Sizing chilled beam w/ DOAS in E+ and Openstudio

asked 2015-08-11 10:46:31 -0500

wangh
395 ●4

updated 2020-03-08 11:28:20 -0500

I always have trouble understanding the Zone Cooling Design Supply Air Temp in the sizing tab. Is that the supply air temperature of the DOAS system (which is usually supplied at 70F or room neutral level) or it is 55 F? I always get an error if I set it to 70 F. This question applies to other DOAS coupled with non-air system which makes the supply air temperature higher than the typical 55 F. Thanks in advance,

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answered 2015-08-11 12:03:52 -0500

Annie Marston
4297 ●4 ●31 http://www.resustain.com

I know that in EnergyPlus when you do zone sizing and you have radiant conditioning with a DOAS, you need to put the supply air temperature as if it was an air system. So for cooling rather than using the 70F that is coming out of you DOAS, in order for the zone equipment to size properly you need to put 50F (20F cooler than room temperature, which i believe is the Appendix G guideline) or similar. This is not very intuitive but over the years of using E+ I have noticed how critical getting the sizing components right is for your systems to run correctly. If in doubt you can always read the input output reference guide that comes with E+ I am sure it is available online as well.

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Thanks that cleared my confusion. So if I put 50F or 60F, what will be the difference? I think it will only change the design cfm but since the air system is sized by ventilation requirement, the cfm should always be the same. So it is really weird to me how E+ will size DOAS with a radiant system correctly.

wangh ( 2015-08-11 15:16:20 -0500 )edit

@wangh so there are three sizing objects, zone sizing, system sizing and plant sizing. The zone sizing object which is the one you are refering to above, sizes all your zone equipment regardless whether it is air or radiant, so it seems a little counter intuitive but to size any equipment in the space, radiators, VAVreheat boxes etc.. you use the supply air temperature even if the piece of equipment is not supplying air, the larger the delta T you use, the smaller the sizing for the capacity of the piece of equipment.