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EnergyPlus Zone Sizing: how to account for load introduced at terminal from central system

asked 2015-03-12 11:01:02 -0600

mleach gravatar image

updated 2015-04-13 13:26:16 -0600

I'm running into a sizing problem with EnergyPlus. I have a multi-zone VAV system with AirTerminal:SingleDuct:VAV:Reheat terminals. When I autosize from design days, I am getting 0 heating load for a particular interior zone, so the reheat coils are getting no power. However, there IS an operational heating load created by the 55 F air getting delivered from the central air handler and the minimum flow rate for the zone terminal. As far as I can tell, this load is NOT accounted for when the coil is sized. I'm by no means an EnergyPlus sizing expert, but it looks to me that the reheat coil gets sized according to the results of Sizing:Zone, which seems to calculate design heating and cooling loads according to the loads in the space (interior, from equipment, people, lights, etc.; exterior, through the zone envelope), WITHOUT accounting for any minimum flow rate from the central system.

First of all, can somebody verify my assumptions about how these reheat coils get sized? Second, if I'm not missing anything, how can I properly account for the impact of the potential heating load generated by terminal minimum flow rates when autosizing (particularly for the case where zone loads are small)?

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After doing a bit more digging, I found some additional info, but still no answer to my question. Here is the additional info, for what it's worth: 1. Sizing:Zone does look at ventilation rates but (it appears) only to verify that zone equipment flow rates are enough for minimum ventilation. This doesn't affect the scenario described above. 2. Under Coil:Heating:Water Sizing (my coil type) in the Engineering Reference, it says that coils in terminals get sized according to the terminal flow rate, although what I'm seeing with autosizing does not seem to be consistent with this...

mleach gravatar image mleach  ( 2015-03-12 12:08:11 -0600 )edit

I don't have a straight answer to give you, but there's ton of info on the Energy Plus Support group, and looking there in order to formulate an answer would be a good idea. This thread for example

Julien Marrec gravatar image Julien Marrec  ( 2015-03-13 09:22:15 -0600 )edit

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answered 2015-03-17 12:24:32 -0600

Archmage gravatar image

A useful trick to use is to add OtherEquipment to an internal zone with negative loads that are scheduled to be on for only the heating design day. This will give some heating load for the sizing routines to work with.

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@Archmage This seems like a reasonable assumption for an all-air system for a core zone, but what happens when there is a perimeter zone with forced air VAV and baseboard heating? I would think that this would impact the sizing of that system.

jmcneill gravatar image jmcneill  ( 2015-04-22 16:48:30 -0600 )edit

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Asked: 2015-03-12 11:01:02 -0600

Seen: 548 times

Last updated: Mar 17 '15