No Mechanical Ventilation in Ideal Air Loads results

Hello,

I have been trying to create a balance graph (according to this example by @chris with new HB.

I noticed there are no mechanical ventilation results for the zones (or rather they all equal ‘0’).
I was wondering what might be the reason for that - i experimented with different flows per area/per person/ach and demand controlled ventilation but nothing seems to change.What might be the reason for such behavior?

I use latest LB/HB build and E+ v. 9.4.0. Please find attached a file with internalized data (i described steps in the file).

MECH_VENT_PROBLEM.gh (103.6 KB)

Thank you for your time and help.

Hey, only thing I did is change 1.1.1 components to 1.1.0 as I have this one installed and it worked. So I am not sure exactly what is wrong in original file but general assumptions are ok I think
MECH_VENT_PROBLEM-v2.gh (104.8 KB)

Hi,

Thank you for your reply.
This analysis runs correctly for me as well and I get same graph.
The problem is that there are no Mechanical Ventilation Loads in the results. As far as I can tell from the picture you posted, neither are they present in your file.

Could you please confirm you get other results than “0” from this panel?

I retracted my previous answer as apparently it was wrong and there still was no Mechanical Ventilation.
However I have now found the solution. The load output requested was set to sensible which is only energy added to the room, and MechVent is energy lost, so changing to all solves the problem:

image

Thank you very much, that fixed the problem.

I still do no understand why Ventilation gains/losses are not accounted when only sensible loads are selected. It is counterintuitive to me to have ventilation gains and losses are not taken into account with sensible loads calculation. Could someone explain that to me?

Hey @Wujo ,

I agree that this is very confusing and I am almost inclined to think that it is a bug in EnergyPlus’s output reporting. But I guess that it’s just that EnergyPlus’s outputs are just weird because the results is still pretty balanced in both the Sensible and the All (or Total) case. The way that we usually compute the mechanical ventilation term is by subtracting the Supply air energy from the Zone Air energy:

And, when we use the total heating/cooling values, the difference between these two gives us the correct term. However, it seems that when we use sensible values, the supply air and the zone air are somehow equal to each other (causing them to cancel out to 0). I guess it’s just that EnergyPlus doesn’t report the sensible balance in a way that includes the Ideal Air System and it can only report sensible values for just the Zone (in isolation of what is coming through the HVAC system). Maybe we could find an explanation if we dig thorough the EnergyPlus output documentation.

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So I think the issue may come down to the fact that you can’t really know if outdoor air provides sensible vs. latent cooling/heating to the zone until it has already traveled through the HVAC system and is then interacting with the zone. For example, you can have warm humid air outside that would have added both sensible and latent heat if it were introduced directly to the room. But then it goes through the HVAC, becomes cool and dry, and then results in sensible and latent cooling to the Room. Or maybe the cooler air introduced to the room is still more humid than the room air so there actually is some latent gain alongside sensible cooling. And I guess it’s just hard to know how much of the original heat of the warm humid outdoor air was actually sensible vs. latent in the end because everything got warped when it passed through the HVAC. So EnergyPlus just throws its hands up and only gives us the sensible balance of the Zone and excludes everything that happened in the mechanical system.

I don’t know if that’s really a satisfying answer but I think something like this may be the reason why we don’t get the outdoor ventilation air in the sensible balance. And we need to request the Total (both sensible and latent) if we want to know the energy that went into heating/cooling the outdoor air.

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I was facing the same problem. But the reason why the mech_vent_load get no value was, I toggled the hvac_energy_use_ to be true.

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