Summary

This report is based on a trend-data review from the Andover Controls BAS (Building Automation System) installed in the Carlisle, MA Town Hall. The BAS system controls an RTU (Roof Top Unit) that provides heating and cooling to the facility through 23 VAV (Variable Air Volume) boxes located on the first and second floors.

We looked at the following systems to see if there are opportunities for improved comfort at the Town Hall and to prepare for an analysis of individual components of electric utility demand costs. It is common in New England for summer monthly demand peaks to be set when the air conditioning equipment is operating at maximum capacity:

1. Stages of DX cooling
2. RTU variable speed drive speed
3. VAV box flow rates / flow rate setpoints
4. VAV box discharge air temperatures/discharge air temperature setpoints
5. space temperatures

Analysis

Stage 2 DX Cooling Calls

There were 8 hours in the months of July, August and September 2015 when both Stages #1 & #2 of DX cooling were energized. These are shown below along with the maximum daily outside air temperatures. July 20 and August 17 were Mondays after a Sunday temperature that reached 88F.

The 2015 utilization of Stage 2 cooling is an improvement over 2013 when Stage 2 cooling was required regularly for up to 12 hours per day when Outside Air Temperatures were above 90F. The persistence of the improvements made to reduce the operation of Stage 2 cooling since 2013 are evident on the heat map below. We do not know the load of the compressor, so we are unable to calculate the savings. But we can estimate that the reduction of 280 hours in July for 2015 versus 2013 represents about $140 of savings in the month, based on 5 cents/kWh and a 10kW average compressor load.

Heat Map

Calls for DX Stage 2 cooling are indicated by red boxes. The darker red boxes indicate that the Stage 2 cooling is required for longer portions of the day.

Space Temperatures and Set Points

The 2015 data from July and August indicates that space temperatures were maintained more comfortable than in previous summers:

• Individual VAV box cooling set point range - 68 to 73 F
• VAV box average Cooling set point – 71F
• Design Cooling Set Point – 72F to 73.5F
• The RTU contains an occupied cooling set point of 76 F, but this is not being utilized in the control sequence.

On days when Stage 2 cooling was energized, it appears that it came on after the RTU was unable to meet the 65F discharge air temperature set point for a period of time. The interesting note here is that the individual VAV boxes were all calling for discharge air temperatures of 61 to 62F for many hours prior to this. There appears to be a disconnect between the discharge air temperature set point at the RTU and at the individual VAV boxes. It is not known if this is by design or an oversight.

If the RTU provided the lower discharge air temperature earlier, it is possible that some spaces such as ones served by VAV 1 and VAV2 would be overcooled. One way of addressing this is by seeing if the minimum flowrates on some of the VAV boxes can be reduced. The other possibile outcome from bringing on the Stage 2 cooling earlier is that it may be able to be held off later in the afternoon when it is possible a monthly peak demand may be reached.

VFD Speeds

Two of the days (July 20 and Aug 17) when Stage 2 cooling was called on and the VFD ran 100% speed for most of the day were Mondays.

Electricity and Peak Demands

We have submitted a registration form to the Town which will be forwarded to EPO in order to access to the utility main meter interval data for both electricity and gas. In our next update, we plan to report on this data in some detail consistent with our approach to Gleason and CPS. To complete that analysis, we would like a copy of recent electric and gas utility bills so that we can understand the rate schedule and pricing methods in use.

Summary

It is reported that Demand Prices within the Carlisle area will be rising in the next couple of years. If this materializes, buildings that are more finely tuned than other buildings will be able to take advantage of the extra work they have invested.

We recommend learning more about:

1. Connection between set points at VAV boxes and the RTU
2. Can minimum VAV box flow rates be decreased on some VAV boxes to prevent overcooling of some spaces while meeting the demands of others.
3. kW associated with Stage 2 cooling
4. kW associated with RTU-VFD speeds between 85 and 100%.

Understanding how the individual VAV box set points and repetitive warm days affect comfort, calls for Stage 2 cooling and maximum VFD speeds may allow pre-cooling earlier in the day and a reduction of the monthly peak demand charges. If it is determined that the peak demands can be predicted, then an analysis can also be done on whether battery storage is cost effective to manage peak demands that cannot be mitigated through controls.

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