Wednesday, November 4, 2015

CO2 is a problem?

I recently received a link to a new paper on the effect of elevated levels of CO2 on the cognitive ability of building occupants. This explains why student test scores are reported to be higher as ventilation rates increase. While LEED gives a point for increasing ventilation 30% above the 62.1 minimums, the implication here is that the ventilation rate needs to be quite a bit higher than that.

Currently, outdoor CO2 levels are approaching 400ppm. The report states that while no ‘critical’ level is defined for indoor spaces, the 62.1 DCV (Demand Controlled Ventilation) suggests that it be “700 ppm above outside”, or no more than 1100 ppm total, which is quite a bit higher than what is found to be “safe”. They seem to believe that 600ppm is a good target.

Achieving this will require an efficient means of introducing outside air into a building.

One option is to simply increase the outdoor air setting of the air handler, but most units are designed to handle only about 30% of the unit’s air flow capacity for the climate in which it is installed, so this may not provide enough ventilation. Another option is to increase the percentage of outside air, but that would likely generate limitations with regard to dehumidification capacity and temperature. I believe the most effective method of introducing ventilation is to vary the quantity of ventilation air to the spaces that need it, which would imply the use of a Dedicated Outdoor Air System, or DOAS.

For this concept to work properly, the DOAS unit would likely need to be at a little larger than typical. It would also require an effective delivery system. For it to be energy efficient, it must only deliver as much outside air to each space as is required, which would involve a measured and controlled air flow damper at each zone. The VAV DOAS unit would then supply dry, cool ventilation. Doing this however comes with the risk of sub cooling spaces if the dehumidified ventilation air is not reheated (resulting from minimum ventilation rates that exceed the load). 

I’m sure there are other possible means of accomplishing this, but it’s likely that they would not be quite as cost effective as the DOAS solution. 

At Krueger, we call our DOAS unit a “Chilled Box”, which is an ECM variable speed, series flow, fan-powered terminal unit with a sensible cooling coil on the induction inlet. I encourage you to read my recent ASHRAE Journal article on this topic, as it not only provides a solution to address this CO2 issue, but it can also satisfy a number of other ventilation challenges you may come across.