Energy Star tax credits
A recent article in ASHRAE Journal, “An Updated Look at DCV Approaches“, October 2009, page 82-84, covers an important component of building energy: HVAC ventilation control, also called “Demand Controlled Ventilation“. It is an overview of the reasons and logic behind one of the more cost-effective methods for controlling air quality while reducing energy usage.
Outside air – Opening a window
Comfortable and clean indoor breathing has always been associated with fresh air. You simply dilute inside air with conditioned outside air. Its the same as opening a window (albeit with more precision). Scientific observation reinforces the correlation between fresh air and a comfortable indoor breathing environment.
Formal design procedures and requirements for this have been part of the building codes for several generations. ASHRAE Standard 62, “Ventilation for Acceptable Indoor Air Quality (IAQ)”, is usually the basis for these. In summary, ASHRAE 62 describes uses several methods which to determine the required volume of outside air. One common method relies on a “prescribed“ outside air quantity (volume/time) as determined by the maximum anticipated number of occupants. Outside air volumes often have a considerable impact on the HVAC energy load, therefore close attention is paid the precise control.
Controls – What is DCV?
There are several common methods for controlling outside air. The most common is the so-called “Demand Controlled Ventilation” whereby the addition of fresh air is only necessary according to the “demand” caused by occupancy. Carbon dioxide (CO2) concentration is a reasonable proxy for overall air quality, and it has been shown that the concentration of CO2 correlates well to the quantity (concentration ) of people in the space.
The Brutal Reality
One point in the article that sticks out was:
…traditional DCV systems use carbon dioxide sensors to indicate per person outdoor airflow rates. To accurately correlate CO2 levels to per-person outdoor airflow rates, sensors need to be installed, calibrated, and maintained properly.
Recent studies indicate that one or more of these steps generally are not followed in buildings with DCV (Fisk) and (Shrestha and Maxwell). One study showed that 80% of CO2 sensors in buildings read high by an average of approximately 40%, resulting in higher-than-needed ventilation rates.
Anecdotal evidence also suggests that it is common for DCV systems to be disabled when not working properly or complaints are received about IAQ.
Statistics can often lie. We need to see details on these results. But if this fact is true, I can offer an observation. Building (HVAC et al) controls tend to be near the back cover of the contract specifications. Heck, out of 27 divisions of the specs, the CO2 sensors are in division 25. The toilet stall accessories come before them!
So for this reason, Division 25 subcontractors often are forced to make do with the absolute low bid. Too often, not enough commissioning and testing is budgeted to sure the controls are robust and reporting correctly. Calibration is always an issue. Hardware needs to be commissioned as well, specs need to be tighter. The new technology can be compromised by mass adoption/application. We professionals should avoid this trap by being careful what we accept.
Recommissioning easily finds Issues
If problems with the DCV are latent, or undetected, the issue should come up during an Energy Star Audit via the measured CO2 levels. The service contractor may discover that room CO2 levels reported by the building sensors are significantly different from his measurements.
Regular HVAC service personnel do not always deal with HVAC controls at this level, but perhaps they should. Management should hire service contractors who can check the controls at least twice a year. In any event, this points up a need for re-commissioning. Hiring a “commissioning” firm that regularly performs such work and can identify issues is a good start.
LEED-EB (Existing buildings) commissioning and Energy Star commissioning practices are becoming more common, and those provide a framework of expectations for the future. Commissioning is no longer a process confined to million square foot projects being done by the likes of AECOM and Turner.
Over-ventilating to be safe?
Over-ventilating wastes energy and costs the building owner money. This is the common result of the failures mentioned above.
Marketing
There is a large swathe of commercial real estate in this country which has not been partly or fully screened for energy conservation measures. Perhaps half of all commercial real estate is operated and maintained as it has for decades. Owners and tenants send a lot of money to the utility companies. I love utilities, I invest in them, but I don’t love them THAT much.
So I say to the owners and operators of these facilities:
“Let us engineers walkthrough your building and find you a way to save on your energy expenses”
If you would like to discuss what services I can provide you and your clients, please feel free to contact me at noel@noelsusskind.com
