Why does my conditioned building fail dry bulb comfort?

This article explains circumstances under which it's possible for a building that is air-conditioned (heated and cooled) to still report a fail in the Comfort - Dry Bulb feature.

What does a failing zone in Comfort mean?

A zone (room / space) fails if it doesn't provide air temperatures (dry bulb temperatures) within the zone for a minimum % of hours that the HVAC system is on. 

The default is 98%. This means that if the zone is not within its temperature range (allowing for tolerances) for 98% of the time when the HVAC is on, then it fails.

My building is air-conditioned, why does it fail dry bulb comfort?

For most models and most HVAC systems, all zones that have HVAC on SHOULD pass the default comfort criteria for temperature. This is because most HVAC systems are sized to achieve the setpoint at both full load and zero load under 0.04 / 99.6 percentile weather conditions.

There are however some circumstances that cause zones to fail. Known issues are summarised in the bullet points below and described in more detail beneath.

  • HVAC System is "Heating + Ventilation" meaning it does not include any cooling. 
  • HVAC System is "Radiant Floor" and by default is aiming towards an operative temperature setpoint rather than an air-temperature setpoint. 
  • Natural Ventilation is on and an option without both Heating and Cooling is selected.
  • HVAC System is "CAV - Return Air Central AHU" and perimeter zones have hours with low internal loads while Core Zones have hours with high internal loads.
  • HVAC System is any DOAS system and operating schedule has lots of hours with zero internal load while the HVAC system is on
  • HVAC System is "Displacement" and core zones have large variation in load either through roof glazing or diversity
  • Setback temperatures are substantially lower than the Heating setpoint and the HVAC system has only one hour to achieve Heating setpoint and the first hour does not include full internal loads.

More details on each of the above items below.

HVAC System is "Heating + Ventilation" meaning it does not include any cooling.

This HVAC system does not have any cooling provided. This means it has no way of cooling the space to keep it comfortable. In almost all applications it should be expected to have hours where the space is too hot.

To avoid this try changing to a system with cooling. If your building doesn't have cooling you may want to try adding Natural Ventilation.

HVAC System is "Radiant Floor".

This HVAC system by default works to maintain an operative temperature setpoint rather than an air temperature setpoint.

Operative Temperature is the average of the air temperature and the mean radiant temperature (average surface temperatures in the space, adjusted for distance from the middle of the zone)

It's very easy, particularly in heating, to have a space condition where the air temperature is lower than the setpoint and the radiant temperature is higher than the setpoint. (eg mean radiant temperature is, say 26C / 80F because the floor temperature is warm and the air temperature is 16C / 60F the operative temperature could still be 21C but the air temperature which is used for this comfort criteria fails).

In most instances air temperature is not a suitable measure of comfort for a Radiant system.

Natural Ventilation is On

If you have turned Natural Ventilation On and you have picked an option other than "Natural Ventilation Plus Heating & Cooling" then either the heating, the cooling or both systems in the zone have been disabled for the building. This will make it much less likely the space will be conditioned within an air-temperature setpoint range.

HVAC System is "CAV - Return Air Central AHU"

This system operates with a central air handling unit where the airflow is constant to each zone and sized for the cooling needs of each zone at the design supply air temperature.

When the system operates, the supply air temperature is decided by whichever zone in the building needs the most cooling. If you have a core zone at 100% load, then the supply air temperature will be the design load.

Although heating coils are sized for all zones to "reheat" the air, the perimeter zones are only sized based on what is needed for a winter day with the central unit providing neutral temperature air to an empty perimeter zone.

When the annual model runs, if there is a lot of heat loss out the facade, low loads in the perimeter zones but high loads in the core zones (causing the air handling unit to supply cold air) there could easily be hours where the reheat coil in the perimeter zones is not capable of reheating the air up enough to meet the space load and uncomfortably cold hours will be delivered.

HVAC System is any DOAS system and operating schedule has lots of hours with zero internal load

DOAS stands for Dedicated Outside Air System. DOAS systems have ventilation air supplied to the zone in a way that is separately controlled to the zone units (could be radiators, fan coil units etc).

Often the air supplied centrally is conditioned (usually cooled but also tempered using heat recovery).

When EnergyPlus does autosizing for DOAS systems, it does not allow for the cooling (or heating impact of the ventilation air from the DOAS system as part of sizing the zone equipment.

For sizing calculations this is not an issue as Sefaira adjusts the zone equipment sizes before publishing them in the results. However for the Energy and Comfort simulation we rely on EnergyPlus autosizing to save simulation time.

This means that if the air supplied to the zone is conditioned (the default is to supply at 13°C / 55°F) the zone heating equipment will be undersized for the energy run (typically by around 20-30% but this can depend on your settings).

If the load profile includes lots of hours where there is very low load in a space, it is therefore possible that the heating unit in the zone will not have enough capacity to maintain the heating setpoint and hours will be reported as failing.

To get around this, increase the tolerance of the thermal comfort calculations. 

HVAC System is "Displacement" and core zones have large variation in load

The Displacement system supplies cooling air through the building, usually at very mild temperatures through a raised floor plenum. The air is usually supplied around 16-18C / 60-65F.

The air is supplied into the floor plenum at a constant pressure. At perimeter zones the system includes Fan Power Terminals that can push more air into the perimeter zone to provide extra cooling as needed (and that include a heating coil to provide heating). In the core zones there are no fan powered terminal boxes or heating coils and therefore there is no way of modulating the diffusers (where the air comes in) so that more or less air can come in to meet the load.

This is somewhat offset by the "displacement upwards" of loads in the core zones which reduces the heating load.

We believe this is an accurate representation of how displacement systems work.

The result is that the core zones are sized based on a space load and cannot cope with large variations in load for core zones.

If your model has large variations in the core zone diversity, then it's quite likely the zone will end up being too cold at times of low internal load.

If you have core zones that in practice actually have large variations in space load and a tight comfort range is expected at all times when those variations happen, it might be worth considering a different system.

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