Duct Dynasty: Are Smart Vents Really That Smart?
The Answer Can be Found in Static Pressure, Fan Motor Efficiency Readings
Many of you have seen the advertisements for the smartphone-controlled dampers that fit into a duct system to control airflow into individual rooms. They seem like cool devices. After all, you can manage them from your smartphone, so they have to be awesome.
It also has to have some credibility because the idea was scooped up on the popular TV show “Shark Tank.” Could this be the latest device to save energy and solve all your comfort problems, or could there be a dark side to it?
What if they cause more harm than good? Let’s look at the potential impact these smartphone-controlled dampers can have on the typical duct systems found across the country.
The concept behind the smart vent goes back to a myth that is decades old. The myth is that if you close the supply registers to rooms you don’t use, you’ll save energy by not heating or cooling those rooms. Sounds logical, right?
To understand some of the interactions these devices can cause with an HVAC system, you have to understand how dampering supply registers affects static pressure and system airflow. It’s also important to know how fans in HVAC equipment respond to these changes in static pressure.
Static Pressure Impacts
Anytime you block off supply register openings, system static pressure goes up. Depending on of how much airflow is blocked off, the static pressure increase could be substantial. When this is combined with a duct system that is already undersized or restricted, you have a recipe
According to studies we’ve done at the National Comfort Institute (NCI), the typical duct system across the country is drastically undersized. The average total external static pressure is found at 0.82-inch of water column (wc) with fan airflow averaging less than 300 cfm per ton in cooling mode. That’s not good.
The majority of systems cannot function when a device of this type is added to the system. This is why testing is so important. You need to verify that a device of this type can be properly and safely added to a system.
Fan Airflow Impacts
The two most commonly encountered fans in residential HVAC equipment are constant-speed permanent split capacitor (PSC) and variable-speed/electronically commutated motor (ECM) devices.
The most common air-handling equipment in the field use a PSC fan as the source for moving airflow through the duct system. As static pressure increases with this type of fan, the amount of airflow it moves decreases. As static pressure goes up, airflow goes down.
ECM fans respond to pressure changes in a duct system differently. As static pressure increases, the amount of airflow a variable-speed fan moves will remain constant up to a certain pressure limit.
Variable-speed fans have another interesting reaction though. As static pressure increases, the amount of energy consumed by the fan also increases. This increase in energy consumption is due to the fan having to do more work to achieve the same amount of airflow. Knowing how these fans respond to changes in pressure is important in realizing potential issues that could be created as registers are blocked on a typical duct system.
Airflow is the medium used to transfer heat in an HVAC system. For equipment to have a long, trouble-free life, airflow has to be within acceptable specified levels for the heating and cooling system requirements. When fan airflow drops below these levels, equipment life is drastically reduced and comfort, efficiency, and maintenance problems begin.
Anytime an undersized duct system is restricted by a device such as this, the amount of airflow a constant-speed fan moves decreases even more. In many instances, open registers in the undersized duct system are the only things keeping the fan moving the minimal amount of airflow needed for borderline equipment operation. Once registers are blocked, fan airflow drops below acceptable levels due to an increase in static pressure.
A constant-speed fan in cooling mode will begin to see increased compressor failures due to refrigerant flood back. Evaporator coils also begin to intermittently freeze-up during periods of extended run time due to a lack of airflow.
In heating mode, heat exchangers in gas furnaces could crack due to overheating from a lack of airflow. Heat pumps trip out on high head-pressure controls and experience longer run times. This leads to secondary electric heat strips coming on to satisfy the building load.
A variable-speed fan will typically move the same amount of fan airflow when a device like this is used. This comes at a price, though, as there could be negative impacts to the equipment and its operation. For example, any energy savings promised from a variable-speed fan will be lost. The variable-speed fan will consume more energy to move the programmed amount of airflow.
Testing these Devices
Some of your customers will absolutely want to have these devices installed. Great news if you know how to handle the situation. Let’s look at how to turn a potentially disastrous add-on into a huge opportunity.
A simple static-pressure test before these devices are installed would be a great way to see if they could work. The equipment’s fan-rated pressure, which is found in the manufacturer’s engineering data, could be compared against the actual measured static pressure as an initial step. This would ensure the ducts have enough capacity to be blocked temporarily by these smart-vent devices.
Start to incrementally block the registers to the rooms these devices are to be installed in and watch the static pressure readings. If they are at an acceptable level when the register is sealed off, then the system could make use of them.
If pressures are excessive when the registers get blocked, the smart vents wouldn’t be a good option. Adding additional supply duct capacity to the system may be needed to handle the new smart vents.
Depending on how you make out with this step will determine whether you progress to the next step, which is checking fan airflow.
If duct pressures are acceptable, then plot fan airflow as another verification method to see if a smart vent could work. If fan airflow falls within an acceptable range, these devices could be an option. If fan airflow is already borderline, other options need to be considered.
Don’t Miss the Obvious
Above all else, make sure you aren’t creating more problems than you correct. It is possible to unknowingly create pressure imbalances in a home by installing smart vent devices that could turn a safely operating combustion appliance dangerous due to pressure changes created by closing supply air registers.
If a customer enquires about these devices, or you encounter them, it is a sign they are experiencing comfort-related issues in their homes. Be aware of the clues that might lead you to an opportunity to serve your customers better and truly solve their problems instead of installing a high-tech device that could slowly kill their equipment.
SIDEBAR: Thumbs Down
Kevin O’Neill, owner of O’Neill Cooling & Heating, Myrtle Beach, South Carolina, is also not a fan of the smartphone-controlled vents. “These are not a good idea,” said O’Neill.
“I agree, it could freeze coils during cooling season, could lead to early compressor failure, and can trip the high-pressure lockout on a heat pump in heat mode, causing more strip heat usage in the heating season. Now you get a high electric bill and a service call instead of saving money.”
O’Neill questions how a homeowner controls system airflow. “It says in the literature that it has a static pressure sensor, but what is it set for? What if it is set too high or too low?”
Ultimately, O’Neill said, if the homeowner has rooms that aren’t being used, perhaps he should have purchased a smaller home.
“Most systems have too little airflow to start with, even when they’re ‘professionally designed.’ Now you are reducing it further under homeowner control. The typical homeowner is not qualified to design his duct system. Now the system is oversized because several rooms are no longer conditioned. Oversized systems tend to be harder to balance, not easier.”
These devices could cycle the limit switch on a gas furnace.
“A limit switch is a safety control, not an operating control. It is not designed to put up with constant cycling. This is likely to cause early failure of the limit control. If you have a power-robbing thermostat, it will run the battery in the thermostat down. I saw this happen with a pleated filter that constantly tripped a limit switch on my brother’s gas furnace.”
If you have a variable-speed blower, it will use more power to overcome the higher static pressure, said O’Neill.
“Using dampers in the registers to cut off airflow makes a lot of noise. Using this to air balance a house would make rooms noisy. Try sleeping at night with the bedroom register whistling. I think these are a bad idea. I give them two thumbs down.”
Publication date: 5/25/2015