a tolerance control).

How Thermal Comfort works in EnergyPlus

Sefaira uses EnergyPlus as the means for calculating thermal comfort and uses the same simulation that provides energy outputs. Here's a very brief overview of how it works:

- Sefaira breaks the building into thermal zones. This link tells you more about how to do that - the default is perimeter / core zoning with a perimeter zone in each cardinal direction.

- For each thermal zone, the heat loads from the interactions at the building envelope are all calculated in 15 minute time-steps, along with internal occupants, equipment, lighting and other space loads. The cooling and heating provided by any HVAC systems are also accounted for.

- EnergyPlus then calculates hourly the air temperature, humidity and the radiant temperature (based on the temperatures of all the surfaces). It does this using the heat balance method every 15 minutes across the whole year.

- EnergyPlus includes some scripts that Sefaira uses to convert these outputs into thermal comfort measurements.

Full details available in the EnergyPlus reference manual chapter on Occupant Comfort available with this link.

Why runs take longer

As described above, EnergyPlus runs 15 minute time-steps when we calculate thermal comfort. We do this because the warmup of the building typically is not done adequately with fewer time-steps.

When Thermal Comfort is switched off, Sefaira runs with 60-minute time-steps. This is because energy results are not substantially different with 60-minute time-steps but simulation times are faster.

Why results are different

Changing the time-step changes the results but in most instances it's not expected to change results substantially. 

The only known instance where results may vary substantially is if Natural Ventilation is on in conjunction with heating in a cool climate. In this instance, heating energy with 4 time-steps will be much lower (and more accurate) because the operation of the windows in Natural Ventilation mode will be smoother and let in less cold air.

Why some air-conditioned buildings still fail dry bulb comfort

Sometimes models will fail thermal comfort even if HVAC is on in all zones.

We've written an extensive article on this - click here.

Sefaira's interpretation of ASHRAE 55 and why most zones fail the default PMV criteria in cold climates

The default PMV criteria are from ASHRAE 55 but are very difficult to pass for cold climates.

This article describes how we interpreted the ASHRAE 55 code.

This article describes why we expect models in cold climates to fail ASHRAE 55 and what work-arounds / interventions may be needed.

 

 

 

 

 

 

 

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