Heating or air conditioning return air ducts & register FAQs:

Questions & answers about air conditioning or heating system return air or make-up air duct registers & ductwork.

This HVAC ductwork article series describes the inspection of the defects in return air registers & ducts for heating or cooling systems (HVAC) to help detect duct defects like 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.

Heating or Cooling Return Air Duct & Register FAQs

These questions & answers about heating & air conditioning return air ducts were posted originally

at RETURN AIR REGISTERS & DUCTS - be sure to see the diagnostic and repair advice given there.

On 2017-12-16 by (mod) - filter at registers?

Craig,

In an air handling system in which the air filtration is accomplished by installing a filter at the return air inlet, we must install an air filter at every independent return air inlet in the system.

Otherwise only partial air filtration is being accomplished, the indoor air quality is diminished, and the remaining house dust blowing through the system will speed the clogging of the blower fan blades and reduce the ability of the air handler to move air.

On 2017-12-16 by Craig

My son recently purchased a home. When changing the primary 20*20 return filter we noticed another smaller 10*20 return with filter inside the primary return. Can you explain what purpose this serves? A smaller return branching off of the larger primary return. Thanks.

On 2017-12-08 by Kevin

My return is located on left lower back wall of the condo. Just outside the room / closet where the air handler is located. I just purchased a server on the adjoining wall. The return filter is 12x24”. The server presently has 16” clearance. How do I know if it’s sufficient clearance?

Question: air conditioning system return air only on first floor - second floor is too cold in winter and too warm in summer.

(July 14, 2014) Mike said:
A central air system was installed in my house with only return air ducts on the first floor. Naturally, it is cold in the winter and warm in the summer on the 2nd floor. Other than installing exposed ducts from the 2nd floor, are there any other options?

[Click to enlarge any image]

Reply:

Difficult but common problem, right Mike?

While it's not so hard to push cool air downstairs from an upper floor furnace, in heating mode it's tough to push warm air down from a second floor to a first floor return.

Accepting the proviso of avoiding new ductwork, see if you can do some air-push from the second down to first floor via fan-operated in-floor register-blowers that might make some improvement.

An alternative is an in-supply-duct booster fan to boost cool air flow into those areas.

Some owners give up and install a second air handler in an attic to more easily push cool air down into that space.

Question: Both supply and return air in ceilings

(Sept 27, 2014) Chip said:
I'm in the process of building a new house. No sheetrock has been installed yet so I'm at a good point for this. The HVAC systems and duct work have just been installed. There is a supply and large return in the rooms where I would expect it.

What has me concerned is that both the supply and returns are in the ceiling.

They are on opposite sides of the rooms, but in the winter, won't warm air just run across the ceiling and leave stagnant cold air around my toes?

Reply:

Chip: you're right that IF warm air finds a too-easy too-short path from the supply register over to the return register then that short circuit can increase heating costs. With supply and return at opposite sides of the room that's some relief from the problem.

I agree that in a design optimized for heating, we'd want our cold air returns on floors or low on walls - where the cooler room air is found during heating season.

But if your system is to be used for both cooling and heating, you can't optimize for both modes unless you add separate zonable or controllable return ducts.

Question: fiberglass insulation in cold air return ducts?

(Nov 10, 2014) Steve said:
I have only lived in my house for a about 3 years. I was cleaning my cold air return registers when I noticed there was fiber glass insulation in one of could air return ducts. The duct work for the house is typical box aluminum. Why would the previous owner put fiber glass insulation in the return duct? I only noticed this one of the return ducts.

The return duct runs between the dinning/kitchen room floor and the garage floor. The house was built around 1976. A new furnace was installed approximately 3 years before I moved in. Should I try to remove it? Is there a reason for the insulation be inside the duct work?

Reply:

Steve,

Fiberglass is commonly used as an HVAC duct liner in OEM installations; the fiberglass is in a coated "board" like form and works perfectly well as an insulator. The coating gives a fairly hard surface on the duct interior intended to protect against fiberglass release and hopefully to slow down the collection of dust and debris on the insulation surface.

I do worry if the inside-duct fiberglass insulation is damaged by mechanical cleaning (which can then send fiberglass dust into the building occupied space) or if it is soaked by water leaks from any source (which can sometimes cause mold contamination).

If the fiberglass is a home-made DIY project using normal building insulation it's an improper installation that is likely to blow more fiberglass dust around than you'd like.

Indeed other duct systems put the insulation on the duct exterior but there's nothing conceptually wrong with in-duct insulation with the caveats I gave above.

Question:

(June 4, 2015) Mary said:
What can happen to an air conditioner unit if an air return register is blocked? Will it Freeze the coils? Can it cause the coils to rust over time & cause a leak in refrigerant?

Question: where should I locate the return duct

(June 4, 2015) Nathan said: [something like the following, original was lost]

Where does the return air inlet go and how big should it be? Isn't it better to have an air return in every room?

Reply: yes

Nathan, without on-site information about the location, climate, occupancy, building and its heating or cooling system specifics one can offer but a general reply to your HVAC design question. Generally we put the return air opening in a room on an opposite wall from the supply air register.

The duct and register sizing in a small room is most likely modest, but again depends on air velocity and the questions I noted above. I agree that it's good design to provide return air in individual rooms as that permits the HVAC system to work effectively even when the room door is closed. Ballpark for a small room at typical air velocities for heating might be 4x8".

(June 13, 2015) Anonymous said:
Thanks,just what I needed to know.

Question: how to solve un-even cooling in the house by fixing a return air balance

(July 9, 2015) GRF said:
I have a central air system in my house blower unit in the attic evaporator coil and compressor outside . it is a three ton cooling capacity two stage system made by Bryant. my issue is the main part of the house cools very good but our family room on the far end of the house is consistently 5-8 degrees warmer . the room has cathedral ceilings. the incoming cool air vents are up high and the single return vent is at same level in between the to incoming vents. my question is if I increased the size of the return air to the system will i get better cooling in this room?

Reply:

GRF if you can make some simple tests to determine if the system is return-air-starved then you'll know what would be the effect of increasing return air.

But separately you describe what could be short circuiting - the entering air may simply dive back into the return air because of where the supply and return vents are located.

To fix that means relocating either supply or return.

Question: return air located at bottom of furnace / air handler in the garage

(Aug 3, 2015) SWB713 said:

I have a 5 ton unit and the air return is located at the bottom of the furnace/air handler located in the garage. The air return is in the family room through the wall and right into the air handler. this is where my filter is located as well.

My issue is that at the end of the run of the ventilation system, these rooms are much warmer. The air filter is a 20x25x4, which is 500 square inches. The grill for the return air in the wall is 29x7, which is only 203 square inches. If I increased this to a 30x20 grill, would this theoretically increase air flow through out my house and better cool the rooms further away from the air handler? (provided that I have adequate ducts)

Reply:

Watch out: the first risk of taking return air right at any furnace, moreso in a garage, is the risk of drawing fatal carbon monoxide or combustion gases into the building air supply: killing the building occupants.

Question: Renovating villa in Dubai

(Aug 6, 2015) Wayne said:

I live in Dubai, we are modifying/renovating a 10 year old villa and replacing the roof mounted units and FCUs', we currently have an open return system that circulates through the suspended ceilings back to the various FCUs', would you recommend to change the return ducting to a closed system back to the FCUs'?

The MEP specialist is concerned that we may create condensation issues in the summer due to 50 degrees Celsius external temps, and especially on the first floor due to the high vaulted concrete roof structure. Please advise.

Reply:

Using the whole open ceiling as a giant return duct or plenum is common in commercial HVAC installations. It allows placement of return ducts and moving them and thus adjusting air flow simply by replacing a suspended ceiling panel with a grid. It's also less costly than adding dedicated, engineered, return ducts for which someone had to figure out the placement of the air inlets.

The down side of this approach is that any contaminants in the ceiling, including dust, moisture, mold, fiberglass, all get blown through the HVAC system.

Question: solving a room that's too cold where the return air is being collected

(Jan 4, 2016) Frances said:
My air return is in my family room. This is the coldest room in my house. The unit is in the attic. The warn air does not get halfway down in this room. What can I do about this?

Reply:

Frances:

1. keep the door to the family room open

2. partly-close the supply air registers in rooms closer to the air handler

3. check that the return duct is adequately sized and not crimped or obstructed

4. Keep your air filters clean - replace monthly

For details search InspectApedia for RETURN AIR IMPROVEMENTS

Question: furnace pulls all the warm air out of the home and I'm cold

(Jan 30, 2016) Margaret Del Castillo said:

My home has a crawl space I recently had a furnace, air conditioner and Venmar EVO5 installed in the laundry room, the furnace is suspended from the ceiling.

The return air duct is above the closet in the hall and faces directly into my kitchen & living room area When the furnace goes on it pulls all the warm all out of the rooms and I never feel warm.

This furnace was installed last month and I don't use it because of this problem I still have my baseboard heaters. Is there a solution, the furnace company says where they put the vent is the only place for it Any suggestions?

Reply:

x

Question: return air inlet never stops sucking air in

(Oct 23, 2016) Confused said:
My return in my house is always sucking air even went set to off, there is always a small amount of air coming out of the vent

Reply:

Confused,

Either the fan is not shutting off (check the fan switch settings at the thermostat and in the air handler at the fan limit switch), OR there are convection currents: warm air is rising into the return air, perhaps because there is a duct opening in an attic or roof space.

...

Continue reading at RETURN AIR REGISTERS & DUCTS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

HVAC Return Air Improvement 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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RETURN AIR REGISTER & DUCT FAQs at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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