Designing HVACs: What should you know about indoor air quality?
Energy efficiency has become the primary focus of HVAC design.
While that’s a good thing, it’s equally important that your design consider the health and comfort of the end users. On average, indoor air quality levels are two to five times more polluted than outdoor air. It’s little wonder, then, that 70% of British office workers complain about poor air quality, according to the Building Engineering Services Association (BESA).
Poorly designed HVAC systems can harm air quality. Another way HVACs can adversely affect indoor environments is through the lack of maintenance. Here again, design often plays a part, making it difficult for personnel to service the equipment. You can learn more in Why you need to design HVACs for regular maintenance.
HVAC can spread dust, bacteria, allergens or other harmful substances. Depending on the type of HVAC, it can be the source of bringing outdoor pollution inside. If it heats or cools the building excessively, then the occupants experience discomfort. Humidity can be another problem with HVACs. An immoderate amount of moisture or dry air also causes discomfort. Moisture has the added problem of being the source of mould.
For a building’s indoor air environment to be healthy, it needs to provide:
- Adequate ventilation – introducing and distributing clean air
- Controlled contaminants
- Comfortable temperature and humidity levels
While some particulate matter comes from within a building, an HVAC system can draw in outdoor air and with it, pollutants, to supply the building. The most common pollutants are:
- Volatile organic compounds (VOCs)
These are organic chemicals emitted as gases from products or processes, such as cleaning agents and air fresheners. Coming from outside, sources of VOC contamination might include chemicals in polluted groundwater, brought inside during water use. The Threshold Limit Value (TLV) is the limit a person can be exposed to a certain VOC without suffering adverse effects. This is measured in ppm (parts per million). Below are VOC indoor air-quality limits.
Common VOC levels ppm around the house
|
VOC |
Common in |
TLV |
|
Ethanol |
Cleaning agents |
1,000 ppm |
|
Formaldehyde |
Moulded plastics, plastic finishes |
0.1 – 0.3 ppm |
|
Acetone |
Furniture polish, nail polish, wallpaper |
750 – 1000 ppm |
|
Benzene |
Glue or paint applied to furniture |
0.1 ppm |
|
Dichlorobenzene |
Mothballs, deodorant |
25 – 50 ppm |
- Carbon monoxide (CO)—Part of the danger of carbon monoxide is that you can’t smell, taste or see it. Varying levels on CO and their effect are shown below, including air quality norms.
Air-quality CO ppm
|
TLV |
Consequences |
|
250 – 400 ppm |
Normal background concentration in outdoor air |
|
400 – 1,000 ppm |
Concentrations typical of occupied indoor spaces with good air exchange |
|
1,000 –2,000 ppm |
Complaints of drowsiness and poor air |
|
2,000 – 5,000 ppm |
Headaches, sleepiness and stagnant air. Also typical: poor concentration, loss of attention, increased heart rate and slight nausea |
|
5,000 ppm |
Workplace exposure limit (8-hours) in most areas |
|
>40,000 ppm |
Exposure may lead to serious oxygen deprivation resulting in permanent brain damage, coma, or death |
- Particulate matter (PM)
PM is a mixture of both solid particles and liquid droplets that hang in the air. Examples include dust, pollen, soot and smoke. The particles vary in size, but it gets especially dangerous when you’re dealing with indoor PM10 levels and below. This is the smaller particulate matter 10 micrometres in diameter or smaller which can be inhaled. The PM2.5 threshold is even worse. This is particulate matter of 2.5 micrometres or less in diameter. It can penetrate the lung barrier and enter the blood system, causing serious health issues, such as cardiovascular and respiratory disease and cancers, even at extremely low concentrations.
Indoor air quality guidelines and standards
The Institute of Air Quality Management (IAQM) is the professional body for the UK indoor air quality industry. While not specific to HVACs, they provide guidance to members for assessing indoor air quality (IAQ). The Chartered Institution of Building Services Engineers (CIBSE) is an international professional association of engineers. They have an alliance with the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).
ASHRAE air-quality standards are the most widely used guidelines in the U.S. Their guidelines don’t set standards for the design of HVACs, but instead define minimum values and acceptable performance within the HVAC industry. For example, ASHRAE Standards 62.1 and 62.2, which are used by CIBSE, define “roles of and minimum requirements for mechanical and natural ventilation systems and the building envelope intended to provide acceptable indoor air quality in residential buildings.”
They specify minimum ventilation rates and other measures with the purpose of reducing a building’s negative impact on health. Building owners and managers can use the ASHRAE 62.1 ventilation spreadsheet to calculate the ventilation rate of current systems that serve multiple zones.
ASHRAE recommended ventilation rates determine minimum fresh air requirements (Ashrae).
|
Site |
Air-change rate standard: ASHRAE, per hour |
|
Home |
0.35 – 1 |
|
Hotel rooms |
1 – 2 |
|
Offices |
2 – 3 |
|
Retail shops |
2 – 3 |
|
Schools, except for lecture halls |
5 – 6 |
|
Sports facilities |
4 – 8 |
|
Restaurants |
6 – 8 |
The Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA) counts CIBSE as a member. REHVA provides the recommended ventilation rates
REHVA: Air ventilation requirements
L/s stands for litres per second.
|
Supply airflow rate L/s |
Extract airflow rate L/s |
Air velocity* m/s |
|
|
Living rooms >15 m** |
8+0.27 L/(s m**) |
0.10 |
|
|
Bedrooms >15 m |
14 |
0.10 |
|
|
Living rooms and bedrooms 11-15 m** |
12 |
0.10 |
|
|
Bedrooms < 11 m**, 3rd and the following bedrooms in large apartments |
8 |
0.10 |
|
|
WC |
10 |
||
|
Bathroom |
15 |
||
|
Bathroom in one-room apartment |
10 |
||
|
Utility room |
8 |
||
|
Wardrobe and storage room |
6 |
||
|
Kitchen*** |
8 |
||
|
Kitchen, one-room apartment |
6 |
||
|
Kitchen, cooker hood in operation |
25 |
|
Average airflow rate of an entire residence L/(s m**) |
0.42 |
|
Staircase of an apartment building, ACH |
0.5 |
*Maximum air velocity values apply at design airflow rate and supply air temperature in heating season conditions, in boost mode higher velocities may be accepted
**Transfer air from bedrooms may be reduced, 12 L/s is the minimum value
***Airflow rate in the kitchen when cooker hood is not in operation
Indoor air-quality parameters
What is acceptable indoor air quality? Organisations pretty much agree on the parameters, listed here.
|
Characteristic |
Conditions to be met |
|
Particulate Matter (PM) |
10 micrometres or less in diameter: 50 ug/m3; 2.5 micrometres or less in diameter: 15 ug/m3 |
|
Carbon monoxide |
Less than 9 ppm |
|
VOCs |
Less than 500 ug/m3 |
|
Formaldehyde |
Less than 27 ppb |
|
Carbon Dioxide |
ASHRAE CO2 levels are about 700 ppm above outdoor air levels (usually about 1,000 to 1200 ppm) |
|
Humidity |
Below 60%, ideally between 30% and 50% (EPA) |
|
Temperature |
68.5°F to 74°F (winter); 75°F to 80.5°F (summer) (ASHRAE) |
How to measure indoor air quality
You can measure air quality in offices and other buildings using IoT sensors placed in various places.
The most accurate indoor air quality monitoring method, IoT sensors use a scale to translate the concentration of molecules of different particulates and substances. You’re alerted when anomalies are detected, enabling you to evacuate an area if pollution or humidity levels reach dangerous levels.
Air filters for the best air quality
A well-designed HVAC system can help improve IAQ levels. Air filters should be carefully considered. These use a rating system called Minimum Efficiency Reporting Value, or MERV.
The rating system is defined in ASHRAE Standard 52.2, which provides the testing conditions and the performance necessary to meet each MERV level. The higher the MERV rating, the more particulates are removed from the air. When you hear, for example, MERV 13 PM2.5, we’re talking about a filter that removes a minimum of 90% of PM10, a minimum of 85% of PM2.5, and a minimum of 50% of PM1.0. Below highlights the rating system.
MERV-rated air filters
|
Minimum % of particles filtered |
|||
|
PM2.5 zone |
|||
|
MERV rating |
0.3 – 1.0 microns |
1.0 – 3.0 microns |
3.0 – 10.0 microns |
|
1 |
- |
- |
>20% |
|
2 |
- |
- |
>20% |
|
3 |
- |
- |
>20% |
|
4 |
- |
- |
>20% |
|
5 |
- |
- |
>20% |
|
6 |
- |
- |
>35% |
|
7 |
- |
- |
>50% |
|
8 |
- |
>20% |
>70% |
|
9 |
- |
>35% |
>75% |
|
10 |
- |
>50% |
>80% |
|
11 |
>20% |
>65% |
>85% |
|
12 |
>35% |
>80% |
>90% |
|
13 |
>50% |
>85% |
>90% |
|
14 |
>75% |
>90% |
>95% |
|
15 |
>85% |
>90% |
>95% |
|
16 |
>95% |
>95% |
>95% |
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