Switch off HVAC when windows are in operation

I am trying to simulate a mixed-mode scenario in which, when the conditions permit, a window is opened and the HVAC is shut off (cooling/heating as well as ventilation).

I have built the following definition based on one of the sample files:

natvent test.gh (115.4 KB)

As you can see, I am trying to compare a purely mechanically ventilated room (top) with a mixed-mode ventilated room (bottom).

In both cases, I have defined set points between 19 and 24C; in the bottom case, I have additionally allowed the window to be opened when the indoor temperature exceeds 22C and the outdoor temperature exceeds 21C (but only if the outdoor temperature is at least 1K cooler).

The overall decrease in cooling energy for the analysis period is 7%.

I had expected this to be higher, and am wondering: is the IdealAirLoads system still providing outdoor air while the window is open (and therefore possibly having to cool it to 12C or whatever that set point is)? If so, how do I prevent this?

Hi @MaxMarschall ,

Yes, the Ideal Air System will continue to supply the minimum outdoor air requirement even when the windows are open since there’s no guarantee that the windows are open enough to meet that requirement. You can tell the system to stop supplying that minimum outdoor air using a ventilation_sch_ and the HB Apply Room Schedules component. But, when the room is in between heating/cooling setpoints like that, the outdoor air isn’t heated or cooled when it’s brought in by the system so I would doubt that you’d see much of a change by using the ventilation schedule.

7% savings with a 24C cooling setpoint is actually pretty good in my experience and I’m almost a little surprised it’s that large when you aren’t changing the cooling setpoint. I would recommend that you change this cooling setpoint because we know that PMV does a poor job at evaluating thermal comfort in naturally ventilated buildings and, as the Adaptive Model illustrates, occupants will likely desire warmer indoor temperatures if the average monthly conditions are warm and they have the freedom to open the windows. If you ask me, I think this change in comfort models is where the vast majority of energy savings from naturally ventilated buildings comes from. As you see with your simulation here, if you don’t account for this comfort model change, the operable windows really aren’t much different than the air side economizers that we already put into most air-based HVAC systems these days.

Depending on the climate, I think you can boost the cooling setpoint up to something like 27C-29C if you want to account for the behavior and desires of occupants under the adaptive model.