1:5:10:365 EcoTip Blog

April 21, 2008

:112 Door Undercuts

Filed under: :112 Door Undercuts — Tags: , , , , , , , , , — John Banta @ 12:26 am

Welcome to today’s 1:5:10:365 Tip for becoming a better steward for our home and planet.

1:5:10:112 EcoTip: Doors are deliberately undercut to provide a path for airflow created by central heat and air systems. If the room has both a supply and return register – and is balanced as described in EcoTip:111 The undercut isn’t necessary. If you only have supplies registers in a room – but no returns – then the air must have a path to follow back to the central system to provide proper ventilation. This is provided by the space under the door. The door undercut may not be enough in which case you need to undercut it more or provide an RAP.


Tamarak Technologies Return Air Pathway


 Additional Information

Suggested Review – :111

The amount of gap under the door is important to allow good air exchange and prevent pressure differences from developing in the house. Many builders cut a standard half-inch to one inch gap without understanding that size does matter. If carpet is installed, it may close the gap as well.

A 30 inch door with a half-inch gap can handle up to 25 cubic feet per minute (cfm) of air coming from the supply register in that room when the door is closed. To handle 75 cfm it would be necessary to have a 1.5 inch gap. If the gap isn’t big enough then back-drafting can occur (more on this in tomorrow’s tip).

Tamarack Technologies has informtion about the necessary door way undercuts or as an alternative you can use their Return Air Pathways (RAPs) instead of needing to undercut a huge gap under the door.

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April 20, 2008

:111 Fill a Bag – Checking HVAC Flow

 Welcome to today’s 1:5:10:365 Tip for becoming a better steward for our home and planet.

1:5:10:111 EcoTip: In order for central air conditioning systems to function efficiently, it is necessary for them to be free of significant leaks and be balanced. This means that the air supply is flowing somewhat equally throughout the system. In an unbalanced system you might have too much air going to one area and not enough in others. There are sophisticated duct balancing systems that can be used to professionally perform these checks, but an inexpensive simple do-it-yourself method was developed by the Canadian Mortgage and Housing Administration using a plastic garbage bag, coat hanger, duct-tape and a timer.

image credit: Lawrence Berkeley National Laboratory


Additional Information

Suggested Review – :110

The CMHC says to use a Glad 66 cm x 91 cm garbage bag. Which is about 2ft X 3ft when it is laid out flat. The coat hanger is bent into a circle, shape that will fit all the way around the heating and cooling supply registers you have in your home. You may need to reshape it for different openings. Tape the edge of the plastic bag to the coat hanger.

To test your duct system – turn your HVAC fan unit to on. It is not necessary to have it actually heating or cooling  –  just blowing. Place the completely deflated garbage bag over a supply register and time how long it takes the bag to inflate completely. Record this time for all the supply registers. Now place the fully inflated garbage bag over each return register record how long it takes for the bag to deflate.

The total inflation time for the supply registers should equal the total deflation time for the return registers. If there is only one return register, you may find it deflates too quickly to measure accurately. In this case you might try using two bags simultaneously over the return register.

If it takes 2 seconds for bag to inflate that is approximately 75 cubic feet of air per minute coming into the room. This is pretty good for a standard size room. Same size rooms should have approximately the same amount of air delivered to them. Half size rooms should have about half the air. Serious deviations should be checked. A duct may have come loose or the system may not have been designed properly.

The following table from CHMC shows ratio between inflation times and airflow rates.

Time to inflate a plastic trash bag (66 cm x 91 cm [26 in. x 36 in.]


Approximate time to inflate bag

5 L/s (10 cfm)

13 seconds

10 L/s (20 cfm)

8 seconds

15 L/s (30 cfm)

5 seconds

25 L/s (50 cfm)

3 seconds

If more air is required, adjust the grille openings at the supply register in the room. Keep in mind that bedrooms require more fresh air when occupied by more than one person. http://www.oee.nrcan.gc.ca/Publications/infosource/Pub/hrv/hrvsystem.cfm?text=N&printview=N#airflow



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