IMC 605 is simple but important: air needs to be filtered before it reaches the coil or heat exchanger. This section is about approved filters, correct filter location, service access, and even airflow across the filter.
Plain English:
605.1 General
Heating and air-conditioning systems must be provided with approved air filters.
Return air, outdoor air, and makeup air must be filtered upstream of any heat exchanger or coil.
Filters must be installed in an approved convenient location.
Liquid adhesive coatings used on filters must have a flash point of at least 325 degrees F.
605.2 Approval
Media-type and electrostatic-type filters must be listed and labeled.
Media-type filters must comply with UL 900.
HEPA filters must comply with UL 586.
Electrostatic-type filters must comply with UL 867.
Dwelling-unit filters must be designed for the intended application, but do not have to be listed and labeled.
605.3 Airflow Over the Filter
Ducts must be constructed to allow even air distribution over the full filter.
In plain English: do not design a filter rack where air slams one side of the filter and bypasses or underuses the rest.
On Plans: Why it matters
A lot of IMC 605 comments come from basic coordination misses: outdoor air tied in downstream of the filter, filter access that is technically possible but not practical, missing filter listing basis, or a filter rack arranged so poorly that airflow is uneven across the filter face.
Code Path: Where to show it
M-001: air-filter code note, filter types, and listing basis
M-101: filter location relative to return air, outdoor air, makeup air, coils, and heat exchangers
M-501: filter rack, access, and filter section details
M-601: equipment / filter schedule with filter type, rating, and UL basis
Equipment plans / sections: service clearance and airflow path through the filter section
Check: Do
Show filters upstream of coils and heat exchangers
Identify whether the filter is media, electrostatic, or HEPA
Show practical access for maintenance and replacement
Lay out the filter section so airflow uses the full filter face
Review Risk: Don't
Don't bring outdoor or makeup air in downstream of the filter section
Don't rely on vague "provide filter" notes with no product basis
Don't show filter access that works only on paper
Don't ignore airflow distribution across the filter bank
Field Tip:
Trace return air, outdoor air, and makeup air to the first coil or heat exchanger. If any of those airstreams can reach that component before passing through the filter, expect a redline.
Masterbuild QA Lens
Fire-resistive duct assemblies (Section 605) must be fully described in the permit set before the structural review begins. Confirm: the rated assembly type (UL design number), the fire rating required for the separation, the duct material and gauge within the rated enclosure, and the access provisions for any dampers within the enclosure. Incomplete fire-resistive duct documentation holds both the mechanical and structural reviews.
Drawing / Submittal Check
For air filters (IMC 605): the filter schedule must show the filter type (fiberglass, pleated, HEPA), the MERV rating, the face velocity at design CFM, and the maximum pressure drop. Confirm the AHU schedule matches the filter schedule - filter MERV and size at the AHU. Show the filter access clearance on the unit arrangement plan. Confirm the bypass air path (if any) is addressed - unfiltered bypass is a common quality and code issue.
Common Review Risk
Air filter MERV rating changes during construction - installing a higher MERV filter than designed - are the most common filter-related commissioning failure. A higher MERV filter has a higher pressure drop, which reduces the system airflow and may overload the fan motor. Confirm the filter MERV rating on the approved drawings is what is installed at the time of commissioning. Substitutions require a design review.
When To Escalate
Escalate when the air quality in an occupied building is being attributed to inadequate filtration - when occupant complaints about dust, odors, or particulates suggest the installed filter MERV rating is insufficient for the actual contaminant load. Upgrading the filter MERV rating after construction requires a fan capacity analysis to confirm the increased pressure drop does not reduce the design airflow.
Duct System Coordination
Air filter requirements affect the duct section housing the filter rack, including minimum face velocity, filter slot construction, and access clearance. Show filter rack location, filter size, and MERV rating on the air-handling unit schedule.
Ventilation Coordination
For air filter installations (IMC 605): the filter performance must be confirmed in combination with the outdoor air rate - a high-MERV filter with a high pressure drop reduces the system airflow and may result in less outdoor air delivery than the design requires. Confirm the design airflow is achievable with the selected filter MERV rating at the maximum filter loading condition.