What you will learn with Supply Vent Myth article:

  1. How modern duct systems are designed
  2. How closing the supply vents effects the static pressure in the duct work
  3. How closing supply vents effects modern HVAC equipment
  4. How to save energy and therefore money
  5. Where to obtain the books and/or software to learn how professionals design duct systems and do load calculations for sizing heating and cooling systems
  6. How heating and cooling equipment ie furnace, air conditioner, or heat pump can be effected by closing off supply vents
  7. Alternative methods used in commercial HVAC
  8. Alternatives for residential HVAC

The Supply Vent Myth – Does Closing Supply Vents Save? The Grand Myth

supply vents and duct work
Supply Vent Myth - Does Closing Supply Vents Save? - A myth many people believe is that they can close off some of their supply registers and save money. This may be true if you only close off less than ten percent. The system installed in your home is designed for a certain amount of airflow. If this airflow is restricted in any way it causes the system to function improperly and could cause system failure. This myth most likely goes back to the early days when people would close off most of their large home in the winter to save on coal and wood usage.

Today’s modern HVAC systems are designed specifically for a certain amount of airflow and if you restrict that airflow, whether it be by a dirty air conditioner filter or closing off supply vents, problems will occur. Open those supply vents, closing too many off is the same as a dirty or restricted filter.

Supply Vent Myth - Does Closing Supply Vents Save? - Static Pressure

By design modern HVAC air delivery systems use static pressure for duct work design. It is how designers size the duct work.

A certain amount of static pressure in a given size duct work system will deliver the appropriate amount of CFM’s needed or required for a given house or apartment. The designer designed the duct work for the air conditioner or cooling system. It is a little complex to describe the process of designing and sizing a duct system as there are several factors in that process either required by code or using the Manual J Residential Load Calculation and the Manual D Residential Duct Systems but the system is sized for a certain amount of airflow and static pressure. When you close off the supply vents you change the static pressure and the amount of air delivered to other vents in the system. Coupling too many supply vents closed off with a slightly dirty filter and you begin causing problems with air conditioner and even the furnace if it is winter time. The final answer to the question of: Supply Vent Myth - Does Closing Supply Vents Save? is no in the long run for a modern HVAC system you are causing problems with your air conditioner and possibly causing problems with your heating system.

Supply Vent Myth - Does Closing Supply Vents Save? - Duct Pressures and Leaks

A good designed and sealed duct system is balanced and does not leak. That is by mechanical code (for new systems but always check with your local code enforcement agency for details) and in ideal situations. In the real world, according to the Department of Energy, the average duct work system leaks more than 20% of the air. That is 20% lost that your air conditioner or heating system worked hard to produce.  Lost energy unless you like paying to heat or cool a crawl space, basement or attic. Most people do not want to heat or cool those places so aside from the air lost the homeowner or business owner is also losing the dollars it takes to produce that 20% of lost conditioned air.

Related Link: What does the Mechanical Code say about duct systems and duct leaks? (opens in a new window)

In most commercial new construction projects and in residential duct work installations some states require a DALT or Duct Air Leakage Test where the newly installed duct work is tested for leaks. This test is typically witnessed by a third party and/or Code Inspectors to ensure the new duct work is leak free and meets all requirements of the code.
Supply Vent Myth - Does Closing Supply Vents Save?
If you close off those supply registers it will increase the pressure inside the duct system and force more air out the leaks in the duct work if in fact your duct does have leaks. When was the last time you had your duct work inspected for leaks? This will also cause more air velocity to flow out of the registers that are not closed. In the summer a slight draft or breeze is good but in the winter a slight draft or breeze makes you feel cooler and most of the time in the winter we do not want to feel colder. Yet another reason why you do not want to close off your supply vents.

Supply Vent Myth - Your Heating and Cooling Equipment and Supply Vents

What happens to your heating and cooling equipment by closing off too many supply vents? It really depends on what you have and if you are heating or cooling:

  • Air Conditioner - low air flow is not good for an air conditioner at either the condenser or the evaporator. Low air flow on the evaporator coil leads to a frozen air conditioner. Whenever I get a call and the customer tells me the air conditioner is frozen I automatically think of two things. Either the system has low air flow across the evaporator coil or the system has a refrigerant leak and has a low charge of refrigerant. Either way, the first thing I check when I arrive at the customers house is the air flow. Sometimes I find the customer closed off too many supply vents in conjunction with having a dirty filter. This problem is easily corrected however if left to continue this way you could possibly cause severe damage to the compressor. And replacing a compressor is not cheap in any regards.
  • Since a heat pump uses the refrigeration process to heat and cool the heat pump can be damaged in much the same way the air conditioner is damaged when the heat pump is operating in the cooling mode. In the winter time when the heat pump is operating in heat mode low air flow will cause the pressures inside the refrigeration system to rise. Another way to possibly cause damage to refrigeration components in your heat pump.
  • a Cracked heat exchanger for a gas furnace

    a cracked heat exchanger for a gas furnace

    A gas furnace on the other hand needs the air flow to keep the temperature inside the heat exchanger at engineered temperatures otherwise you will begin causing problems with the limit switches and causing the furnace to quit operating altogether. Other possible problems from this is damage to the heat exchanger in the form of cracks because it was operating outside of it’s designed and engineered temperature ranges. Blower motors are also sized for a certain amount of air flow. When the air flow is reduced the motor will operating at a higher than normal temperature which means it will reduce the life of the motor.

Another aspect other than what is mentioned above is cold/warm air infiltration through tiny cracks in windows and other places. When you close the vent in a particular room the cold air (winter time) and the warm air (summer) slowly creeps into the room making it cooler or warmer than normal. Since it is unlikely that the inner walls of the building (home or office) have insulation then the that will cause a higher than normal load in the house if you would have otherwise left the supply vent in service and kept the room at normal temperatures.

So you can see by trying to save a few dollars on the heating or cooling bill you can actually cause bigger problems that will end up costing much more than you can save by closing off your supply vents.

Supply Vent Myth - Alternatives Used in Commercial HVAC to Modulate Supply Vents (Supply Air)

In commercial HVAC there are older systems called VVT’s or Variable Volume Terminals and these systems used modulating dampers and rudimentary controls along with barometric dampers to modulate dampers to supply ducts and registers. The problem with these systems was everything was hooked up to a constant volume air handler or rooftop package unit. So while the supply dampers modulated open and closed the air conditioner or gas pack continued to run at 100 percent. So when the system really did not save anything for energy efficiency purposes but it did reduce air flow to supply vents based on the control variable in the controller and what the thermostat input was to the controller. When the dampers closed off enough the excess coming from the air handler or rooftop unit was vented out a barometric damper typically in a plenum above the ceiling. These systems went by the wayside because they really did not save energy.

Next, comes the pneumatic VAV or Variable Air Volume Systems. These systems did save money but required special HVAC equipment to work properly. An air handler with modulating vanes on the blower so air flow could be reduced along with either several compressors that would unload so when the system was calling for less air the compressors would unload and the modulating vanes would reduce the air flow. The compressor unloading was designed to match the air flow so the evaporator coil would not freeze. Vice versa for when the system needed more air. The compressors would stage up and load the vanes would modulate to increase the air flow. While these systems can be old and clunky they worked to save energy and some continue to work in some buildings. This is simply an example of one of the types of systems I have worked on. In commercial systems, there are several other ways to modulate the cooling and the air flow.

Modern VAV or Variable Air Volume Systems use direct digital controls and variable frequency drives along with complex algorithms inside the microprocessor controller to modulate the systems up and down based on demand and these systems save a lot of money. None of these alternatives are currently viable in residential systems unless you have a huge house or can afford to pay several thousands of dollars to have a system like this installed in your home. While it would save energy and which correlates on the money you spend monthly on energy the return on investment would be several years and there are other residential alternatives to installing a commercial type system in a residence.

Supply Vent Myth - Residential Alternatives for Saving Energy Versus Closing Supply Vents

Quick and simple but expensive options for you are:

  • Geothermal heat pump - these systems are very efficient especially the modulating systems. You can even find one that will heat and cool plus make hot water for you. There are special requirements for having a geothermal heat pump system and this includes either having lots of land (more than an acre), a large body of water adjoining your land or a couple of dependable wells on your property. If you are interested call a company that specializes in geothermal systems.
  • A few manufacturers are making very high SEER completely modulating air conditioners, heat pumps and gas furnaces. These types of systems have the ability to run between 20% and 100% based on demand and this is really what you are looking for when you are thinking of closing off the supply vents.
  • A lot of ductless mini split air conditioners or heat pumps are fully modulating and I have to say this is how they do it in most of Europe. If you decide on this option make sure you get the fully modulating type as some of the cheaper units are not fully modulating and therefore not as energy efficient.

So open up those supply vents so you don’t create new problems with your HVAC system. Unless of course, the system is more than 30 years old but then again I would caution against restricting the airflow on any given type of system even if it is ancient.

High Performance HVAC

Supply Vent Myth - Does Closing Supply Vents Save?

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The Typical Home Duct Work Air Delivery

According to EnergyStar the average duct work system leaks 20 to 30 percent of the air inside the ducts. Since your air conditioning and heating systems use more energy than any other appliance in the home, you are losing 20 to 30 percent because of leaky ducts. That's wasted money. If you are a DIY handy person then there are tons of resources to show you how to fix your duct work.

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Typical Home Duct Work Air Delivery

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