ASHRAE Standard 170 for healthcare ventilation

ANSI/ASHRAE/ASHE Standard 170, Ventilation of Health Care Facilities, is the most widely cited North American document for healthcare HVAC design. It sets minimum air change rates, outdoor air quantities, pressure relationships, filtration and temperature and humidity ranges for spaces from operating rooms to patient bedrooms. Many hospital owners, design teams and infection prevention staff use it as a shared reference.

In Canada the picture is a little different. The governing national standard for healthcare HVAC is CSA Z317.2, Special requirements for heating, ventilation, and air-conditioning (HVAC) systems in health care facilities, and most provincial health authorities require it. Standard 170 still matters here: designers use it for comparison, US-based equipment vendors and consultants work from it, and some private clinics and outpatient facilities reference it. This guide explains what Standard 170 contains, how its tables are used, and how to handle it alongside Z317.2.

What Standard 170 covers

Standard 170 applies to hospitals, outpatient facilities, nursing homes and similar healthcare occupancies. It is developed jointly with the American Society for Health Care Engineering (ASHE) and is incorporated by reference into the FGI Guidelines for Design and Construction used across the United States. The standard is maintained under continuous revision, with new editions and addenda published every few years, so always confirm which edition a project or contract cites.

Its main content falls into four areas:

• Design requirements for space ventilation, set out in a large table listing each space type with its required values.
• System requirements, such as filtration efficiency for different space categories, outdoor air intake separation from exhaust sources, and humidification and duct lining limits.
• Specific spaces with extra rules, including operating rooms, airborne infection isolation (AII) rooms, protective environment (PE) rooms and pharmacy compounding areas.
• Planning and commissioning expectations, including the use of an infection control risk assessment during design and construction.

Reading the ventilation table

The core of Standard 170 is its space ventilation table. For each room type it lists:

1. Pressure relationship to adjacent areas (positive, negative or not required).
2. Minimum outdoor air changes per hour.
3. Minimum total air changes per hour.
4. Whether all room air must be exhausted directly outdoors.
5. Whether air may be recirculated by room units such as fan coils.
6. Design relative humidity range.
7. Design temperature range.

The table gives minimums. A designer may supply more air where cooling loads, equipment or clinical needs require it. Air change rates are calculated from the supply airflow and the room volume, so ceiling height matters: a tall room needs more airflow for the same rate.

Typical values for key spaces

The following values are representative of recent editions of Standard 170. They are shown here to illustrate how the table works, not as design values; always use the current edition and the version adopted for your project.

Operating rooms have a design temperature range around 20 to 24 C (68 to 75 F) and a relative humidity range with an upper limit of 60 percent. Lower humidity limits were relaxed in recent editions after debate over energy use and clinical needs, which is a good example of why the edition matters.

Pressure relationships and differential pressure

Standard 170 expresses pressure relationships as directions rather than airflow offsets. For AII and PE rooms it sets a minimum pressure differential of 2.5 Pa (0.01 in. w.c.) between the room and the corridor or anteroom. Maintaining that value reliably usually requires tight room construction, a deliberate airflow offset, and in many cases a pressure monitor with a local alarm.

Achieving these relationships is a controls and airflow problem as much as a design one. Constant volume terminals, venturi air valves and room pressure monitors are common tools. Our guide to room pressurization control explains the tradeoffs between offset control and direct pressure control.

Filtration requirements

Standard 170 assigns minimum filter efficiencies by space category using ASHRAE 52.2 MERV ratings. General patient care areas typically require two filter banks, with a prefilter upstream of the supply fan and a higher efficiency final filter downstream. Operating rooms and PE rooms require higher efficiency final filtration, and PE rooms require HEPA filtration at the supply. Filter frames must seal well, because bypass around a filter undermines its rated efficiency. See filtration, MERV ratings and HEPA for how these ratings compare.

Standard 170 and CSA Z317.2 in Canada

CSA Z317.2 serves the same purpose as Standard 170 but is written for Canadian codes, climate and health system practice. It is published by CSA Group and referenced by provincial health authorities and many hospital design guidelines. Where a provincial authority or the owner requires Z317.2, it governs, and Standard 170 is at most a reference.

Some practical differences that design teams notice:

• Scope and structure. Z317.2 includes broader requirements on system design, redundancy, maintenance access and infection control during construction, while Standard 170 focuses more narrowly on space ventilation parameters.
• Climate. Z317.2 addresses humidification and heat recovery in a way suited to cold Canadian winters.
• Values. Air change rates, pressure requirements and temperature ranges for similar rooms are broadly comparable but not identical. Never assume a Standard 170 value meets Z317.2 without checking the Z317.2 table.
• Referenced codes. Z317.2 sits alongside the National Building Code of Canada, provincial building codes, CSA Z8000 for healthcare facility planning, and CSA Z317.13 for infection control during construction.

Our guide to hospital HVAC and CSA Z317.2 covers the Canadian standard in more detail.

Applying the standard on a project

A clean process prevents most compliance problems:

1. Confirm which documents govern: the provincial authority, owner standards, Z317.2 edition and whether Standard 170 is referenced at all.
2. Build a room data schedule listing every space with its required pressure, outdoor air, total air changes, exhaust and temperature and humidity criteria.
3. Size terminals and systems from the room data schedule, adding margin for cooling loads where needed.
4. Specify controls that can hold the required pressure relationships, with monitors and alarms where the standard or owner requires them.
5. Verify through testing and balancing and functional testing that every room meets its scheduled values, and record the results.

Frequently asked questions

Is ASHRAE Standard 170 mandatory in Canada?

Not by itself. It is not adopted by the National Building Code of Canada. In most provinces healthcare HVAC must meet CSA Z317.2 as required by the health authority or the owner. Standard 170 becomes contractual only when a project specification cites it.

Does Standard 170 apply to medical clinics?

Its scope includes outpatient facilities, and some clinic types have specific entries in its table. In Canada, smaller clinics are often governed by the building code and ASHRAE 62.1 unless they perform procedures that bring them under Z317.2 or a provincial health authority standard. Check with the authority having jurisdiction.

How is compliance verified?

Through air balancing to confirm airflows and air change rates, differential pressure measurement across room boundaries, and functional testing of controls and alarms. Ongoing verification is important because pressure relationships drift as filters load and doors and ceilings are disturbed.

Key takeaways

• ASHRAE Standard 170 sets minimum ventilation, pressure, filtration and environmental criteria for healthcare spaces.
• In Canada, CSA Z317.2 is usually the governing healthcare HVAC standard, with Standard 170 used as a reference.
• Values in the two standards are similar in intent but not interchangeable, so check each room against the governing table.
• Edition matters: requirements such as humidity limits have changed between editions.
• A room data schedule and thorough air balancing and functional testing are the backbone of demonstrating compliance.