Ventilation Systems

Ventilation is the fresh air we bring in mechanically, via fans.  There are five things to consider when designed a ventilation system (1) how the system will interface with outside air (2) whether the system includes any method to distribute the air within the building (3) whether there is filtration and (4) whether the ventilation system is integrated with a forced air heating system (5) how the ventilation system is controlled.

Do you need a Ventilation System?

Depending on how tight your building is and how much pollutants are expected to enter it, you may not need a ventilation system at all, although even in moderately tight buildings you are taking some risk if you don't have one.  Always keep in mind that the infiltration rate for a building will vary from close to zero in mild weather to close to the ACH50 value in extreme winter weather even though the yearly average will be between 1/15ACH50 and 1/25ACH50 depending on local climate. See the full discussion in the ventilation and filtration section.  Another consideration is the energy use tradeoff between air tightening and ventilation, which is covered in the infiltration discussion.

Types of Ventilation Systems

There are three types of whole house mechanical ventilation systems, and a strategy for passive natural ventilation.  Unless the house is built extremely tight, it has some degree of natural ventilation during some of the year, that amount may be sufficient to meet the house's fresh air needs during that time.  Mechanical ventilation has the advantage of supplying a known amount of  fresh air, in addition to be able to provide filtration by circulating the air in the house (although with an energy cost and possibly additional up-front cost).  With all whole house ventilation systems, there may be additional spot ventilation, for example in bathrooms and kitchens.  If spot ventilation exists, the only difference between it and an exhaust only system is that the control is fully manual.

Exhaust Only Ventilation
exhaust only

In the simplest form of this system a spot ventilation fan (typically the bathroom) is fitted with a timer that turns the fan on for part of the day every day. This timer typically has a manual control also so the fan can be used for spot ventilation. This system forces air out of the house, de-pressurizing it so that outside air will leak in to return the pressure to zero (see diagram at right: blue is the exhaust line, grey are the infiltration paths, which are essentially everywhere).  In this case the amount of air leakage into the house is above what already leaks via infiltration, and as with infiltration, this air can pick up particulates and VOCs on its way.  Some builders add fresh air ports in the vicinity of windows to avoid this problem, but this solution only helps if the building is quite tight, but in that case, the vents will also raise the overall infiltration rate.1  The air that leaks in will come from the path of least resistance, which means that if the attic or basement aren't sealed as well as the walls it will come from those place.  It also means that the leaks nearest to the exhaust port will leak more air than those farthest away, so some areas of the house may get a lot of air and others not so much.

While typical exhaust-only ventilation systems use only one fan and one duct, you can also use multiple fans and multiple ducts, for example you can remove air from the kitchen, home office and other bathrooms.   Exhaust only systems can also be combined with, or run in parallel to, a forced air furnace or other air distribution system.

The major advantage of exhaust only systems is cost and low operation energy: for more details on the energy cost of ventilation see the infiltration section. Note that exhaust only systems, especially in bathrooms, should have a noise rating of no more than 1 sone.

There are three main disadvantages of this system:

  1.  Fresh air comes thru leaks, which may pick up pollutants. In addition the air moving thru the wall could increase condensation, particularly in hot-humid climates, although it may reduce condensation issues in cold climates.
  2. Unless there is a distribution system, fresh air is not likely to be evenly distributed.
  3. The air that leaks in must be heated/cooled to room temperature.

Many states, including Washington have ventilation codes that specify exhaust only systems.  A recent study2 indicates that exhaust only systems are likely not that effective and in fact may actually introduce more pollutants than they remove, although this depends on how many pollutants the occupants produce and the leakage path that makeup air takes to get in.

Supply Only Ventilation
exhaust only

This system is the opposite of exhaust only: rather than forcing additional air into the house, supply only increases the pressure in the house relative to outside, and so air is forced out.  While this prevents back-drafting and drawing pollutants from the attic, crawl space or wall, during the heating season, this air is also likely carrying excess moisture, increasing the chance of condensation in insulated cavities.  In cooling climates, supply-only will have the opposite effect: it will reduce the chances of condensation.   As with the exhaust only system, vents can be added to give the air a way out, with all the same caveats and the additional one that wind pressure can overcome the interior pressure difference and also drive air inside, forcing more air out the leeward side and creating a ventilation imbalance in the building.  Supply ventilation can also be done with multiple fans or with a multi-port fan (ie one fan, but multiple ducts).

 As with exhaust only ventilation, all the air that enters will have to be heated or cooled depending on the temperature outside.  While with exhaust only ventilation the air comes in thru many small openings, and hence is usually undetected, supply ventilation is more likely to come thru only a small number of opening, and hence when the outside temperature is far out of the comfort range, supply ventilation creates a large potential for discomfort in whatever room the air is supplied to and as a result supply only ventilation is usually installed a part of a forced air system (see below).

Balanced Ventilation
exhaust only

This is the system of choice as it (hopefully) has no effect on the air pressure in the house.  Its biggest drawback is that it has a higher initial installation cost.  In this system, air is pulled out of one or more rooms, and then exhausted from one or more rooms.  In the simplest version, a supply fan is installed to run at the same time as an exhaust fan, but at a distant location. As with all other mechanical ventilation systems, if a fan creates a pressure difference between indoors and out, air may actually leak in or out thru the envelope: its just with a balanced system this is much less likely because the path of least resistance is more likely to be via the other fan.

HRVs and ERVs are inherently balanced systems in that the units are built with two matched fans.

Although balanced systems generally do a good job of equalizing the pressure between indoors and out, it does not necessarily do a good job of distributing fresh air throughout the building.

 Air Flow and Air Pressure

Air moves from areas of high pressure to low pressure until the pressure difference is zero, unless there is a barrier to prevent it. Like water, air will go through all susceptible openings, and only a specially designed barriers will prevent it doing so.  Air pressure differences are created due to a wind, stack effect or by mechanical fans.  Areas in a house are often at different pressures, and the house can be at higher pressure than outside or lower.  In stack effect, by definition the upper floor of the building is at higher pressure than outside and the lower floor is at lower pressure.  A ventilation or heating air supply duct in a room will create a positive pressure relative to the room with a return duct.   Likewise an exhaust fan creates negative pressure both in the room and in the building.  Likewise a fireplace, woodstove or furnace the gets its air supply from inside the house will create a large negative pressure because they act like large exhaust fans sending air up the flue.

Ventilation distribution

No matter what type of ventilation system is used, typically not every area of the building will get fresh air distributed to it unless it is specifically designed to distribute the air evenly, which generally means there is an air inlet in every room.  There are two general approaches here: either the air inlets are part of a recirculation system, or the ventilation system itself supplies or removes air from every room.

Recirculation system
recirc

Typically this is done by using a special thermostat to control the fan on a forced air heating system.  In this system, the forced air furnace's fan is used to recirculate air, often combined with a supply-only ventilation system or HRV (see integrating ventilation and heating, below). In the diagram at right, a forced air system's air patterns is shown: the dark blue are supply ducts and the light blue is the return air thru the rooms.  Even if the ventilation system is physically tied into the heating system, the two logically separate since they serve separate purposes.  Ventilation air is not shown in the diagram, but could be of any type introduced into any point in the system.

If the heating system is not forced air, you could use a separate recirculation fan in which case you can potentially use much smaller ducts than a forced air system would use, however as the duct length and number of bends in each run goes up, so does the recirculation fan energy, so there is a tradeoff there.   Unfortunately in order to get good mixing the recommended recirculation rate is .7ACH, which for most houses means more than 100CFM, and hence the recirculation fan is likely to use more energy than the ventilation fan.

In a recirculation system there are multiple supply points and one or more return point.  The air flows between the supply and return via doors and hallways, so if doors are potentially closed, they need to have a 1" gap at the bottom for the air to flow thru.  As with all air flows, the path air will take is that of least resistance, so for example if a supply duct is right near a door that leads to a room with a return duct, the air flow will short circuit right out the door.  However, air mixes very easily, so as long as the short circuit isn't very short, you'll still get some mixing. The bigger issue here is with stratification: supply air that is cool fed by ducts low on the wall with returns low on the wall will tend to stay near the floor; likewise warm supply air fed and returned high on the wall will stay high.  Forced air heat ducting has low supply and returns because the supply air is much warmer than room air so it rise, allowing only the cool air into the return and hence forcing some air mixing from floor to ceiling, although it does tend to leave the ceiling air much warmer than the floor air.

The advantage of a recirculation system, especially one with filtration is that you can likely reduce the ventilation rate, possibly by as much as 50% which reduces the associated heating/cooling energy of that fresh air as well.

Vent air in every room
recirc

Even distribution of air to all rooms can also be done with the ventilation system itself if the system supplies and/or removes air from all rooms.  While it would be theoretically possible to do this with a exhaust-only or supply-only system, its typically done with a balanced system and usually an HRV or ERV.  In diagram at right, the dark blue represents only ventilation air: there are 3 supply points and 2 returns.  The light blue is the path of air moving thru the rooms: note that the airflow between the two halves of the building is ambiguous--it depends on the relative pressure between the two halves--if the two sides are balanced, no air will move thru that opening.

 The catch with this system is that it's a challenge to balance this system so that all rooms receive adequate ventilation unless the system uses dampers and is calibrated, although it may be possible to get a decent distribution thru careful duct layout and using a central location for the fans. It is also the case that not every room has the same ventilation requirements, so you may want to design a system where some rooms get more air than others.

Keep in mind that the typical ventilation rates per room are likely to be very low, often not more than 10CFM.  If rather than using one big return duct as is done with forced air systems, if you have as many return ducts as you have supplies, the vent rate will go up because now you have less supply ducts delivering the same total quantity of air.

Integrating Ventilation and Heating

If your heating system is already ducted, its tempting to use those ducts for ventilation as well. The big issue with this is that ventilation air requirements are generally much lower than heating air requirement, especially in cold climates.  For example a typical house with a maximum heating load of 40kBTU and 140°F air would need a total of 529CFM.  Even a low energy house with  a 10kBTU maximum heating load would still need 132CFM of 140°F air.  If cooler air is used, say from a heat pump or fan-coil, where the air temperature is likely closer to 100°F, then the air flow requirement goes up to 309CFM.  On the opposite extreme, heating load can go very low, especially as insulation levels go up.  In all cases ventilation rates stay the same, typically in between 45 and 150CFM depending on the size of the house.

In order to match these two inherently mismatched requirements, the typical solution is to introduce fresh air into the return duct at the needed ventilation rate.  This can be done with either a supply only system or a balanced system via an HRV.  For the situation where the heating system doesn't run enough, a special thermostat turns on the heating system fan (but not the heat source) for how ever long is necessary to get the right amount of ventilation.  Since the heating system fan almost always has a higher flow rating than is required for ventilation, this means that the fan only runs for some portion of every hour., or if the heater fan is variable speed it can be run at a lower flow rate. One problem with this setup is that furnace fan motors are generally not as efficient as ventilation fans, so this solution often uses more fan energy.  There does seem to be trend toward higher efficiency heater fan motors, so that would solve the problem.  Look for units with fans using no more than 1w/CFM, or preferably .75w/CFM.

Another solution to this is to lower the energy requirements of the building by increasing insulation, which is the approach taken by the Passive House program (PassivHaus, is the original German standard: the US website is http://www.passivehouse.us).  In this case, the insulation level of the building is designed so that it needs no more than 3.164BTU/SF, which for a 1000SF building is 3164BTU/hr, and for a 2000SF building is 6328BTU/hr.   Assuming the 100SF building requires 45CFM ventilation, the supply air must be 135F, and if the supply air is only 100F then you need to bump the ventilation rate to 100CFM.  While this still isn't an exact match, it is possible to use the same ducts for both in this case.3  Passive house designs often put a heat source in the supply duct, so the thermostat only turns on the heat source; the fan is already running.

One other challenge of combined heat and ventilation systems is that the duct placement of the two is not the same.  Heat needs to be supplied to all rooms, but ventilation is often taken out rooms that typically introduce pollutants (kitchens, baths closets etc) and supplied to other rooms.  Likewise heating ducts are usually near the floor, cooling ducts are up high, and ventilation ducts can be either, depending on the assumed relative temperature of the ventilation air.

Temperature of ventilation air

If ventilation air is supplied directly into a room rather than thru a return duct, the air can be cold enough to be a discomfort to occupants.  Even using an 70% efficient HRV can still have a problem if the air temperature outside is quite cold, but no matter how cold is it outside, the air coming in the house is never warmer than room temperature.  To prevent this from happening, either the incoming air must be heated before put into the room, or put into the room in a remote enough location so that no one notices, or spread around enough rooms so the flow rate is low enough that no one notices.

Ventilation Control

The key to a good ventilation system is getting the right amount of air, but given the variability of infiltration, this is a non-trivial task.  Most systems either run continuously or intermittently on a fixed schedule, leaving the building over-ventilated some days, and possibly under-ventilated others depending on the ventilation rate the system is set to.

Some have suggested using CO2 sensors or humidity sensors as a control mechanism, but since ventilation rates are set more by dilution air requirements, neither of these is a good proxy for air needs.  In cold climates, since infiltration air is mostly a product of outside temperature, you could potentially use outside temperature as a proxy.4

Some activities create greater needs for ventilation than others; cooking for example, but also showers and inviting a group of friends over.  The basic exhaust only systems using a bath fan and timer usually allow for both running at a regular interval plus a manual mode for running for an additional amount of time.  An even simpler option is to open a window a crack, but it is very difficult to get the right amount of air this way.  In the case of whole house ventilation systems, if the house isn't too tight, extra spot ventilation can be provided in kitchens and bathrooms via a manual switch or a crank timer.  If spot ventilation is undesirable the ventilation system can also be designed to run at a higher speed.

Duct layout

Whether or not ducts are used for delivering heat, they should be kept inside the building's envelope to prevent heat loss.  They also should be sealed tightly (mastic works best, duct tape often fails) to prevent dust from wall cavities from entering them, and so no wall cavity should be used as a duct without a metal liner.  Smooth metal ducts are preferred since they provide the least resistance to air flow, and so reduce the strain on the heating and ventilation system fan motors.  It is also important to size the ducts to allow for sufficient air flow as well as to avoid bends, especially sharp bends, whenever possible as they also increase the back pressure making the motors work harder.

In a forced air system the ducts are designed to deliver enough heat to each room, while in a ventilation system they have to deliver enough fresh air.  The air movement requirement for heat delivery is much greater than that for ventilation (5-10 times greater: the colder it is out, the bigger the difference), so any duct system capable of delivering heat will deliver fresh air effectively.  In forced air systems, there is usually only one or two air returns, which if placed so that warm air is delivered evenly, will also deliver fresh air evenly.  The disadvantage of having limited returns is that closets and offices may not be ventilated enough.

In ventilation only systems, air is supplied to the rooms that are most occupied and removed from ones that cause the most contamination.  While this sounds good in theory, the two largest sources of water vapor (kitchens and bathrooms) will not be well served by a whole house ventilation system because they need a much higher volume of air, and so those locations will need spot ventilation fans anyhow.  The counter theory to this is that as long as you run the fans continuously, you can avoid spot ventilation. This is key for extremely tight houses such as Passive House compliant designs because the house is too tight for spot ventilation. Closets and offices could be well served by a whole house system.

Getting the right amount of air in each room is accomplished by installing dampers on the path to each duct and adjusting them so that the right amount of air comes out.

Where ventilation is needed
Some fresh air is needed everywhere, but pollutants and excess moisture are usually confined to specific areas.  As a rule of thumb, fresh air is generally supplied to rooms that are most frequently occupied (bedrooms, living rooms) and exhausted from rooms that generate the most pollution (kitchens, bathrooms, offices).

  • Kitchens - cooking can create both excess moisture and odors, and so a range hood that vents to outside is normally used to eliminate these sources. Circulating fans (ie no duct to outside) are not effective at all. Any ventilation in the kitchen needs to have a grease filter in it to avoid gumming up the ducts and fan.  Downdraft cooktops work by sucking large quantities of air (fighting to capture the warm waste air from cooking, which wants to rise), and because of that can remove all the air in a house in short order.  A 600CFM downdraft fan will remove all the air in a 2000 sq ft house in about 30 minutes.  It is not advisable to use these, but if it is used a window should be opened, or other source of make-up air used to prevent the house from de-pressurizing.  Since there are no sealed combustion gas ranges available yet, the use of a gas range will consume oxygen and produce carbon monoxide and possibly polyaromatic hydrocarbons.

  • Bathrooms - bathing produces significant excess moisture which generally requires 30-60 minutes of a fan running at 50CFM to remove it. Because this amount of fresh air is much greater than the amount of fresh air normally needed a separate exhaust fan should be installed and connected to a crank timer which allows the fan to automatically turn off when it is done.

  • Laundry - dryers produce excess moisture and should be vented to outside.  Since most people hang some clothes to dry, a spot exhaust fan should be installed.

  • Utility - Furnaces and hot water heaters should be sealed combustion units, which are self venting to outside.  

 

Resources

Understanding Ventilation, John Bower, Healthy House Institute, 1997

Builders Guide Series, Joe Lstiburek, EEBA, 2000.

The Health House Workbook, American Lung Association, 1995

Healthy by Design, David Rousseau & James Wasley, Hartley & Marks, 1997


Notes

1: although vents in a tight house will increase infiltration rates, at least the air coming in is as clean as outside air, so although tightness provides no energy benefit, it does provide a health benefit. 

2: Ventilation System Effectiveness and Tested Indoor Air Quality Impacts -- Building America Report 1309,  Armin Rudd and Daniel Bergey, March 2013.

3: there is still a difficulty in that design temperature (in the US anyhow) is not the absolute coldest temperature, but the temperature such that 98% of days don't get colder, but of course that means that 2% of days do.  You can still use a combined ventilation and heating system, but you supplement it with something else, for example electric radiant panels.

4: I've not seen this suggested anywhere, so I maybe wrong about it.