In my living room I had two metal return vents which are both 14 x 12 = 168 square inches, not including the vent slats which are narrow.

As part of a renovation one of those vents is being replaced by a wooden grill, that dues to the limitations of working with wood, has less surface area. If I count up all the holes in the wooden grill, it comes to 62 square inches of hole.

Should I be concerned about this reduction in return vent size?

  • Reducing area of one vent by two-thirds could cause a hissing sound if the system was maxed-out. That is to say, if the two vents were pulling the maximum air through the grilles without hissing, then reducing the size of one grille could cause a noticeable hiss. What is the capacity of the unit? – Lee Sam Jan 22 '20 at 7:31
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    when they installed the system how much money do you reckon they wasted on oversized grilles? – Jasen Jan 22 '20 at 8:34
  • How many square inches are the supply vent grills? The supply should match the return or else your system will have inefficiencies, simple. – MonkeyZeus Jan 22 '20 at 13:38
  • @MonkeyZeus The supply vents in the room are much smaller, but there are no returns in other rooms so I think the large size of the returns in the living room is meant to compensate. – Tyler Durden Jan 22 '20 at 14:25
  • Correct. If your supplies's total square inches is larger than your returns then you are going to notice that your furnace runs longer in order to bring things up to temp. So you will be wasting energy and placing unnecessary strain on your furnace. However, it is perfectly fine if the return is larger than the supply. – MonkeyZeus Jan 22 '20 at 14:33

Yes you should be concerned. When we size the duct work it is usually the minimum size needed. By reducing this opening you are affecting the furnace. Depending on the type you can cause enough excess heat to cause the safety’s to trip. Even if the safety’s do not trip the reduced flow can cause the fire box heat exchanger to run hotter (gas or electric) and fail sooner. So your plan needs to be modified a slight change in size 10% can cause problems but this huge change is asking for problems and I did not even mention it will cost more to heat the home because the flow is restricted.

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    Plus adding strain to the blower+ – JACK Jan 22 '20 at 15:09
  • Limited inlet area reduces efficiency significantly ; The builder put in 20 X 30 , I added a 16 X 20. – blacksmith37 Jan 22 '20 at 15:16

To check this yourself you will need an anemometer. Your furnace/AC system will have airflow requirements listed in the manual. Using an anemometer you can verify that the draw from your returns is providing sufficient flow for the requirements of the system. Typically you would make this measurement near the furnace intake by inserting the anemometer into the duct supplying cold air to the unit.

After replacing the intake grill, you can again measure the airflow. If it is reduced to the point of being insufficient for the requirements of the furnace then you would need to either improve the conductance of the new grill or install additional cold air return feeds. Balancing an HVAC system can be tricky, however, so consulting an HVAC professional is not a bad idea if you're not confident about doing this.

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