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Lights Sensible Heat Addition - Return Air Fraction

asked 2018-05-09 11:23:41 -0600

MA_Meilleur gravatar image

updated 2020-01-26 11:43:27 -0600

Hi guys, I'm modeling an office building with multiple levels on OS 2.2. When I noticed that heat addition from lights were quite high, I wanted to set a fraction of that heat would go in the return air of a plenum, but that field in the Loads 'Light Definitions' (Return Air Fraction) is blocked.

What are my options to send a fraction of the heat from my fixtures out of my thermal zones ? Do I have to make a plenum ? Other Ideas ?

Cheers ! !

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answered 2018-05-09 11:56:24 -0600

updated 2018-05-09 12:35:24 -0600

This was a bug that has been fixed in v2.5.1:

Part of the problem was the App was setting the value shown for "Fraction Visible" to the "Return Air Fraction" field in the osm. If you don't want to upgrade to the latest development release, that would be one workaround. The other is to fix it with a measure.

To change the fractions of all lights definitions in a measure, the code would look like:

# assign fractions to variables
frac_radiant = 0.3
frac_visible = 0.2
return_frac = 0.5

model.getLightsDefinitions.each do |light_def| # loops through all OS:Lights:Definition in model
  # set fractions
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Great to know thanks !

By curiosity, what measure would you use in that context ?

MA_Meilleur gravatar image MA_Meilleur  ( 2018-05-09 12:20:44 -0600 )edit

None exist that I know of. See my edit above. More info about writing measures is here.

Eric Ringold gravatar image Eric Ringold  ( 2018-05-09 12:34:45 -0600 )edit

@Eric Ringold, any idea why even with the version 2.5.1, adding a return air fraction does not have an impact on 'Lights Sensible Heat Addition [GJ]' in the 'Annual Building Sensible Heat Gain Components' report ? (See my Answer bellow)

Thanks !

MA_Meilleur gravatar image MA_Meilleur  ( 2018-05-14 12:51:29 -0600 )edit

All the electrical energy to the lights contributes to the building heat gain, the fractions just denote what amounts contribute to different terms in the heat balance: radiant to the long-wave radiation term, visible to the short-wave radiation term, return to the zone return air.

Eric Ringold gravatar image Eric Ringold  ( 2018-05-14 13:10:28 -0600 )edit

That's interesting... I would of though that a fraction of the heat gains from the lights would be lost in the plenum and ventilated outside, specifically for systems without heat recovery.

So there is no way to keep a load but to reduce its heat gains ?

MA_Meilleur gravatar image MA_Meilleur  ( 2018-05-14 15:40:59 -0600 )edit

answered 2018-05-10 10:02:06 -0600

MA_Meilleur gravatar image

Quick update, I have downloaded OS V2.5.1, the 'Return Air Fraction box' is now available under 'Lights Definitions'.

I would have excepted that the sum of 'Lights Sensible Heat Addition [GJ]' in the 'Annual Building Sensible Heat Gain Components' report would have lowered, but it didn't.

Any idea why the heat gains due to lights did not reduce after having setting a 'Return Air Fraction' ?

Thanks !

Lights Definition Screen Shot

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Asked: 2018-05-09 11:23:41 -0600

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Last updated: May 10 '18