Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
I picked one of the systems to look at (they all appear to be similar) It is a single-zone system with a constant volume fan 0A_FC -- Packaged Rooftop Heat Pump Thermal Zone: 0A_FC
From the HTML table you posted: v_bz = .00236 m3/sperson * 72.15 ppl + .00032 m3/sm2 * 268.31 m2 = .256 m3/s e_z = 1.0 (OS defaults e_z zone air distribution effectiveness of 1.0) v_oz = v_bz / e_z = .256 m3/s v_ot = v_oz = .256 m3/s (for single zone systems) v_ou = sum (v_ot) = .256 m3/s (because it's a single zone system)
v_ps_heating = 0.256 m3/s v_ps_cooling = 1.814 m3/s
x_s_heating = v_ou / v_ps_heating = .256 m3/s / 0.256 m3/s = 100% OA x_s_cooling = v_ou / v_ps_cooling = .256 m3/s / 1.814 m3/s = 14.1% OA
This is a system that serves a zone with a very low heating demand. If this were a VAV system, during heating mode, minimum OA flow is higher than the required heating airflow, so you'd need 100% OA to meet the min OA requirement at that flow rate. You could increase the flow rate by lowering the heating supply air temperature. However, this is a CAV system; the fan is one speed and the system is designed for that flow rate. In this case, the cooling flow rate is the driver. Based on my understanding of the VRP calcs, the %OA should be 14.1%, not 100%. I think there might be a bug in E+ VRP calcs for single-zone CAV systems, and I suggest switching to ZoneSum for single-zone systems for now.
| | 2 | No.2 Revision |
I picked one of the systems to look at (they all appear to be similar)
It is a single-zone system with a constant volume fan
fan
From the HTML table you posted:
0A_FC -- Packaged Rooftop Heat Pump
Thermal Zone: 0A_FC From the HTML table you posted:
0A_FC
v_bz = .00236 m3/sperson m3/s*person * 72.15 ppl + .00032 m3/sm2 m3/s*m2 * 268.31 m2 = .256 m3/s
e_z = 1.0 (OS defaults e_z zone air distribution effectiveness of 1.0)
v_oz = v_bz / e_z = .256 m3/s
v_ot = v_oz = .256 m3/s (for single zone systems)
v_ou = sum (v_ot) = .256 m3/s (because it's a single zone system) system)
v_ps_heating = 0.256 m3/s
v_ps_cooling = 1.814 m3/s m3/s
x_s_heating = v_ou / v_ps_heating = .256 m3/s / 0.256 m3/s = 100% OA
x_s_cooling = v_ou / v_ps_cooling = .256 m3/s / 1.814 m3/s = 14.1% OAOA
This is a system that serves a zone with a very low heating demand. If this were a VAV system, during heating mode, minimum OA flow is higher than the required heating airflow, so you'd need 100% OA to meet the min OA requirement at that flow rate. You could increase the flow rate by lowering the heating supply air temperature. However, this is a CAV system; the fan is one speed and the system is designed for that flow rate. In this case, the cooling flow rate is the driver. Based on my understanding of the VRP calcs, the %OA should be 14.1%, not 100%. I think there might be a bug in E+ VRP calcs for single-zone CAV systems, and I suggest switching to ZoneSum for single-zone systems for now.
