Eliminate Over Active Blower

 Control Humidity by defeating the OAB syndrome                                     

Over Active Blower Syndrome is the major cause of poor humidity control. cool 2Blasting enough coolingproducts furnaces ac 3 or heat into the home in ten minutes to make up for a twenty minute cooling gain or heat loss can’t help but screw things up. On and off blower operation has been accepted to be the biggest contributor to the far too common HVAC humidity, comfort, efficiency and IAQ problems. 


Temperature Responsive Blower Speed Modulation provided by the HVAC Whisperer (aka, the ecMModulator and high endECM4ZPackage ecm motor combination) solves those problems. Your first time watching Temperature Responsive Blower Speed Modulation perform on an air conditioning system is like entering into a new, interesting and rewarding world of HVAC the way it should be. You’ll immediately recognize the cure for Over Active Blower Syndrome.


Temperature Responsive Blower Speed Modulation has been around for over 70 years. A simple way of describing it is to picture the air in the home as Italian salad dressing. You have to shake it to mix it up. Once it is mixed, all you have to do is to jiggle it now and then to keep it mixed.

The modern, convenient and correct way to view the HVAC system is to include the entire home as a complete and integrated system. Not just the equipment and ductwork.

The Whisperer’s form of jiggling takes place during the periods when there is no call for heating or cooling. The constant but gentle input from a slow turning blower (say 300 rpm (adjustable) (That’s 28% of full speed). That’s enough silent, draft free air motion to unnoticeably move the air through the entire the HVAC/home system. The system’s air is always mixed and the air flow through the HVAC/home system is always moving in the proper direction. Room temperatures stay balanced and there is never an uncomfortable or noisy draft. The thermostat is no longer the isolated, out of the way and sluggishly unresponsive troublemaker.

Air is flowing constantly at a much more gentle speed.  This constant and gentle flow keeps all IAQ devices in the system working constantly at peak efficiency. A call for heating or cooling results in the blower’s speed responding to the temperature of the air that is being delivered. During a heat cycle the blower’s speed starts from the low (300 rpm) and increases as the supplied air temperature gets warmer. During cooling the blower’s speed increases as the temperature of the supplied air becomes cooler. Air reaches all the crooks and crannies that might harbor little pockets of air that might not have gone through the filter, through the electronic air cleaner, past the UV light or through and around any other IAQ product that needs air circulation at a sensible speed to properly and efficiently do its job. 

The HVAC Whisperer’s Temperature Responsive Blower Speed Modulation has revolutionized HVAC humidity removal and control. Here are four fundamental HVAC facts that will shine a bit more light on advanced humidity control.  

The first fundamental: Condensate is both collected by and adheres to the AC coil in inverse proportion to the temperature of the coil. I.e. the colder the coil, the more water it collects and the tighter the water sticks to the coil.

The second fundamental: The slower the air movement across the coil, the colder the coil's fins will be and the colder the air coming off the coil will be. Thus, the colder the coil, the more condensate that will be collected and the tighter it will hold to the coil.

A fixed-speed (non-modulating) blower moves air through the coil at a high speed regardless of the load. This results in a warmer coil and much less humidity removal.  

The third fundamental: Coil bypass factor is an important, inconvenient and mostly ignored fact of HVAC life.  It is a very important physical measurement that has a goodly number of effects on the success of any AC system. Coil Bypass Factor is the percentage measurement of the air that travels through the coil that is not affected by the coil. It bypasses (doesn't touch) any cold surfaces of the fins, tubes, etc. The discharge air temperature from the coil is a mixture of air that is essentially return (room temperature) air and the cold air that impacts the fins. The bypass factor tells us the percentage of return air that makes its way through the coil without changing temperature. The Face Velocity of a coil is calculated by dividing the cfm that the blower is delivering divided by the face area of the coil. Here are four examples of the bypass factor for a 3 tube coil with 14 fins per inch. Face Velocity (FV) 200 ft./min = (BPF) 0.07 which cool 5means that only 7/100 or (7%) of the delivered air is not cooled. (FV) 300 ft/min = (BPF) 0.11 or (11%) is not cooled. (FV) 400 ft/min = 0.14 or (14%) is not cooled. (FV) 0f 500 ft/min = (BPF) of 0.18 or (18%) is not cooled and water starts being stripped from the coil and blown into the ductwork.

Another point in the AC cycle that causes huge problems is when the blower is run full speed for a period of time after the AC shuts down. This is an attempt to get the slight amount of cooling left in the coil. Here is what happens: There is a lot of water held in the coil. The temperature of the coil rises very quickly. The combination of a warming coil (water doesn’t stick to a warm coil) and high speed air results in the water being blown down the ductwork. This can be a disastrous cause of mold growth over a short time period.

The fourth fundamental: Is that when the face velocity of a fully cold coil approaches 500 ft./min, water is stripped from the coil and blown past the drain pan and down the duct. A couple reasons for this is that the faster the air, the warmer the coil, the warmer the coil, the less adhesion it has for holding onto the condensate. So unnecessarily high speed air whether caused by a high speed fan or a zone bypass duct is doing two things to any AC installation. It is decreasing the efficiency of the coil (BPF) and sending condensate into the ductwork. Moisture and mold go together and the wet duct also adds to what is often mistakenly called re-evaporation from the coil and high humidity.

The HVAC Whisperer’s Temperature Responsive Blower Speed Modulation both responds to and at the same time controls the temperature of the coil.

DRY CLIMATE OPERATION: When the compressor starts and the relative humidity is low, the coil instantly becomes cold and because there is little latent load, the blower quickly gains speed and acts like any other AC system. The dry climate advantage is the ability to comfortably and silently run the fan continuously between the AC cycles so that the home stays in temperature balance and all the IAQ equipment that can be attached to the system also operates continuously at peak efficiency.

HIGH HUMIDITY OPERATION: When the compressor first comes on and the blower is running at its adjustable minimum speed. The coil becomes very cold very quickly. The HVAC Whisperer’s Temperature/Speed Response senses a change in the delivered air temperature and immediately begins to increase the blowers speed. In the few seconds when the coil is super cold and the volume of air traveling through the coil is increasing, the coil is rapidly gathering moisture. This tilts the sensible/latent ratio towards latent. The result is that the coil seizes and reserves a register blue smalllarge portion of the compressor’s capacity for latent removal. The compressor is working at full capacity but is now dedicated to latent removal. The blower smoothly increases in speed as it follows the drop in delivered air temperature. This begins a regulated shift from almost all latent load, towards sensible load. Depending how humid the air is, determines where this shift will stop shifting towards and capturing the sensible load. The more humid the air, the longer it will take to reach a normal and sensible latent balance. Over time, the latent tilt continues removing moisture which in turn releases more of the compressor’s capacity to remove heat.

On a hot day and a high humidity situation (say 75% indoor rh), when the HVAC Whisperer is first installed, you’ll swear that the blower is moving way too slow. However, if you watch the gauges, you’ll see that the compressor is operating at full capacity. It might take a day or so to drop the humidity level into the 50’s or high 40’s, but all that time, the humidity level is dropping. The sensible latent ratio is constantly tilting towards sensible. The delivered air is getting colder and the blower speed is creeping up towards full speed.

SHATTERERING THE “OVERSIZED EXCUSE”: OVERSIZED”, is an excuse that is used far too often instead of simply fixing what is wrong. What is wrong with the present equipment is Over Active Blower Syndrome which can be cured by the ecMModulator’s Temperature Responsive Blower Speed Modulation kit.

To maximize indoor humidity control requires a precise, nimble, accurate and self-adjusting balance between running a very cold coil without frosting the coil and running blower speeds that create a warm coil and a high bypass factor which strips water off the coil. Setting to the “20 degree rise” rule of thumb and calling it a day might keep you out of mechanical trouble most of the time. However, it isn’t precise, accurate, nimble or self-adjusting. It does not provide sufficient humidity control.

Every air conditioning system is oversized over 90% of the time. HVAC systems are designed for peak output and pretty much continuous operation at the design weather condition for the area where they are located. At any time the weather conditions are above or below design conditions, the system is oversized. The HVAC Whisperer takes “oversized” and uses it to deliver even more dramatic results. Imagine a cold and oversized coil latching onto the latent load and not letting go till the condensate is down the drain. That’s the biggest, thirstiest, no holds barred dehumidifier available. The ecMModulator says to oversized: “Bring it on!!!!!

WHAT ABOUT WHEN THE COMPRESSOR SHUTS OFF? Within a few seconds after the compressor shuts off, the temperature of the air leaving the coil increases. The HVAC Whisperer’s controlled temperature/speed responds slows the blower down. It takes about 30 seconds to a minute for the blower’s speed to drop from near full speed to about half speed which is slow enough to prevent water from being stripped from the coil. At that point, the drop in speed is much slower because of the refrigeration effect of pressure equalization. On some cap tube units, it might take 10 minutes to reach minimum speed. All the while there is significant air cooling and humidity removal going on. That equals energy savings that would be lost if the blower were off.

HVAC SUPER STARS in the hot and humid southland love the HVAC Whisperer’s Temperature Responsive Blower Speed Modulation. Now, they can run the blower full time and still easily reduce their indoor relative humidity by at least 10% and usually a lot more. 10% is easy while 20 to 25% is common. A 10% reduction in relative humidity is the comfort equivalent of a three degree drop in temperature.


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    Shawn Woodrow

    A. Ralph Woodrow, Inc.
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    Wilmington DE 19805
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    Your service company since 1950. Visit us on the web at www.aralphwoodrow.com