Hello all,
I would kindly ask for an information please.
My HVAC multizone system, looks like this. VAV AHU system (with sensible heat recovery, no mixing) + Fan Coil Units (FCU). Primary cooling/heating is achieved via FCU (eliminates sensible cooling/heating loads). Conditioned air supplied by AHU serves for ventilation purposes only and is controlled by volume control dampers (regulate airflow rates based on amount of fresh air needed for each zone). AHU is selected based on total airflow as per ASHRAE 62.1 for all zones. I am introducing 30% more fresh air than min. required by referenced standard to achieve IEQc2 (Increased ventilation). Several CO2 monitors installed in densely occupied areas , sense high CO2 levels (higher than set values). It will alarm occupants and via BMS (building monitoring system) trigger appropriate corrective action (e.g. opening dampers, or AHU's external louvers) to increase outdoor fresh air delivery to the zones to eliminate high levels of CO2.
My concerns are:
1) How can HVAC system deliver more fresh air when AHU is selected based on total airflow rate as per ASHRAE 62.1. Does it mean that I have to add a safety margin e.g. 10-15% to a design flow rate to accommodate such scenario ?
How can I calculate the value of safety margin that will mitigate CO2 levels below 800ppm for example?
2) How can I deliver more air in case of constant volume systems? The air rates are fixed.
3) How to approach the problem in case of all air VAC systems (where air supplied by AHU is used to eliminate loads)?
Thanks muchly.
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
June 1, 2011 - 11:07 am
CO2 sensor issue
Those are indeed top notch questions.
1)
If you have automated variable volume control dampers elsewhere in the building, you could program them to turn down airflow to non-CO2 effected zones to say 50% to give you enough air to shortly "flush-out" CO2-effected zones. Remember that normally it takes a long time for CO2 to build up in a large space.
If you have only constant dampers, you could ask the manufacturer if you could increase the max fan speeds for a short flush-out period on the main AHU.
How can I calculate the value of safety margin that will mitigate CO2 levels below 800ppm for example? ---Luckily LEED doesn't ask you to calculate this. There are simulation programs like CONTAM and EnergyPlus that can do this with considerable effort, but I would go on the fact that the AHU / system in place is only capable of delivering an absolute maximum of X and would not over design a system for a scenario that is likely never to happen. That's what smoke exhaust systems are for. The CO2 sensor credit is about alerting to a problem, more than over designing a AHU to fight smoke.
2)see the second part of 1)
3)If you do the american thing and try and combat loads with air from the AHU, you usually end up with huge ducts transporting a mixture of fresh and the much larger volume of recirculated air, meaning the AHU is large enough for transporting 800% the required fresh air if it had to run on 100% outdoor air...so basically you'll tell the system to ignore space loads during the "flush period" and just run as much outdoor air as you can suck in through the ducts to outside.