Dear Dr. Li,
I think you are asking me a question about VAV system use in my experience.
I see a significant number of VAV systems for the commercial buildings that I model with EnergyPlus. These buildings are normally larger than 50,000 square feet. All of the VAV systems use variable speed drives.
p.s., VAV systems have been around since the 70s or perhaps earlier! I am starting to see more zone-based HVAC systems, such as VRF + DOAS or WSHP + DOAS. There are clear benefits including greatly reduced reheat, lower fan power and often heat recovery is used for further savings.
The Building Performance Team
James V. Dirkes II, P.E., BEMP , LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of YuanLu Li
Sent: Friday, April 13, 2012 12:18 PM
To: EnergyPlus_Support
Subject: RE: [EnergyPlus_Support] Re: Heating/Setpoint Controller
Since you are commenting on my reply to another user, I would like to know what you are doing with VAV.
As far as I know, there are not many HVAC system running VAV. (or using variable speed motors) Modular system is common. VSD may be low cost now, but the controlling algorithm is not easily applied.
VAV is not new. I was involve with one system in 2007. It was more complex than the system simulated with EPlus.
VAV in the system simply means that you are using a variable speed fan. How this is controlled make a lot of difference in perfornance In EPlus, it is simply the sum of all the VAV dampler air volume.
When the VAV damper is at maximum, the problem is set-point not met. When the VAV dampler is at the minimum set-point, the reheat heater comes on, so that you have cooling and heating at the same time. . If you lower the minimum damper position to the desined value 0.3, the DX coil may freeze.
Without VAV and fixed damper with zone temperature control, the DX coil may be turned off when the temperature is low and cooling is not needed.
Set-point manager is to free up the thermostat control of individual zones. If you have zone thermostat to control the common system equpment, the DX coil is turned off when the cooling reached the desired zone thermostat set point value.
If you do not have thermostat control in each zone connected to the same AHY, the configuration is the master/slave temperature control of the zones. No set-point manager is required. This is how the EPlus worked up to version 4.0. The examples, have seen been "up-dated".
What I have discribed in my last reply was to use a real system for comparison.
EPlus is simulating the variable damper with reheat heater as the equipment for the VAV AHU.
For this configuration you may run into the problems listed in the paragraph above. A real system has more controls which are not simulated. If you know how to use EMS for these extra controlls, go head. If not, just leave the VAV alone and use it with care. A simple work around is to cool the return air by increasing the fan speed, so that the over cool situation can be minimized. I understand this is not what the user want to do to satisfy the ASHRAE guide line.
If you remove the reheat and set the minimum fan speed low, the DX coil would freeae.
Therefore, the set-point temperature need to be raised to 16 to 18°C so that the fan speed at 0.3 would not be too cold. If the internal added heat load is low, you may rase the set-point temperature to 2°C below the zone thermostat set-point.
Individual zone temperature control can be achieved with other configurations. (Zone fan coil system)
Window units, etc.
Dr. Li
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: ecoeficiente@xxxxxxxxx
Date: Fri, 13 Apr 2012 13:30:05 +0000
Subject: [EnergyPlus_Support] Re: Heating/Setpoint Controller
Hi, I am stuck on the same problem.
Is Energyplus EMS the only way? Or which 4-5 variables can you adjust?
Also, what do you mean by not using VAV? I am using an AHU and VAV or not, there is the same problem, since you have to use a Setpoint manager for the coils.
It is a very common HVAC configuration, which I cannot find how to configure...
Any idea?
Regards,
Germán.
--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, YuanLu Li <yli006@...> wrote:
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> If yu do not have pre-heat and reheat and there is not need to use VAV, you should not model using VAV. The zone thermostat should be used to control the cooling and heating coils directly. When you use VAV dampler, the theremostat controls the damper which change the flow rate which results in different zone temperature. The process is more complex. You need to adjust four or five variables to work together to make the system work. ============================In a real VAV system, a computer control system is used to control the fan speed. Pressure sensors are also required. Duct temperature, diffuser temperature and room temperature are modified to make the desission on whaat to do. Dr. Li
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> From: matthew.sverre.anderson@...
> Date: Thu, 10 Nov 2011 15:13:58 +0000
> Subject: [EnergyPlus_Support] Heating/Setpoint Controller
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> Hello Everybody, I have a question that likely (or at least hopefully!) has an easy answer and I am simply too foolish to know it.
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> So I have a building which I modeled as a VAV with no reheat, and no preheat coil. I want the coils to operate as needed and simply ignore any frost warnings. However, I currently have no schedule for either since there will be many times during the fall/spring where we might need to heat/cool during the same day or during the week.
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> The problem I am currently having is that the heating/cooling coils are operating year long at the same time. I want the HC to operate when the building needs heating, the CC to operate when the building needs cooling (neither to operate at the same time). Am I thinking about this incorrectly?
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> Also is there a way to implement a dead band above/below the set point where the system is "satisfied" to prevent the system from "slamming" on/off?
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