Dear Shane,
There are MANY possible reasons!
Are the unmet loads for cooling? Heating? Occupied or unoccupied hours? (I don’t worry as much about unoccupied hours)
You NEED to report a lot of variables and review them carefully in order to understand where things are going wrong. Once you know where the problem is, a solution often becomes apparent.
If you are autosizing, review the design discharge temperature setpoints. For example, in a VAV / reheat system, the zone reheat coil discharge temp must be greater than the air handler discharge temp or you’ll never do any heating.
I always create an RVI file so that output variables are organized in a manner that is organized and makes sense to me. Then I dump the data into a spreadsheet which has appropriate display settings for the variable type. It makes the review much easier! Example shown below�.
The Building Performance Team
James V. Dirkes II, P.E., LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Shane Watson
Sent: Sunday, February 13, 2011 7:12 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Sizing of System
Thankyou James,
I am sizing the HVAC system as per the ASHRAE 90.1 specifications, but still the unmet hours are high. Could you throw some light as how to reduce these setpoint not met hours.
From: "James V Dirkes II, PE" <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Sent: Sat, 12 February, 2011 8:46:22 PM
Subject: RE: [EnergyPlus_Support] Sizing of System
Dear Shane,
EnergyPlus normally responds favorably to assigning a Sizing:Parameter Sizing Factor greater than 1.0. ASHRAE 90.1 requires a factor of 1.15 for Cooling systems and 1.25 for heating systems.
There are many other reasons why you may be experiencing the high number of unmet load hours, but that’s a start!
The Building Performance Team
James V. Dirkes II, P.E., LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Shane Watson
Sent: Saturday, February 12, 2011 4:37 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Sizing of System
When on simulating the idf model on design days, i am getting a large number of timeset point not met hours. What would be the correct way to reduce the setpoint not met hours and upto what limit.
Thanks in advance.
From: Drury B Crawley <dbcrawley@xxxxxxxxx>
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Sent: Thu, 10 February, 2011 6:51:31 PM
Subject: Re: [EnergyPlus_Support] Sizing of System
It is better to use design conditions from Chapter 14 of 2009 ASHRAE Fundamentals. If there are design data for that location, the .zip file will contain a .ddy file with that design data (design days). Otherwise you will have to create design day objects with the data from ASHRAE. You can read about design days in the ASHRAE document.
On Thu, Feb 10, 2011 at 8:17 AM, Shane Watson <mechenergy@xxxxxxxxx> wrote:
Hi everyone,
I am simulating a VAV system in an office building, Sizing of the system is done with weather file by using the objectSizing period:Weather file days. Is it a correct way to size the system on the weather file and let the software decide the design condtions. What impact it will have on the HVAC energy consumption.
Thanks in advance.
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