[Equest-users] HVAC Occupancy Sensors

Thu Mar 16 15:01:02 PDT 2017

I commonly explore the incremental savings and costs of layering occupancy sensors alongside DCV.  When you are already providing new lighting occupancy sensors, the extra incremental costs/labor can be attractive because you can cross-coordinate disciplines and consider the sensor hardware/installation labor a sunk cost to perform double-duty between lights and terminal boxes.

DCV (CO2) sensors and “box occupancy sensor” savings, while interrelated, are NOT THE SAME THING!  They can be complementary, but are definitely interactive ECM scope (cutting into each other’s savings potential) so don’t assume the savings of one in isolation adds up with the savings of the other in isolation.  If this isn’t intuitive, I’ll try explaining box occ. sensor savings through the rest of this email, but advise googling the fundamentals behind DCV sensors immediately after reading this (so my contribution doesn’t need to be twice as long)!

A differentiated pair of Zonal MIN-FLOW-SCH schedules can easily “push” the terminal boxes from a minimum flow rate fraction determined by design ventilation calculations down to ZERO(*).  A few caveats:

·         (*) Think hard about what “ZERO” really means:

o   If you tell a terminal box’s damper actuator to fully close but the central fan upstream is still pushing/pressurizing air behind it… there will be non-zero leakage around the damper and through the box seams.  I think a few percentage points is probably a fair ballpark for new spec-grade equipment (like 5% of design), unless your specification/review happens to lock down on this metric.

o   If we’re talking existing/old boxes, short of measurements to prove me otherwise I might assume leakage around 10-15% of design however to cover myself.

o   Not every mechanical designer responsible for answering calls re: comfort will permit near-zero flow during regularly occupied hours in their controls sequencing.  Be cognizant this can be an energy vs. comfort pain-point and be sure you’re on the same page with your design team / owner.

o   If you’re reading this far and thinking “Nick, I want to shunt off power to my terminal boxes entirely!” … bear with me and stick around for my thoughts regarding hotel rooms.

·         Allowing the boxes to drop way down without also providing a setback temperature during those periods may not look so attractive: MIN-FLOW-SCH only permits lower minimums, not maximum flows.  If your controls specification/installation is up to snuff, you’ll want to also specify a new set of zonal thermostat schedules (HEAT-TEMP-SCH, COOL-TEMP-SCH) alongside the new zonal MIN-FLOW-SCH to pull back the conditioning needs during the same unoccupied hours.

·         Consider potential for “warmup/drift” comfort issues during generally occupied hours.  I make a point for terminal occ sensors to not set back temperatures quite as aggressively during normal occupancy hours as would be appropriate overnight.

A very analogous but instructively distinct situation is hotel rooms, where you see local hotel room HV&C shut entirely off during untenanted periods.  Similar core concepts, similar technical approach.  Major difference is that you will sometimes see the unitary equipment entirely shut off instead of set back, making the rooms literally unconditioned/unventilated when not in occupancy.  Great for energy and reducing operating costs, but can make for musty/off-smells during hotel stays – I’d think really hard about applying that approach in other commercial building occupancies where it isn’t already normal/standard practice for the given region.

Rounding back to the other half of the original query:  how many hours you should model to determine savings, that depends first on the point of your model.  I think there are 2 effective tiers of effort:

·         If you’re doing LEED or compliance modeling, then it would be easiest to follow the % reductions prescribed for lighting occupancy sensors, between 10 and 15% depending on project size.

·         If you’re interested in estimating the “actual” savings (to inform whether this is a wise/responsible recommendation to make for your clients’ dollars), then toss those prescriptions aside and think about the actual likely human occupancy behavior through the day/evening for the spaces in question.

o   The correct % of unoccupied hours could be much higher/lower than 15% on a case by case and building by building basis.

o   I’m not advocating a unique set of thermostat/fan schedules per zone, but it isn’t much extra effort to determine and document the answer for the building in-aggregate to create & apply a uniform set of schedules.

o   Zones/systems with extremely different usage might benefit from a unique set however.

o   Same advice applies to DCV explorations.

~Nick
[cid:image001.png at 01D29E6B.FE0E3940]
Nick Caton, P.E., BEMP
  Senior Energy Engineer
  Regional Energy Engineering Manager
  Energy and Sustainability Services
  Schneider Electric

D  913.564.6361
M  785.410.3317
F  913.564.6380
E  nicholas.caton at schneider-electric.com<mailto:nicholas.caton at schneider-electric.com>

15200 Santa Fe Trail Drive
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From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Michael Mantai via Equest-users
Sent: Thursday, March 16, 2017 11:00 AM
To: Manuela Mengue <mmengue at dagherengineering.com>; John Aulbach <jra_sac at yahoo.com>; 'equest-users at lists.onebuilding.org' <equest-users at lists.onebuilding.org>
Subject: Re: [Equest-users] HVAC Occupancy Sensors

I’ve never tried to account for this in a model, although we’ve had several projects connect HVAC to occupancy sensors.  I’m wondering if you could adjust the zone min. airflow setpoint to the lower unoccupied setpoint for 10% of the occupied hours, to kind of match the 10% reduction in LPD per Appendix G.  If the zone temperature setpoint is also reset based on the occupancy sensor, it seems like you could also adjust the temperature setpoint schedules in eQuest to the lower/higher unoccupied settings for 10% of the occupied hours.  My guess is this would have negligible effect on energy savings, but maybe not.

From: Equest-users <equest-users-bounces at lists.onebuilding.org<mailto:equest-users-bounces at lists.onebuilding.org>> on behalf of Manuela Mengue via Equest-users <equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>>
Reply-To: Manuela Mengue <mmengue at dagherengineering.com<mailto:mmengue at dagherengineering.com>>
Date: Thursday, March 16, 2017 at 10:59 AM
To: John Aulbach <jra_sac at yahoo.com<mailto:jra_sac at yahoo.com>>, "'equest-users at lists.onebuilding.org'" <equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>>
Subject: Re: [Equest-users] HVAC Occupancy Sensors

John,

See Appendix G – Table G3.2 (2) Occupancy sensor for Non-24h and <5000 ft2.

My question pertained to the HVAC component which achieves additional savings for installing occupancy sensors.

Best regards,

Manuela Mengue
Design Engineer

From: John Aulbach [mailto:jra_sac at yahoo.com]
Sent: Thursday, March 16, 2017 10:38 AM
To: Manuela Mengue <mmengue at dagherengineering.com<mailto:mmengue at dagherengineering.com>>; 'equest-users at lists.onebuilding.org' <equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>>
Subject: Re: [Equest-users] HVAC Occupancy Sensors

Somewhere, in the mysteries of space, I have always been told to use 15% less lighting for the proposed case for occupancy sensors. I am unsure of the source. ASHRAE ??

John R. Aulbach, PE

On Thursday, March 16, 2017, 6:02:34 AM PDT, Manuela Mengue via Equest-users <equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>> wrote:

Hi all,

Has anyone succeeded to model HVAC occupancy sensors in eQuest? In this project, supply air from AHU units feed into VAV boxes in classrooms. The VAV boxes modulate airflow in accordance to occupancy sensor readings. The ASHRAE Table G3.2 allows us to take credit for lighting controls, however, I could not find guidance on HVAC occupancy sensors for energy models. My first approach would be to adjust occupancy levels in Proposed (how?), but this raises another question: doesn’t LEED require to use the same schedules in Proposed and Baseline?

Thanks in advance for your suggestions,

Manuela Mengue

Design Engineer

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