CO2 Controls
In today's "Green Architecture and Engineering" concept, the use of CO2 controls in the AHU fresh air intake is advocated as an energy efficiency feature. This short article will explain why Energy IR thinks its otherwise - read on!
Based on standard control parameters for CO2 control, fresh air dampers open and close according to the amount of CO2 the sensor reads and feedbacks to the control loop. At CO2 levels above 900 ppm, the dampers are requested to open, thus allowing fresh air into the building to dilute the CO2 levels - this is said to "save" energy by not allowing hot and humid fresh air into the building unless necessary.
In our audits we have come across glaring issues that is usually ignored by most consultants or designers...
- CO2 sensors require frequent calibration. The sensors need to be effective to provide effective feedback! Calibration means higher operating cost and is a responsibility of every building owner. In reality calibration is thrown out of the door in most buildings unless it is a cleanroom type environment.
- ASHRAE recommends at least 15 CFM of fresh air per person. This rule should be complied to at all times to ensure sufficient dilution of air in a space area. By using a CO2 damper control, this might not be achieved unless a minimum damper opening is specified. This action will mean less effective controls by the BAS system.
- If an office has fixed number of staff, the fresh air intake can be pre-determined. A confirmed CFM of fresh air is better as it ensures dilution of air-conditioned air at all times; It provides comfort for users if the AHU design and cooling load is done correctly. Most importantly fresh air acts as a constant safeguard against contamination.
- Dampers require frequent maintenance as it is motorized - again cost will be incurred to maintain the system.
A recent audit on a office building found that:
1. The return air CO2 reading was below 900 ppm at all times, thus the dampers were always closed - expensive sensors and controls was never operated in a week of constant measurement at one-minute interval.
2. There were office workers generating CO2 but the the occupancy was averaged at 15m2 per person which is very much above the norm. Without fresh air intake due to closed dampers, the office workers were not provided fresh air for dilution in the air, thus increasing exposure to contamination. If we assume that there wiill be leakages into the building, it defeats the purpose of "Energy Savings" through CO2 controls! Depending on leakages to provide good indoor air quality is not good engineering practice!
3. Most of the dampers were jammed and almost 40% of the sensors where showing CO2 skewed readings - thus defeating the whole control objective and expensive equipment simply doing nothing but adding to potential IAQ problems!
The findings above are believed to be typical in most installation due to the overall lack of understanding on the requirements for fresh air.
Energy Savings using CO2 Controls - read the following to see why it does not save energy....
Advocates of CO2 controls maintain one major mantra : Energy Savings due to lower fresh air infiltration.
At Energy IR, we call it ENERY AVOIDANCE - similar to switching off lights when not in use - it is NOT ENERGY EFFICIENCY!
We dont see fresh air as a culprit but rather lack of design principles and concepts as the main cause of higher energy consumption due to fresh air. The fact remains that a building requires fresh air to maintain Indoor Air Quality (IAQ) -period.
IAQ can never and should not be compromised at any time in any design even though it may mean higher energy use. If an owner and/or designer puts energy consumption a higher priority against renovations, new furnitures etc, it is fine. But putting indoor air quality behind energy savings is not the right attitude!
IAQ is NEVER COMPROMISED in ENERGY IR's Design
If you are in an office building, chances are that you may be breathing low quality indoor air as you read this!
Energy Savings - Energy IR's method
Energy Savings is achieved using a dedicated Fresh Air AHU or Outside Air AHU (OAHU). The job of the this OAHU is to dry and cool the air but primarily to dry the air. This avoid mixing hot and humid outside air into the AHU system which in turns stresses the cooling coils which has to handle a mix of humid and dry air.
By using an OAHU and low face velocity design - low face velocity means lower fan energy and lesser chilled water flow - we can reduce the overall energy usage. This method is the Pure Engineering way where energy efficiency is pursued by design.