Modeling a 100% OA blow-thru space heater in eQuest?

asked 2017-07-19 14:52:14 -0500

13saw13
85 ●4

updated 2017-07-20 12:51:43 -0500

I'm trying to model a warehouse (heating-only) in eQuest that uses three blow-thru space heaters. The heaters take 100% outdoor air and are constant volume systems. Does anyone have any experience modelling warehouses and if so, what is the best method to model this scenario? I'm currently using the unit ventilator system because it seems to be the heating-only option that doesn't have a return path (it was also the default from the DD wizard). Does this make the most sense or is there a more viable HVAC option? I have the system's MIN-OUTSIDE-AIR set to 1.0 in an attempt to ensure that the unit only uses outdoor air. I have also modeled exhaust fans at the zone level. Upon checking the SV-A report for the system, it seems the outside air ratio is only 0.35, which can be attributed to the fact that outside air flow has defaulted to the same value as the exhaust flow. Is there any way to keep the system at 100% outdoor air while still accurately modeling the usage of exhaust fans? Is a different system type better suited for this scenario? Any help is greatly appreciated!

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answered 2017-07-19 15:39:30 -0500

Molly Curtz
1348 ●3 ●12

updated 2017-07-19 15:40:52 -0500

In eQuest, outdoor air and exhaust air requirements specified at the zone level take precedence over values specified at the system level. So, although you tried to make your system 100% OSA with the system input for MIN-OUTSIDE-AIR, this is being ignored due to your input of an exhaust flow at the zone level. From the help (right click on field) for MIN-OUTSIDE-AIR:

If you enter MIN-OUTSIDE-AIR as well as the ZONE keywords OUTSIDE-AIR-FLOW, OA-CHANGES, OA-FLOW/PER or EXHAUST-FLOW, the ZONE values take precedence.

So, to correct: you should specify that there is 100% outside air flow at the zone level probably by specifing OUTSIDE-AIR-FLOW in cfm for each zone.

Also note:

The unit ventilator system type does have a return air path, but it does not have a return fan, so I think you will be modeling your described actual system just fine.
The unit ventilator system is a zonal system, so to confirm the system is operating as desired use the thermal zone hourly report variables, where you can report zone unit supply cfm, zone exhaust cfm, and fraction of OSA to supply air for the zone unit. Additionally, if you have multiple zones assigned to a single system you will be simulating one unit ventilator per assigned zone.

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Thank you very much! This confirmed my understanding of what I was seeing in the SV-A reports. I do have a follow-up question for you: If I specify OUTSIDE-AIR-FLOW at the zone level to be the same as my design supply flow, will eQuest interpret that as meaning that the system must receive the maximum supply flow at all times to meet the specified ventilation requirements? (1)

13saw13 ( 2017-07-20 06:56:43 -0500 )edit

What I've tried to do is combine three blow-thru heaters into one system. In July (when there is no heating requirement), only one of these units will run (without heating) to supply the ventilation requirement per ASHRAE 62.1. Do you think I need to separate this into three systems in order to model this accurately? My problem then becomes the fact that I have to create three zones (one zone for each system) when in actuality the heaters work in unison to heat/ventilate one large zone. Any workarounds or suggestions you can think of? Thanks again! (2)

13saw13 ( 2017-07-20 07:04:25 -0500 )edit

A1: Yes. Furthermore, the UVT system is a constant volume system. As far as I know, the flow when operating can't vary. It will always run at the design CFM if operating. The best way to review these things, and check what is happening in simulation is to create hourly reports: On the Project & Site Tab, right click on Hourly Reports to create a report and on Hourly Report Blocks to create a block of variables, then assign that block to your hourly report.

Molly Curtz ( 2017-07-20 11:15:46 -0500 )edit

A2: It depends on what you think is most important to get right. If you split the system into three zones, each served by one UVT, you can turn off two during the summer. But, you won't get any vent flow into the zones with their UVTs OFF. I don't know if you care about modeling the impact of OSA on the zone temp, for example. You can try to get zone temp to equalize across the zones by using "air walls" as the partition walls....

Molly Curtz ( 2017-07-20 11:20:20 -0500 )edit

...A2 cont.: . Alternately, use one zone & one system and run different eQ models for different run periods (Rt. click on Simulation Options on the Project & Site Tab). In the winter periods your system would reflect all three units. In the summer, the system would reflect the one unit that operates to provide ventilation. You'd need to add up the results for the different run periods to get your total annual energy. This would change your system based on a pre-set schedule. If the real units stage based on htg. load, then, you could look at the heating hourly rpt. results to set run periods

Molly Curtz ( 2017-07-20 11:25:50 -0500 )edit

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