Mechanical ventilation stops spontaneously

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Mechanical ventilation stops spontaneously

Postby Daniello » Fri Mar 23, 2018 9:45 pm

Hi everyone,

I'm simulating an office building, with just one thermal zone. I'm using detailed HVAC with detailed HVAC data. The design supply air rate is about 1.7 ac/h and infiltration about 0.10 ac/h, so total fresh air without natural ventilation is about 1.8 or 1.9 ac/h. The AHU works when the office is occupied, from 8:00 to 13:00 and from 15:00 to 18:00. Mixed mode is activated, with a minimum temperature limit of 19 degrees and a maximum wind speed limit of 1.5 m/s (although i've already increased it and the problem still occurs). Natural ventilation is calculated, and the window operation is always on in weekdays (24 hours on, at 100%), and off in weekends (24 hours off, at 0%).

When checking the total fresh air hourly outputs I found out something weird. In occupied periods, when AHU should be working, the ac/h values sometimes decreased from 1.8-1.9 (which is the correct value) to 1.2 (for example). So, to figure out what was going on, I checked the sub-hourly values, and saw that in some steps (set to 4, 6 and 10 in different simulations, always happening the problem) the ac/h values decreased to 0.10, which means that mechanical ventilation was not working. Consequently, the hourly average (the aforementioned 1.2 value) was lower than 1.8-1.9 ac/h.
In the same way, I found out that sometimes (again with no pattern) windows didn't open when temperature and wind speed limits where satisfied, thus not allowing for natural ventilation. So mixed mode sometimes don't work properly.

Does anyone know what's the reason of this? If some of you need the .dsb or .csv files, just let me know.

Thanks in advance.
Luis Sousa
Posts: 364
Location: Stroud

Re: Mechanical ventilation stops spontaneously

Postby Luis Sousa » Fri May 11, 2018 4:46 pm


From your description, the Mixed mode is giving priority to Nat Vent and this turns off the AHU.

In this case you will need to check your model inputs to find out which input is not allowing the window to open when the conditions are suitable for natural ventilation.


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