[Equest-users] Fwd: Hours floating when fans on

James Hess JHess at tmecorp.com
Tue Jul 28 19:46:48 PDT 2009

**warning: long email response**
(more of a blog :)

I concur with Steve.

It's likely that you have excess simultaneous heating and cooling going on in the building. To better align your model with the simultaneous heating and cooling that is going on in the building, you first need to get the actual programmed VAV box min airflow setpoints from the EMS and input into eQuest.  Also find out if these go to zero during unoccupied hours (if not, that would be a great ECM).  This should get your model a lot closer to the bills.

But high minimum airflow settings are not the sole cause of excess simultaneous heating and cooling in VAV systems.  You have some investigative work ahead of you. I would recommend that you get some trends going on the VAV boxes and look for things that could cause a lot of unnecessary simultaneous heating and cooling.  Variables to trend include box airflow, damper position, reheat valve position or status (ie on/off, electric resistance), discharge temp, zone temp, etc.  Scenarios to look for that cause excess simultaneous heating and cooling include VAV boxes fighting each other (ie open spaces, uncoordinated boxes), malfunctioning airflow readings, broken dampers, too tight of temperature dead bands between cooling and heating modes,  etc. A broken damper that is wide open causes the reheat coil to go wide open to prevent overcooling of the space. A faulty low airflow reading causes the damper to go wide open, causing the reheat coil to go wide open to prevent overcooling of the space. Uncoordinated VAV boxes serving large open spaces can get locked into situations where one box is trapped in full heating mode while another box is trapped in full cooling mode.   All of these situations lead to excessive simultaneous heating and cooling.

A new situation I recently ran into is where the VAV box has programmed min and max airflow settings for heating mode. Having two airflow settings for heating may or may not be a good idea, I'm thinking it's not a good idea.  It's definitely not a good idea when the default max heating airflow value is set by default in the controls system to a value of 2200 CFM. What I found was when a box slipped into heating mode, it could not escape. This was because the control action if the box wasn't meeting the zone heating setpoint was to open the damper and provide more "cold" air, up to the max airflow or damper position, whichever comes first. Because the max airflow setting was 2200, the damper position usually maxed out first. Reheat valve doesn't open and modulate until the damper is at max position. What happened is that the damper opened 100%, which obviously didn't allow the box to meet the zone setpoint and in fact made the situation worse. To compensate, the reheat valve opened 100% and that was still not enough to bring the zone back to setpoint. The zone setpoint was 73. The actual temp was 69 to 70. The occupants were complaining that the area was too cold and we had 100% simultaneous heating and cooling.  This is the kind of stuff we run into in the real world. You have to get into the details to find the problems and solutions.  This particular situation could have been avoided if the max heating airflow setting was set equal to the min heating airflow setting. Then, the only control action in heating mode is the reheat valve itself, which would not have to fight the damper.

This example just illustrates that you could have your work cut out for you in discovering all the reasons for why the building is using more heat in the summer while your "ideal" model shows otherwise.  VAV systems can be efficient, but also have tremendous potential to waste significant amounts of energy.

But back to the min airflow settings.  These can have such a huge impact on overall building energy usage. In my experience, just improper setpoints (high) and non-existant occupancy programming can lead to double the energy usage in the building from what it could be if simultaneous heating and cooling were minimized (ie what your ideal model assumed). That may sound crazy, but I think many experienced energy engineers have seen this and know that it's true.

If all we accomplished in our profession is getting rid of unnecessary simultaneous heating and cooling in existing buildings, we'd be accomplishing a lot of energy savings.  Steve Doty wrote a great article recently on minimizing unnecessary simultaneous heating and cooling in the Journal of Energy Engineering.  Highly recommended reading.

Hope all of this helps for your situation and is also useful to all other energy engineers out there using eQuest.

Cheers! :)

James A. Hess, PE, CEM
Energy Engineer
TME, Inc.
Little Rock, AR

From: Steven Gates <stvgates at pacbell.net<mailto:stvgates at pacbell.net>>
Date: July 28, 2009 5:47:58 PM CDT
To: "James.Souffrant at tac.com<mailto:James.Souffrant at tac.com>" <James.Souffrant at tac.com<mailto:James.Souffrant at tac.com>>, "equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>" <equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>>
Subject: Re: [Equest-users] Hours floating when fans on
Reply-To: "steve.gates at doe2.com<mailto:steve.gates at doe2.com>" <steve.gates at doe2.com<mailto:steve.gates at doe2.com>>

Make sure the minimum flow ratio matches what the boxes actually do, rather than being allowed to default.  Also, if hydronic reheat, are you simulating the piping losses?  In many cases the thermal losses of piping can be 1x-3x larger than the actual reheat end-use load.

From: equest-users-bounces at lists.onebuilding.org<mailto:equest-users-bounces at lists.onebuilding.org> [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of James.Souffrant at tac.com<mailto:James.Souffrant at tac.com>
Sent: Tuesday, July 28, 2009 12:01 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Hours floating when fans on

Would anyone have any suggestions for how to decrease the amount of time a system spends floating?  I am modeling a VAV reheat system, with 24 hr heating availability, space heating set point 74 occupied /unoccupied, throttling range of 1 degree.  I am unable to simulate enough heating during the summer to reflect actual usage and I believe most of it is being lost during these time periods.  I have attached a pdf of the specific system SIM file.  Specific months of reference are May-Nov.


James Souffrant, EIT
Engineer I, Project Development and Engineering

Energy Solutions

4431 North Front Street
Harrisburg, PA 17109  USA
Direct:  +1 (717) 232-8182 x264
Fax:      +1 (717) 232-8187
Mobile:  +1 (717) 982-3691
Email:    James.Souffrant at tac.com<mailto:James.Souffrant at tac.com>
Web:     <http://www.tac.com> www.tac.com<http://www.tac.com>
P  Please consider the environment before printing this e-mail.

TAC is becoming Schneider Electric on October 1, 2009.
Notice: This email message, together with any attachments, contains information of TAC, which may be confidential, proprietary, copyrighted and/or legally privileged. This email is intended solely for the use of the individual or entity named on the message. If you are not the intended recipient, and have received this message in error, please immediately return by email and then delete it.

Equest-users mailing list
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.onebuilding.org/pipermail/equest-users-onebuilding.org/attachments/20090728/56b8cdac/attachment-0002.htm>

More information about the Equest-users mailing list