Does occupancy schedule influence ventilation in Ideal Air Loads? Influence on sizing

Hello,

I would like to check the annual heating and cooling loads in an office building.
There are two ventilation values assigned - per floor area and per number of people in HB.
As far as i understood - first is constant, second one is variable (the amount of air supplied would depend on occupants in a zone at a specific time).

I created a single-zone test model and experimented with numerous schedules and the differences seem to be very small - does occupancy schedule influence the amount of air supplied at each simulation timestep?

I would also like to ask you if it makes sense to include ventilation in initial model. What I would like to find out are solely heating and cooling loads (+peak loads) depending on facade layout. I do not want to include any systems at this stage.
As this is an office building it seems safe to assume that HVAC system would combine ventilation with cooling and heating, thus supplied air temperature would match the zones needs.
I have doubts setting ventilation up with Ideal Air Loads as supplied air would furtherly cool the space in heating season and heat it in cooling season.

On the other hand, as far as i can see in this example by @chris, the ventilation was taken into account into sizing calculation.

Thank you for your time and help

Hi @Wujo ,

To answer your question, the occupancy schedule does NOT influence the outdoor air supplied through the ideal air system UNLESS you have set the demand controlled ventilation option to True. By default, it is False so, if you did not touch that (or the ventilation schedule), then the outdoor air flow rate through the system is a constant value equal to the sum of the flow/floor are and the flow/person.

The ventilation schedules will change the outdoor air flow rate through the system, though. If you were to plug the zone’s occupancy schedule in as the ventilation schedule you can think of this as another way of simulating demand controlled ventilation.

And I would say that it definitely makes sense to account for ventilation in any model of a commercial building. Residential buildings can be a bit of a wild west when it comes to ventilation standards and so there are some cases where people get away without providing any ventilation through the mechanical system. But commercial buildings tend to be pretty tightly regulated with ventilation standards.

In fact, for some buildings like hospitals or laboratories it’s critical that you model ventilation correctly. Otherwise, you might end up making the completely wrong design decision based on your model. Program types like labs and hospitals have such high ventilation rates that the façade really does not impact the energy use any more than a few percent at best (though the façade can still have important implications for the size/cost of the HVAC system in these cases). In any case, you’ll just have to believe me when I say that I have seen lab buildings built with passive house walls and the cruddiest heat recovery system available on the market, which is effectively akin to throwing millions of dollars and tons of CO2 out the flue every year.

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Hello, thank you for your answer @chris

You said that the outdoor flow rate is sum of flow/floor + flow/person and is constant if i do not change “demand controlled” to true. As flow/floor is pretty straightforward to me, It is unclear to me how flow/person is treated on default settings (false). Will it take maximum number of people/m2 set in a zone and treat it as constant despite occupancy schedule? As if occupancy was set to “1” at all times for ventilation?

As for your last paragraph - I just started another discussion yesterday here about ventilation demand in office spaces that is presented in ASHRAE vs European standards. It struck me that for a office 100m2 space the demand is almost 2x higher in Europe (and that is on the lower side of spectrum, engineers usually design systems for 50m3/h per peron - around 0.139 m3/s)

Making an initial, sizing model, as I have to follow european guidelines, I was wondering if I do not artificially cool/heat the space in periods where i want to do opposite and in which I would condition the space with ventilation air (so in winter I would actually supply warm air, while in Ideal airloads the air supplied would be cool).

Doesn’t this artificially cause higher cooling/heating loads (and peak loads) than it would be in reality? Wouldn’t this cause systems oversizing?

Thank you for your time and help,
Have a nice day

Yes, that’s exactly what happens. It just takes whatever value of people/ floor area is assigned to the zone and uses that to determine the air flow to add. I know it seems wasteful and you might be surprised by how many opportunities there are like this to save energy in typical buildings. It’s not that demand controls really cost that much and, a in a lot of cases, they can pay for themselves in under a year. It’s just that the building industry is not the fastest to make changes and, particularly when people expect their buildings to last for a long time, it often takes a lot of effort to convince people to build something other than “what we’ve always done.”

I’ll respond to your question about European standards over there.

You can think of the ideal air system as essentially recirculating the air of a zone. So it takes in air at the room temperature and spits out warmer or cooler air as needed to meet the setpoint. While it’s doing this, it also mixes in some outdoor air into the stream that it’s supplying back to the room (if ventilation air has been specified). So it’s not artificially oversizing the system. If no outdoor ventilation is specified, it’s only supplying hating/cooling to offset the zone loads and, if outdoor air is specified, it also accounting for the the fact that the hot or cold outdoor air adds or removes heat from the zone. Does that answer your question?

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Thank you for in-depth explanation @chris, this is exactly what i needed to know.