Comments on: The Sucking and the Blowing — A Lesson in Duct Leakage

By: David Butler

David Butler — Wed, 10 Dec 2014 10:08:50 +0000

@Greg, I'm still not clear on
@Greg, I'm still not clear on your use of the term "airflow loss" so I'll answer it both ways.

When technicians air-balance a system, they should also adjust blower speed to get the airflow right. This may require reconfiguring the duct system (if it's overly restrictive). In cooling mode, 400 CFM per ton across the evap coil is considered nominal, but it's appropriate to go as much as 10% lower in very humid climates and 25% higher in arid climates. With furnaces, the acceptable airflow range is determined by the manufacturer. Within that range, there's little impact on performance. I can't be more specific than that.

Now, if by "airflow loss" you're referring to supply duct leakage (air is literally lost to the attic, unconditioned basement or crawl), we need to also include return side leakage, which is "airflow gain" in your vernacular. The 2012 energy codes specify a maximum leakage rate (return + supply) of 4 CFM per 100 ft2 of conditioned floor area, at a test pressure of 25 Pascals. This is waived if ducts are fully inside conditioned space.

A better metric in my opinion is percentage of system airflow. In new construction I typically specify 5% max, but for an older home, 10% may be a more reasonable goal. Whether or not that can be achieved with retrofit duct sealing depends on how accessible the ducts are. The good news is that a lot can be accomplished by sealing leaks close to the air handler. These are more consequential because pressures are much greater near the blower than out at the grilles.

BTW, return side leakage often imposes a much larger performance hit than supply side leakage, especially if you have any return ducts in the attic. It doesn't take much 130F air to cripple an otherwise capable AC. Return leaks in a damp crawl space can overwhelm the AC's ability to pull moisture out of the house.

In any case, you may want to have an energy auditor or HERS rater perform a duct leakage test. Some air balancing technicians have the equipment to test duct leakage.

By: greg

greg — Tue, 09 Dec 2014 19:08:41 +0000

Thank you David.
Thank you David.

I'm still curious to know what percentage of airflow loss is reasonable to expect in a balanced system? I read somewhere that 10% was a target. That seems optimistic given potential leaks and duct configuration.

By: David Butler

David Butler — Tue, 09 Dec 2014 09:53:26 +0000

@Greg, I suspect you
@Greg, I suspect you misunderstood the contractor. If the air measured at the vents was literally 50% of the airflow at the furnace, that would mean half of the supply air is leaking from your ducts! What he probably meant is that closing off that many registers caused the blower to slow down, thus reducing the amount of air moving through your system. Airflow in = airflow out.

In heating mode, reducing the airflow doesn't materially impact a furnace's output. The supply air temperature will rise, but as long as it doesn't exceed the maximum spec, there's no concern. Otherwise it would trip the high limit and shut off. Changing the airflow can also have a small impact on efficiency, plus or minus.

Cooling mode is a different story. If airflow across the evaporator coil is reduced 50% as you said, this would significantly reduce sensible efficiency and could cause the evaporator coil to freeze up.

Your duct system clearly needs to be balanced and the system airflow adjusted to ensure proper evaporator temperature in cooling mode. Depending on the particulars, it may be necessary to enlarge or add a return duct and/or modify your filter configuration. You should also consider getting your ducts checked for leakage. Unfortunately, these are common problems with residential HVAC.

I recommend hiring an HVAC technician experienced in airflow diagnostics. NATE or NCI certification is a good indicator.

By: David Butler

David Butler — Thu, 17 Jul 2014 21:17:29 +0000

@Steve C: sounds like you
@Steve C: sounds like you understand most of the downsides to duct leakage to the inside. One energy related impact you missed is if duct leakage occurs in the floor system or any other relatively closed cavity, it will likely induce envelope leakage (e.g., to the outside). If you don't test, you're guessing.

As Allison described in the article, this is easy to diagnose (and to demonstrate) with a manometer.

By: Steve C from the D

Steve C from the D — Thu, 17 Jul 2014 18:56:29 +0000

Thanks for the info on duct
Thanks for the info on duct leakage outside the building envelope. Here in Michigan I need some heavy duty ammo to convince folks that it is important to seal all ducts, even ducts inside the building envelope. Because it is, isn't it? ENERGY STAR Qualified Homes thinks so with its very tight total leakage requirement. I try the comfort argument, I try the consistent temperature argument, I try the balanced air flow and pressure argument, but folks don't care cuz they are stuck on "it is all leakage inside the thermal envelope so who cares". Help me with this cuz I am beginning to believe them after four years back in an area with most of the ducts inside the envelope. The big problem occurs when a builder does have ducts outside the BE and the contractor doesn't know how to seal properly. Otherwise, folks don't care about sealing ducts here in Michigan.

By: Allison Bailes

Allison Bailes — Thu, 17 Jul 2014 18:27:25 +0000

John R.:
John R.: Happens all the time. See the link in my last comment. See you at Summer Camp?

Bob S.: Ah, how did you know where I was leading with all this?

Daryl: No, actually flex duct is easier to get airtight than rigid duct. I've seen old hardpipe systems that fall apart when you take the insulation off. Flex duct has an airtight inner liner, so if you seal the connections, trunks, plenums, and air handler (and don't do stupid things), it's very tight. Of course, flex isn't often installed well, but getting proper air flow is usually a bigger problem than duct leakage.

By: Bob Seaton

Bob Seaton — Thu, 17 Jul 2014 18:09:29 +0000

I assume that shutting a
I assume that shutting a bedroom door (with no air egress) leads to a positive pressured bedroom and negative pressured house. What about just closing an unused bedroom's supply register (leaving the door open)?

By: John Ring

John Ring — Thu, 17 Jul 2014 17:49:25 +0000

I have a dear friend who
I have a dear friend who lives in a 7 year old building filled with 60 very expensive condos -about 60 of them.
They all have their heat pumps - one or two, depending on the size of the condo - in closets with fully louvered doors. The louvers are so close together that the air flow is so restricted that the door has to be open about 18" or it will slam shut. It is maddening that this was done in the first place and apparently approved by the mechanical inspect and the building owners will do nothing about it.
And, there are no return air paths from the bedrooms and most of the units are oversized. And people are paying over $1,000,000 for this!!
We've still got a LOT of educating to do.

By: Allison Bailes

Allison Bailes — Thu, 17 Jul 2014 17:24:32 +0000

David E.:
David E.: Indeed, that can put a serious dent in air flow and also cause the coil-freezing, compressor-blowing, heat exchanger-cracking problems I mentioned above. We wrote about that issue a few years ago and one way to solve it:

Helping an Asthmatic HVAC System to Breathe

.

By: David Eakin

David Eakin — Thu, 17 Jul 2014 17:19:51 +0000

I recently saw a different
I recently saw a different form of unbalance. I was visiting my daughter who has an air handler in a closet (very common for non-basement homes) and noticed that the closet door would pull shut when the air handler was running. This system only had supply ducts to the rooms and a central return to the side of the closet - which was obviously much too small. We cut an additional hole in the drywall and installed an additional, larger return grill to better balance the air flow.