Examples of 'energy efficient' control strategies that often waste energy - CO2 control

I often review BMS functional descriptions that read: The AHU outside air damper is modulated between minimum damper position (set at commissioning) and 100% open, as the maximum return air CO2 increases from 600 to 800 ppm.

This control strategy is designed to maintain indoor air quality and not to improve energy efficiency. It does not meet the intent of demand control ventilation. 


Demand control ventilation (CO2): 

Mechanical engineers (consultants) calculate the minimum amount of outside air required to meet Australian Standards (code); we usually call this outside air minimum damper position (e.g. 30%). This is the minimum amount of outside air that we must supply to be code compliant. 

The purpose of installing CO2 sensors is to reduce the amount of minimum outside air below code when the floor is not fully occupied and the measured CO2 is reducing. Remember that many new premium grade buildings have 150% of code minimum outside air, so the minimum outside air damper position could be 60% open, or more. 

Even if the CO2 on the floor is 1000 ppm, to be code compliant we only need to supply the originally commissioned minimum outside air amount; we do not need to supply more than the code minimum amount. Therefore, we should never see an outside air damper open more than the minimum amount, unless economy mode is enabled and we are providing free cooling. 

When mechanical engineers design the size of the chillers, their design calculations allow for the AHU's to be delivering minimum code outside air, not 100% outside air. If it is 35°C outside and the AHU outside air dampers are 100% open due to an incorrect CO2 control strategy, then the chillers will be running much harder than they need to and will likely not be able to achieve design indoor air conditions. It is a common misunderstanding, but CO2 control does not take priority over temperature control. 

CO2 sensors are generally very inaccurate. As the BMS selects the highest CO2 sensor to control the outside air damper, an incorrectly programmed CO2 control loop is greatly affected by inaccurate sensors and will almost certainly result in delivering significantly more outside air than required, because at least 1 out of the 20 sensors the AHU is monitoring will be reading high. If the CO2 control loop is correctly programmed, then the worst that can happen is that you provide code minimum outside air, compared to providing 100% outside air. 

Other than inaccurate sensors, any densely populated floors will drive all the AHU's serving that rise into 100% outside air. 

When you have AHU's unnecessarily delivering 100% outside air or just more outside air than required, it puts unnecessary increased demand on the chillers. 

When one system is incorrectly controlling, it often negatively affects other systems. On a 35°C day, the chillers will not be able to supply design chilled water if the AHU outside air dampers are 100% open. This then results in the AHU's delivering a higher supply air temperature, which in turn causes the temperature on the floors to increase, the VAV dampers to open and the AHU supply fans to ramp up. 

An incorrectly programmed CO2 control loop doesn't only increase chiller power consumption in summer but also boiler gas consumption in winter.         

How should it work? 

I prefer to change the name from 'minimum outside air' to 'design outside air', because it's not really the minimum when you implement demand control ventilation. When the measured CO2 reading is high (above 800 ppm), we modulate open the outside air damper to 'design outside air' position (code minimum amount). 

When the measured CO2 is low we modulate the damper closed to a point that I like to call 'minimum make up air'. This is the amount of outside air required to keep the building positively pressurised, taking into consideration other extracts, for example, toilet and kitchen extracts and leakage etc. 

You can see below that a CO2 control loop programmed to maintain indoor air quality will modulate the outside air damper between 60% and 100% open, compared to a CO2 control loop programmed for demand controlled ventilation, that will modulate the damper between 25% and 60% open. 

  • Full economy mode: damper 100% open
  • Design outside air: damper 60% open
  • Minimum make up air: damper 25% open
  • AHU disabled: damper 0% open

Why does this happen?

I worked in London for 9 years, from 2001, when BMS manufactures still had large training budgets. I remember driving up to Manchester and spending 5 days in a hotel learning how to program an AHU controller (I probably did about 10 courses over 7 years). That type of training, from what I see nowadays, doesn't exist. Depending on the size of the company you may get a webinar that you log into for a couple of hours a week or 'here is the manual, go work it out'. Either way, at best you will only get some sort of product training - how to write software, build databases, graphics, alarms and trends. But what BMS technicians generally do not get training on is the following:

  • HVAC control fundamentals
  • Efficiency 
  • Lessons learnt
  • Valve sizing
  • Instrument selection
  • Network design
  • Project management
  • Etc      

The class room based training wasn't just about how to connect process variables to PID control modules. The trainer with 20+ years of experience gave you tips and passed on their knowledge, lessons learnt and mistakes made. Thank you Peter Marshall, you made a difference :-)