Home Ventilation

Exhaust Ventilation Systems

Have you ever noticed the extra light switch in your bathroom that operates the small exhaust fan located in the ceiling? This is an example of an exhaust ventilation system in its simplest form which serves to remove moisture, odors, and indoor air pollutants generated by the bathroom in order to provide indoor air quality benefits - The fan pulls air, odors and contaminants out of the bathroom, pushes it outside through the roof of your home, and fresher and cleaner air then rushes into the bathroom through air gaps around the bathroom door. However, exhaust ventilation systems can grow to be much more complex than a simple bathroom exhaust fan, applying the same depressurizing concept to entire homes and buildings depending on the outdoor climate and scenario.


Exhaust Ventilation Systems work by depressurizing a home, building or specific area - An indoor fan creates a negative pressure within the occupied space causing higher pressured air outside the occupied space to migrate into the occupied space through dedicated vents, open windows, and unfortunately in many cases through air gaps and air leaks in a building's exterior envelope. Exhaust Ventilation Systems utilize a rotating fan or impeller to create an air pressure difference and directional flow of air out of the areas they serve and essentially depressurize the areas they operate within. On the back end of the rotating fan blades the air pressure is low, causing higher pressured indoor air to rush toward the rotating fan blades of the exhaust system where it collides with the fan blades and gains velocity and directional flow. This air flow is then propelled and directed outside of the home, building or designated area(s) the exhaust system serves to depressurize, carrying along with it contaminants that are suspended within the air stream. As the exhaust system directs air and contaminants out of the area it serves, this area loses air pressure, which causes high pressured make up air from the surrounding areas to rush in and replace or 'make up' the air in area the exhaust system is operating within. Therefore, the main concern with exhaust systems is the make up air and what the make up air is composed of, and this is the primary factor that dictates if an exhaust system is applicable or not.


Whole Home Exhaust Systems are popular in moderate outdoor climates and cooler parts of the country with low humidity and where central air conditioning is in many times not even installed or needed. If you've ever been in one of these northern homes without central air you may have noticed a rather large fan device installed in the ceiling called a 'Whole House Fan', also known as an 'Attic Fan'. These Whole Home Exhaust Systems consist of a fan that pulls outdoor air into the home through the home's windows, throughout the home, and finally into the home's attic, either from a central location or from several locations at once. Once air enters the attic it naturally drifts away by temperature differences and draft by the wind penetrating air gaps in the home's eve and breathing soffits. These whole home exhaust systems are designed to be activated with the home's windows open and in cooler climates so they can draw in cool, crisp and fresh air into the home. In essence providing comfort benefits to homes without the need to run a central air conditioning system. Air sources from Whole Home Exhaust Systems such as these are also unfiltered and can also bring in outdoor air pollutants such as plant allergens like pollen and other air borne contaminants, so their fresh air intake properties can also provide IAQ problems, especially for those with asthma or allergy issues.


Since Whole Home Exhaust Systems are typically found in moderate to cool climates with low humidity, Exhaust Ventilation Systems in Houston residential applications are generally reserved to servicing 'spots' or areas such as bathrooms and kitchens for indoor air quality purposes. Because Houston's climate is so hot and humid, whole home exhaust ventilation systems are impractical ventilation strategies. Installing a whole home exhaust ventilation system in a Houston home would allow the heat and humidity from the outdoors to enter the home through windows, soffits, and other air gaps in the home's outer envelope. This would inevitably cause the central air conditioning system to constantly run in order to remove this heat and humidity pulled into the home by the exhaust system, in turn providing no comfort or energy efficiency benefits. Other than bringing in fresh air, whole home exhaust ventilation systems in Houston also provide little to no indoor air quality benefits because high pressured outdoor air saturated with heat and moisture is drawn into the home by the negative air pressure created by the exhaust system. Moisture problems would then occur throughout the home, windows may sweat with condensation when the HVAC system allows indoor temperatures to reach the dew point, and moisture would ultimately collect in crawlspaces and in between walls where it would provide a perfect breeding ground for fungi, mold, mildew, and bacteria. This basically limits home exhaust systems to servicing specific rooms or spots within Houston homes for specific indoor air quality benefits, such as in kitchens and bathrooms. Let's sum up the pros and cons of Home Exhaust Ventilation Systems:

* What is Backdrafting? Combustion appliances like water heaters, fireplaces and wood burning stoves have their own natural exhaust systems that carry small particles and toxic fumes such as carbon monoxide generated by the appliance out of the home via natural draft due to heat rising. Careful consideration has to be taken when installing Home Exhaust Ventilation Systems so they don't over-power an appliance's own natural exhaust system. When home exhaust ventilation systems are too powerful they cause Backdrafting, which means they essentially 'fight' an appliance's natural exhaust ventilation system, 'pulling', and backdrafting the dangerous particles and toxic fumes back into the home where they can cause health issues.

Supply Ventilation Systems

Supply Ventilation Systems are the exact opposite of exhaust ventilation systems - Unlike exhaust ventilation systems which negatively depressurize an occupied space, Supply Ventilation Systems positively pressurize occupied spaces. In homes, Whole Home Supply Ventilation Systems in their simplest form utilize the home's central air conditioning system's indoor blower motor as a fan to supply fresh outdoor air directly into the house. These systems work by drawing fresh outside air into the interior living space through a supply vent that penetrates an exterior wall with a duct that feeds into the return ducting of the home's central air conditioning system, essentially giving the return side of the HVAC system access to the outside air. This ventilation strategy allows the occupied space's air pressure to reach higher pressures than the surrounding unoccupied space's air pressure. Which, unlike exhaust ventilation systems that negatively depressurize a home in turn 'drawing' air into the home through windows, dedicated vents, and unsealed air gaps of a homes exterior. The positive pressure created by a supply ventilation system forces air out of such areas and gaps, essentially creating a pressurized vapor barrier that prevents outdoor heat, moisture, and contaminants from entering into the home. Since whole home supply ventilation systems connect directly to the home's central air conditioning system's return air ducting, the fresh air that is supplied to the occupied space by the HVAC system's indoor fan usually goes through the same process that air would normally go through when the HVAC system is running in cooling mode and is filtered and conditioned before being distributed into the home via the HVAC system itself (Although some supply ventilation systems do incorporate controls that will periodically run the HVAC systems blower motor independently while the air conditioning system is not running, or incorporate dedicated supply fan accessories, more about that below).




Although the general concept described above remains the same, supply ventilation systems can incorporate different features including mechanical dampers, barometric dampers, dedicated supply fans, dedicated filtering systems, sensors and controls. These accessories give the system improved control over the air being supplied into the occupied space. A whole home supply ventilation system's installation process is relatively simple and inexpensive - A hole is typically punctured through an exterior wall somewhere in the attic of the home close to the home's HVAC system. The exterior end of the new puncture is then covered with a weather-proof hood to prevent any water infiltration from rain and also to prevent any critters from crawling into the system itself. A duct is then ran from the hole in the exterior wall to the home's HVAC system and connected to the HVAC system's return air ducting, giving the HVAC system fresh air accessibility. It is here in this ducting between the HVAC system and the exterior wall that various features, other than barometric dampers, may accompany the system. Generally and at bare minimum a manually operated air damper or motorized actuator controlling an air damper should then be inserted inline with the new fresh air intake duct in order to control the rate of outdoor air being pulled into the home by the HVAC system's indoor blower motor. If the fresh air damper is controlled by motorized actuator or accessories such as a dedicated supply fan are being added to the system, they are then fed power and hard wired into the HVAC systems controls, which allow the air damper and accessories to operate accordingly based on certain factors such as temperature, humidity and whether the home's recommended ASHRAE air exchange rate has been satisfied. And finally with all supply ventilation systems it is a good practice to install an adjustable barometric air damper on the HVAC systems return air ducting somewhere before the fresh air intake ducting from the supply ventilation system, which will serve as an air pressure relief valve. This is especially true for tightly sealed homes without adequate exhaust systems or fireplaces to relieve pressure generated by the supply ventilation system, which would allow the indoor air pressure to reach unwelcome levels. A barometric air damper opens when air pressure within the home gets too high and becomes strong enough to force the damper open which essentially vents air directly into the attic, in turn relieving and dropping air pressure within the living space. Once the air pressure is relieved the barometric damper closes and stops venting air into the attic. Too much air pressure within a home's living space can cause doors and cabinets to fly open or close on their own, make opening and closing doors a very difficult and superhuman hulk-like task, create strange explainable noises, cause windows and cabinet doors to shake and rattle, and in general cause other annoying and spooky phenomena that may lead someone to believe their home is being haunted by ghosts.

Advantages of whole home supply ventilation includes the ability to control where incoming air is coming from, treat and filter the incoming air, and minimize the humid air that is pulled into the living space from the outdoors. Supply ventilation systems, unlike exhaust ventilation systems, prevent backdrafting of combustion appliances due to the positive air pressure they create within a home. And since supply ventilation systems pressurize the house, this generates a pressurized vapor barrier that keeps outdoor air pollution, moisture and heat out in hot and humid climates although it may cause moisture to condense in walls in cold climates if warm air escapes through unsealed air gaps to the outdoors.


Whole home supply ventilation systems are typically found in hot and humid climates as a means to introduce fresh air into homes and improve indoor air quality by positively pressurizing the house, which prevents outdoor air pollutants, heat and humidity from entering the home through air gaps. Installing a supply ventilation system can add latent and sensible heat and moisture from the outdoor environment directly into the home's HVAC system, where it will have to be removed by the air conditioning system and this translates to increased run time of the HVAC system which increases energy consumption. This is exactly why manufacturers of these systems have started incorporating humidity and temperature sensors and other accessories into their designs. These sensors monitor outdoor humidity and temperature and affect how far open or closed the fresh air damper is, and controls the rate of fresh air intake in a more energy efficient manner while still providing IAQ benefits. Supply Ventilation Systems may not be the perfect answer when it comes to energy consumption, but they are applicable in Houston's hot and humid climate and are relatively inexpensive indoor air quality solution to fresh air. Let's sum up the pros and cons of using this ventilation strategy:

Balanced Ventilation Systems

Unlike whole home exhaust-only and supply-only ventilation systems, which rely solely on natural ventilation created by internal and external pressure differentials delivered by a single fan to move air and pollutants. Whole Home Balanced Ventilation Systems utilize 2 fans in order to execute both exhaust ventilation strategies and supply ventilation strategies at the same time and introduce fresh outdoor air into a home at the same rate that stale and polluted indoor air is exhausted from the home. This equal exchange of opposing supply and exhaust air streams neither pressurizes nor depressurizes living spaces which allows balanced ventilation systems to provide superior indoor air quality benefits and, as we'll soon learn, take advantage of energy efficiency benefits as well when compared to conventional supply-only and exhaust-only systems.


It's easy to assume that with balanced ventilation systems, since they incorporate both supply and exhaust ventilation processes, that these systems would also incorporate the same flaws that accompany supply-only and exhaust-only systems such as the fact that supply-only and exhaust-only systems can raise heating and cooling costs because they force conditioned air out of a home and introduce unconditioned air into a home, in turn making air conditioning systems have to make up for this excessive heat loss or heat gain (which increases energy consumption). Although this is true to a certain extent, whole home balanced ventilation systems also incorporate energy transferring mechanisms called Energy Recovery Devices which help alleviate the handicaps of excessive heat loss or heat gain from ventilation. Balanced systems with energy recovery devices, unlike supply-only and exhaust-only systems, actually 'recover' a portion of the thermal energy that is normally gained and lost by exhaust-only and supply-only ventilation processes by exchanging the temperature and humidity between the opposing air streams (see below).

Energy Recovery Theory
(100% Air to Air Energy Exhange)

Hypothetically speaking - There are two opposing air-streams traveling in and out of a home. One at the temperature of 0ºF, and the other at 70°F. What happens to the temperature after it passes through an Energy Recovery Device? Many would assume each air stream would equalize and balance to the average temperature of 35ºF each. However, with recovery devices the thermal energy in the air-streams actually switches and the 70°F stream of air from the inside of the home now becomes 0ºF, and the 0ºF stream from the outside of the home becomes 70°F. This is the fundamental theory behind balanced systems with recovery devices. The case described above transfers 100% of all total energy, which has yet to be accomplished with traditional heat recovery ventilators (HRVs) or enthalpy or energy recovery ventilators (ERVs) - An energy recovery rating of 75%-85% much more realistic with HRVs and ERVs.


In order to achieve this energy recovery technique, balanced systems with recovery devices implement unique features within the system such as heat exchangers, enthalpy exchangers and other methods of transferring sensible (temperature) and latent (humidity) thermal energy which allow the balanced system to recover the heat and moisture being lost or gained through the exhaust and supply ventilation processes. For example, in Heat Recovery Ventilators (HRVs) a sensible heat exchanger is incorporated into system's design that allows these HRV systems to recover a large portion of the sensible heat energy (temperature) that is being lost or gained through the system's exhaust and supply processes. And, in Enthalpy or Energy Recovery Ventilators a sensible and latent heat enthalpy exchanger is used to recover portions of both heat and moisture from the supply and exhaust ventilation processes. There are also specialized energy recovery ventilators with cooling and heating functions which are balanced ventilation systems that utilize air source heat pumps to transfer air to air energy, which provide the greatest energy exchange benefits (100% and more) when compared to other forms of balanced home ventilation. Let's take a moment to look inside a typical Balanced Ventilation System that utilizes an Enthalpy or Energy Recovery Ventilator to transfer both heat and moisture between the supply and exhaust air streams and explore how it works.

Inside an Energy Recovery Ventilator

Inside Whole Home Balanced Ventilation Systems such as Heat Recovery Ventilators, Energy Recovery Ventilators, and Energy Recovery Ventilators with Cooling and Heating functions, there are opposing 2 fans which are responsible for the supply and exhaust processes of the system - One fan supplies fresh air into the home while the other exhausts polluted air out of the home. These fans direct the opposing incoming and outgoing air streams through various forms of energy exchanging components depending on the desired energy to be transferred and savings outcome, and this is where balanced systems with recovery ventilators begin to differ internally.


Heat Recovery Ventilators transfer only portions of Sensible Heat Energy, or Air Temperature between supply and exhausted air streams. In colder and less humid climates where heating a home dominates the HVAC process the majority of the year (and the utility bill) making it important to focus on preserving heat within the house, such as northern parts of the country, Heat Recovery Ventilators are primarily used in order to exchange only the heat energy that is gained or lost during the balanced air exchange process. During the winter time with HRVs, a large portion of the heat in the indoor air being exhausted from the home is removed and transferred into the cold outdoor air being supplied into the home, in turn preheating the fresh supplied air as it enters the home and in turn saving energy by reducing heating system run time and providing comfort and IAQ benefits. No moisture is transfered during this process with HRVs and only energy in the form of temperature is exchanged within the air which is why these systems are particularly useful in colder climates with low humidity. During the summer months with HRVs the exact opposite is true and a large portion of the hot outdoor air being supplied into the home gets cooled down by the conditioned indoor air exiting the home.



Enthalpy or Energy Recovery Ventilators take the energy transfer process a step further than HRVs by exchanging both sensible heat (air temperature) and latent heat (humidity). In hot and humid climates where cooling a home dominates the majority of HVAC usage during the year, such as here in Houston Texas, Enthalpy or Energy Recovery Ventilators are utilized in order to minimize both heat and humidity gained or lost during the fresh air exchange process. ERVs are essentially HRVs with the bonus feature of transferring latent heat energy or moisture. During the summer with ERVs a large portion of the heat and moisture from the outdoor air being supplied into the home is removed and transfered to the cold and dry indoor air being exhausted out of the home, essentially precooling and dehumidifying the supplied outdoor air as it enters the home and by doing so taking the load off of the air conditioning system. This transfer of energy provides energy efficiency benefits by reducing the run time of the cooling system when compared to the other conventional ventilation methods. During the wintertime with ERVs, like HRVs, the process reverses and the heat and moisture in the warm and humid indoor air being exhausted out of the home is removed and transfered into the cold and dry outdoor air as it is supplied into the home, essentially preheating and humidifying the incoming air taking the load off the home's heating system, which again saves energy and provides IAQ and comfort benefits.




The main difference between enthalpy or energy recovery ventilators and heat recovery ventilators is the type of heat exchanger located inside of the system located at it's core which dictates how much of what type of energy is actually transferred between supply and exhaust air streams. HRVs simply exchange thermal temperatures via a sensible heat energy exchanger whereas ERVs utilize a sensible and latent heat energy enthalpy exchanger which transfers both heat and moisture between the two opposing supply and exhaust air streams. Both HRVs and ERVs typically (but not always) make use of counter-current heat exchangers in one form or another due to their ability to efficiently transfer heat energy without the air streams every actually mixing. These types of heat exchangers are essentially metallic tubes, plates, or cells placed close in proximity to each other that the opposing exhaust and supply air streams are channeled through, allowing sensible heat (temperature) to transfer across air streams by means of convection. The difference between then between ERVs and HRVs that utilize counter-current heat exchangers is that the ERV's version of the counter-current heat exchanger also incorporates a permeable membrane of silica, or some specialized synthetic micropourous material which also allows latent heat (moisture) to be transfered beween the air streams through evaporation and condensation processes. Some ERVs utilize a rotating heat-wheel instead of the counter-current method described above. These types of heat exchangers are found in higher-end models of ERVs because they have a higher energy exchange ratio when compared to counter-current heat exchangers. However, heat-wheels also mix a percentage of the air streams, which means a certain portion of the air leaving the living space re-enters the living space. Typically this ratio of air portions mixed is very low (around 10%). Heat wheels simply recover more energy at the cost of indoor air quality, so using them to ventilate areas such as close to kitchens or bathrooms should be re-considered.


Energy Recovery Ventilators with Cooling and Heating functions take the energy recovery process even further than the previously discussed balanced ventilation systems by incorporating an air sourced heat pump into their design which precools and preheats opposing supply and exhaust air streams beyond the energy recovery ratio. Since the heat exchanger inside both ERVs and HRV's fails to completely transfer 100% of the thermal energy contained within the exhaust and supply air streams, there is always some amount of energy being lost or gained through the balanced ventilation processes. Typical HRV's or ERV's can achieve a 75% energy recovery rating, with the most advanced models effectively recovering upwards to 95%-96%. But, even at this impressively high level of energy recovery, a small percentage of heat or humidity is still not being recovered, and in some climates where humidity is often high and a contributing factor to comfort (such as here in Houston) an HRV or ERV can actually cause humidity levels to rise indoors. Let's take a look at how this might happen: For example, in a tightly sealed Houston home where the air conditioner rarely runs because the house is so well insulated, running an ERV will slowly add humidity to the home on a very humid day. The air conditioner may only occasionally turn on to cool the home's temperature down which occurs rather quickly due to the home's superb insulation factor. However, this short run-time doesn't give the air conditioning system enough time to effectively remove moisture from the air within home and with the ERV slowly adding small amounts of humidity to the home despite all its recovery efforts, the temperature may remain cool from the air conditioner occasionally running but humidity can continue to rise indoors - This is why manufacturers have developed energy recovery devices with heating and cooling functions. Inside these systems, the manufacturer inserts a sensible and latent heat exchanger along with a heat pump in between the exhaust and supply air streams, essentially achieving 100% of energy transfer and more so that even on a hot and humid day, the air supplied into a home will never raise indoor temperatures or humidity.

Inside an ERV with Heating and Cooling

How does it work? On a summer day, A supply fan channels the incoming hot and humid outdoor air over the supply side of a sensible and latent heat enthalpy exchanger, which removes some heat and moisture from the supply air as with an ordinary ERV. The now-energy-recovered supply air is then directed over the internal heat pump's evaporator coil, which cools and dehumidifies the supply air even further and beyond the 100% recovery ratio before distributing the air into the home. At the same time on the exhaust side of the ventilator, the cool and dehumidified exhaust air from the home is channeled over the exhaust side of the sensible and latent heat enthalpy exchanger, where it picks up the heat and moisture removed from the supply air as with an ERV system. This exhaust air is then directed through the heat pump's condenser coil, which adds the heat to the exhaust air that it removed from the supply air when it was directed over the heat pump's evaporator coil, and expels it outdoors. During winter months, the same process takes place in reverse and provides the home with warm air. This precooling and preheating technique takes the heat and moisture load off the HVAC system no matter what the outdoor climate is and provides all the benefits of the other balanced ventilation systems without all the flaws or the possibility of increasing indoor humidity in very humid climates.

With 2 exhaust fans and a small heat pump, you would be correct in assuming these systems consume the most electricity when compared to the other ventilation systems we've discussed. So why would anyone have one installed? Other than their dehumdification properties, these systems are designed to ventilate homes that are very well insulated, so well infact that an air conditioning system rarely has to run in order to maintain the temperature in these types of superinsulated homes. Air leaks and air infiltrations are rare in homes that are insulated very well, and like a well insulated Yeti cooler, these superinsulated homes maintain their temperature for long periods of time. In these applications running 2 variable speed exhaust fans and a small heat pump often consumes less electricity than running the entire air conditioning system, which in turn reduces energy consumption. Lets go over the pros and cons of a system like this: