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Ideal Loads outputs: Supply Air or Zone energy demands?

asked 2020-09-19 17:20:09 -0500

Mafalda Correia gravatar image

updated 2020-09-19 17:31:28 -0500

Greetings. I have read the I/O Reference on this and did a quick search here, I'm still not sure of what to use.

I have a building with an Ideal Loads HVAC system. Since I use Airflow Network for natural ventilation (there's no simultaneity in my hybrid model, I use EMS), I inserted my mechanical ventilation rates through the Outdoor Air field of Ideal Loads (and modelled infiltration through AFN: Crack). I've got no Heat Recovery and no economizer on my Ideal Loads. I need to obtain the useful (thermal) heating and cooling energy demands for my conditioned zones, to compare to an energy certification scheme's thresholds, and from there I'll use a theoretical Heat Pump with a global efficiency of 0.2 to calculate the heating and cooling COPs the theoretical way in order to determine the electrical loads (this is only a virtual, theoretical house).

After a search here, I think the outputs I want to obtain from EnergyPlus are the "Supply Air" ones, but doing a preliminary run I get (for my dining-living room zone):

1) Zone Total Cooling Energy pretty much at every hour that I get Zone Total Heating Energy (that doesn't happen for Supply Air);

2) Annually I have: larger Supply Air than Zone for Heating, and the inverse for Cooling; the latter would mean I have negative Outdoor Air cooling annually, attending to the 'Supply Air + Heat Recovery = Zone + Outdoor Air' expression on I/O Ref??

3) I only get 12 h annually of active Supply Air cooling, what? My HVAC has intermittent operation and I have optimised natvent and shading models, but still I'm simulating on a warm temperate climate with high radiation, 12 h is very strange... (this is for a 20-25ºC constant thermostat)

4) Summing each heating and cooling annual pair, I get a little bit more Supply Air than Zone total HVAC demands, for far less operating hours (less than half).

Well, I'm still confused.

Can someone please enlighten me on what is the variable I want to obtain for my thermal loads? Thanks in advance!

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answered 2020-09-25 18:40:59 -0500

Mafalda Correia gravatar image

updated 2020-09-26 07:52:40 -0500

In case anyone is wondering, the best response I found for this is here:

In the case that outdoor air is considered, Ideal Loads handles the treatment (pre-heating or pre-cooling) of this outdoor air (in my case, necessary to oblige to minimum ventilation rates) separately from the conditioning of the indoor Zone air. So, Supply Air will have bigger annual sums than Zone for heating (because pre-heating for ventilation is included in Supply Air), and smaller for cooling because the outdoor air provides some (free) cooling, i.e.: when the outdoor air is cooler than indoor air, the outdoor air in the mix results in less energy required (to be taken) to cool the mix to supply conditions than it does without the outdoor air included (Zone).

Thus, Supply Air is the closest output to the actual thermal energy the HVAC system uses.

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Asked: 2020-09-19 17:20:09 -0500

Seen: 235 times

Last updated: Sep 26 '20