Ashrae handbook natural ventilation

Some particles can be removed with various types of air filters. Gaseous contaminants with higher molecular weight can often be controlled with activated carbon or alumina pellets impregnated with a substance such as potassium permanganate. Persily et al. The filtration and air cleaning options have the advantage of always being operational as long as the systems are properly designed, installed, and maintained.

However, the lack of standard test methods is a critical issue in application of some air-cleaning technologies. Building envelope air sealing and pressurization can be quite effective in protecting against outdoor releases as long as effective filtration against the contaminant of concern is also in place. Protection provided by operational changes such as system shutdown and purging depends heavily on timing; if timing is inappropriate, occupant exposure may be increased.

Isolating vulnerable zones and other system-related modifications depend on building layout and system design, and careful implementation is necessary for effectiveness under the range of conditions that exist in buildings. Finally, many retrofits can also increase energy efficiency and improve indoor air quality, which should be included in a life-cycle cost comparison of different options to the degree possible. Outdoor air introduced into a building constitutes a large part of the total space-conditioning load, which is one reason to limit air change rates to the minimum required.

The effect on heating loads tends to be much larger than on cooling loads McDowell et al. Chapters 17 and 18 address thermal loads in more detail. First, incoming air must be heated or cooled from the outdoor air temperature to the indoor or supply air temperature. The rate of energy consumption by this sensible heating or cooling is. Equations 33 and 34 are known as the sensible heat equation.

HVAC designers typically assume sea-level air pressure for locations with altitudes of ft or lower. A method to adjust for elevation is provided in Chapter Air exchange also modifies the moisture content of the air in a building. The rate of energy consumption associated with these latent loads, to add or remove water from the air and neglecting the energy associated with any condensate, is. Equation 35 is known as the latent heat equation.

A makeup air unit MAU is to condition cfm of outdoor air in the winter for a building in Atlanta, Georgia. Solution: From the weather data tables provided on the CD included with this volume, Atlanta is at an elevation of about ft. Because this is below the rule-of-thumb cutoff of ft for assuming sea-level conditions, air density is assumed to be 0.

However, for humidification design, a dew-point temperature of 9. However, both high- and low-temperature psychrometric charts are available from ASHRAE, as is a table of moist air properties at standard conditions in Chapter 1. With cfm of outdoor air to be conditioned, and using the sensible and latent heat equations for sea level, the energy needed to condition this outdoor air is.

Humidification can be provided by cold water, warm water, or steam, so a more precise psychrometric analysis is needed to size the heating coil correctly after the humidification method is selected and it is decided whether the humidifier will be placed before or after the heating coil.

As Example 1 shows, ventilation loads are substantial. When cooling outdoor air, substantial moisture usually must be removed from the ventilation air; reheat or regenerative heat recovery may be required in all but dry climates. Airflow through insulation can decrease the thermal load through heat exchange between infiltrating or exfiltrating air and the insulation.

Conversely, air moving in and out of the insulation from the outdoors can increase the thermal load. Experimental and numerical studies demonstrate that significant thermal coupling can occur between air leakage and insulation layers, thereby modifying the heat transmission in building envelopes.

In particular, research Bankvall ; Berlad et al. A literature review by Powell et al. The effect of such airflow on insulation system performance is difficult to quantify, but should be considered.

Using a computer simulation, Kohonen et al. Several experimental studies e. Judkoff et al. Buchanan and Sherman performed two- and three-dimensional computational fluid dynamics CFD simulations to study the fundamental physics of the IHR process and developed a simple macro-scale mathematical model based on the steady-state one-dimensional convection-diffusion equation to predict a heat recovery factor.

Their results show that the traditional method may overpredict the infiltration energy load. Heating and cooling degree-day values are based on sensible temperature data, but infiltration loads are both sensible and latent.

Infiltration degree days IDDs more fully describe a climate and can be used to estimate heat loss or gain from infiltration in residences Sherman Total infiltration degree-days is the sum of the heating and cooling infiltration degree-days and is calculated from hour-by-hour weather data and base conditions using weather weighted by infiltration rate.

The selection of base conditions is an important part of the calculation of the IDDs. Under some circumstances, it can effectively control temperature, contaminants, and possibly airborne moisture in mild climates, but it is not considered practical in hot and humid climates in the summer or in cold climates during the winter.

Temperature control by natural ventilation is often the only means of providing some cooling when mechanical air conditioning is not available. The arrangement, location, and control of ventilation openings should combine the driving forces of wind and temperature to achieve a desired ventilation rate and good distribution of ventilation air through the building. However, intentional openings cannot always guarantee adequate temperature and humidity control or indoor air quality because of the dependence on natural wind and stack effects to drive the flow Wilson and Walker Axley a and the Chartered Institute of Building Services Engineers CIBSE reviewed natural ventilation in commercial buildings, including potential advantages and problems, natural ventilation components and system designs, and recommended design and analysis approaches.

Natural ventilation openings include 1 operable windows, clerestories, doors, and skylights; 2 roof ventilators; 3 stacks; and 4 specially designed inlet or outlet openings such as OA louvers and EA grilles, ventilation penthouses or shafts, chimneys, windcatchers, and roof monitors. Operable windows transmit light, and provide ventilation when opened. They may open by sliding vertically or horizontally; by tilting on horizontal pivots at or near the center; or by swinging on pivots at the top, bottom, or side.

The type of pivoting used is important for weather protection and affects airflow rate. Exterior doors, often with insect screens, can also provide a path for natural ventilation; as with operable windows, security concerns must be considered.

Roof ventilators provide a weather-resistant air outlet. Natural-draft or gravity roof ventilators can be stationary, pivoting, oscillating, or rotating. Selection criteria include appearance, ruggedness, corrosion resistance, stormproofing features, dampers and operating mechanisms, noise, cost, and maintenance. Natural ventilators can be supplemented with power-driven supply or exhaust fans; their motors need only be energized when the natural exhaust capacity is too low.

Gravity ventilator dampers can be manual or controlled by thermostat or wind velocity. A natural-draft roof ventilator should be positioned so that it receives full, unrestricted wind.

Inlets can be conical or bell-mouthed to increase their flow coefficients. The opening area at any inlet should be increased if screens, grilles, or other structural members cause flow resistance. Building air inlets, most effectively installed at the lowest levels, should be larger than the combined throat areas of all roof ventilators. Stacks or vertical flues should be located where wind can act on them from any direction. Without wind, stack effect alone removes some air from the rooms with inlets.

In buildings that use natural ventilation, floor-to-ceiling heights are often increased well beyond the normal 8 to 10 ft. Higher ceilings, as seen in buildings constructed before air conditioning was available, allow warm air and contaminants to rise above the occupied portions of rooms. Air is then exhausted from the ceiling zones, and outdoor air is introduced near the floors; a degree of floor-to-ceiling displacement airflow is thus desirable when using natural ventilation during the cooling season.

The relationship describing the airflow through a large intentional opening is based on the Bernoulli equation with steady, incompressible flow. The general form that includes stack, wind, and mechanical ventilation pressures across the opening is.

The discharge coefficient C D is a dimensionless number that depends on the geometry of the opening and the Reynolds number of the flow. Aspects of wind that affect the ventilation rate include average speed, prevailing direction, seasonal and daily variation in speed and direction, terrain, and local obstructions such as nearby buildings, hills, trees, and shrubbery.

Liddament reviewed the relevance of wind pressure as a driving mechanism. A multiflow path simulation model was developed and used to illustrate the effects of wind on air change rate. Average wind speeds may be lower in summer than in winter; directional frequency is also a function of season. Natural ventilation systems are often designed for wind speeds of one-half the seasonal average. Equation 37 shows the rate of air forced through ventilation inlet openings by wind or determines the proper size of openings to produce given airflow rates:.

Air intakes should be placed in exterior high-pressure regions, and air reliefs should be placed in exterior low-pressure regions, but because of wind variations, these static locations will, at times, not be optimal.

Other considerations include flow control when wind speed is high, and security. Air intakes should face directly into the prevailing wind. If they are not advantageously placed, flow will be less than that predicted by Equation 37 ; if intakes are unusually well placed, flow will be slightly more.

Desirable air relief locations are 1 on the leeward side of the building directly opposite the intake; 2 on the roof, in the low-pressure area caused by flow separation; 3 on a side perpendicular to the windward face, where low-pressure areas occur; 4 in a dormer on the leeward side; 5 in roof ventilators; or 6 by stacks. Chapter 24 gives a general description of the wind pressure distribution on a building, but transient computational modeling will likely be required.

If building internal resistance is not significant, flow caused by stack effect can be expressed by. If there is thermal stratification, use an average temperature for T i. The discharge coefficient C D accounts for all viscous effects such as surface drag and interfacial mixing.

For this condition, flow through the opening is bidirectional i. Interfacial mixing occurs across the counterflow interface, and the orifice coefficient can be calculated by Kiel and Wilson :. If enough other openings are available, airflow through the opening will be unidirectional, and mixing cannot occur. Additional information on stack-driven airflows for natural ventilation can be found in Foster and Down The greatest flow per unit area of openings is obtained when the intake and relief areas are equal; Equations 38 and 39 are based on this equality.

Increasing the relief area over intake area or vice versa increases airflow but not in proportion to the added area. When openings are unequal, use the smaller area in Equation 38 and add the increase as determined from Figure 8. Several general guidelines should be observed in designing for natural ventilation. Some of these may conflict with other climate-responsive strategies such as using orientation and shading devices to minimize solar gain, with building codes that encourage compartmentalization to restrict fire and smoke movement, or with other design considerations.

In hot, humid climates, use mechanical cooling. If mechanical cooling is not available, air velocities should be maximized in the occupied zones of rooms.

In hot, arid climates, consider evaporative cooling. Airflow throughout the building should be maximized for structural cooling, particularly at night when the outdoor air temperature is low.

Building and surroundings characteristics. Topography, landscaping, and surrounding buildings should be used to redirect airflow and give maximum exposure to breezes.

Vegetation can funnel breezes and avoid wind dams, which reduce the driving pressure differential around the building. Architectural elements such as wing walls, parapets, and overhangs should be used to promote airflow into the building interior.

If there is no prevailing direction, openings should be sufficient to provide ventilation regardless of wind direction. Windows should be located in opposing pressure zones. Two openings on opposite sides of a space increase ventilation flow. Openings on perpendicular sides of the building force air to change direction, providing ventilation to a greater area.

If openings are at the same level and near the ceiling, much of the flow may bypass the occupied portion of a room and be less effective in removing contaminants from there.

Vertical distance between openings is required to take advantage of stack effect; the greater the vertical distance, the greater the ventilation rate. Openings near the NPL are least effective for thermally induced natural ventilation. If the building has only one large opening, the NPL tends to move to that level, which reduces pressure across the opening.

Greatest airflow per unit area of total opening is obtained by having openings of nearly equal areas. An inlet window smaller than the outlet creates higher inlet velocities.

An outlet smaller than the inlet creates lower but more uniform airspeeds through the room. Openings with areas much larger than calculated are sometimes desirable when anticipating increased occupancy or very hot weather. In one room, horizontally separated windows are generally better than vertically separated windows. They produce more airflow over a wider range of wind directions and are most beneficial in locations where prevailing wind patterns shift.

Window openings should be accessible to and operable by occupants, unless fully automated. For secondary fire egress, operable windows may be required. Intake and exhaust openings should not be obstructed by draperies, furniture, or nearby indoor partitions, for example.

Vertical airshafts or open staircases, where allowable by fire code, can be used to increase and take advantage of stack effects. Enclosed staircases intended for evacuation or safe haven during a fire must not be used for ventilation. Methods and tools have been developed in recent years to move the art of designing natural ventilation systems beyond the application of simple rules of thumb to engineered design Axley et al.

Successful application of purely natural ventilation systems for cooling may be very limited in hot or humid climates, such as in much of the United States, by thermal comfort issues and the need for reliability. However, hybrid or mixed-mode ventilation systems or operational strategies offer the possibility of saving energy in a greater number of buildings and climates by combining natural ventilation systems with mechanical equipment Emmerich The air-side economizer is one form of hybrid ventilation control scheme, and enjoys wide use in commercial, industrial, and institutional buildings in appropriate climates.

Integrated multizone airflow and thermal modeling is recommended when designing natural and hybrid ventilation systems Axley a; Dols et al. Most infiltration in U. A fan pressurization test is relatively quick and inexpensive, and it characterizes building envelope airtightness independent of weather conditions; some jurisdictions now mandate that buildings be tested and rated. The airflow required to maintain this pressure difference is then measured. The airflow rate is generally measured at a series of pressure differences ranging from about 0.

Depressurization tests are also used. The results of a pressurization test, therefore, consist of several combinations of pressure difference and airflow rate data.

An example of typical data is shown in Figure 9. This relationship is called the leakage function of the opening. The form of the leakage function depends on the geometry of the opening. Background theoretical material relevant to leakage functions may be found in Chastain et al. Figure 9. Openings in a building envelope are usually not uniform in geometry, and wind varies, so generally flow never becomes fully developed. Each opening in the building envelope, however, is often described by Equation 40 , commonly called the power law equation :.

Sherman a showed how the power law can be developed analytically by looking at developing laminar flow in short pipes. Additional investigation of pressure and flow data for simple cracks by Chastain et al. Walker et al. A typical value for n is about 0. Values for c and n can be determined for a building by using fan pressurization testing.

In some cases, the predicted airflow rate is converted to an equivalent or effective air leakage area as follows:. All openings in the building shell are combined into an overall opening area and discharge coefficient for the building when the equivalent or effective air leakage area is calculated. The air leakage area of a building is, therefore, the area of an orifice with an assumed value of C D that would produce the same amount of leakage as the building envelope at the reference pressure.

An airtightness rating, whether based on an air leakage area or a predicted airflow rate, is generally normalized by some factor to account for building size. Normalization factors include floor area, exterior envelope area, and building volume. With the wide variety of possible approaches to normalization and reference pressure difference, and the use of the air leakage area concept, many different airtightness ratings are used. Reference pressure differences include 0.

Reference pressure differences of 0. Although this may be true, they are below the typical range of measured values in the test; therefore, predicted airflow rates at 0. This uncertainty and its implications for quantifying airtightness are discussed in Chastain , Modera and Wilson , and Persily and Grot b.

Round-robin tests by Murphy et al. Some common airtightness ratings include the effective air leakage area at 0. Air leakage areas at one reference pressure difference can be converted to air leakage areas at another reference pressure difference by. Air leakage area at one reference pressure difference can be converted to airflow rate at some other reference pressure difference by. Flow coefficient c in Equation 40 may be converted to air leakage area by.

Finally, air leakage area may be converted to flow coefficient c in Equation 40 with. Equations 42 to 45 require assumption of a value for n , unless n is reported with the measurement results. When whole-building pressurization test data are fitted to Equation 40 , the value of n generally is between 0.

Therefore, using a value of n in this range is often reasonable. In reality, unless wind and temperature differences during the measurement period are sufficiently mild, pressure differences induced by weather during the blower door test cause measurement errors. Modera and Wilson and Persily studied the effects of wind speed on pressurization test results. This figure shows the large range of measured envelope tightness but also shows typical and extreme values in the housing stock.

These levels are in terms of effective annual average infiltration rate Q inf , which is based on normalized leakage area NL:. The fan pressurization procedure discussed in the section on Envelope Leakage Measurement allows whole-building air leakage to be measured. Additional test procedures for pressure-testing individual building components such as windows, walls, and doors are discussed in ASTM Standards E and E for laboratory and field tests, respectively. Dickerhoff et al.

The following points summarize the percentages of whole-building air leakage area that they found associated with various components and systems. Values in parentheses include the range determined for each component and the mean of the range. Both interior and exterior walls contribute to the leakage of the structure.

Leakage can occur between the sill plate and the floor or foundation; through cracks below the bottom of the gypsum wallboard, around and through electrical boxes, and through plumbing penetrations; and into the attic at the top plates of walls. Holes drilled through the top plates into the attic for passage of wiring are often unsealed. Leakage across the top ceiling of the heated space is particularly insidious because it reduces the effectiveness of insulation on the attic floor and contributes to infiltration heat loss.

Ceiling leakage also reduces the effectiveness of ceiling insulation in buildings without attics. Recessed lighting, plumbing, and other penetrations leading to the attic are some particular areas of concern, as are intentional openings such as access hatches or for whole-house fans.

The location of the heating or cooling equipment, air handler, or ductwork in conditioned or unconditioned spaces; the venting arrangement of a fuel-burning device; and the existence and location of a combustion air supply all affect air leakage. Modera et al. Field studies also showed that in situ repairs can eliminate one-quarter to two-thirds of the observed leakage Cummings and Tooley ; Cummings et al.

More variation in window leakage is seen among window types e. Windows that seal by compressing the weather strip casements, awnings often show significantly lower leakage than windows with sliding seals. Leakage around the frames of windows also can be significant if not properly sealed, typically with controlled-expansion foam, during their installation. When a fireplace is not in use, poorly fitting dampers allow indoor air to escape.

Glass doors may not seal the fireplace structure more tightly than a closed damper does. Chimney caps or fireplace plugs with signs that warn they are in place effectively reduce leakage through a cold fireplace but may not be allowed by fire code. The gap between a metal fireplace insert and the surrounding wall, tiles, or brick is often another leakage path if not properly sealed. Exhaust ducts going from conditioned spaces to the outdoors frequently have either no dampers or dampers that do not close properly.

The gap between the duct and the wall or roof penetration often needs sealing, as well. The ASHRAE Design Guide for Natural Ventilation is meant to assist owners, architects, engineers, facilities personnel and building design professionals to explore the feasibility of natural ventilation for their project during the early phases of design, taking into account such considerations.

Natural ventilation takes a wider amount of dialogue with the design team, ownership team and stakeholders to ensure its long-term success, especially in a world with greater weather extremes. The progress of a design team from conception to fruition for a natural ventilation scheme is one of collaborative review of critical analysis results.

Natural ventilation is reliant on a variable source of air movement and cooling capacity. Owners and tenants must be aware not only of the average expected results, but also the frequency of the extreme conditions for the current day and in a future climate-changed condition. Openings between rooms such as transom windows, louvers, grills, or open plans are techniques to complete the airflow circuit through a building. Code requirements regarding smoke and fire transfer present challenges to the designer of a natural ventilation system.

For example, historic buildings used the stairway as the exhaust stack, a technique now prevented by code requirements in many cases. Natural ventilation, unlike fan-forced ventilation, uses the natural forces of wind and buoyancy to deliver fresh air into buildings. Fresh air is required in buildings to alleviate odors, to provide oxygen for respiration, and to increase thermal comfort. However, unlike true air-conditioning, natural ventilation is ineffective at reducing the humidity of incoming air.

This places a limit on the application of natural ventilation in humid climates. Wind can blow air through openings in the wall on the windward side of the building, and suck air out of openings on the leeward side and the roof. Temperature differences between warm air inside and cool air outside can cause the air in the room to rise and exit at the ceiling or ridge, and enter via lower openings in the wall.

Similarly, buoyancy caused by differences in humidity can allow a pressurized column of dense, evaporatively cooled air to supply a space, and lighter, warmer, humid air to exhaust near the top. These three types of natural ventilation effects are further described below. Wind causes a positive pressure on the windward side and a negative pressure on the leeward side of buildings. To equalize pressure, fresh air will enter any windward opening and be exhausted from any leeward opening.

In summer, wind is used to supply as much fresh air as possible while in winter, ventilation is normally reduced to levels sufficient to remove excess moisture and pollutants.

An expression for the volume of airflow induced by wind is:. The coefficient of effectiveness depends on the angle of the wind and the relative size of entry and exit openings. It ranges from about 0. Sometimes wind flow prevails parallel to a building wall rather than perpendicular to it. In this case it is still possible to induce wind ventilation by architectural features or by the way a casement window opens. For example, if the wind blows from east to west along a north-facing wall, the first window which opens out would have hinges on the left-hand side to act as a scoop and direct wind into the room.

The second window would hinge on the right-hand side so the opening is down-wind from the open glass pane and the negative pressure draws air out of the room. It is important to avoid obstructions between the windward inlets and leeward exhaust openings.

Share This. Significant updates to the edition include the following: The scope is changed to remove commentary and to more specifically identify occupancies previously not covered. Informative tables of ventilation rates per unit area are included for checking existing buildings and design of new buildings. The Ventilation Rate Procedure is modified with a new simplified version for determining Ev and a more robust option for determining values of Ez.

The Natural Ventilation Procedure is significantly modified to provide a more accurate calculation methodology and also define the process for designing an engineered system. Natural ventilation now requires considering the quality of the outdoor air and interaction of the outdoor air with mechanically cooled spaces.