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What constraints are placed on Ideal Air Loads that would cause it to be unable to meet setpoint?

asked 2019-01-29 15:47:00 -0500

updated 2019-01-29 15:47:13 -0500

I am using ideal air loads in a somewhat unique zone and am getting 8369 unmet cooling hours. I had thought that ideal air loads was designed to meet temperature setpoints no matter what.

Zone characteristics are as follows:

  • Below grade
  • All adiabatic boundaries except with the floor above (this was done to avoid additional heat gains from the OS-assumed ground temperature of 18C)
  • Very large volume
  • Setpoint of 41F

Could the issue be the outlandishly low setpoint temperature?

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If you plot the zone temperatures for the zone, what does it look like is happening? Would be interesting to see what temperature it is typically getting down to. Also is your heating setpoint set somewhere below 41f. Normally I would expect a fatal error if you don't have a deadband, but I can't say I've confirmed that with ideal air loads. Thought I'd ask just to confirm.

David Goldwasser's avatar David Goldwasser  ( 2019-01-29 19:16:14 -0500 )edit

I built in a 1F deadband. For most of the year the system get the zone at exactly 55F. In the coldest of months, it was able to the temperature below 55F (even meeting the 41F setpoint at some times).

jbatt's avatar jbatt  ( 2019-01-30 12:57:35 -0500 )edit

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answered 2019-01-30 13:01:42 -0500

As a follow-up. There is indeed a constraint placed on Ideal Air Loads (alluded to in the following error):

  ** Severe  ** InitPurchasedAir: For ZoneHVAC:IdealLoadsAirSystem = UNDERGROUND IDEAL LOADS AIR SYSTEM serving Zone UNDERGROUND
   **   ~~~   ** ..the minimum supply air temperature for cooling [13.00] is greater than the zone cooling mean air temperature (MAT) setpoint [10.00].
   **   ~~~   ** ..For operative and comfort thermostat controls, the MAT setpoint is computed.
   **   ~~~   ** ..This error may indicate that the mean radiant temperature or another comfort factor is too warm.
   **   ~~~   ** Unit availability is nominally ON and Cooling availability is nominally ON.
   **   ~~~   ** Limit Cooling Capacity Type=NoLimit

There does not seem to be a way to change this constraint on minimum supply air in the OS interface; however, I reduced it to 1C in the idf and it seemed to solve the issue.

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That's good to know, thanks for the update.

David Goldwasser's avatar David Goldwasser  ( 2019-01-30 13:05:28 -0500 )edit

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Asked: 2019-01-29 15:47:00 -0500

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Last updated: Jan 30 '19