Does Closing Register Save Energy

Your air conditioner, estrus 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 rut pumps using electricity generated in fossil-fuel fired ability plants, the amount you employ at home may exist merely a third of the total. A question I get asked frequently is whether or not it's OK to shut vents in unused rooms to save coin. The answer may surprise you.

The photograph above shows a typical vent for an ducted HVAC organization (air conditioner, heat pump, or furnace). On the return side, yous'll typically see plain grilles, just on the supply side, where the conditioned air gets blown back into the house, well-nigh HVAC contractors install registers like the one to a higher place. Information technology has a lever of some sort that allows you to adjust the louvers backside the grille.

You lot'd think that since information technology's adjustable, it must exist OK to open or close it to suit your needs, right?

The blower and the blown

The blower in your HVAC arrangement is the heart of the air distribution. It pulls air from the firm through the return ducts then pushes it back into the house through the supply ducts. In high-efficiency systems, the blower is powered past an electronically commutated motor (ECM), which can adjust its speed to varying conditions. The bulk of blowers, however, are of the permanent split up capacitor (PSC) type, which is not a variable speed motor.

In either case, the arrangement is designed for the blower to push confronting some maximum force per unit area deviation. That number is typically 0.five inches of water cavalcade (iwc). If the filter gets as well dirty or the supply ducts are besides restrictive, the blower pushes confronting a higher pressure.

In the instance of the ECM, a loftier force per unit area will crusade the motor will ramp upwardly in an endeavor to maintain proper air flow. An ECM is much more efficient than a PSC motor under ideal conditions, only as information technology ramps up to piece of work against higher pressure, you lose that efficiency. You still get the air flow (maybe), merely it costs you more than.

hvac air handler return supply resized 600

The PSC motor, on the other mitt, volition keep spinning but at lower speeds as the pressure goes up. Thus, higher pressure means less air flow, and, as nosotros'll see below, depression air flow can cause some serious problems.

The of import matter to remember here is that no matter which type of blower motor your HVAC organisation has, it's not a skilful thing when information technology has to push against a higher pressure.

Closed vents increase pressure

In a well-designed system, the blower moves the air against a pressure that'south no greater than the maximum specified by the manufacturer (typically 0.five iwc). The ideal system also has low duct leakage.

The typical system, withal, is far from platonic. Although most systems are rated for 0.5 iwc, the National Comfort Institute, which has measured static pressure and air menstruation in a lot of systems, finds the typical arrangement to exist pushing against a static pressure level of virtually 0.eight iwc. At present we're fix to address the question of closing vents.

When you first closing vents in unused rooms, you brand the duct system more restrictive. The pressure increases, and that means an ECM blower will ramp up to continue air flow upwardly 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 than duct leakage, as shown below.

hvac air handler duct system pressure flow closed vents 1 resized 600

The more than vents you close, the higher the pressure in the duct organization goes. The ECM blower will use more and more than energy every bit yous practice so. The PSC blower will work less but non move as much conditioned air. In both cases, the duct leakage volition increase further.

hvac air handler duct system pressure flow closed vents 2 resized 600

What about heat?

In add-on 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 estrus exchanger and either gives up heat or picks up estrus.

In a fixed-capacity arrangement—and almost are—the amount of heat the scroll or heat exchanger is capable of absorbing or giving up is fixed. When the air menstruum goes down, less estrus exchange happens with the air. Every bit a event, the temperature of the coil or heat exchanger changes.

If air menses is low, information technology'll dump less estrus into the coil in summer, and the ringlet volition become colder. If in that location'southward water vapor in the air, the condensation on the ringlet may start freezing. You might even end up with a block of ice, as shown in the photo beneath. And ice on the coil is actually bad for air flow.

It's also bad for the compressor as not all of the refrigerant evaporates and liquid refrigerant makes its fashion back to the compressor. If y'all want to accept to buy a new compressor, this is a proficient way to exercise it.

frozen evaporator coil for air conditioner

Same matter if y'all have low air flow over a heat pump coil in winter. You lot could get a actually 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, and so, let exhaust gases to mix with your conditioned air. When that happens, your duct organization can get a toxicant distribution organisation as it could exist sending carbon monoxide into your dwelling house.

ix unintended consequences of closing vents

Let me now summarize the issues I've described above that can result from closing vents in your domicile. The first affair that happens is the air force per unit area in the duct organization increases, which may requite rise to these negative consequences:

Increased duct leakage
Lower air flow with PSC blowers
Increased energy utilisation with ECM blowers
Comfort problems because of low air menstruation
Frozen air conditioner gyre
Dead compressor
Cracked rut 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 wintertime due to lower surface temperatures in rooms with airtight vents

You're not guaranteed to get all the problems that employ to your system, but why take the chance.

A Kickstarter projection to avoid

I recently wrote about all the IT folks who are trying to follow in Nest's footsteps and profit from the home free energy efficiency movement. I used the Aros smart window air conditioner as the case of companies that think you can solve problems simply by creating a product with a smartphone app.

hvac e vent kickstarter supply duct system

Well, meet a more malignant idea: the E-vent. (You lot tin find information technology easily enough by searching on the term "Kickstarter E-Vent.") It'south just a Kickstarter project right now, and perhaps it won't go funded. If it does go funded, however, it will be discipline to all the problems I described above. It doesn't affair whether you close the vents by getting up on a ladder in your habitation or from the embankment in Cozumel. Information technology's withal a bad thought.

The Eastward-Vent page on Kickstarter says they monitor the air temperature and open vents if the temperature gets too cold while air conditioning or too hot while heating. Of grade, that's not going to piece of work unless they monitor the temperature right at the curl or heat exchanger. And that still probably wouldn't piece of work considering in that location's a wide range of adequate temperatures for dissimilar systems.

This is an HVAC product adult past people who don't know some very important principles of heating and air workout. Permit's hope they don't impale anyone.

The merely way endmost vents could work

The fundamental problem here is that closing supply vents in your HVAC system changes what comes out in particular locations. Information technology doesn't change what the blower is trying to do. Nor does it modify the amount of heat the air conditioner, heat pump, or furnace is trying to move or produce.

It's possible you may be fine endmost a vent or two in your domicile, simply it will depend on how restrictive and leaky your duct organisation is. If it's a typical duct system with 60% higher static pressure than the maximum specified, closing even 1 vent could ship it over the edge. If it's a well designed organisation with low static pressure level and sealed ducts, yous shouldn't have a trouble as long as you don't endeavor to close too many.

The simply way something like this could work is if closing a vent signaled the blower to move less air and the air conditioner, estrus pump, or furnace to motility or produce less heat. (Properly designed zoned duct systems practise this by using variable speed ECM blowers with multi-stage systems.) Otherwise you're subject to those 7 unintended consequences, one of them potentially deadly.

Related Articles

The Sucking and the Blowing — A Lesson in Duct Leakage

What Is Pressure? – Understanding Air Leakage

4 Means a Bad Duct Organization Tin can Lead to Poor Indoor Air Quality

Thanks to Curt Kinder, David Butler, John Semmelhack, Eric Sandeen, and Dale Sherman for suggestions in the comments beneath that made this article better and more complete.

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