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outdoor air minimum limit type

asked 2017-08-29 17:36:58 -0500

Matt Koch gravatar image

updated 2017-08-29 21:16:21 -0500

I am wondering about the OA Minimum Limit Type in OS:Controller:OutdoorAir vis-a-vis ASHRAE 62.1 in a run-of-the-mill VAV system. The Ventilation Rate Procedure in OpenStudio/EnergyPlus calculates my minimum OA air flow rates, and I can follow all steps of that calculation. Unless the occupant count changes, those should be the minimum OA flow rates at all times, except when unoccupied.

As such, I am tempted to use the FixedMinimum for said Minimum Limit Type. Of course, when using the ProportionalMinimum, my EUI is reduced compared to FixedMinimum, which is desirable. However, if I understood the manual correctly, then for ProportionalMinimum, the OA flow rate is reduced proportionally to the ratio of supply air flow rate to design supply air flow rate, which could easily drop below the ASHRAE 62.1 minimum OA flow rate. Therefore my question: Is the ProportionalMinimum in violation of ASHRAE 62.1 and must I not therefore always use FixedMinimum if I want to follow ASHRAE 62.1?

Also, is the Minimum Limit Type applicable regardless of whether I run an economizer, or does it only apply when I run an economizer?

Furthermore - and loosely associated with my primary question above - even if I do not use Demand Controlled Ventilation (DCV), ASHRAE 62.1 includes a component that varies with occupant count. As the occupancy count may change throughout the day, should not the ASHRAE 62.1 minimum OA flow rate change throughout the day also? Or is the ASHRAE 62.1 minimum OA flow rate calculation done only once during sizing, and thus a changing occupant count cannot be accounted for "dynamically" (unless one uses DCV, of course)?

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answered 2017-08-30 08:00:29 -0500

updated 2017-08-30 14:48:14 -0500

I would suggest that the ProportionalMinimum represents a cycling fan system where the average outdoor air over the time step is reduced. FixedMinimum is better in this case and would represent an increase in outdoor air during the fan on cycle where the average over the time step is higher than the ProportionalMinimum case. And VRP is of course a better calculation and IMHO should be used with a non-cycling fan system. I'm not sure what happens with VRP if the fan cycles.

The FixedMinimum input will maintain a minimum outdoor air flow rate (multiplied by the Min OA schedule if one exists) regardless of economizer. There is also a hierarchy, FixedMimimum is higher than exhaust air flow rate is higher than Ventilation:Mechanical.

Your last question "ASHRAE 62.1 includes a component that varies with occupant count" and then say "should not the ASHRAE 62.1 minimum OA flow rate change throughout the day", that sounds like DCV to me. If you don't use DCV, you can use FixedMinimum and the Min OA schedule (or occupancy schedule as a surrogate) to get this response.

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Thank you, rraustad, I appreciate the insight. I don't think I can follow the Fixed/ProportionalMinimum explanation, but your last paragraph is intriguing. Nevertheless, only one part of the minimum OA flow is driven by occupants, the other by area. So if I used the Minimum OA Schedule, I would affect both when really I would only want to affect the part driven by occupants. I do think this is different from DCV, even though DCV might be a good approximation of what I want to accomplish. Academic perhaps - my primary concern is satisfying ASHRAE 62.1 while saving energy - DCV may be best.

Matt Koch gravatar imageMatt Koch ( 2017-08-30 08:40:20 -0500 )edit

Your right about the occupancy schedule not including area. So you could take the occupancy schedule and offset it by a fraction that represented area. Then the FixedMinimum would represent total OA based on floor area + occupancy and when multiplied by the Min OA schedule could get close if you did this right.

rraustad gravatar imagerraustad ( 2017-08-30 08:57:58 -0500 )edit

Let me be a little more clear. Calculate OAtotal (area + people) and OAarea. OAareaFrac = OAarea/OAtotal. Then copy the occupancy schedule to one named MinOASchedule. Then correct MinOASchedule values by Value = ( Occupancy schedule fraction * (1 - OAareaFrac) ) + OAareaFrac.

rraustad gravatar imagerraustad ( 2017-08-30 11:12:06 -0500 )edit

That is very clear, thanks @rraustad

Chris Jones gravatar imageChris Jones ( 2017-08-30 11:31:27 -0500 )edit

This is rather interesting, though I am thinking more along the following lines. At full load, we have OA0 = OAa0 + OAp0, while at part load we have OA = OAa + OAp with OAa = OAa0. I think these two expressions can be combined to OA/OA0 = 1/(1+OAa0/OAp0)OAa0/OAp0 + 1/(1+OAa0/OAp0)OAp/OAp0, whereby OA/OA0 represents the (new) minimum OA schedule and OAp/OAp0 represents the occupancy schedule? I suppose this would have to be done somewhere on a spreadsheet as OpenStudio does not seem to have a corresponding scaling mechanism.

Matt Koch gravatar imageMatt Koch ( 2017-08-30 13:39:29 -0500 )edit

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Asked: 2017-08-29 17:36:58 -0500

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Last updated: Aug 30