How to "switch off" an HVAC system at certain times

ladybug-tools
1

I have a project where I want the HVAC system to not operate at certain times (e.g. at night).

The way I have tried to do this is by creating custom set point schedules. I’ve simplified my model to be a single room (without windows) and these are the schedules/loads I’m using for this example:
set points.gh (89.3 KB)

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That is, for the times where I don’t want heating or cooling, I pick unrealistically high/low set points (99C and -99C). Is that common practice or is there a better way?

The system I am using is “DOAS with fan coil air-cooled chiller with central air source heat pump”. Since I only defined ventilation “per person” (per area/zone/ACH are 0), to prevent airflow during the night hours I simply set it to demand-controlled ventilation:

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From the results, I see that the set points appear to come through correctly:

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However, I still appear to have cooling/heating energy consumption during those night hours:

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This is what the system looks like in OpenStudio:

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The way I understand this system (the Mech engineers at our firm told me to pick it, I’m no Mech engineer myself) is that outdoor air is either cooled (the blue thing) or heated (the red thing) before it is pushed by those fans through a loop that is connected to each of the rooms (in this case just one room). Each room has a terminal with a fan where the air enters the room, but no heating/cooling occurs at this point (hence the “NoReheat” terminology).

And yet, I’ve noticed that EnergyPlus is giving me a zone-level air terminal heating and cooling rate:

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What do these outputs mean, and why are there again values greater than 0 during those night hours?

Furthermore, as per the OpenStudio schematic I’m getting 3 fan outputs (supply and exhaust for the loop, plus the air terminal for the single zone):

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While the zone ventilation rate drops to zero at night, I can see that there is still a constant airflow through the DOAS loop. Some questions on that:

  1. Is that supposed to happen?
  2. Is that what is causing the additional heating/cooling at night?
  3. Why doesn’t the DOAS airflow energy vary based on the demand of the room?
  4. Why is the DOAS exhaust fan energy lower than the supply?
  5. Do I have access to more HVAC parameters, or would I need to do that through Ironbug? The reason I ask is, I was checking out the OpenStudio model with one of the Mech engineers and he wanted me to change one of the values related to one of the equipment pieces (I think it was the inlet air temprature of the DOAS heating coil), but I had no idea how to do that in GH.

Apologies, I know this is quite an essay with a lot of questions but I’m appreciative for any pointers/insights at all!

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That’s likely the result of your DOAS. The demand controlled ventilation is only going to put in some dampers over the outdoor air controller to ensure no fresh outdoor air gets mixed in when there are no occupants in the space. But the DOAS fan is a constant fan and the DOAS air setpoint is effectively a constant setpoint. So those keep running over night. People do this a lot of times for humidity control at night and I imagine that’s kinda what’s happening for your model since it’s primarily the chiller that’s running at night.

If you’re not concerned about night time humidity control, you can shut off the DOAS completely at night by using the doas_avail_sch_ on the HB DOAS HVAC component. That will shut off the fan and essentially stop that cooling coil in the DOAS from running over night.

I know people often consider this nighttime DOAS shutoff as an energy conservation strategy when there isn’t anything precious in the space that has to be protected from humidity.

3

Thanks @chris, that helps to explain why the DOAS is consuming energy at night (i.e. the “Hot water loop central air source heat pump electricity consumption” and the “Chiller electricity energy”).

However, I’m still not understanding why there is a “Zone air terminal sensible heating rate” and “Zone air terminal sensible cooling rate” - does this mean that the zone air terminal is consuming energy for cooling and heating? I thought it only has a fan. If it is cooling and heating, why is it doing so at night?

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For context, I have a project where there are some large spaces that are only sporadically occupied (e.g. exhibition hall) and others that are regularly occupied (e.g. office). We are looking at a strategy to encase the exhibition hall in its own thermal envelope so that we can shut its energy consumption off entirely during long unoccupied periods, while not having it affect the neighbouring spaces when in “off” mode.

In other words, we’ll probably want to keep the DOAS going but just make sure that the “off” spaces aren’t locally ventilating/heating/cooling.

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Hi @MaxMarschall MaxMarschall,

You can make different doas loops for one building.

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different doas loops.gh (38.3 KB)

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Interesting, thanks @Erikbeeren. Forgive my ignorance on the topic but are you suggesting this simply to facilitate the modelling, or are you saying that in the scenario I described it would be common practice to literally have separate loops in the building?

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With this configuration you will get different air loops feeded by 1 chilled waterloop and one hot waterloop. For DOAS systems it is quite common to use different airloops in one building. Often you can safe energy by grouping rooms with the same thermal load caracteristics to one airloop. South facing rooms for example have different load caracteristics then north facing rooms. The heating and cooling demands will be quite different. Sometimes we use 8 different airloops for one building.

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Those EnergyPlus outputs are not actual energy usage like fuel or electricity. They’re just a record of the sensible heat that is being added/removed to the Zones as a result of air being blown through the terminal. Over night, the DOAS fans, setpoint manager, and DOAS heating/cooling coils are still running and blowing “neutral temperature air” through those zone terminals. Given that this “neutral temperature” air is typically at a different temperature from the current zone air, it adds or removes heat from the space. Does that make sense?

1 Like
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Hello @MingboPeng,

The doas_avail_sch on HB DOAS HVAC works. However, I don’t have much control of the other systems parameters such as the DOAS system’s supply air temperature. That’s why I’d like to build a detailed system with IronBug.
But the system I built has two problems that I can’t figure out:

  1. The DOAS fan runs all the time, despite the availability schedule
  2. The FCU fan is supposed to be constant airflow, but the result shows the air flowrate varies across the day
    Could you please help? Script attached. Thanks!
    20240804 IB DOAS+FCU forum.gh (226.1 KB)
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Have you tried to adjust the room’s occupancy schedule to test if DOAS response to it?

You probably need to check the controls. For example, the FCU’s CapacityControlMethod to meet zone’s cooling and heating load.

11

Have you tried to adjust the room’s occupancy schedule to test if DOAS response to it?
Yes I tried, but it doesn’t work. It appears neither the availability schedule nor the DCV has any impact on the VAV fan that supplies OA to the system.

You probably need to check the controls. For example, the FCU’s CapacityControlMethod to meet zone’s cooling and heating load.
It works if I change the capacity control method to constant fan variable flow. Which raises a new question: what does the default method, cycle fan, mean? How can the fan cycle off while there is cooling demand?

Thank you!