HVAC system not applied in Simulation

I am trying to complete a thermal comfort analysis for an office building with over 700 rooms. Out of approximately 260 occupied and air-conditioned rooms, I have obtained 79 rooms that are uncomfortable with regard to the PMV values from the ISO7730 standard.
When I analysed the Psychometric and Monthly Data Chart, I noticed variable temperatures, even reaching above 30°C. Each room has a specific program, nevertheless all conditioned rooms should receive similar temperature data during assigned schedule. I applied the DOAS HVAC system with setpoints set to 21-23.5 degrees and schedule for office hours with occupancy between 9AM-6PM. It seems to me that applied setpoints are not respected in the psychometric chart.

To clarify the results:

  • I checked several times if the rooms that don’t meet the PMV standard are: conditioned, have a proper HVAC system, lightning, people and the schedule program.All seem fine.

  • I recreated the simulation with only 5 overheating rooms plus the floor above and under. The results of thermal comfort were correct, and the rooms were in the range of operable temperature.

  • The overheating occurred in adjacent rooms with an internal windows, so to verify if it could be the design problem I unplugged the internal windows and change it to regular interior wall. Unfortunately the final thermal comfort data haven’t changed, I received the exact same number of overheating hours for the rooms.

At this point any ideas of what data should I recheck will be extremely valuable. In addition, I attached to this topic the screenshots:

1. Visualization of Energy Room Attributes (form u-Is conditioned, HVAC Type, Cooling setpoint)

2. Psychometric chart of the exemplary overheating room.

3. Schedule.

If needed I will attach the .gh file, but it has a lot of data so the file may be big.

@mostapha @chris @AbrahamYezioro Please, check this post. I would appreciate your opinion, I´m simply running out of ideas. I did another small simulations with several rooms that are overheating and in that case there is no problem with the HVAC at all, works perfectly.

Hi @karnas ,

Sorry for the late reply here. It sounds like you are discovering the same thing that freaked me out the first time I learned about it, which is that all HVAC systems that heat and cool spaces with hot/cold air are only conditioning the room air to be at the thermostat setpoint.

However, human comfort is more complex than just the air temperature and, in the vast majority of cases, the air temperature accounts for less than half of the total heat loss from human subject. Most of that “air based” heat loss is happening as convection from the human to the air (though this can go up a lot with increased air speeds) and the rest happening through respiration/perspiration (which is affected by both the air temperature and relative humidity, as well as the air speed in the case of perspiration).

But, as I said, all of that “air based” heat loss is usually under half of the total human heat loss in indoor conditioned environments. Under most indoor cases, the mean radiant temperature (MRT) of the room will account for a little more than half of all the heat loss from the subject. It is for this reason that we tend to use the Operative Temperature, which is the average of the room air temperature and the MRT, when trying to evaluate human comfort on a psychrometric chart. And, if you look at the X-axis of your psychrometric chart, you will see that this is what you are plotting.

So, if you plotted the “Zone Air Temperature” on your chart instead of the “Zone Operative Temperature”, you would see your thermostat setpoints are perfectly respected for all of your conditioned rooms. And, in reality, a lot of HVAC engineers will do this plot with air temperature and say that their job is done. But this very wrong in my opinion and in the opinion of many comfort experts. Clearly, you can see in your case that, even though the air temperature is at the setpoint, the human subject can still be uncomfortable thanks to the MRT and the temperature of the surfaces in the room.

With this said, I don’t totally blame HVAC engineers for trying to do this because many of the factors that drive the MRT of a room are more in the domain of the architect than the engineer. For example, if you have a room that is fully glazed, facing west, with high solar heat gain coefficients, the temperature of the Room surfaces is going to be much warmer than the room air when the sun is setting in the afternoon. And, even if you have a very low air temperature setpoint around 70F, the warm surfaces will almost certainly push you out of compliance with 10% PPD. Similarly, if you have a fully-glazed north-facing room with windows having a poor U-Value, then the interior temperature of the glass and overall MRT of the room is going to be much colder than the heating setpoint in Winter, even if the air is at a toasty 75F. Engineers can try to make the situation work with tools like perimeter heat and cool air curtains over the façade but this is never going to be as comfortable as a case where the architect just used their design tools wisely, limiting their use of glass on sensitive orientations, choosing windows with good SHGC and U-Value, and using shading in cases where it’s appropriate.

So what you are witnessing here is a current problem in the building industry that no one has really taken full responsibility for the MRT in our buildings. And, as a result, occupants are largely dissatisfied with the thermal environment of our buildings, even when they are conditioned. For example, in this survey of over 80,000 occupants in conditioned buildings, less then half of them said they were satisfied with the thermal environment:

Hope that helps explain what is going on here.