Undersized return air duct problem diagnosis & cure: this HVAC ductwork article describes the problem undersized return air openings - a problem that can be observed during inspection of the defects in return air ducts for heating or cooling systems (HVAC).

We also discuss duct troubles such as such as missing air conditioning cool air supply or return air registers, undersized air conditioning duct openings, improper cooling duct routing, cooling (or heating) air duct corrosion, leaky air duct connections, defective heating or cooling ductwork materials.

The photograph above shows a small HVAC return air duct located on an interior building wall. Whether or not this return air duct is adequate depends on how many such returns were installed in the building and whether or not the return air capacity is balanced with the air handler and air supply ducting.

UNDERSIZED OPENINGS - Under-sized Return Air Openings/Ducts

AIR CONDITIONING & HEAT PUMP SYSTEMS, of which this is a chapter, describes the inspection of residential air conditioning systems (A/C systems) to inform home buyers, owners, and home inspectors of common cooling system defects. Asbestos in or on HVAC ducts is a possible hazard for which we provide links to a separate document - see "Asbestos HVAC Ducts" at below left.

Under-sized single central air return:

when we see a comparatively small single air return in a building, such as a 12X18" grille in a ceiling or wall, we suspect that insufficient return air ducts are present. This concern is elaborated next.

The photograph shows an under-sized single air return for a duct system serving an entire two story home. We increased the size of the return duct to match the air handler and observed a doubling of the air flow in the system from this single improvement!

Central return air openings are very likely to be undersized when we see that the opening measurements are much smaller than the opening for air provided right at the blower fan.

At another example installation, the blower fan or return air plenum opening at the furnace was 20"x18" while the return air opening at the door to the utility room was 13"x13" (reduced further in effective size by a louvered screen).

When we see amateur attempts to add supply or return air to a system, such as observation of unanticipated openings in building walls, rough-sawn door undercuts, additional openings cut in the return duct or plenum near the air handler, we suspect system design problems. It is possible that previous occupants have found that the warm air supply was not adequate.

Should you confirm problems with inadequate heat delivery in some areas you'll want to consult with an HVAC engineer to make accurate measurements.

Door undercuts, central cool air return:

If room doors are shut and not under-cut to permit air flow out of the room, the system's ability to cool the room may be reduced, sometimes significantly.

Try running the cooling system for 10 minutes with all room doors shut. Then open a room door slightly and see what happens!

When a building uses a central return air grille and doors are not undercut heating some rooms may be difficult - system cannot push cold air from the cooled or heated room back to the furnace.

While modest amounts of new duct work (to take return air from the living area) are usually not costly, extensive re-ducting of air flow may involve significant expense

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Continue reading at AIR FLOW IMPROVEMENT, HVAC - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these RETURN AIR INCREASE FAQs questions and answers about how to improve the return air supply to the air handler in your building.

Or see these

Recommended Articles

• AIR FLOW IMPROVEMENT, HVAC
  • AIR FLOW TOO WEAK
  • AIR LEAKS in RETURN DUCTS
  • LEAKY DUCT CONNECTIONS
  • REGISTER & DUCT LOCATION
  • RETURN AIR, INCREASE
  • RETURN AIR REGISTERS & DUCTS - home
  • UNDERSIZED RETURN DUCTS

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UNDERSIZED RETURN DUCTS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to HVAC DUCT SYSTEMS

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