Issue overview

I tried the autosize feature on the reheat box min fraction and it does work but I’m still not getting enough OA out of the system. Here is the problem: when the system OA is upsized based on 62.1, i.e., the system OA is greater than the zone sum, the minimum damper positions required to support the higher OA are the ones that should be used. Further, the max supply for some zones may need to be upsized to support the higher system OA.

In the Primary School example, the system OA is sized to 3.82 m3/s by the VRP sizing, which is much higher than the zone sum OA, but the autosized minimum damper positions ensure only the breathing zone OA will be provided (assuming those rates were input in the zone sizing object). This means under certain conditions, zones could receive less OA than the breathing minimum.

Regarding your question about an example of where the zone maximum is not upsized, see the zone MULT_CLASS_2_POD_1_ZN_1_FLR_1. It’s max supply should be upsized to equal the breathing zone OA divided by 0.6. This means it’s maximum supply will be greater than what it was sized during zone sizing. Minimum supply m3/s = 0.9086/0.6 = 1.51 m3/s, which is greater than the zone max supply of 1.123 m3/s calculated earlier.

We need to configure the minimum damper positions to ensure that the mixed flow rate is at least equal to the autosized VRP 62.1 system OA flow rate.

Here is my suggestion to allow this operation in V8.7:

1. Check if the current minimum damper positions can support the new system OA calculated using autosized VRP and reset the positions as necessary. Carry forward this information to the reheat box minimum.
   Currently, autosizing of the min flow rate for a VAV damper does not pay any attention to OA requirements. It is based on the Sizing:Zone input fields ”Cooling Minimum Air Flow per Zone Floor Area”, ”Cooling Minimum Air Flow”, and ”Cooling Minimum Air Flow Fraction”. If there is no sizing calculation a default of .000762 cubic meters per second per square meter of zone floor area (0.15 cfm/ft2) is used.

2. Check if there are zones that need more supply than the maximum and reset the maximum as necessary.

3. When DesignSpecification:OutdoorAir is entered in the VAV terminal unit, the control does not account for the ventilation efficiency. Need new input(s) in the terminal unit?

4. Also look at impact of Sizing:System 1.0, !- Zone Maximum Outdoor Air Fraction {dimensionless}

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