CO2 sensors and outside air reduction. LEED for schools & ASHRAE 90.1-2004

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Previous posts have confirmed my suspicions that increasing the R-value of
walls and roofs has little to no effect on energy performance, due to the
outside air requirements.

This raises a few questions:

1. What is the most effective way to use CO2 sensors to dramatically
reduce the outside air requirements?

2. Is it permissible to vary the CFM between the base case and design
case in the ASHRAE 90.1-2004 Appendix G model, and consequently in the LEED

3. What is the most effective way to use CO2 monitoring in a building
with 40 classrooms and other large spaces. If one sensor is triggered,
won't the entire zone get fresh air and defeat the energy savings?

4. If every classroom has a sensor, does any one have experience with
the replaceable / Mail in sensors to assure their accuracy.

5. If I model this in Equest, do I need a zone for every classroom.

Thank you for any feed back.

James F. Geers

James Geers's picture
Joined: 2011-09-30
Reputation: 0

First of all, changing the R-value of walls and roofs can have a significant
impact on overall energy use, especially in locations with extreme outdoor
conditions. Granted, with more ventilation air required, the envelope is
responsible for a smaller piece of the overall energy use of the building.

So, to reduce ventilation air energy use:

A. Use energy recovery ventilation! In moderate climates, energy recovery
ventilation might not make a whole lot of sense, but climates where heat is
needed much of the year, ERV's make a lot of sense!

B. Turn off the ventilation when it's not needed! This includes time
clocks and CO2 sensors. I've seen alot of schools with old unit ventilators
in each classroom. These unit ventilators often stay on all night, bringing
cold air into the classroom and wasting tons of heat!

Regarding your CO2 questions:

1. Decoupling the ventilation air distribution system from an airside air
conditioning system allows for the most options. Installation of CO2
sensors in each space with corresponding ventilation air VAV supply results
in ventilation air being supplied only where it's needed. But, this can be
cost-prohibitive, and where lots of spaces see similar occupancy, completely

2. It is permissible to vary the CFM between the base case and design case
for ASHRAE 90.1-2004 Appendix G (G. Exception), some programs,
including eQuest model DCV, so no change in ventilation rate is required,
just activation of DCV.

3. In a building with 40 classrooms, you could create one branch of each
type of space (classrooms, offices), and control ventilation to each block
of spaces. For high-use areas with varying schedules (caffeteria, gymnasium
and auditorium), have each space have it's own demand controlled

5. If you model this in eQuest, you can still do blocks of classrooms,
especially if your occupancy schedule is the same for each classroom.

Hope this helps,


No Username provide's picture
Joined: 2011-09-30
Reputation: 200

People can make an argument that the total energy usage due to wall/roof insulation levels can at times seem like a small percentage of the total energy use in some buildings.

This might also be true of the value of a green roof on a 40 story building - of course the effect is only 1/40th of the effect on an identical footprint one story building. But for that one level the effect is just as important as it would be for the top level (only level?) of the shorter building.

There still is an absolute amount of energy involved that can be directly affected by the selection of an optimal insulation value. Just because the building is dominated by ventilation loads doesn't mean there can't be a real savings by choosing the best insulation levels.

The idea might be worth one or two parametric runs of your simulation.


David S Eldridge's picture
Joined: 2011-09-30
Reputation: 2000