Inside
Insights
When Did It Become
Too Trendy To Trend?
Steve Tredinnick, P.E., Mechanical Systems, Affiliated Engineers Inc.
Editor’s Note: “Inside Insights” is a column designed
to address ongoing concerns of building owners and
managers who use district energy services.
We are now in the era of powerful desktop computers that can
store years’ worth of instrumented data either on hard drives or burn
the data onto other types of media for
archiving. In the past three months, I have
run across three clients that compile a
plethora of historical instrumented data
from their building/campus energy management and control systems (EMCSs). If I
wanted to know the cooling coil valve position of AHU- 3 at 12: 45 p.m. on Aug. 13,
2004, that information was logged. If I
inquired about the outside air temperature
at that specific time, that was logged too.
Similarly, if I asked about the supply air
temperature or the space temperature
served by AHU- 3 on the time and day in
question – you guessed it – these data
were also logged. This would be true for
AHU- 1 through AHU- 20, as well as the
chilled-water temperatures, pump pressures, etc. Outstanding! A wealth of information that I could use to determine why
the cooling system is not performing
properly. Right? Wrong!
My next request to the operators was,
“Let’s see the trend log report of all those
points over the last four months.” “Uh,
sorry, Steve. We do not trend log the information into a common file. Each point of
data is just saved in database format.”
What? Hmmm. No problem – I can easily
compile the trend log myself. Just hand
over the data files, and it is just a quick cut-and-paste job in a spreadsheet. Wrong
again! I now had 255 files from the archive
database, and they all had the same file
name: “128.dbs.” To quote television’s
“Crocodile Hunter,” Steve Irwin: “Crikey!”
All the tools were there to trend the data,
but the dots weren’t connected to write a
subroutine compiling all the pertinent
points in a common trend log file.
If this were just one client, I would pass
it off as just a missed opportunity or mis-communication between the client, his
maintenance staff and the controls vendor.
But three clients in three months? It became
apparent that in spite of the ability and
storage capacity of EMCSs to trend large
amounts of data, these features often
receive little or no use, since operators are
generally not trained in data management,
visualization and analysis. The operators truly
have to keep the building up and running
and the occupants happy – that is where
they focus their energy.
I find trended data to be crucial to the
success of my projects when troubleshooting an event or occurrence that I do not
want to occur again or I want to know why
it occurred. Should you initiate trend logging in your facility? To decide, it is essential to answer the pertinent “who, what,
when, why and how” questions.
Starting with the basics, with “what”:
A trend log is “a log of data that is collected
through an energy management system…
consisting of time-series or change-of-value
data that can be collected for digital points
such as temperature, pressure, or status”
(according to Portland Energy and
Conservation, Inc., O&M Best Practice
Series Operation and Maintenance
Assessments – A Best Practice for Energy-Efficient Building Operations, September
1999). Also, what should be trended? The
more information, the better. As a minimum, depending on each system being
monitored (heating, cooling, lighting,
security, etc.), I suggest the following
timestamped data:
● Outside air temperature or enthalpy
● Water and air temperatures
● Space temperature and humidity
● Speed of fans or pumps (variable
frequency drive info)
● Pressures and pressure drops
● Flows (if available)
● Equipment status (on/off) and run time
● Amperage draw
● Position of valves and dampers
Why should one trend data? Having
multiple points of data to analyze from a
specific point or time period can assist in
troubleshooting why something occurred
so a reoccurrence can be anticipated or
eliminated. Common problems such as
clogging or fouling surfaces can be diagnosed by trending the temperatures and
pressure drops of filters, strainers or cooling
coils. Trend logs can also be used as calibration tools for energy modeling or hydraulic
modeling of water systems, verifying operating schedules and assisting in commissioning activities. Another important tool is
Having multiple points of data
to analyze from a specific point
or time period can assist in
troubleshooting why something
occurred…
benchmarking before and after energy retrofits to determine if the return on the investment was met in the expected timeframe.
The easy answer to when to trend log:
“It depends.” It depends on the size and
complexity of the system(s) that the ECMS
is monitoring. However, if the system deals
with occupant comfort or uses energy, the
safe bet is to trend the data.
How often should a point be trended? Every hour to half-hour is usually adequate for most trending; however, every
15 minutes is common. Every minute is
excessive unless the actual process demands
this amount of data. For the most part, it
is limited by the communication network
or data highway technology and what it
can withstand.
