Ventilation schedule not affecting HVAC

I am working on a shoebox model for a lab and using a DOAS VRF HVAC system with heat recovery to estimate the energy loads. I applied a simple ventilation setback to turn down the ventilation to 0.43 during unoccupied hours:

When I run the setback scenario using ideal air loads, I see a significant reduction in heating loads which makes sense. However, I am getting the exact same energy loads with and without the ventilation setback when I apply the DOAS VRF HVAC system:

I am trying to understand why I’m not seeing reductions when I run the model with the HVAC system. Am I missing something?

This issue, @eeshakhanna , is that the HVAC templates of the OpenStudio standards gem are just set up to not react to individual zone ventilation schedules. I guess that the OpenStudio team made that decision because an HVAC with central air loop has one central outdoor air mixer for several zones on the loop. So, if you diminish the total outdoor air being mixed at this central exhaust/intake based on an averaged schedule across multiple rooms, you might end up distributing the “ventilation reduction” unevenly (and consequently end up not meeting ASHRAE 62.1 in some rooms).

But I know that ventilation schedules on central air loops like this are used to model important ECMs so we’ll have to expose it somehow. I think the way that we will end up doing it is to add an option for ventilation_schedule_ on the components used to assign detailed HVAC templates to Rooms. Feel free to let me know whether you agree with this strategy on this Github issue:

In the meantime, you can account for your schedule in the OpenStudio application by going to the DOAS loop’s outdoor air mixer and applying a “Minimum Outdoor Air Schedule Name” on this object. For DOAS systems, you will probably need to change the “Minimum Fraction of Outdoor Air Schedule Name” to be blank:

Then, you can save the OSM and simulate it with the “HB Run OSM” component.

If you need a fully Grasshopper-based solution, you can also just model the whole system in IronBug, where you have full control over all HVAC properties. You can apply your IronBug HVAC to the OSM exported from LBT Honeybee using the workflow here:

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Thanks Chris for that explanation. All makes sense. I do want a Grasshopper solution since I am running a parametric sensitivity analysis on my model. I have not used Ironbug before but trying to get more savvy with it. I’m using the HVAC template that comes with the install along with the Re-run OSM workflow in Mingbo’s screenshot, but I am running into this error:

All looks fine inside the HVAC cluster:

Any leads on what’s causing the error?

Thanks again.

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It’s hard to know what the issue is without the Grasshopper file but it’s clearly an E+ failure. Make sure that your Honeybee Rooms are unconditioned so that they don’t interfere with the Ironbug HVAC. Perhaps @MingboPeng might know more.

I’ll try to prioritize the outdoor air schedules on the templates among the energy capabilities to add.

Hey @eeshakhanna ,

I just wanted to let you know that I finished implementing the ability for the LBT HVAC templates to have outdoor air ventilation schedules. I know that I originally said that I might expose this as an option on the HVAC template component but I found it was more intuitive and straightforward to just stick to the following principle:

If a Template HVAC is capable of meeting the Room-assigned criteria, it will do so automatically

This means that all of the templates that support ventilation (the All-Air and DOAS systems) will now always try to meet the room-level outdoor air ventilation specification as it is described. Ventilation schedules will be respected as long as all of the Rooms assigned to a given air loop have the same ventilation schedule (otherwise, the ventilation is set to be constant at the maximum level needed for the Rooms, like it was previously). For ventilation schedules assigned to Rooms with PTAC systems, the schedule will only have an effect if a given hour of the schedule drops to zero since such systems use on/off fans to supply the ventilation air.

The principle of meeting Room-assigned criteria also means that HVAC templates capable of meeting humidification setpoints (eg. for hospitals, nursing homes, or certain laboratories) will automatically include a humidifier in the air loop if a humidification setpoint is found on any of the Rooms. I will try to be consistent about enforcing the principle going forward.