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PMV too low when there is no cooling

asked 2021-01-15 06:41:04 -0500

davis gravatar image

updated 2021-04-17 13:51:48 -0500

image description

Hi, attached image is the PMV data graph from my experiment.

There is only cooling in the afternoon as seen in the image and I can understand why the PMV is much lower. However, in the morning and at night when there is no cooling, the room have negative PMV which suggest there is some cooling means in it even though all cooling coils are off.

May I check if you guys know what is the problem causing it?

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I would recommend reviewing this post while you look over your thermal comfort inputs like activity schedule, work efficiency schedule, etc. that are used to calculate PMV. Which thermal comfort model (Fanger, KSU, etc.) are you using?

Aaron Boranian gravatar image Aaron Boranian  ( 2021-01-15 09:46:02 -0500 )edit

thank you for the reply. I have check this post. My air velocity is 0.1m/s, clothes insulation is 0.5 clo, work efficiency is 0 and people activity level is 126W/person = 1.2 met and all these are set constant throughout the 24 hours.

I am using fanger PMV. For extra information, I have 14 different thermal zones which uses all the same load definition including the 4 above schedules (air velocity, clothes insulation, work effiency and people activity level)stated for PMV. The graph shown is for 1 thermal zone but all the other 13 zones have the same result which morning have low PMV

davis gravatar image davis  ( 2021-01-15 20:53:11 -0500 )edit

Attach is my idf file and I hope you can advice me on this. Thank you.

https://drive.google.com/file/d/1EcCd...

davis gravatar image davis  ( 2021-01-15 20:53:59 -0500 )edit

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answered 2021-01-16 16:33:33 -0500

updated 2021-01-19 11:19:11 -0500

The schedule applied to your heating setpoint temperature setting of dual setpoint thermostats in all zones is Alwas 0 degree[JTH], which is always 0$^\circ$C.

image description

If people are inside of these zones, then that is much too cold and should be more like 22$^\circ$C. My results for PMV are in the -6 to -14 range, so maybe this is an old IDF that you shared? The [Tuck]Thermostat Cooling Temperature schedule applied to the cooling setpoint temperature setting of dual setpoint thermostats in all zones is ~24$^\circ$C, which seems correct.

image description

I altered your IDF to apply a constant 22$^\circ$C heating setpoint, and now the PMV outputs are closer to the image in your post (see below) but still having the same quick drops in PMV around 9 AM on weekdays. Weekends actually have better PMV ranges than weekdays.

image description

This made me think that your heating equipment does not have enough capacity to meet the loads in these two weeks of January. It looks like the only thing directly connected to each zone is a VAV terminal unit without reheat, so all the heating in zones comes from the supply air provided by the central air system. However, it looks like each central air system does not contain a heating coil -- only an outdoor air system, constant volume fan, and chilled water cooling coil. So, the central air system is running on the weekdays and mixing in colder outdoor air. Without a heating coil to heat that mixed air, the central supply fan is providing cold supply air to the zones through the terminal units.

It looks like you're using OpenStudio based on how the EnergyPlus IDF that you shared is organized. You apply the thermostat temperature in the Thermal Zones tab and define the HVAC components of central air systems in the HVAC Systems tab.

UPDATE

After more investigation following the comments below, it looks like the central air systems are all over-cooling the zones because they are forced to operate with constant volume fans and a scheduled temperature setpoint of 12$^\circ$C, so you're essentially providing constant cooling capacity to the zones each time the air system operates. This capacity is "autosized" for a specific cooling design day defined in the model -- 0.4% drybulb with mean wetbulb condition for Singapore, in your case. However, that is an extreme case and the system should be allowed to change air flow rate or setpoint temperature to accommodate lower loads that occur in annual simulations. You could fix this by applying other setpoint manager types in addition to the current scheduled-only options (warmest, e.g.) or by changing fans from constant volume operation to variable volume operation.

I have plotted the temperatures at each node as it progresses through this configuration to illustrate this issue.

image description Outdoor Air System of Air Loop HVAC 15, which serves Admin Office Zone

image description Supply Side of Air Loop HVAC 15, which serves Admin Office Zone ... (more)

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Thank you for your help.

The following are the pointers to reply to your comments.

  1. This is the same idf file that I used to plot my PMV. How I did was I output the variable in excel for the zone "admin office"

  2. The heating setpoint was left 0 because the simulation is not suppose to have any heating because the outdoor air temperature throughout the year range from 24 to 32 degree so the just by cooling it should be able to let the room reach 23.5 degree based on theory.

davis gravatar image davis  ( 2021-01-17 03:09:50 -0500 )edit
  1. Each air loop including its fresh air intake and plant loop only operates from 9am to 5.30pm and does not operate in the morning. Therefore in the morning from 12am to 9am and from 5.30pm to next day 12am there should not be any supply air coming from each air loop to the zone and thus the zone should be hot and should have a higher PMV of maybe around >+3.

Thank you and I look forward to your reply.

davis gravatar image davis  ( 2021-01-17 03:15:09 -0500 )edit

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Asked: 2021-01-15 06:41:04 -0500

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Last updated: Jan 19