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Constant volume system, design temperature influence.

asked 2016-10-28 18:43:17 -0500

kbk78's avatar

updated 2017-05-06 08:32:02 -0500

I have a very simple single zone model served by a heating only constant volume system. There is no outdoor air and the fan pressure drop is zeroed out to reduce fan heat influence.

I run the model with zone sizing of 85 F heating supply and then with zone sizing of 95 F. I anticipated the heating results to be identical as the only impact this would have would be on the fan volume and fan power. However we see a 7% change in the heating energy. Could someone hint what could be causing this.

I energyplus V8.5 idf files I used: C:\fakepath\85.idf.jpg C:\fakepath\95.idf.jpg

Here are the HTML output files: C:\fakepath\85Table.html.jpg C:\fakepath\95Table.html.jpg

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Sizing:Zone calculates the zone design air flow rate which assumes a variable amount of supply air at a fixed temperature and humidity. As you set different supply air temperatures, it results different zone design air flow rate with a different fan energy consumption.

TianZhichao's avatar TianZhichao  ( 2016-10-29 02:50:42 -0500 )edit

@Tian, As I indicated I zeroed out the fan energy. the outputs attached indicate no fan energy consumed. For some reason there is a 7% change in the heating subcategory.

kbk78's avatar kbk78  ( 2016-10-31 08:51:22 -0500 )edit

Are you hitting the thermostat setpoint in the 85F case? Did you make absolutely sure that there is no OA being brought in by your Outdoor Air System?

Julien Marrec's avatar Julien Marrec  ( 2016-11-02 07:59:47 -0500 )edit

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answered 2016-11-02 08:33:45 -0500

updated 2016-11-02 08:35:14 -0500

By having a clearly improbable model with no infiltration and no outside air, you're getting temperatures you might not be expecting...

To illustrate this point, here are the hourly temperatures for your zone, all in degrees celsius. The blue series is your sizing:zone object at 85°F and the green is the 95°F sizing.

Hourly temp

Here's the daily mean of the temperature:

Daily Mean

And here's the monthly mean:

Monthly mean

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Sorry, I don't understand what your answer intends to convey. Why is anything other than the fan power changing.

To your point about improbable model, I originally had a proper LEED model with heatpump heating and cooling. Found the savings jump 10% with changes just to the zone sizing temperature. To debug the same I created a very simple electric heating only model to minimize influence of other parameters.

Moreover, Why should it matter weather the model is probable or not?

kbk78's avatar kbk78  ( 2016-11-07 09:45:29 -0500 )edit

Your thermostat is 21C, and you're getting like 32 in one case and 35 in the other one. Is that what you really want?!

Julien Marrec's avatar Julien Marrec  ( 2016-11-07 13:53:42 -0500 )edit

The two models are Identical except for the change in the zone sizing temperature. There is no cooling in the model.

I do not know why it would control to different temperatures in either case. Maybe there is some numerical instability and there is some impact as to what end of the deadband it is controlled to. A small change may be acceptable. But a 7% change does seem quite high, Would you agree.

kbk78's avatar kbk78  ( 2016-11-07 17:18:02 -0500 )edit

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Asked: 2016-10-28 18:43:17 -0500

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Last updated: Nov 02 '16