Project SCL3: Containment. Hot or Cold?

The following is a guest post from Tim Guarnieri, Principal, Critical Facilities Practice at Q Builders, Inc. Tim’s been providing assistance and guidance to Mozilla in building out SCL3.

In laying out racks in a data center, you can either contain the cold air or contain the hot air (or do nothing). In this post, Tim talks about both options and why we’re using cold aisle containment.

What is “containment” and why is it important in a data center?

The containment of cold air supply or hot air return in data centers is a practice used primarily to maximize the efficiency of the cooling supply and heat rejection systems.  This helps minimize the cost of running and maintaining these systems. As this cost is being passed on to Mozilla, minimizing it as much as practicable is an important goal of this project.

Containment is also a means of ensuring that the cool air required by today’s high-density server installations is supplied in the correct volumes and at the correct temperature for these servers to operate as efficiently as possible. Since power is expensive, ensuring optimal server efficiency is also a very important goal of this project.

Why “cold aisle” containment?

The data center provider Mozilla chose, Vantage Data Centers, engineered the data center floor to deliver cold air from below and return hot air from above to the central cooling plant. Further, they provided the perforated tiles required to support a cold aisle installation. This saved Mozilla a good deal of money but, equally as important, it is the right solution for the power density Mozilla intends to have in each of its cabinets (i.e. up to 12 kW per cabinet).


In high power density server installations (e.g. blade servers) , it is imperative that cool air be delivered at the right volume and temperature to the gear. Cold aisle containment ensures that:

  • Hot and cold air aren’t mixed
  • Cold air temperature can be monitored and controlled
  • Cold air supply volumes can be monitored and controlled

As Mozilla’s needs grow and change over time, we can monitor and control the environment to meet the needs of not only today’s server and network infrastructure but tomorrow’s as well.

What about “hot aisle” containment?

Hot aisle containment would work best in a data center environment completely dedicated to Mozilla. In other words, containing hot air implies that the cold air supplied to the data center floor must be somehow “contained” by the boundaries of the data center itself (i.e. ceilings, walls, doors, floors, etc).


In a data center environment shared with other tenants, as is the case with Mozilla’s space at Vantage (we’re sharing with two other tenants), Mozilla would not be able to monitor or control the temperature or volume of the cold air being supplied to the servers.

Further, Mozilla has no way of knowing (or influencing) the efficiency of other tenant’s designs and installations so the ambient air temperature for the data center module serving all three tenants (we’re only separated by cage walls) is very likely to fluctuate and be higher than the temperature ranges recommended by ASHRAE [edit: ASHRAE helps define recommended data center temperature limits].

ps. Special thanks to for their hot & cold aisle graphics.