- Thermal Audit
Indoor Air Quality
ANSI/ASHRAE Standard 62.1
Indoor air quality is a complex issue. Pure earth atmosphere, at 70°F with 50% relative humidity and devoid of smoke particles, mold spores, air-bourne pathogens, plant pollen, respirable dust and with oxygen and carbon dioxide levels that remain constant despite our breathing - would be ideal, but terribly impractical. Compromises are needed, and the complex trade offs are the subject of the field of indoor air quality.
Attempts have been made to create universal guidelines which can be incorporated into building codes. Some techniques are simply prescriptive, using static formulas. If these calculations tell you that your house needs 100 ft3 of fresh air per hour, will this give you have a healthy place to live? The answer depends on whether the air outside is fit to breath and not contaminated with automobile exhaust, ozone, pollen etc. If so then you are trading one set of problems (low oxygen, high carbon dioxide, cooking fumes) for another (a code-purple air pollution day perhaps). For an asthmatic individual, one air exchange per hour could be great one day, and a mistake the next day. To mitigate this, standards require that the quality of the outdoor air be studied first before design decisions are made. While common in commercial buildings, this kind of analysis is seldom done for residential homes. A better approach is to determine and monitor contaminants of interest, and have a ventilation system that works to maintain the air within acceptable limits. The only limitation of this approach for home owners is that the measurement equipment is currently cost prohibitive, although this need not be the case because the technology to produce low cost sensors exists.
One of the more commonly cited references on indoor air quality (IAQ) is ANSI/ASHRAE Standard 62.1. ASHRAE is the American Society of Heating Refrigeration and Air conditioning Engineers. In the section below, we will analyze it in great detail and relate it to single family home constriction.
ANSI/ASHRAE Standard 62.1
This standard sets minimum ventilation rates for indoor or enclosed areas that have human occupants, and assumes that the outside air is free of "unusual" contaminants. Human comfort is not considered; for example, humidity levels that are uncomfortable aren't a problem unless they encourage the growth of mold.
They also acknowledge that what is acceptable for the majority (80%) of people, can be dangerous to sensitive populations (elderly, sick, young children); that odors are perceived differently by different populations and context sensitive (toilet odor in the kitchen may not be acceptable) and not considered by the standard; nor are outdoor pollutants like carbon monoxide.
- The status of regional compliance with national air quality standards must be determined, which in the USA means “attainment” or “non attainment” with National Ambient Air Quality Standards.
This is simply a legal status. You can check out your area and drill down into the detailed maps to see how your building area is rated.
- Outdoor air quality must be investigated before a ventilation system design is completed. The standard is concerned only with "criteria pollutants", namely:
- Sulfur Dioxide
- Particles (PM10)
- Carbon monoxide
- Oxidants (Ozone)
- Nitrogen dioxide
The goal here is simply to make a determination as to whether the outdoor air is to be classified as "attainment" or "non attainment".
- Local air quality is to be studied during times when the building is likely to be occupied, and should consider adjacent building (e.g. exhausts from restaurants) and their relationship to outdoor intakes along with the shape of the building and prevailing winds.
- A report outlining the regional and local (site) specific outdoor air quality must be presented to and reviewed with the building owners, although the format or content is not specified.
The purpose of this is to ensure that the designers of the ventilation system are aware of air quality problems in the design phase, as it might determine the need for air filters, electrostatic precipitators, or special procedures.
They mention that studies of naturally ventilated buildings (e.g. windows that open) have fewer "sick building syndrome" complaints overall. The guidelines are as follows:
- Rooms need to be open to, and within 25 feet or 8 meters of operable openings to the outside (e.g. windows), and the area of the opening should be at least 4% of the surface area of the floor.
For example: A 8'x8' kitchen with no windows opens to a living room 8'x14', and the living room has window on the far wall that opens 32" by 16". The total floor area of the two rooms is 176 sq ft. The window area window is 3.5 sq ft. The distance between the far ends of the living room and kitchen are 24 feet, which just makes the specification. However, if there is a kitchen door, then the kitchen is not permanently open and fails the specification. The window area is 1.9% of the total floor area, so it fails. The window is 3% of the living room floor area, so it fails too. To pass, the living room window needs to be a little larger and the kitchen needs a window or the kitchen door needs to go and the living room window needs to be about twice as large.
- The area of the open window used in the 4% calculation is to be the unobstructed area, i.e. factor in louvers, grates and insect screens.
I will add a link later, but I suspect that many residential insect screens reduce the air flow significantly.
- For any room without windows that ventilates through an adjoining room, the opening between the rooms must have an area of at least 8% of the floor area, and be greater than 25 sq ft or 2.3m2.
A large 36" doorway that is 8' high, is 21 square feet, so you need something more along the lines of an archway or completely open wall. In the kitchen example, 8% of the floor area is only 2.9 sq ft.
- The openings to the outside need to be operable by the people inside.
It is interesting that the 4% and the 8% in the equations above were chosen "to be consistent with model building codes in the United States". I would like to see gas diffusion studies to back up these figures as being optimum.
Engineered natural ventilation
The standards do not cover this area, but it is worthwhile noting that clever design can take advantage of thermal currents (heat rising up through a vaulted ceiling and out from a central vent), or other design features that provide a constant movement of air, resulting in a design that is not compliant with the ASHRAE standards, yet totally acceptable. Their take is that if the appropriate authority with jurisdiction signs approves, then it complies.
- Exhaust ducts should have negative pressure, or be sealed.
This is to make it impossible for exhaust air to leak back out into the living area. If the exhaust fan is at the building exit, on the distance between the fan blades and the exhaust louvers are pressurized. In the case of a residential heat exchanger, this is probably 4 feet of flexible ducting and many of these do indeed leak. If the exhaust fan is deeper inside the house, then the pressurized exhaust ductwork needs to be sealed in accordance with accordance with SMACNA Seal Class A.
- Mechanical ventilation systems must have controls that allow the fan to operate whenever spaces are occupied.
- All airstream surfaces must be resistant to microbial growth and not be subject to peeling for flaking or any other form of erosion.
- Outdoor are intakes, including ducts, windows, and doors (if they are part of a natural ventilation system) must maintain a separation distance from sources of pollution (exhaust vents, garbage bins, etc.).
The intake of a ventilation system must be keep away from source of pollution. This of course is obvious, the question is rather -- how far away?
|exhaust (smelly, irritating)||5|
|chimneys and flues||5|
|automobile area, parking garage||7.5|
|parking lots, street, driveways||1.5|
|high volume traffic||7.5|
|distance from surface to intake, factoring in snow depth||0.30|
|Cooling tower intake||5|
|Cooling tower exhaust||7.5|
These types of things need to be adjusted based on a more through investigation. For example, if the smelly exhaust was from a curry house, stating that its not your problem because the intake is 5m away, is not likely going to be well received. This of course, should have been described thoroughly in the initial outdoor air quality report.
Rain Intrusion and Entrainment
- Intakes must not allow water to enter
This is to minimize the growth of mold, and needs serious consideration in colder climates. There are many well engineered rain hoods that are totally useless against blowing snow, and I have seen many residential air exchangers in Canada that are soaked much of the winter. The standard leaves the problem of snow entrainment up to the designer, which means for most residential owners of heat exchangers, they will need to fix the problem themselves.
- Louvers should not allow more than 3g of water per square meter of louver during a 15 minute test at maximum air velocity.
- Bird screens must prevent birds from nesting in the intakes.
Birds are a particular hazard because bird feces caries many diseases, and the are usually heavily parasitized with fleas and ticks which could be sucked into the intake.
- Local sources of pollution (a clothing dryer for example) must be exhausted directly to the outside.
The ventilation of combustion appliances, such as heaters, is not addressed directly, other than specifying that local building codes and the manufacturer's instructions be adhered to.
- Air filters with a MERV greater or equal to 6 are required upstream of wetted surfaces
This prevents dirt accumulation and the growth of unwanted things. Residential heat exchangers often fill with filth in this manner because of condensation and need to be washed out every few months. For commercial buildings, it would be an issue with cooling coils and evaporative coolers.
- Maximum humidity is 65%, provided there is a method to dehumidify.
This is not a level designed for comfort, rather it is a point where condensation on cooler surfaces and subsequent mould growth is a possibility.
- Intake air flow must be greater than exhaust outflow for air-conditioned buildings that are dehumidifying.
Obviously this is only possible if the difference is exhausted through cracks and leaks. The resulting mild pressurization prevents moist outside air from entering and condensing on cooler surfaces.
- Drain pans under dehumidification equipment must prevent standing water, and be positioned under the entire width of the device.
- Drains must have a p-trap
This is accomplished with pan slope and drain size. Residential heat exchangers have this designed into them, and have a drain tube that is inserted into a drain or to the outside. The tube usually is looped to create a water trap that creates a seal against drawing air in from the drain.
- Humidification systems must use potable (drinkable) water
This makes sense because if is contaminated, after evaporation, the contaminates will be sent through the system as a power.
- Liquid water penetration of the building envelop is not allowed
- A vapor retarding barrier is required to prevent condensation on colder parts of the building
- Exterior joints and seams must be caulked to limit infiltration of the building envelop
Soil gasses like radon are not covered, and are likely covered by your local building codes.
- Ducts, pipes and other surfaces cool enough to cause condensation, must be insulated.
A typical residential example in colder climates is the incoming domestic water pipe. The ground water is often quite cold, and water droplets form rapidly when the water is running, and wet the surfaces below which are often ceilings. They give an example of cold water pipes in a commercial bathroom as being exempt because they can be cleaned, and defer to local building codes.
Air Classification System
CLASS1 Nothing objectionable, can be recirculated (living room air)
CLASS2 Mildly offensive (pool or gym locker room)
CLASS3 Irritating and offensive (autopsy room and other horrible things)
CLASS4 Dangerous or harmful (kitchen hood, paint spray booth)
- You can recirculate air within a class, or to a higher class provided the contaminants (classes 2-4) are similar.
- Air can be passed through cleaners to upgrade its classification
- Mixing classes of air results in a classification to the higher class
- Energy recovery resulting in less than 10% cross-contamination with CLASS2 air or 5% cross-contamination with CLASS2 air does not affect the classification of CLASS1 air.
The last point is applicable to heat recover ventilators, such as units from Venmar that are common in northern homes. There are tight seals around the heat recovery core resulting in mixing far less than 10%. I would question the wisdom of mixing 5% autopsy air with the air in a living space.
The procedures for determination of adequate air-exchange rates are based on achieving an 80% satisfaction rate. This isn't a problem for a public space where the majority rules, but is a problem for a home owner that is one of the 20%, perhaps with allergies, and excellent sense of smell or low tolerance to sensory assault.
Ventilation Rate Procedure
One procedure is to treat the room type and occupants separately, and sum them together to get the flow rate needed by a room.
In the table below, the human component is categorized by the type of activity, and the rate in Liters per second required due to having people in the building.
|Category||L/s per person||Description|
|0||0||A storage facility where human activity is negligible compared to the ventilation needs of the building.|
|1||2.5||Quiet office work|
|2||3.5||Lobby (more people around)|
|3||5||classrooms and schools|
These are the building components, categorized, with liters per second of air per square meter of floor space.
|0||.3||Conference room, lobby|
|2||.9||Art classroom with paints and glues|
They assume that people are breathing the air in a zone that starts 3 inches from the floor, up to six feet (unless you are a basketball player), and two feet out from the walls. This is referred to as the breathing zone.
The significance is that ventilation air needs to flow through this zone, and not high up in a ceiling, to be effective. With cooling, this is not a problem because the cool air settles forcing the stale warmer air upwards. It can however be a problem in the heating season. In a residential setting, air mixing can be solved with ceiling fans.
Residential heat exchangers are not likely to be affected during the heating season because their outputs will be colder (having come from outside will always be cooler than the outgoing air), and drop to the floor.
- Determine if air cleaning is required (particles and ozone)
- Sum up the demand by room using the tables above, e.g. Room 1: (6 people*2.5L/s demand) + (30m2*0.6L/sm2), and so forth for each room.
- Factor in the zone mixing efficiency (probably 1, or totally mixed in most houses)
- If the PM10 standards are exceeded in your region (as is the case in many large cities), the incoming air needs to be filtered
- If the second highest daily maximum one-hour average monitored ozone levels exceed 313 μg/m3, then filtering is required. See the EPA for discussions of ozone standards.
The Indoor Air Quality Procedure (IAQP)
The previous procedure was formula based, whereas IAQP is based on controlling the concentrations of contaminants. This requires:
- Identifying the contaminants of concern
- Determining acceptable limits
- Specifying the indoor air quality criteria
- Designing an system to meet the objectives
In a residential setting, the contaminants of concern are likely CO2, excessive humidity, cooking exhaust, and small particles. The concentration of total volatile organic compounds is an excellent indicator if you have a way to measure it.
There is a lot more in the specification, including scenarios specific to multi zoned HVAC systems in large commercial buildings that are unlikely to apply to the average home owner.
This specification has a lot of worthwhile considerations for the designer of residential dwellings, In particular. The following are things I would consider important:
- Use the formulas to calculate the minimum flow rate required, and compare this to the specification of your heat exchanger or fan system. It is possible that you already have more airflow than is required. Even a small air exchanger can pump more than twice the air required for a typical small family home.
- Existing homes are seldom air tight. A figure of 0.15 air exchanges per hour is typical for something built in 1982, with considerably higher infiltration rates on windy days and in the winter. (see Measured Air Leakage of Buildings, ASTM for examples).
- Check government sources for pollutants prevalent in your area. If desert dust, or smog is a problem in your area, consider some type of central air cleaner.
- Ensure that incoming water pipes aren't dripping water, and if you have a heat exchanger, that the incoming vent isn't perpetually wet with condensation or melting snow. You really do want to minimize the growth of mildew and mold for health reasons.
- The sensor based approach is the way to go, with air flow automatically reacting to demand or special needs. However, until someone produces an inexpensive control modules that measures dust, organic compounds, carbon dioxide and humidity which can be used to control the system, it is not practical for the homeowner. The technology does exist to make these cheaply once consumers demand it. (see update below)
- Windows that open are a low cost solution in any region that has good air outside. If people feel unwell due to poor air, they will simply open the window and solve the problem.
- There is an obvious long term problem with sealing a building, and having central control over the ventilation - namely complexity and needing electric power to keep it all going forever. A vastly better approach would be to engineer materials that have the required permeability to allow air exchange through them naturally, working to exchange heat and air without needing electricity or moving parts. That, coupled with careful architectural features to encourage air currents would make for a comfortable, and sustainable home.
Update: February 2011 - A new article on residential On-Demand ventilation has been added, where gas sensors are being used to continuously adjust the ventilation.