High-end zone control systems — the kind that works with communicating variable capacity equipment — typically manage airflow by monitoring the blower’s RPM. There’s a direct, known correlation between a particular blower’s RPM and the CFM it delivers, and since the zone stats know what’s going on in each zone, the zone controller can make good decisions about how best to distribute the air while maintaining the right CFM across the evaporator coil.

Short of that type of Cadillac system, the only way to do this properly without making things worse is to have a professional re-balance the duct system, or design, install and commission an after-market zone control system. This stuff isn’t rocket science but it is hard, which is why the hvac industry as a whole has done such a poor job getting it right in the first place. Anyone who comes up with a way to fix this stands to make a fortune, but they must first respect the challenge for what it is.

Thing is that there are 3 things going on at once here. Pressure across airhandler is increasing, duct leakage is increasing along with flow to rooms and house depressurization is increasing in absolute terms.

Ok, here are some numbers for you:

Total air handler flow 500 CFM, 100 CFM x 4 registers + 100 CFM into the attic as a leak. This represents a total air flow loss of 20% of the total. Typical.
Assuming attic leaks are from the supply plenum, we can use K value there of 10 which would give a total K of 80 for 4 open registers. CFM= K x square root of pressure. Supply and return plenum pressure of 40 and 100 Pa will be used giving total of 140 Pa or 0.56 inches across the system blower.

When one register is closed, system pressure goes up to 0.66 inches, with supply plenum at 130 Pa, duct leakage goes up to 114 CFM which is now 25% of the total. Pressure at the registers goes up to 31.4 Pa for a flow increase from 100 to 114 CFM per register which might satisfy the thermostat 7% faster but during that time we are losing energy to duct leaks 14 % faster for a net energy loss. During this time for a 1000 sf house at 6 Airchanges, our CAZ has gone from 2.5 to 3.1 Pa putting it into the danger zone. Airhandler pressure drop has gone over the magic 0.5 inches putting the blower on an inefficient part of the curve at 0.66 inches and total system air flow has reduced by 50 CFM meaning we are running our system less efficiently because we are no longer stripping as much heat or cool from our system. As David has pointed out, you may be increasing the likelihood of short cycling a furnace or freezing up an AC unit where both cripple efficiency and will damage your system.

If this house was say 4 Ach, you’d be pushing the CAZ to over 5.1 Pa where you’d be backdrafting a typical hot water heater constantly.

So on all three counts, you’ve lost efficiency, put your system at risk and increased the likelihood of backdrafting which will make a home unhealthy or even deadly. Now, what was it that you were saying was good about closing vents?
This is not to say that people don’t close vents and get away with it, they do. Just like we got away with no seat belts in 1960 but now with deaths per mile reduced by 80%, we realize that Ralph Nader was right, “unsafe at any speed”. We can say our systems are unsafe right now, closing vents only makes it worse.

The fact that registers can be closed by homeowners evolved from a day when real sheet metal ducts were actually oversized. Now that they’re undersized, those closeable registers are still with us.

None of the HVAC equipment mfr take total responsibility for an installed system. If their box puts out heat or cool, their job is done. Their lawyers would never allow them to take responsibility for the entire system which includes its connection to the house which is why these freakish HVAC systems are so prevalent. A group of dedicated pros know about how to make houses work as a system and are fighting an upstream battle against this ignorance.

I cannot help thinking that you must be working for these gadget manufacturers under cover.

BTW, a pressure measuring transducer sensitive enough to measure the pressures at the register and not have zero drift in excess of the pressure they’d need to measure would cost many $100 each and they sell their “smart vents” for $80 all in. We’ve been making super low pressure gauges for over 30 years and think we’d know about such a device.

Further, you may not understand “consumer gadgetry” and what it is capable of doing. As an example, we all know that if we have 3 data points, we can plot a curve. The boundary conditions of that curve are not known, but we can definitely plot it with three points. Therefore several single room adjustable vents, each with simple pressure sensors, are capable of detecting problematic airflow issues by way of an algorithm running on, say a cell phone app. The optimization of the result is a set of linear equations which can easily be done by a cell phone app. Whether this is done by any of the various companies making these products is not my concern, however there is zero difficulty with this technically.

Clearly this has nothing to do with industry practitioners, codes and so forth. Call it a priori diagnostic testing if you want, I am only pointing out that this is plausible.

While you can justifiably point to exaggerated marketing claims by some of those marketing these devices, your claim of an inability of an average consumer to close a duct and improve his situation with his airflow is equally flawed. My opinion is that one making such a claim should show his work, not rely on appeals to authority or fright scenarios such as damage to coils and compressors.

Anybody can and does change the settings on the AC vents in a house. And you know what? That’s what the adjustment level on the vents is for.

In conclusion, if you want to claim that closing one duct out of 4 will cause damage or inefficient operation, be my guest – show your work. I posted a simple scenario including a leaky duct system equal to another duct, creating a total of 5.

Respectively,

Mike Hayes

Simple. Because such limits cannot be defined for an individual home without diagnostic testing. In other words, it all depends on the condition of the as-built duct system.

In particular, there’s a wide variation in the number of vents. Many smaller homes and apartments only have 5 or 6. More to the point… studies have consistently shown that the majority of central air conditioners are already starved for air due to undersized and/or restricted ducts, so even closing a couple of vents will make the system even less efficient and risk coil freeze-up and/or damage to the compressor.

If you want to modify the air balance in a home, fine. But hire a professional who understand air balance and airflow diagnostics.

Poor duct design and installation practice is probably the leading cause for comfort and energy waste in the home. The only way to solve this is to hold industry practitioners to a higher standard through codes and 3rd party commissioning, not through promoting consumer gadgetry that’s likely to make things worse.

Your point seems to be that close a fair number of ducts is counterproductive, but why not clearly define the limits of applicability of this? The example that everyone can relate to is closing vents in a car in the summer. There’s no one in the passenger seat, so the driver closes those vents and gets more airflow. Duh!

Taking a home system with 4 equal size vents and an attic “leakage” equal to one vent, each moves 20% of air. Closing one increases attic leakage by 5%, and increases flow to the other 3 vents by 5% each.

Wasted cool air in the attic moves from 20% to 25% of the total, but the closed off room is considered worth it. Yes this simple method does not calculate air mass but it is close enough to make the point. The static pressure increases and effect of those on the entire AC system is negligible. Further, in your article you discuss overly (IMHO) the airflow issues (essentially air is pushed by FANS) and do not discuss compressor and duty cycle effects. These are > 95% of the consumer of electricity in the AC system.

In the example above cited duty cycle will change proportionately and there will be a savings in electricity equal to perhaps 15-20% of total energy. If there is no savings this means duty cycle did not change, and that implies the closing of vents introduced inefficiencies as you claim.

But your claims, admittedly having validity at the lunatic fringe of closing vents (say 50-75% of vents closed) simply do not have validity in the arena of closing 1-20% of vents. Without this qualification I feel your analysis is very misleading.

Regards.

My response:
Our exchange was happening on
http://energyvanguard.com/blog-building-science-HERS-BPI/bid/76258/Can-You-Save-Money-by-Closing-HVAC-Vents-in-Unused-Rooms#comment-388593915

Nate did not respond there and I don’t know how he could have sent me an email since I never gave out my address. Whoever is in charge of this testing should talk to me about the exact nature of the tests they are performing so I can get them the correct gear. I think they should need 4 channels of pressure, duct leakage and house leakage measure ment to do the job. The layout of your test house will dictate what is needed. I suggest you call me at 604 732 0142 ext 112

Performing these tests in a meaningful way would require and understanding of the range of leakages in existing ducts so those leaks could be simulated in the test house to determine the range of results. The LBL study could provide some guidance but you may have to go a step beyond even that because I believe they were only looking at increased energy loss and reduced air handler flow but were not so concerned about CAZ. Not sure about that though.

You will also have to deal with your own biases. Tricky. If your purpose is to prove what you have been saying all along is correct, you will certainly miss the dangerous conditions. If you want to prove for example, to your insurance company that there is no risk, then you'll need to test over a range of conditions that truly represents the probable range of houses that your units will be installed in. That will be much tougher. You might find for example that the only safe application is to find ways to increase flows to areas that need it rather than to restrict flows to areas that don't which is the paradigm your running now.

Then you can determine the existing CAZ pressures for different combinations of exhaust appliances running and door closures. Then install your Vents and repeat.

There are a lot of wise people in this Group that may add or disagree with my procedure. This is where I go to fine tune ideas I have since there are hundreds of years of experience in this Group and most of them really care that the work gets done right.

Bear in mind, if you don't get this right, your vents do have the ability to kill people and can certainly make a lot more of them sick. Yes, they can close their registers now but the difference is in your case the product that may have caused the problem has your name on it.

Overheated or over cooled rooms are the ones closest to the supply plenum and will also have the most affect on: increasing pressure at the air handler, increasing pressures over downstream duct leaks which will depressurize the house and increase energy losses.

Good luck.