Bath fans and kitchen range hoods are examples of exhaust-only ventilation. The fans pull air from the room and exhaust it outside of the house. These systems are a good idea for every house because bathrooms and kitchens can generate a lot of moisture and potential indoor air pollutants. Exhaust-only systems are not great solutions for whole-house ventilation, though. Even in leaky old houses, the air drawn into the house through cracks and gaps while the exhaust fans are running is unreliable, and we can\u2019t filter it or choose where it is distributed. Even when makeup air is provided, we miss the opportunity to capture any of the energy of the exhaust air. Like any ventilation system, it is important to test the fan flow during installation to make sure it is running properly.<\/p>\n Supply<\/strong><\/p>\n A supply ventilation system brings outdoor air into the house. In some areas, it is common for outdoor air to be introduced to the return side of the air conditioning ductwork. In this type of system, the incoming air can be filtered and distributed throughout the house. These systems are not particularly energy efficient, though, and can make it challenging for the air conditioning system to keep relative humidity at comfortable and healthy levels. Another supply-only option is a ventilating whole-house dehumidifier, which can solve a few problems. It can introduce fresh air while dehumidifying, distributing, and filtering indoor air. Keep in mind that supply-only ventilation strategies pressurize the building, so air will be forced out somewhere, which can present the risk of condensation issues in some building assemblies.<\/p>\n Balanced<\/strong><\/p>\n Though there are other ways to configure balanced ventilation, the most common systems are ducted ERVs and HRVs. They bring in and exhaust equal volumes of air, capturing some of the energy or heat from the outgoing air to precondition the incoming air. They also include filters, and the air can be exhausted from and delivered to anywhere in the house\u2014exhaust is commonly taken from kitchens, bathrooms, and laundry rooms, and fresh air is typically delivered to bedrooms and other living spaces. Most experts agree that it is best if these systems have their own ducts, separate from the heating and cooling systems.<\/p>\n <\/center>How Much Ventilation Do You Need?<\/strong><\/p>\n Building scientist Joe Lstiburek says that for many houses the flow rates for continuous ventilation found in the International Residential Code are adequate. There is a 30% reduction in these rates for houses where a continuous balanced ventilation system is installed and provides supply air to each bedroom as well as the living room, dining room, or kitchen. Balanced ventilation does not relieve the need for exhaust ventilation in kitchens and bathrooms.<\/p>\n Some experts, and the Department of Energy, prefer the ventilation rates in the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standard 62.2, which are often higher than the prescriptive IRC. But Lstiburek has concerns with over-ventilating homes and causing humidity issues. He suggests a compromise. \u201cYou can size a system according to 62.2, commission it to run at the building code rate, and educate the occupant on the difference,\u201d says Lstiburek. \u201cIf they\u2019re going to be rebuilding their carburetor at the kitchen sink or stripping furniture in their living room, they\u2019re going to need a much different ventilation rate than people who are not doing stupid stuff in their homes.\u201d<\/p>\n Even with balanced ventilation, we still need exhaust fans in kitchens and bathrooms. Though the IRC does not require exhaust fans in bathrooms with windows, nor does it require a ducted exhaust fan in kitchens, most experts agree that bathroom exhaust is critical to remove excess moisture and recirculating kitchen range hoods are not adequate. Particularly in houses with gas stoves, a kitchen hood fan that exhausts to the exterior is important. When a bath fan is installed, the code requires a minimum of 50 cfm intermittent\u2014bath fans that we turn on only when the bath is in use\u2014or 20 cfm continuous. In a kitchen, the requirements are 100 cfm intermittent and 25 cfm continuous. Large range-hood fans over 400 cfm require makeup air to avoid incidentally creating indoor air quality issues, like back-drafting combustion appliances.<\/p>\n With minimum requirements met, cfm is less important than effective design and installation. \u201cIn new construction today we\u2019re often putting in kitchen range hoods that are between 500 cfm and 600 cfm of exhaust,\u201d says Lstiburek. \u201cCan you get the same performance with 100 cfm of exhaust over your range? The answer is yes, of course, if the range hood is wider than the cooking area and has a lip. It\u2019s a matter of capture efficiency.\u201d<\/p>\n If you live in a house with a ducted heating and cooling system, you\u2019re in luck\u2014the system most likely includes an air filter. If that\u2019s news to you, you have a problem: Your filter is dirty. Within the context of an HVAC system, filters have two purposes. The first is to protect the components of the HVAC equipment from debris. The second is to clean particulate matter from the air for homeowner health. Dirty filters impact both purposes. They prevent equipment from operating as designed and can eventually fail to clear the air we breathe of particulates.<\/p>\n Particulates, or what we commonly call dust\u2014things like human skin, pet dander, pollen, mold, and even fine debris from building materials, including lead paint\u2014can float in the air, and are unhealthy, to varying degrees, to inhale. Particulates in the air can cause numerous health issues from asthma to allergies to developmental issues in children (in the case of lead paint). PM2.5 is a particularly small and harmful indoor air pollutant. \u201cA lot of the PM2.5 we find in our homes is the product of combustion, whether it\u2019s your cookstove, or candles, or diesel fumes, or wildfires,\u201d says Harrod, \u201cThat\u2019s the really dangerous stuff. It\u2019s so small that it can affect all the organs in your body.\u201d<\/p>\n Dust, and particularly PM2.5, will only be captured by a high-quality and high-\u00adefficiency filter. The most common rating system for the filters you can install in your HVAC system and will find inside an HRV or ERV is the minimum efficiency reporting value, or MERV. In the MERV rating system, the higher the number the better, and better means that the filter not only catches smaller particles but catches them more of the time. Many experts recommend MERV 13 filters, which are rated to capture 85% of PM2.5.<\/p>\n Upgrading to a higher MERV filter is not always as easy as buying a better filter. A more efficient filter will generally need to be larger or deeper than a less efficient filter, otherwise it could affect performance, and even damage your HVAC equipment. \u201cIf you choose to upgrade your furnace filter,\u201d says Harrod, \u201cyou need to make sure to pick one that is compatible with your furnace equipment and that won\u2019t create so much resistance that it will cook your blower motor.\u201d<\/p>\n The solution is often to install a new filter rack, which could cost a few hundred dollars. And of course, it doesn\u2019t matter what filter you use if it is dirty. Harrod says that how often you need to change your filter depends on the filter type, how dusty your house is, and the size of the filter. A 1-in. filter will likely need to be changed every one to two months. A 4-in.- or 6-in.-deep pleated filter can be changed one to two times a year. Keep in mind that it is also important to make sure that your filter is fitting properly in the filter box. Otherwise, air and particulates will bypass the filter.<\/p>\n What if you don\u2019t live in a house with a ducted HVAC system? In many homes with hydronic or electric-resistance heat and no air conditioning, the air is not being filtered. In fact, this could be the case in homes with ductless minisplits, too, if the HVAC designer doesn\u2019t consider the need for ventilation and filtration. Rokicki suggests three possible solutions: add a balanced ventilation system to the house, build a DIY air purifier, or run an off-the-shelf air purifier.<\/p>\n In houses with unfinished basements, crawlspaces, or attics, it can be relatively easy and affordable to run the ducts for an ERV or HRV retrofit. However, Rokicki cautions that the house needs to be relatively tight for the ERV or HRV to perform effectively (7 ACH50 or less is her preference). Therefore, she recommends first making building enclosure improvements by air-sealing and insulating, verifying tightness with a blower door, completing the calculations to determine the necessary flow rate for the house, and properly commissioning the system to make sure it is performing as designed.<\/p>\n Ideas for DIY air purifiers abound online (check out engineer Ross Trethewey on \u201cAsk This Old House\u201d explaining how to make a DIY air filter). Most are some version of a box fan blowing air across furnace filters. Harrod says that he has run one of these DIY air filters while monitoring for PM2.5 and has seen the level in the house diminish in a matter of minutes. As for air purifiers you can buy, Rokicki recommends Blueair products, and says to avoid ultraviolet air purifiers, ionizers, and ozone generators, and focus on filtration. Keep in mind that both DIY and manufactured air purifiers are localized solutions, likely to only be effective in the room where you run them.<\/p>\n As mentioned earlier, too high or too low relative humidity levels can be unhealthy in our homes. When relative humidity is too high, there is more potential for condensation and mold growth. Dust mites also thrive in high humidity. Low relative humidity levels can be a factor in respiratory infections and ozone production. Both high and low relative humidity can be a catalyst for asthma, allergies, and bacterial growth. Healthy and comfortable indoor relative humidity generally falls between 30% and 60%, though experts don\u2019t always agree on how tight that margin should be.<\/p>\n \u201cIf you look at the Sterling Chart (facing page, top), you\u2019ll find that the sweet spot for minimizing dust mites and viruses and most health risks is 50% relative humidity. That\u2019s ideal for health and for comfort,\u201d says Nikki Krueger, director of marketing and business development at Santa Fe Dehumidifiers. \u201cBut I live in Wisconsin and if I try to get to even 40% relative humidity in my house in January, when it\u2019s 10\u00b0F outside, the inside of my house is going to be dripping. So, we also need to be thinking about the house and what we might be creating as far as microbial growth when we shoot for that level of winter\u00adtime humidity.\u201d<\/p>\n Though building enclosure issues can cause relative humidity levels to spike, it\u2019s our mechanical and ventilation systems that bear most of the responsibility for maintaining the sweet spot. A properly sized and installed air conditioning system, for example, considers both sensible and latent cooling capacity. Sensible capacity is the system\u2019s ability to lower the temperature. Latent capacity is the system\u2019s ability to remove moisture from the air. Likewise, both an HRV or ERV could provide the adequate flow rate for ventilation, but one may be chosen over the other for its effect on humidity levels. HRVs only exchange heat between incoming and outgoing air while ERVs exchange both heat and moisture.<\/p>\n First published in a 1986 paper titled \u201cIndirect health effects of relative humidity in indoor environments,\u201d what has come to be commonly referred to as the Sterling Chart in building science circles shows the risks of too low or too high relative humidity on human health. The authors\u2019 suggested range of 40% to 60% is often not reasonable in homes, but staying between 30% and 60%, depending on the season, is often possible.<\/p>\n Where dehumidification is a concern that is not being addressed by the HVAC systems, there are options from stand-alone dehumidifiers to ventilating dehumidifiers. Similar to air purifiers, stand-alone dehumidifiers are a local solution and can work as such. If a basement is the source of high humidity, for example, the right size dehumidifier running in the basement may do the trick. However, if the house has generally high levels of relative humidity and a need for ventilation, a ventilating dehumidifier can solve a few issues.<\/p>\n A Santa Fe ventilating dehumidifier, for example, can be used to keep relative humidity levels in check, to distribute air throughout the house\u2014filtering it through an MERV 13 filter in the process\u2014and to introduce fresh outdoor air into the building.<\/p>\n \u201cOne of the common misconceptions about ventilating dehumidifiers is that they are automatically going to dehumidify the outdoor air they bring inside, but that\u2019s not what a ventilating dehumidifier is doing,\u201d explains Krueger. \u201cThe fan comes on in the dehumidifier when there is a call for ventilation, and then when the humidity controller senses that the humidity is too high in the living space, the compressor comes on and dehumidifies the space.\u201d<\/p>\n Rokicki recently taught a Fine Homebuilding online learning course called \u201cA Path to Better Indoor Air Quality.\u201d The class covers designing and building new homes with healthy indoor air quality in mind, improving indoor air quality while remodeling, troubleshooting indoor air quality issues in existing homes, and the maintenance it requires to keep indoor air quality healthy over the life of a home. Throughout, Rokicki stresses that we need to consider the homeowners\u2019 habits, present and future, as well. For example, if you design a house with a recirculating range hood because the homeowners plan to install an induction cooktop and mostly eat takeout, you\u2019re not considering the next homeowners, who may like to cook with gas. If you expect your clients\u2019 teenage children to run the bath fan you installed during a remodel and they don\u2019t, you\u2019ll be getting a quick call back to address the peeling paint and moldy ceiling.<\/p>\n While we can\u2019t make homeowners use their range hood, a ducted vent is the most effective solution in the kitchen and should be common practice. And if a bathroom will be used by teenagers, an exhaust fan on a motion sensor and timer will go a long way toward preventing high humidity levels and the problems they cause. Designers and builders today have the knowledge, tools, and products to provide everything a homeowner needs for healthy indoor air quality. Educating them on the dangers, and the solutions, is the final step. If my experience is telling, consider handing them an indoor air quality monitor when you hand them the keys to their house. That red circle is alarming!<\/p>\n \u2014 Brian Pontolilo; senior editor.<\/em><\/p>\n From\u00a0Fine Homebuilding\u00a0<\/em>#323<\/p>\n RELATED STORIES<\/strong><\/p>\n![\"\"](\"https:\/\/images.finehomebuilding.com\/app\/uploads\/2024\/04\/16155254\/01232362-exhaust-ventilation-diagram-700x425.jpg\")
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\nChange Your Filters<\/h2>\n
Don\u2019t Forget Dehumidification<\/h2>\n
\nFinding the Middle Ground for Relative Humidity<\/h3>\n
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\nEducate Your Clients<\/h2>\n