RE: [EnergyPlus_Support] fixed pulse HVAC size + thermostat w/hysteresis

At the moment, all energy analysis programs that I am aware of are intended to simulate HVAC and control systems which are operating properly. You can simulate some faults (e.g., a setpoint that is off by a fixed value), but that is not the intention of the modeling software.

I think you will need to tap your creativity in order to simulate and unstable (“hunting”) control loop.

James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of jeannieboef@xxxxxxxxx
Sent: Tuesday, December 17, 2013 2:50 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Cc: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] fixed pulse HVAC size + thermostat w/hysteresis

Modern building controls are pretty good. And minute wize simulation is uncommon for anual energy consumption simulations. Therefore e+ assumes perfect control as a good approximation.

In TrnSys I have simulated complex systems with PID controlers where my dual heating/cooling thermostats are setup with 4 points, e.g. 20 (heating on), 22 (heating off), 26 (cooling on), 24 (cooling off). But it doesn't stop there...the cooling and heating coils have their own setpoints depending on whether the AHU is in heating or cooling mode. And even more complicated because the coil setpoints have to dynamically adjust within the range from design outlet temp to design outlet temp + tolerance which depend on also the temperature sensor in the return airstream. And similarly the other componants like mixing valve (with economizer function), heat exchanger (with bypass), etc. must all be controlled and work together. This seems to work with very small timesteps 0.025 hr. In TrnSys you must set all this up yourself. I'm glad I'm not a controlls engineer. It is quite difficalt.

E+ does have new thermostat types which turn the heating on and keeps it on untill the upperbound is reached. They are thought for specific applications (see io ref), and should not be generally used. I have not tested them and am wary of overall system and numerical stabillity issues due to my experience in TrnSys.

Goodluck.

Mit freundlichen Grüßen- Sent from my iPhone (excuse the brevity)

i. A.

Jean Marais

b.i.g. bechtold

Tel. +49 30 6706662-23

On 17.12.2013, at 03:29, Troy Harvey <taharvey@xxxxxxxxxxxxxxxx> wrote:

I am trying the get a EnergyPlus model to model a fixed sized HVAC system with a thermostat having hysteresis.

I typically use EnergyPlus for annualized energy demand, in which the standard purchased air approach is fine � however for my current project, I need to loook at realistic control conditions. When you look at a given day with EnergyPlus's HVAC output is an unrealistic variable output supply heat into the zone to meet the instantaneous demand rather than a pulse-modulated fixed output you'd see in a real CAV system . And the zone air temperature is perfectly tracking the set point without any sawtooth pattern caused by thermostat hysteresis.

I have set E+ to a 1 minute simulation interval
The HVAC heat output is like in the red line below, using a compact system with a fixed zone BTU sizing, where what I want is like the blue line:

Any thoughts?
<PastedGraphic-18.png>

thanks,

Troy Harvey
---------------------
Principal Engineer
Heliocentric
801-453-9434
taharvey@xxxxxxxxxxxxxxxx

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