MS Thesis by Ryan Militello-Hourigan
Directed by Professor Shelly L. Miller
Mechanical Engineering, University of Colorado Boulder
Low-energy home design is becoming more common in new and retrofitted homes, and energy-efficient designs often sell at a premium . This perceived and realized value of reducing energy use in homes is important as the need to reduce fossil fuel use becomes increasingly critical. Energy efficiency measures, like tightening the envelope of a home saves energy, but can impact the indoor air quality. We monitored the indoor air quality of nine tightly constructed homes, one tightly constructed public library, and one conventionally constructed home, and performed a repeatable cooking activity to observe the impact and response to the resulting fine particulate matter (PM2.5) emissions. We compared the PM2.5 concentrations from the cooking activity while operating the mechanical ventilation systems at default rates (~0.1-0.3 1/h) and in a temporary boost mode (~0.3-0.8 1/h). We also measured the concentrations of total volatile organic compounds (TVOCs), formaldehyde, radon, and bedroom carbon dioxide (CO2) levels. Results show that fine particulate matter concentrations are generally low indoors, but a cooking event drastically increases concentrations and levels are slow to decay. Median time above 35 μg/m3 after cooking was 4.7 hours. Overall, there was not a significant difference between operating the ventilators at standard rates and utilizing the temporary boost. Completely-mixed flow reactor models of select tested homes show that installing and using a directly-exhausting range hood could reduce peak PM2.5 concentrations by 75% or more. Current ventilation practices in these buildings may not be adequate for these common polluting events. TVOC concentrations were generally low with a median average of 459 μg/m3. Formaldehyde was above the California Office of Environmental Health Hazard Assessment (OEHHA) chronic limit of 9 μg/m3 across all tight buildings with a median value of 30 μg/m3. Carbon dioxide levels in bedrooms were high at night (>1000 ppm) in six of the homes, indicating that current bedroom ventilation practices are not consistent nor adequate. Bedroom air exchange rates (AER) determined from carbon dioxide decay ranged from 0.08 to 0.30 1/h with a median value of 0.12 1/h.
Summary of Lessons Learned:
Ventilation Rates: Overall, we noticed that most homes were capable of meeting ASHRAE 62.2 whole-house rates and the 0.3-0.4 ACH passive house recommendation, but many were not typically operated at this rate. This may be okay if the occupants are minimally active, but any pollutants generated will linger for a long time.
We think a good approach for this would be to have three settings on an ERV/HRV: Away/Home/Active. So you would still see energy savings during the day, but ensure proper ventilation when people are home. We saw that many residents only use Boost for cooking and showering.
Bedroom ventilation rates are a different story. We saw that bedroom CO2 was above 1000 ppm for about half of the homes. It’s important to make sure that all units are properly balanced so that bedrooms all receive adequate airflow.
Hoods: We didn’t test any of the recirculating hoods in the homes, but the general consensus in the field is that they don’t work very well. We highly recommend using a ducted hood, and it really should be mandatory if using a gas range.
On the subject of ranges, we think induction is the way to go. Gas burners generate NOx and carbon monoxide, and though electric resistance is a little better, these still generate a good number of ultrafine particles. Induction units produce fewer ultrafine particles, and, in my opinion, cook as well or better than a gas burner.
Radon: If designing a house in a high radon area, we’d recommend assuming it will need an active mitigation system and designing for the openings/pipes needed. The fan can be added after if the home tests high.
Four of the homes we tested were fairly high, and three of those have since installed mitigation systems. Installing it after the fact is probably more costly.
Formaldehyde and TVOCs: These are somewhat harder to nail down. Some homes in the remote areas had lower TVOC levels, so it may partially depend on location. We didn’t notice a trend associated with the formaldehyde. We think it’s good to choose low VOC materials and make sure all wood products are low formaldehyde, but beyond that we can’t add much. We think higher ventilation rates can help with keeping levels low, but there are other factors at play.
Ventilators: We like the idea of monitoring the air like the CERV units do, but a qualm we have is that they don’t currently measure particulate matter. We noticed that cooking the eggs emitted a ton of PM2.5, but in at least one home the CERV unit didn’t respond because it only measures CO2 and VOCs. They are currently working on that though, and it could be set to manually vent if needed.
Note that we are not trying to recommend one manufacturer over the other, but we like the idea of being able to schedule home and away rates with a boost in the kitchen/bathroom. People can’t always be trusted to adjust things manually, so anything automatic is probably better, in our opinion.
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