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Constant Volume Fan not Constant

asked 2021-01-15 17:06:21 -0600

updated 2021-01-20 11:23:22 -0600

I have hardsized everything on the airloop at 500cfm. Constant volume fan. However, when viewing the volumetric flow rate output variables, it appears that the flow rate is not constant. Why?

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And, why does this binary control point (AvailabilityManager:HighTemperatureTurnOn) show other than binary values?:image description

Here is the idf file:

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answered 2021-01-19 12:24:10 -0600

updated 2021-01-19 13:35:36 -0600

It appears your output variable frequency is different than the model's timestep. For example, if your Timestep is set to 4 (15 min timestep), and your output variable is set to hourly, then your output data is not capturing the intra-hourly changes. For example, your AvailabilityManager should be either 0 or 2 at all times; the fact that it's showing different values is due to the values being averaged over whatever your output timestep period is (if hourly, it's averaging the 4 data points that occurred that hour). Now if your fan is also cycling, the same situation could be occurring: fan turns on for 15 min, off for 45 min --> output variable shows 500 CFM *(15/60) = 125 cfm for that hour.

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This is a good point Eric, and I had considered it, but my Timestep is set to 1 per hour. I have tried setting the output variables to either "timestep" or "hourly", but it yields the same result. In many instances, this partial flow continues over stretches of several hours.

Ski90Moo gravatar image Ski90Moo  ( 2021-01-19 15:07:19 -0600 )edit

What kind of air terminal serves the zone? I suspect something is happening at the zone that's causing the fan to respond that way. Either occupancy-based ventilation requirements, VAV terminal unit, or something like that. Can you post the IDF to look at?

ericmartinpe gravatar image ericmartinpe  ( 2021-01-19 16:21:32 -0600 )edit

Thanks for the help Eric. Please see link above. The terminal is constant volume. No DCV.

Ski90Moo gravatar image Ski90Moo  ( 2021-01-20 11:24:00 -0600 )edit

The Zone Timestep may be an hour, but there is an HVAC System timestep that can still be less than an hour. Set your output variables to "detailed" and that may clear up the confusion. Also read this EnergyPlus documentation, especially towards the end, where it describes the system vs zone timestep and the use of "detailed" for output variables.

shorowit gravatar image shorowit  ( 2021-01-20 12:10:24 -0600 )edit

Thank you Scott! That is it. I have my Minimum System Timestep set to 1 minute.

So, Eric was correct in that it was averaging the system timesteps 1/min over the simulation timesteps 1/hour.

Ski90Moo gravatar image Ski90Moo  ( 2021-01-20 13:14:01 -0600 )edit

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Asked: 2021-01-15 17:06:21 -0600

Seen: 40 times

Last updated: Jan 20