It seems many users forget to size on "Ventilation Rate" instead of "Sensible" in the sizing:system object for systems where the main zone loads are dealt with by zone equipment and the job of the AHU is only to precondition the sum of the minimum outdoor air rates to hospitable temperatures.
Re: Humidity
If, like with most DOASs, you use a heat recovery (HR) devise that has both sensible AND latent transfer, you should consider bypassing the HR at times where zone humidity levels exceed the outdoor air stream humidity levels prior the HR. I have posted before how one can do this.
As designers keep pushing down the outdoor air rates, it becomes apparent that pollutant control is not always the limiting factor...sometimes humidity is the limiting factor. E+ does not model the mold growth on the "cold spots" occuring where the thermal bridging occurs. Keep that in mind when assigning radiant cooling to the correct active surface area (and not simply on the entire ceiling).
ATUs:
As far as I understand, ATU coil/heatexchangers are sized on the air flowrates and supply temperatures specified in the sizing:zone (and associated) object which usually uses the "designday" method (in other words, the equivalent of "Sensible" as in the sizing:system object, i.e. to satisfy the zone load entirely with air)...unless the sizing:system object specified "Ventilation Rate" as the load to size on.
Zone Equipment (simplified explanation)
Except ATUs, zone equipment is sized based on the load calculatedin the zone. The load in the zone is calculated from the energy balance regarding retaining the zone air node at its current thermostat setpoint temperature. The air node itself only has energy transfered to it via air, i.e. convection from all (including non-active) surfaces, and mass flows in (mass flow out is always implied and at the previous timestep's temperature) from various sources such as fan coils, ATUs, infiltration.
Whatever the remaining load is, must then be picked up by the equipment to keep the setpoint.
Mit freundlichen Grü�en- Sent from my iPhone (excuse the brevity)
i. A.
Jean Marais
b.i.g. bechtold
Tel. +49 30 6706662-23
On 25.06.2014, at 08:56, "puranik.sanket@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
Hi all,
It seems that many people are encountering similar problem while implementing Radiant or VRF system with DOAS. The setpoint are not being met while using auto-sizing option. I have been modelling both and have not succeeded in meeting the set points. I believe I am missing something in sizing option and controller:outdoorair, so I am attaching screenshots of these fields.
Please see the attached screenshots and let me know if I am going wrong somewhere. There are other issues as well like controlling humidity but I would first like to meet the temp set points.
Kind Regards,
Sanket
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