Appendix B. Diagnostic Plots

We have identified ten diagnostic plots that can be used to analyze building and cooling system performance.

Whole Building Characteristics

    1.  Whole building electrical power over time
    2.  Hourly whole building electrical power vs outdoor temperature
    3.  Hourly whole building electrical power vs outdoor temperature, for each 24 hour period
Cooling System Characteristics
    4.  Hourly cooling system efficiency vs cooling load (tons)
    5.  Cooling system electrical power vs outdoor temperature
Chiller Characteristics
    6.  Chiller efficiency vs cooling load (tons)
    7.  Chiller efficiency over time
Cooling Tower Characteristics
    8.  Cooling tower electrical power over time (This plot was not included in the original list of nine plots.)
    9.  Cooling tower approach temperature vs cooling produced (tons)
    10.  Cooling tower cooling produced (tons) vs condenser flow
    Prototype Description
    The plots above have been developed for a prototypical Northern California office building. This building is based on 74 buildings and their average energy use. The prototype is assumed to be located in the Alameda area and has 100,000 square feet of floor space in five stories. The exterior walls have R-22 insulation, and the roof has R-25 insulation. Windows are single-pane, with an average shading coefficient of 0.43 and conductance of about 1.0 Btu/hr/sqft/deg F. About 43% of the exterior wall area and 29% of the building wall and roof envelope is glass. The building is occupied by 365 people (about 274 sqft/person). The maximum lighting demand is assumed to be 1.455 W/sqft, and maximum electrical equipment demand is 0.705 W/sqft. The building is assumed to be almost fully occupied, lighted and equipped between 9am and 5pm weekdays, with tapered loads during night and weekend hours. The building is assumed to generate positive pressure whenever the perimeter fans are on, but allows infiltration of 0.07 cfm/sqft at all other times.
    The building is cooled by two electric-powered centrifugal chillers, using two cooling towers with variable-speed fans as heat sinks. The full-load efficiency of the chiller is 0.71 kW/ton. The building is heated by 2 natural gas-fired hot water boilers with a heat input ratio of 1.33, hooked up to a single-zone reheat system.
    The building is heated to 69.6° F from 6am to 8pm on weekdays and from 9am to 7pm on weekends from November through February. The building perimeter is cooled to 72.4°F for the rest of the year during the same hourly periods. The building central core (about 57% of the total building floor area) is cooled as needed 24 hours a day, every day. Likewise, the fans run in perimeter areas during the weekday and weekend periods specified above for heating, but the core area fans run all the time.
1.   Whole Building Electrical Power over Time
    Whole building power is first normalized by dividing by the total floor area of your building to get a Watts per square foot (W/sqft) value. You can then examine this value over time in different ways to see how it varies. Figure B1a looks at an entire year's worth of whole building power on a three-dimensional "carpet" plot. The turquoise x-axis shows the day of the year, and the yellow z-axis shows the hour of the day, while the vertical y-axis plots the value of whole building power. This plot is able to show the variations in electricity use during the year. From this plot you can make several checks. Are your peak usage values at reasonable levels, and do they show up when you expect them? Are there any unexpected peaks? How about your minimum electricity usage? Does it correspond well with unoccupied hours? Can it be reduced further, or kept lower for longer hours?
    The prototypical Northern California Office Building data shown below is an idealized version of a well-controlled building. Power use peaks slightly during the summer (days 180 to 300), when electricity is used to run the chillers, and tapers off in the winter. This annual profile would show more variation in a harsher climate than the Northern California Bay Area's. This building's peak usage of about 4.5 W/sqft occurs during summer afternoons. The power use drops fairly cleanly to a minimum of 1.25 W/sqft at night. There are no unexpected usage patterns in this prototypical 3-D carpet plot.
    Figure B-1a. Whole Building Power Carpet Plot

    Another way to look at the whole building electrical power use is to use a simpler two-dimensional plot of power (y-axis) versus time of day (x-axis). This plot can be viewed in conjunction with, or instead of, the 3-D carpet plot. In Figure B-1b below you can see peak usage, baseline usage and the daily load shape more clearly. You can also see clearly a number of low usage days along the bottom of the plot.

    Figure B1b. Whole Building Power vs. Time of Day All Days

    It's useful to make 2-D plots of only weekdays, weekends, or a specific day of the week to evaluate the its power profile. Figure B1c shows whole building power on weekdays only, but including holidays. Notice that most of the low power days have dropped off the plot, signifying that almost all occupied days use significant power. A plot of weekends & holidays only would also be useful, in order to ensure that power is kept to a minimum when the building is unoccupied.

    Figure B1c. Whole Building Power vs. Time of Day Weekdays
2.   Whole Building Electric Power vs Drybulb Temperature
    Looking at whole building power versus the outdoor drybulb temperature is a good way to determine your building's dependence on weather conditions. (Remember to normalize your power use by floor area, to plot W/sqft). If your building is electrically cooled but not electrically heated, as is the case with our prototypical building, you should see your maximum power correspond to the hottest drybulb temperatures. If you use electric heaters, you would expect a more U-shaped curve, with high power use at both low temperatures and high temperatures.

    In Figure B2a, there are three data "clumps". The lowest clump occurs when no chillers are running to cool the building. The middle clump shows power use when one chiller is on, and the highest clump shows power use when both chillers are in use. Note the increasing slope of each clump, defining the increasing dependency of power use on temperature as more cooling equipment is used. Watch out for stray data points, or small clumps points in unexpected locations.

    Figure B-2a. Whole Building Power vs. Drybulb Temperature All Days

    One can also look at specific days, weekdays only, or weekends only. Figure B2b below of the Northern California prototype looks at only weekdays. The most noticeable difference between the weekday plot and the all day plot is there are fewer low-power use days.

    Figure B-2b. Whole Building Power vs. Drybulb Temperature Weekdays
3.   Whole Building Power vs Outdoor Temperature, for each 24 hour period
    Another meaningful way to examine whole building power is to look at each hour of the day separately. This way you can clearly see hourly trends in power use, and quickly determine where your energy use might be lowered. Figure B3 below is a sample plot from the prototypical Northern California building of power versus outdoor drybulb temperature. There are a number of things to be observed here.
    Figure B3. Whole Building Power vs. Drybulb Temperature All Days
    First, you can see that the minimum energy use occurs during the nighttime and early morning hours, when the building is unoccupied. The flat nighttime load shape shows that little if any cooling is used at night. During hours 5 and 6 you can see power use separate into two lines, the lower line for unoccupied days and the higher for occupied days. The power increases during hours 5 and 6 occur as lights and appliances begin to come on in the building. Note that the power use of these non-cooling related appliances is generally not sensitive to outdoor temperature, as shown by their non-sloping appearance. At hour 7, power use not only rises further, but starts to become temperature dependent. This signals that fans and/or chillers are now used to cool the building. Also at hour 7, the lower unoccupied line begins to split into 2 separate lines as the building core area may need some cooling on some of the minimally occupied days.

    Through the morning and into the afternoon, the power versus temperature curves spread further apart as the power used for lights, appliances, cooling and ventilation peaks. At hour 15 the power use begins to taper off, and the curves slowly coalesce as the end of the day is reached.

4.   Cooling System Efficiency versus Cooling Load
    A plot of your cooling system efficiency versus the delivered load is a useful tool for system analysis. This plot can be compared to theoretical efficiency curves for your system. It can also be monitored over time to check for degradation of your cooling system performance. Cooling system performance is dominated by the chiller, so the shape of this curve should look a lot like your chiller performance curve. Overall system efficiency should lie somewhere between a low 2.0 kW/ton and a high 0.7 kW/ton, with newer systems having higher efficiencies. Figure B4 shows the efficiency curve for our prototypical building. This idealized curve is very well defined. An actual curve will have more scattering of points. But every curve will have an area where the curve generally flattens out and the optimal efficiencies are reached. The curve below flattens out above 30% of the maximum load. To maximize system efficiency you want to run your system at these loads as much as possible, and avoid the lower-load, steeper part of the curve where the system is less efficient.

Figure B4. Cooling System Efficiency vs. Cooling Delivered
Comparison of your efficiency curve to system specifications or to historical performance of your system can be used to find many problems. Your efficiency curve may not be as high as the specs, or as high as it was a year ago. This may be caused by a number of things, including:
5.   Cooling System Electrical Power versus Outdoor Drybulb Temperature
    Another interesting plot is cooling system power versus the hourly outdoor drybulb temperature. This curve shows the temperature dependence, with power use increasing with outdoor temperature. Figure B5 below, for our prototypical Northern California office building, also clearly shows two lines of system operation. The lower line shows operation with only one chiller in operation, and the upper line is when both chillers are running.

    Cooling system power is highly dependent on outdoor temperature. But it also depends on building loads, such as solar loads, lighting and appliance use, and people. In the prototypical plot below there is quite a bit of load scatter at any given outdoor temperature. In general, the load is met by a single chiller at low outdoor temperatures, and by both chillers at high outdoor temperatures. But at non-extreme temperatures one or both chillers may be needed to meet the building loads.

Figure B5. Cooling System Power vs. Drybulb Temperature All Days
6.   Chiller Efficiency versus Chiller Cooling Load
    It is valuable to track of the efficiency of your chillers, either through continuous monitoring or by making periodic checks of performance for various loads. It is useful to compare their performance to the manufacturers specifications, as well as to track performance over the chiller's lifetime. Figure B6 shows chiller efficiency in kW/ton versus the cooling load in tons for our prototypical Northern California office building. Note that this curve reaches an optimum efficiency plateau at a load of about 100 tons. For best overall performance, you want to operate your chiller at or above this level. The newest, most efficient chillers plateau at full-load efficiencies of 0.6 kW/ton or better. Older, less efficient chillers may have full-load ratings of 0.9 to 1.0 kW/ton or higher.

Figure B6. Chiller Efficiency vs. Chiller Cooling Produced
Loss of efficiency over time may be caused by:
7.   Chiller Efficiency over Time
    Looking at chiller efficiency over time will help you to see if you're using your chiller effectively. Efficiency can be plotted on a three-dimensional carpet plot (efficiency vs day of year vs time of day) or on a simpler two-dimensional plot of efficiency vs time of day. Either plot can be used to pinpoint where chiller efficiencies are lower than expected. These plots can be used to fine tune your operating schedules, or to trouble shoot for chiller problems.
    Figure B7a shows the 3-D carpet plot of efficiency for our prototypical Northern California office building. You can see the best efficiency is achieved in the middle of the day when the chiller is running at full load. Seasonal variations can also be observed. The full load "channel" is wider during the summer months, when chiller loads are higher for a longer part of the day. Part-load efficiencies are also better on the "shoulders" during the summer months. During the winter, shorter periods of full-load operation and longer periods of less efficient, part-load operation are observed.
    Figure B7a. Chiller Efficiency Carpet Plot
    The 2-D plot of the prototypical Northern California office building is shown in Figure B7b for all days of operation for a year, then in Figure B7c for weekdays only. In these two plots you can see that weekend/weekday operation is slightly different. On weekdays chiller operation begins at 5 or 6 am, while on weekends operation doesn't begin until 8 am. Weekends are less likely to reach the most efficient full-load conditions, since only the core of the building is being cooled and there are lower building loads.
    Figure B7b. Chiller Efficiency vs. Time of Day All Days
    Figure B7c. Chiller Efficiency vs. Time of Day Weekdays
8.   Cooling Tower Power over Time
    Looking at cooling tower power over time will help you to see when you're using your cooling tower. This plot can be a three-dimensional carpet plot (power vs day of year vs time of day) or a simpler two-dimensional plot of power vs time of day. These plots can be used to fine tune your operating schedules, or to trouble shoot for cooling tower problems.

    Figure B8 shows the 2-D plot of cooling tower power for all days of the year in our prototypical Northern California office building. Looking at this plot together with plots of chiller power use can ensure that the cooling tower is always working when the chiller is running. The prototypical cooling tower for our building runs at two flow levels, one using about 0.8 kilowatts over a full hour of run time and the other using about 1.6 kilowatts to pump double the flow level. If the cooling tower was undersized, as is common, this plot would show significantly more operation time at the 1.8 kilowatt level.

    An undersized cooling tower can seriously impact the chiller efficiencies! A too-small cooling tower will deliver warmer than necessary water to the condenser, not allowing the chiller to release as much heat and worsening the chiller's efficiency. This problem will degrade chiller operations when you most need optimal performance - on hot days and during afternoon load and energy-rate peaks.

    Figure B8. Cooling Tower Power vs. Time of Day All Days
9.   Cooling Tower Approach Temperature vs Cooling Produced
    Another useful plot for evaluating cooling tower performance is the cooling tower approach temperature versus the tons of cooling produced. The approach temperature is defined as the condenser water supply (or cooling tower exit) temperature minus the wet bulb temperature. As more energy is extracted from the cooling tower flows, the condenser water supply temperature "approaches" its limit of the wet bulb temperature, and the approach temperature becomes smaller.
    Figure B9 is a 2-D plot of cooling tower approach versus the cooling tower cooling produced for all days of the year in our prototypical Northern California office building. The approach temperature is reduced as more cooling is produced. This plot can be watched to see if the approach temperature is increasing over time at different tonnage levels - an indication of fouling of the cooling tower and a signal to clean it. More efficient cooling tower operation can deliver cooler water to the condenser of the chiller, for better chiller efficiencies and lower energy use.
    Figure B9. Cooling Tower Approach vs. Cooling Produces All Days
10. Cooling Tower Cooling Produced vs Condenser Flow
    A plot of cooling produced versus condenser flow allows you to keep track of the amount of flow through the cooling tower and how much cooling is produced from the cooling tower alone.

    Figure B10 is a 2-D plot of cooling tower cooling produced versus the condenser flow for all days of the year in our prototypical Northern California office building. This theoretical cooling tower runs at two flow levels, 190 or 380 gallons per minute. Since this is a theoretical building, the condenser flows remain exactly the same all year, with no flow reduction due to fouling of the piping or the cooling tower, and no piping leaks.

    In an actual building, you must watch for flow reductions over time as the cooling tower becomes clogged and dirty. You must also watch for higher than normal flows through the cooling tower, which could indicate problems with pump operations.

Figure B10. Cooling Tower Cooling Produced vs. Condenser Flow All Days

Start of Report
Executive Summary

Section 1. Project Overview
Section 2. Pilot Site Selection and Technology Innovation Findings
Section 3. IMDS Description and Accuracy
Section 4. Building Performance and Findings from the IMDS
Section 5. Automation of Diagnostics
Section 6. Economic Issues and Related Technology
Section 7. Conclusions and Future Plans
Section 8. References

Appendix A. Web-Based Performance Analysis Tools
Appendix B. Diagnostic Plots
Appendix C. IMDS Points, Sensors, and Data Production Systems
Appendix D. IMDS Findings Report Log

Project Home  |  Commercial Building Systems  |  Building Technologies
This page last updated 7/10/99 by SKhalsa