Should minimum flow ratio (air side HVAC) be specified in the baseline model if it is specified in proposed (In Detail Edit, within zone parameters/properties, in Air Flow tab)?
All thermostats are set to reverse action.
Baseline currently shows same min design flow as proposed (.50 cfm/ft2).
When I look in zone properties, basic specifications tab...
proposed model shows 1.0 minimum flow ratio, baseline shows n/a.
Same thing in AIR FLOW tab for each zone...
Proposed currently shows 1.0 min flow ratio, baseline currently shows n/a.
I'm assuming the best option would be to tweak proposed to remove the 1.0 min flow ratio to match baseline?
Also, Within Air-side HVAC system parameters/properties (Fans Tab)...
my proposed model specifies static in WG (1.0) and Total Eff Frac (.33)
my baseline model specifies Design KW/CFM and Delta T inputs instead.
Should the proposed model match the same supply KW/CFM and Delta T inputs in baseline model? (Instead of modeling Static in WG and Mech Eff).
My concern is that by specifying fan power differently between the two models, the proposed fan power may result in using more energy than the baseline.
Chris Baker
CCI CAD Drafter
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Hi Chris,
The baseline system type determines if the minimum flow ratio is to be specified. For Baseline Systems 5 and 7, the minimum flow rate is 0.4 cfm/ft2. For Systems 6 and 8, the minimum fan powered box flow is to be 30% of the peak design flow. All other Baseline systems have a minimum flow ratio of 1.
Thermostats in the Baseline case are proportional. For the Proposed design model you would only use reverse action thermostats if that is what the design is. A reverse action thermostat is used for VAV boxes where the heating maximum flow is greater than the minimum flow. For example, in full cooling mode the VAV box is fully open. As the cooling load drops, the VAV box starts closing until it reaches the specified minimum. If heating is called for, the box starts to open again up to the heating maximum flow. This is explained in the DOE22Vol2_Dictionary manual, page 472 under THERMOSTAT-TYPE.
The fan power for the Proposed case must be set to realize the fan power shown in the Mechanical Schedules for each air handler (or on the shop drawings). You can set this using Static and Efficiency or kW/cfm which ever works best. For the Baseline fan power, you have to follow Appendix G3.1.2.9. And yes, you will often have different fan power in the Proposed and Baseline models. (I am assuming you are following Appendix G and not Section 11 of 90.1).
Christopher Jones, P.Eng.
Tel: 416.644.4226 * Toll Free: 1.888.425.7255 x 527
Thank you Chris.
This system is type 3. I will keep the inputs as is in this case but set the min flow in baseline to 1.
My proposed was changed to reverse action thermostats to limit any underheated hours (very few anyway). My assumption was that baseline would require the same configuration, which is incorrect in this case but depends what you are modeling.
There were forum posts about underheated hours in proposed model and that one way to resolve was to use reverse action thermostats. It worked but I now know my issues were more of a sizing issue rather than anything else.
Thank you!
-Chris