Image Credit: Energy Vanguard
Image Credit: Energy Vanguard The air flow in an ideal HVAC air handler has the same amount of air being pulled in through the return ducts as it sends back to the house through the supply ducts. The ducts are sealed tight so they leak as little as possible and are sized correctly to keep the static pressure low.
Image Credit: Energy Vanguard Closing a register raises the pressure in the duct system. Most homes don't have sealed ducts, so higher pressure in the duct system will mean more duct leakage. With an ECM blower, higher pressure means more energy use, and with a PSC blower, higher pressure means less air flow.
Image Credit: Energy Vanguard The more registers you close, the higher the pressure in the duct system goes. The ECM blower will use more and more energy as you do so. The PSC blower will work less but not move as much conditioned air. In both cases, the duct leakage will increase further.
Image Credit: Energy Vanguard Low air flow can lead to a frozen coil in an air conditioner. And that really kills the air flow.
Image Credit: HVAC Hacks and Other Screwups The E-Vent project on Kickstarter gives you a smartphone app to control your air conditioner registers. Let's hope that the project doesn't get funded, because it could lead to big expenses for unsuspecting homeowners.
Image Credit: Circle Design Technology
More Building Science
Your air conditioner, heat pump, or furnace probably uses a lot of energy. Heating and cooling makes up about half of the total energy use in a typical house. For air conditioners and heat pumps using electricity generated in fossil-fuel fired power plants, the amount you use at home may be only a third of the total. A question I get asked frequently is whether or not it’s OK to close vents in unused rooms to save money. The answer may surprise you.
The photo on the right shows a typical supply vent for an ducted HVAC system (air conditioner, heat pump, or furnace). On the return side, you’ll typically see plain grilles, but on the supply side, where the conditioned air gets blown back into the house, most HVAC contractors install registers like the one above. It has a lever of some sort that allows you to adjust the louvers behind the grille.
You’d think that since it’s adjustable, it must be OK to open or close it to suit your needs, right?
The blower and the blown
The blower in your HVAC system is the heart of the air distribution. It pulls air from the house through the return ducts and then pushes it back into the house through the supply ducts. In high-efficiency systems, the blower is powered by an electronically commutated motor (ECM), which can adjust its speed to varying conditions. The majority of blowers, however, are of the permanent split capacitor (PSC) type, which is not a variable-speed motor.
In either case, the system is designed for the blower to push against some maximum pressure difference. That number is typically 0.5 inches of water column (iwc). If the filter gets too dirty or the supply ducts are too restrictive, the blower pushes against a higher pressure.
In the case of the ECM, a high pressure will cause the motor will ramp up in an attempt to maintain proper air flow. An ECM is much more efficient than a PSC motor under ideal conditions, but as it ramps up to work against higher pressure, you lose that efficiency. You still get the air flow (maybe), but it costs you more.
The PSC motor, on the other hand, will keep spinning but at lower speeds as the pressure goes up. Thus, higher pressure means less air flow, and, as we’ll see below, low air flow can cause some serious problems.
The important thing to remember here is that no matter which type of blower motor your HVAC system has, it’s not a good thing when it has to push against a higher pressure.
Closed registers increase pressure
In a well-designed system, the blower moves the air against a pressure that’s no greater than the maximum specified by the manufacturer (typically 0.5 iwc). The ideal system also has low duct leakage.
The typical system, however, is far from ideal. Although most systems are rated for 0.5 iwc, the National Comfort Institute, which has measured static pressure and air flow in a lot of systems, finds the typical system to be pushing against a static pressure of about 0.8 iwc. Now we’re ready to address the question of closing registers.
When you start closing registers in unused rooms, you make the duct system more restrictive. The pressure increases, and that means an ECM blower will ramp up to keep air flow up, whereas a PSC blower will move less air. Most homes don’t have sealed ducts either, so the higher pressure in the duct system will mean more duct leakage, as shown in Image #3 below.
The more registers you close, the higher the pressure in the duct system goes. The ECM blower will use more and more energy as you do so. The PSC blower will move less conditioned air. In both cases, the duct leakage will increase further.
What about heat?
In addition to moving air, your air conditioner, heat pump, or furnace is also cooling or heating that air that flows through the system. The air passes over a coil or heat exchanger and either gives up heat or picks up heat.
In a fixed-capacity system — and most are — the amount of heat the coil or heat exchanger is capable of absorbing or giving up is fixed. When the air flow goes down, less heat exchange happens with the air. As a result, the temperature of the coil or heat exchanger changes.
If air flow is low, it’ll dump less heat into the coil in summer, and the coil will get colder. If there’s water vapor in the air, the condensation on the coil may start freezing. You might even end up with a block of ice, as shown in the photo below. And ice on the coil is really bad for air flow.
It’s also bad for the compressor, as not all of the refrigerant evaporates and liquid refrigerant makes its way back to the compressor. If you want to have to buy a new compressor, this is a good way to do it.
The same thing happens if you have low air flow over a heat pump coil in winter. You could get a really hot coil, high refrigerant pressure, and a blown compressor or refrigerant leaks.
Similarly, low air flow in a furnace can get the heat exchanger hot enough to cause cracks. Those cracks, then, allow exhaust gases to mix with your conditioned air. When that happens, your duct system can become a poison distribution system as it could be sending carbon monoxide into your home.
Nine unintended consequences of closing registers
Let me now summarize the problems I’ve described above that can result from closing registers in your home. The first thing that happens is the air pressure in the duct system increases, which may give rise to these negative consequences:
- Increased duct leakage
- Lower air flow (with PSC blowers)
- Increased energy use (with ECM blowers)
- Comfort problems because of low air flow
- Frozen air conditioner coil
- Dead compressor
- Cracked heat exchanger, with the potential for getting carbon monoxide in your home
- Increased infiltration/exfiltration due to unbalanced leakage , as I described last week
- Condensation and mold growth in winter due to lower surface temperatures in rooms with closed registers.
You’re not guaranteed to get all the problems that apply to your system, but why take the chance?
A Kickstarter project to avoid
I recently wrote about all the IT folks who are trying to follow in Nest’s footsteps and profit from the home energy-efficiency movement. I used the Aros smart window air conditioner as the example of companies that think you can solve problems just by creating a product with a smartphone app.
Well, meet a more malignant idea: the E-vent. (You can find it easily enough by searching on the term “Kickstarter E-Vent.”) It’s just a Kickstarter project right now, and maybe it won’t get funded. If it does get funded, however, it will be subject to all the problems I described above. It doesn’t matter whether you close the registers by getting up on a ladder in your home or from the beach in Cozumel. It’s still a bad idea.
The E-Vent page on Kickstarter says that the device will monitor the air temperature and open registers if the temperature gets too cold (while air conditioning) or too hot (while heating). Of course, that’s not going to work unless they monitor the temperature right at the coil or heat exchanger. And that still probably wouldn’t work, because there’s a wide range of acceptable temperatures for different systems.
This is an HVAC product developed by people who don’t know some very important principles of heating and air conditioning. Let’s hope they don’t kill anyone.
The only way that closing registers could work
The fundamental problem here is that closing supply registers in your HVAC system changes what comes out in particular locations. It doesn’t change what the blower is trying to do. Nor does it change the amount of heat that the air conditioner, heat pump, or furnace is trying to move or produce.
Yes, your system may be fine if you close a register or two in your home, but the results will depend on how restrictive and leaky your duct system is. If it’s a typical duct system with 60% higher static pressure than the maximum specified, closing even one register could send the system over the edge. If it’s a well designed system with low static pressure and sealed ducts, you shouldn’t have a problem — as long as you don’t try to close too many registers.
The only way that something like this could work is if closing a register signaled the blower to move less air and the air conditioner, heat pump, or furnace to move or produce less heat. (Properly designed zoned duct systems do this by using variable speed ECM blowers with multi-stage systems.) Otherwise you’re subject to those nine unintended consequences, one of them potentially deadly.
Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.
Thanks to Curt Kinder, David Butler, John Semmelhack, Eric Sandeen, and Dale Sherman for suggestions that made this article better and more complete.
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15 Comments
Mini splits is a good option
Mini splits is a good option
Yet another unitended consequence...
...is that closing registers will increase the total amount of energy use (heating OR cooling), and not just in air-handler power use:
http://epb.lbl.gov/publications/pdf/lbnl-54005.pdf
The most concise summary is a paragraph on p25 of that document:
"In every case the closing of registers led to increased energy consumption, even for the low leakage configuration. This trend occurs for all three climate zones: it was not possible to save energy by closing registers; and closing more registers led to increased energy usage."
In other words, the tighter the ducts and the tighter the house, the less pronounced that increase in energy use might be, but it's always a net increase.
Response to Dana Dorsett
Thanks for that link, Dana. It's amazing how many papers Iain Walker has written on the kind of stuff I write about. I ought to just get a list of all his papers and make sure I take a look before I publish anything.
That would be a lot of looking...
...given that it's a lot of papers! :-)
This one stuck in my memory because it isn't intuitively obvious, and countervailing to common perception (and counter common recommendations by energy conservation folks!)
There are multiple products out there for automatically closing duct registers touted as a means of micro-zoning a house for the express purpose of reducing energy use (eg: https://www.theactivent.com/about.html or http://www.coolest-gadgets.com/20081229/the-vent-miser-helps-you-save-money/ or http://econetcontrols.com/EV100.php or http://www.smarthome.com/30254WH/Jackson-Systems-AZR-1004-W-Automatic-Zone-Register-10x4-White/p.aspx ) where in fact LBNL modeling & testing would indicated they do the exact opposite. E-Vent is a Johnny-come-lately to that party.
It's not too surprising that lots of folks would come to the same wrong conclusion then try to make a product out of it. I wonder how many patent violations the E-Vent folks are getting them selves into with the existing bad-idea product implementations? They might be the first to do it with the smart phone app (or not), but the rest of it has prior art, some of it patented.
Dampers
Closing vents is bad, but installing and adjusting dampers is good. Am I dense or do those seem like very similar things? Why is the advice so different between the two?
Iain Walker's research on the effect of closing registers
Allison,
I cited Iain Walker's research on this topic in my November 2011 blog on "More Energy Myths."
In that blog, I wrote: “Closing hot-air registers in unused rooms saves energy. To debunk this myth, [Michael] Blasnik quotes a study performed by Iain Walker, a staff scientist at LBNL: ‘The results of this study showed that register closing led to increased energy use for a typical California house over a wide combination of climate, duct leakage, and number of closed registers. The reduction in building thermal loads due to conditioning only part of the house was offset by increased duct system losses, mostly due to increased duct leakage.’ ”
Response to Nick Welch
Nick,
If your duct system is well designed, with generously (properly) sized ducts, static pressure will be under recommended limits, and balancing dampers will be useful to fine-tune air flows as needed.
If your duct system is more typical -- that is, not really designed at all -- the ducts will be too small, and static pressure will be above recommended limits. Some rooms will be too hot, and some will be too cold. The contractor will probably use the balancing dampers to try to even out the uneven air flow -- which may work, at the cost of limiting airflow more than ever, and raising the static pressure.
Response to Dana Dorsett
After I wrote the article, someone told me about another one that's already out there - the Keen Home Smart Vent. It uses a smartphone app like the E-Vent, so they are indeed late to the party.
Dampers, Zoning etc
The only real difference with closing vent registers vs balancing dampers is the increased potential for losing conditioned air to an unconditioned space by closing vent registers. Particularly if the registers are not sealed to the drywall opening. Usually the balancing dampers are attached to the supply plenum before the flex duct.
Regarding single system automatic zoning, there are good solutions but most involve two speed compressors, variable speed blowers and quality zoning controllers.
Typically balancing dampers are only partially closed, which is a strategy to adopt if you are closing off room registers. Often zoning systems employ automatic dampers with a stop prior to full close. Zoning controllers can also be set for a "dump" zone to compensate for excessively reduced air flow.
If you really want elaborate hvac zoning, try a VRF system with single compressor that has capabilities to heat one zone while cooling another, sometimes used in commercial situations where equipment rooms need different hvac modes than occupants need.
Perhaps a better product...
...would be a thermostat (or phone-app) operated self-modulating register that could never fully close, keeping it 40%-50% open no matter what. The micro zoning would work-mostly, and the energy use hit relatively small (except in leaky duct leaky house situations.)
But the non-closing aspect is so non-intuitive it would be a hard sell. It would be tough to come up with a compelling elevator speech that would cover it, and who the hell (other than energy nerds) has time to read LBNL testing writeups?
Zoning with Modulating Dampers
Dana - I'm not certain, but I think what you're describing already exists. Carrier's Infinity zoning system has fully modulating dampers which can partially open to non-calling zones when the system airflow is excessive.
http://dms.hvacpartners.com/docs/1009/Public/07/ZONE-01DG.pdf
Modulating zone dampers exist, but they still don't get it fully
On p2 they state:
"Comfort vs. energy saving:
If energy saving is an objective, areas which will be unoccupied at different times need to be separated so they
can selectively be setback. If maximum comfort is desired, areas with different heat loss/gain as well as different levels need to be separated."
In fully modulating systems with high turn-down ratios there may be some energy-savings, to dampering down, since the ECM drive air handlers & compressors modulate to reduce the system output without raising duct pressures or lowering delta-T across the coil. These are all sensed managed and controlled by a sophisticated control algorithm.
I was talking about dumb or smart retrofit registers that have a lower negative impact on 1 & 2 speed systems that aren't under the direct control of the air conditioner unit (the way manual registers are uncontrolled by the unit.)
Smart retrofit registers
Now I understand what you mean. Seems like it would be risky without the register having some means of monitoring static pressure. Not that it's out of the question. Lintalert is pretty inexpensive and uses what I imagine would be a similar technology (cheap magnehelic gauge with a limit).
Residential equipment is interesting, and there seems to be a fair amount of obfuscation about what equipment actually does. AFAIK, Carrier/Bryant has within the past several months released the first fully modulating residential AC from a major equipment manufacturer. Prior to this, I believe the only other major offering with fully modulating refrigerant was the Infinity/Evo Extreme heat pump. The zoning system I linked relies on the ECM motor and the latitude within 2 stage equipment to perform, but that's why it potentially needs to bleed to non-calling zones. Of course fully modulating gas furnaces have been around for a while, so zoning for heat would be pretty slick.
A couple questions
If they are dynamically balancing vs strictly closing and opening, couldn't that work? The problems they are trying to deal with is more that a single thermostat on its own can't get room level temperature control, at least that's how I read it.
Also, aren't all registers made with Louvres closing capabilities? If you are only closing off less than 40%, there are at least a handle full of blogs suggesting it is probably fine.
From what I have read, it looks like these units will measure pressure hence can try to understand static pressure buildup and even problems in the system as they start to happen.
I agree that done wrong it could be disastrous but it seems like if it's done carefully and thoughtfully there might be something to it.
Thoughts?
Duct booster fans?
This is very interesting an timely for me. I have an HVAC contractor who has been helping me with a number of problems, but while I'm confident in his abilities working with the equipment itself, I really don't think he knows as much about air flow and design as he seems to think. He actually advised me that, rather than installing manually-adjustable dampers at the plenum for my two main duct supplies, just close off the register vents to force more air through the other areas. I disagree.
My main problem is comfort: getting cold air to the upstairs in summer (Kansas City climate) from a 1991 HVAC system in the basement.
To put some questions to you all:
1. A manually-adjustable damper at the source of main supply ducts, tilted at an angle (rather than vertically restricting, like a wall in the duct) to restrict flow by a variable percentage: this should channel the flow from the plenum up the other line, preventing turbulent "charging" of the restricted trunk line and takeoffs, right? At the cost of some increased pressure--but is it less increased pressure if done at the plenum instead?
2. What about enlarging duct boots, particularly where they are constricted (narrow 2"x12" openings, constricted to a mouth from the 6" takeoff duct--always seemed like a bad idea to me)? This would decrease pressure, increasing the size of the outlet into the room, right?
3. What about a duct booster fan in the trunk line (particularly for the upstairs trunk that has to push the air so far) be a better solution? It would constrict air flow if off, I understand, but should reduce pressure while on, right? If this, or register vent fans, were used in combination with the at-plenum dampers, wouldn't this help reduce the pressure increase problem?
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