Air changes per hour

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Air changes per hour, abbreviated ACPH or ACH, or air change rate is the number of times that the total air volume in a room or space is completely removed and replaced in an hour. If the air in the space is either uniform or perfectly mixed, air changes per hour is a measure of how many times the air within a defined space is replaced each hour. Perfectly mixed air refers to a theoretical condition where supply air is instantly and uniformly mixed with the air already present in a space, so that conditions such as age of air and concentration of pollutants are spatially uniform. [1]

Contents

In many air distribution arrangements, air is neither uniform nor perfectly mixed. The actual percentage of an enclosure's air which is exchanged in a period depends on the airflow efficiency of the enclosure and the methods used to ventilate it. These systems range from a conceptual system of perfect displacement, which removes and replaces all air in a space, to a short circuit flow in which very little of the existing air is replaced. [2] The actual amount of air changed in a well mixed ventilation scenario will be 63.2% after 1 hour and 1 ACH. [3] In order to achieve equilibrium pressure, the amount of return air (air leaving the space) and the amount of supply air (air entering the space) must be the same.

Definitions

Age of air
The average time elapsed since molecules of air in a given volume of air entered the building from outside.
Concentration
The quantity of one constituent dispersed in a defined amount of another.
Concentration, tracer gas
The volume or mass of tracer gas divided by the volume or mass of air plus tracer gas.
Outdoor air
Air outside a building or taken from outdoors and not previously circulated through the system.
Perfect mixing
A theoretical airflow distribution pattern within a ventilated space where the supply air is instantaneously and uniformly mixed with the air in the space such that the concentration of all constituents in the air, and the age of air, are spatially uniform.
Return air
Air extracted from a space and totally or partially returned to an air conditioner, furnace, or other heat source.
Supply air
Air entering a space from an air-conditioning, heating, or ventilating apparatus.

Source: [1]

Formulas

Air changes per hour

In Imperial units:

where:

In metric units

where:

For a given room or building size and number of air changes per hour, the volumetric flow requirement is usually specified in cubic meters per hour when using metric units. [4]

Ventilation rates

Ventilation rates are often expressed as a volume rate per person (CFM per person, L/s per person). The conversion between air changes per hour and ventilation rate per person is as follows:

where:

One cubic meter per minute = 16.67 liter/second

Air change rate recommendations

Air change rates are often used as rules of thumb in ventilation design. However, they are seldom used as the actual basis of design or calculation. For example, residential ventilation rates are calculated based on area of the residence and number of occupants. [5] Non-residential ventilation rates are based on floor area and number of occupants, or a calculated dilution of known contaminants. [6] Hospital design standards use air changes per hour. [7] Recommended air changes rates can be found in relevant standards. [5] [6] [7]

Methods of measurement

Air changes per hour is a measurement intended to communicate the air change effectiveness of a space’s ventilation system. Discussion around the ASHRAE standard 62, Ventilation for Acceptable Indoor Air Quality led to the development of a more direct method of measuring air change effectiveness with the use of a tracer gas. [1] A tracer gas is a small amount of easily-detected gas which is mixed with air in order to study airflow patterns. This method directly compares the age of the air where occupants breathe to the age of air that would occur were the air of the space perfectly mixed. The method is designed to more accurately understand the effects of air-flow patterns within a space, to identify or avoid such occurrences as short-circuiting airflow patterns, in which most of the supply air goes directly to the exhaust, and fails to mix with the air already present. [2] [1]

There are two ways to apply this method; the most commonly used technique, Tracer Gas Decay (step-down), consists of a short burst of gas injected into the space to establish a constant concentration within the building, then injection is stopped and concentration decay at a certain position is recorded, and second Tracer Gas Step-Up, where tracer gas is injected at a constant rate and the concentration response at a certain position is recorded [8]

Airtightness in building

The most common technique to measure airtightness is the fan pressurization method, also known as the blower door test. It is measured by the number of air changes per hour (ACH) that occur when there is a differential pressure of 50 pascals between outside and inside the building. If an air volume equal to the inside volume of the building flows across the envelope in one hour, then ACH = 1. [9] Airtightness improves buildings' energy performance since low airtightness levels imply high infiltration rates, draughts, risk of condensation, and moisture accumulation, and other effects. [10] For this reason, the Passive House standard established performance requirements for airtightness requiring be less than 0.6 ACH with a pressure difference between inside and outside of 50 Pa. [11]

Effects of ACH due to forced ventilation in a dwelling

Forced ventilation to increase ACH becomes a necessity to maintain acceptable air quality as occupants become reluctant to open windows due to behavioural changes such as keeping windows closed for security. [12]

Air changes are often cited as a means of preventing condensation in houses with forced ventilation systems often rated 3–5 ACH though without referencing the size of the house. However, where ACH is already greater than 0.75 a forced ventilation system is unlikely to be of use at controlling condensation and instead insulation or heating are better remedies. [12] Seven out of eight houses studied in NZ in 2010 had an ACH (corrected for ventilation factors) of 0.75 or greater. [12] The presence of forced ventilation systems has been shown in some cases to actually increase the humidity rather than lower it. [12] By displacing air inside a dwelling with infiltrated air (air brought in from outside the dwelling), positive pressure ventilation systems can increase heating (in winter) or cooling (in summer) requirements in a house. [12] [13] For example, to maintain a 15 °C temperature in a certain dwelling about 3.0 kW of heating are required at 0 ACH (no heat loss due to warmed air leaving the dwelling, instead heat is lost due to conduction or radiation), 3.8 kW at 1 ACH and 4.5 kW are required at 2 ACH. [12] The use of roof space for heating or cooling was seen as ineffectual with the maximum heating benefits occurring in winter in more southerly regions (being close to the South Pole in these southern hemisphere reports) but being equivalent only to about 0.5 kW or the heating provided by about five 100 W incandescent light bulbs; cooling effects in summer were similarly small and were more pronounced for more northerly homes (being closer to the equator); in all cases the values assumed that the ventilation system automatically disengaged when the infiltrating air was warmer or cooler (as appropriate) than the air already in the dwelling as it would otherwise exacerbate the undesirable conditions in the house. [13]

Related Research Articles

<span class="mw-page-title-main">Heating, ventilation, and air conditioning</span> Technology of indoor and vehicular environmental comfort

Heating, ventilation, and air conditioning (HVAC) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. HVAC system design is a subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer. "Refrigeration" is sometimes added to the field's abbreviation as HVAC&R or HVACR, or "ventilation" is dropped, as in HACR.

<span class="mw-page-title-main">Ventilation (architecture)</span> Intentional introduction of outside air into a space

Ventilation is the intentional introduction of outdoor air into a space. Ventilation is mainly used to control indoor air quality by diluting and displacing indoor pollutants; it can also be used to control indoor temperature, humidity, and air motion to benefit thermal comfort, satisfaction with other aspects of the indoor environment, or other objectives.

<span class="mw-page-title-main">Blower door</span>

A blower door is a machine used to perform a building air leakage test. It can also be used to measure airflow between building zones, to test ductwork airtightness and to help physically locate air leakage sites in the building envelope.

A sound attenuator, or duct silencer, sound trap, or muffler, is a noise control acoustical treatment of Heating Ventilating and Air-Conditioning (HVAC) ductwork designed to reduce transmission of noise through the ductwork, either from equipment into occupied spaces in a building, or between occupied spaces.

<span class="mw-page-title-main">Dilution (equation)</span> Chemistry concept

Dilution is the process of decreasing the concentration of a solute in a solution, usually simply by mixing with more solvent like adding more water to the solution. To dilute a solution means to add more solvent without the addition of more solute. The resulting solution is thoroughly mixed so as to ensure that all parts of the solution are identical.

Displacement ventilation (DV) is a room air distribution strategy where conditioned outdoor air is supplied at a low velocity from air supply diffusers located near floor level and extracted above the occupied zone, usually at ceiling height.

<span class="mw-page-title-main">Variable air volume</span> Heating or air-conditioning system

Variable air volume (VAV) is a type of heating, ventilating, and/or air-conditioning (HVAC) system. Unlike constant air volume (CAV) systems, which supply a constant airflow at a variable temperature, VAV systems vary the airflow at a constant or varying temperature. The advantages of VAV systems over constant-volume systems include more precise temperature control, reduced compressor wear, lower energy consumption by system fans, less fan noise, and additional passive dehumidification.

Thermal comfort is the condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation. The human body can be viewed as a heat engine where food is the input energy. The human body will release excess heat into the environment, so the body can continue to operate. The heat transfer is proportional to temperature difference. In cold environments, the body loses more heat to the environment and in hot environments the body does not release enough heat. Both the hot and cold scenarios lead to discomfort. Maintaining this standard of thermal comfort for occupants of buildings or other enclosures is one of the important goals of HVAC design engineers.

<span class="mw-page-title-main">Passive ventilation</span> Ventilation without use of mechanical systems

Passive ventilation is the process of supplying air to and removing air from an indoor space without using mechanical systems. It refers to the flow of external air to an indoor space as a result of pressure differences arising from natural forces.

Infiltration is the unintentional or accidental introduction of outside air into a building, typically through cracks in the building envelope and through use of doors for passage. Infiltration is sometimes called air leakage. The leakage of room air out of a building, intentionally or not, is called exfiltration. Infiltration is caused by wind, negative pressurization of the building, and by air buoyancy forces known commonly as the stack effect.

HVAC is a major sub discipline of mechanical engineering. The goal of HVAC design is to balance indoor environmental comfort with other factors such as installation cost, ease of maintenance, and energy efficiency. The discipline of HVAC includes a large number of specialized terms and acronyms, many of which are summarized in this glossary.

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Underfloor air distribution (UFAD) is an air distribution strategy for providing ventilation and space conditioning in buildings as part of the design of a HVAC system. UFAD systems use an underfloor supply plenum located between the structural concrete slab and a raised floor system to supply conditioned air to supply outlets, located at or near floor level within the occupied space. Air returns from the room at ceiling level or the maximum allowable height above the occupied zone.

<span class="mw-page-title-main">Dedicated outdoor air system</span>

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<span class="mw-page-title-main">Ventilative cooling</span>

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References

  1. 1 2 3 4 "ASHRAE Standard 129-1997 (RA 2002): Measuring Air-Change Effectiveness". Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2002.{{cite journal}}: Cite journal requires |journal= (help)
  2. 1 2 Fisk, William J.; Faulkner, David; Sullivan, Douglas; Bauman, Fred (1997-06-01). "Air change effectiveness and pollutant removal efficiency during adverse mixing conditions". Indoor Air. 7 (1): 55–63. doi: 10.1111/j.1600-0668.1997.t01-3-00007.x . ISSN   0905-6947. OSTI   803749.
  3. ↑ Bearg, David W. (1993). Indoor Air Quality and HVAC Systems. CRC Press. p. 64. ISBN   0-87371-574-8.
  4. ↑ § 13-2. Ventilasjon i boligbygning - Direktoratet for byggkvalitet
  5. 1 2 "ANSI/ASHRAE Standard 62.2-2022: Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings". Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2022: 58. ISSN   1041-2336.{{cite journal}}: Cite journal requires |journal= (help)
  6. 1 2 "ANSI/ASHRAE Standard 62.1-2022: Ventilation and Acceptable Indoor Air Quality". Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2022: 90. ISSN   1041-2336.{{cite journal}}: Cite journal requires |journal= (help)
  7. 1 2 "ANSI/ASHRAE/ASHE Standard 170-2021: Ventilation of Health Care Facilities". Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2021: 54. ISSN   1041-2336.{{cite journal}}: Cite journal requires |journal= (help)
  8. ↑ Van Buggenhout, S.; Van Brecht, A.; Eren Özcan, S.; Vranken, E.; Van Malcot, W.; Berckmans, D. (October 2009). "Influence of sampling positions on accuracy of tracer gas measurements in ventilated spaces". Biosystems Engineering. 104 (2): 216–223. doi:10.1016/j.biosystemseng.2009.04.018.
  9. ↑ Jelley, Nick (2017-01-19), "airtightness", A Dictionary of Energy Science, Oxford University Press, ISBN   978-0-19-182627-6 , retrieved 2022-11-16
  10. ↑ Gillott, M.C.; Loveday, D.L.; White, J.; Wood, C.J.; Chmutina, K.; Vadodaria, K. (January 2016). "Improving the airtightness in an existing UK dwelling: The challenges, the measures and their effectiveness". Building and Environment. 95: 227–239. doi:10.1016/j.buildenv.2015.08.017. S2CID   56109406.
  11. ↑ Building a Passive House. Green Energy and Technology. 2018. doi:10.1007/978-3-319-69938-7. ISBN   978-3-319-69937-0.
  12. 1 2 3 4 5 6 Pollard, AR and McNeil, S, Forced Air Ventilation Systems, June 2010, Report IEQ7570/3 for Beacon Pathway Limited
  13. 1 2 Warren Fitzgerald, Dr Inga Smith and Muthasim Fahmy, Heating and cooling potential of roof space air: implications for ventilation systems, May 2011, Prepared for the Energy Efficiency and Conservation Authority (EECA)
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