CO2 Sensor Placement

EMAIL: I've been curious for a while about the discrepancy between the recommendations for sampling locations expressed in ASTM D6245 and the ASHRAE and US GBC requirement for being in the breathing zone. After reviewing some of monitoring data for a nearby school, I'm finding that there are significant problems associated with this sampling location requirement, so I thought I'd pass it along.


RESPONSE: Thanks for the paper (indicating temporary CO2 spikes of over 1,000 ppm due to people exhaling near the sensor). I think CO2 sensor placement is like occupancy sensor or thermostat placement in that it must be located intelligently. For an occupancy sensor, it has to sense the people coming in but not be falsely triggered by people in the halls or miss people behind obstacles. For thermostats, they need to be close enough to the occupants to represent their conditions w/o getting direct sunlight to artificially read hot. For CO2 sensors, they need to be in the breathing zone to represent the air that people are actually breathing but not so close to people as to get artificially high readings from nearby exhalation. You always hear about stratification of air and this is why I’d want the CO2 sensor in the breathing zone “strata”. In a well mixed room, I guess it wouldn’t matter, but I don’t know how many rooms are well mixed or how you’d be sure yours was one. I’ve seen a number of engineers place them in a return air duct which is fed from a ceiling mounted grate, but in these same rooms the warm supply air is provided from ceiling diffusers and it seems like fresh warm air could stay along the ceiling and go straight to the exhaust, giving the CO2 sensor an artificial low reading. In the scenario I just described, ASHRAE 62 gives that style of room a 0.8 zone air distribution factor and assumes the room isn’t adequately mixed. In those rooms I’d much rather see it in the breathing zone. We’ve worked on a couple of day cares and talked about demand control ventilation for children and felt the CO2 sensor should be even lower to get the air quality where the kids were actually breathing.

Of course, I’m not the IAQ expert, so I’d defer to you as to the likelihood of adequate room air mixing. Maybe a similar study with CO2 sensors at different elevations would help, though most building owners aren’t going to want to pay for this.

Roof Evaluation for PV

While working with the nice folks at Taza Chocolate in Somerville, MA as part of a class project, I was able to go up on the roof of the 561 Windsor Street building and evaluate it for photovoltaics (note it is sunny because I did this in November and am a bit slow putting the pictures up on the blog). The building has a large flat roof with relatively no mechanical equipment. There are also no tall buildings to the south that might shade the roof. Though it was obvious that the building received direct sun most of the year, I used a Solare Eye camera to verify the path of the sun in relation to the building and determine what times of the year, if any, the roof would be shaded. The picture above (taken with a timer) shows me on the roof, leveling the camera and orienting it due south. Since I’d already entered our location into its computer, it knew our latitude and longitude and could overlay the path of the sun (as seen in the image at the end of this entry).


According to the Solar Eye results, the roof receives direct sunlight 99% of the time sunlight is available. Assuming the 21,761 square foot roof can hold at least 600 SunPower 305 watt panels (shown to the right), which are 62” by 41” each, the PV Watts program from the National Renewable Energy Lab predicts that the roof would produce 210,251 kWh per year in Somerville, Massachusetts. The first cost for this project would be $2,196,000 at an estimated $12 per watt. Assuming a 30% tax credit of $658,000, this brings the project down to a 19 year payback. This payback would be significantly improved when the accelerated depreciation and any potential revenue from renewable energy credits are included. The state of Massachusetts is expected to unveil a new set of renewable energy incentives in January, which will further improve the payback, which would likely be less than ten years when all revenue sources are considered. While Taza does not currently have roof rights to their building, if they are considering a long-term lease they may elect to negotiate access to the roof. Be sure to check out Taza Chocolate if you're in the mood for some great chocolate. Taza is a small bean-to-bar chocolate maker, and the only producer in the US of 100% stone ground, organic chocolate. If you're purchasing in bulk and live within 5 miles or so of the factory, they'll even deliver it with one of their custom tricylces.

Roca Dual Flush Toilets

For the last month I've been traveling in Morocco and Spain. In both countries, I saw a number of dual flush toilets (though still less than half of the toilets I saw). The most common brand seemed to be Roca. All Roca toilets have been dual flush since 2001 and many are 5/3 liters (in addition to the more common 6/4 liters or 1.5/1 gallon). The company is ISO 14001 certified for having an environmental



management system. Haven't seen their products in the US, but they seemed to work well. The picture above is a typical floor mounted tank type toilet from a Tryp Hotel in Madrid, Spain. The one below is a wall mounted tank type with the tank recessed between the wall studs from an Isis Hotel in Essouira, Morocco.