Posted by Dan Koloski on 20 Dec 2006 at 08:29 pm
Most heating (and all cooling) systems in our area use forced air blower fans to move hot or cold air throughout our homes. (Examples of heating systems which do not utilize forced hot air are hot water or electric baseboard heating systems and radiant floor heating systems). The comfort of your home in winter and summer, your utility bills, the amount of moisture in your home, and the very health and safety of your family from combustion byproducts are all affected by the ductwork in your HVAC (Heating Ventilation and Air Conditioning) system. This article is merely an introduction to a very important and complex topic.
The quality of your HVAC ductwork is something that can actually be measured and quantified. This article will explain why and how this is done. Leaky ductwork hurts your household budget, and actually puts the health and safety of your family at risk. Read on to understand how this all works. By the time you are done reading this article, you will probably understand this topic better than the builder who constructed your current home.
Your heating and cooling system includes a blower fan which pushes air into the supply ductwork, and pulls air through the return ductwork. Ideally, your ductwork system is perfectly sealed, so that the only air pulled by the blower fan is the air coming into the return grilles, and the only air coming out of the supply grilles is coming directly from the blower fan. This does not happen in your, or in any, home. All ducts have leaks, and a great duct system has no more than 5% leakage. Your home probably has 20% to 40% leakage. If you do not have a very high-efficiency heating system (if your home is more than a few years old, you don’t), you may be wasting up to half of your heating dollars trying to heat your attic. That sounds alarmist, but there are documented cases in which this is true.
So how do you tell if you have a tight ductwork system? With the use of a digital micro manometer and a calibrated fan. A manometer is a very precise scientific instrument which measures air pressure. The unit of measure for pressure for our purposes is the Pascal. Without turning this into a high school science class, just understand that atmospheric pressure at sea level is 14.7 pounds per square inch, and that there are about 100,000 Pascals in one atmosphere. So a Pascal is a pretty precise measurement unit for air pressure. A calibrated fan is a very precise fan which can tell us how much air it is moving. The unit of measure for the fan is CFM, or Cubic Feet per Minute. Let’s talk about how this all works.
When your blower fan is running, it is creating positive pressure in your supply ducts. Imagine that all your supply air registers are closed and sealed tight. The blower fan would be trying to push air into your ductwork, but the air would have nowhere to go. It would be like blowing up a balloon. Except that your supply ducts are like a balloon with a pinhole. If the pinhole is small your ducts are tight. If the hole is large, or if you have many holes, your ducts are leaky. When your fan comes on, it is blowing some of that conditioned air out through the holes and leaks, into unconditioned spaces (like your attic). The blower fan is creating positive pressure inside your supply ducts. In like manner, if you have holes and leaks in your return ductwork the blower fan is sucking air from the unconditioned spaces surrounding your return ductwork. The blower fan is creating negative pressure inside the return ducts. Air always moves from an area of high pressure to an area of lower pressure.
Now imagine that we could seal all the registers in your HVAC system, except one. Now imagine that we can temporarily install a very scientific, precise fan at the open register. We then could use that precise fan to blow air into the ductwork system. The sealed ducts are like that balloon, right? Unfortunately, your ducts are like the balloon with pinholes, so instead of trying to push air into a duct system with nowhere for the air to go, instead the forced air is going out through those holes and leaks. It is not going out through the other registers because we sealed them all very tightly. Now the fan is calibrated, and a digital readout tells us how many CFM (Cubic Feet per Minute) are going through the fan, and of course therefore through the holes and leaks in your ductwork (because unlike the balloon, the ductwork is not going to expand, the air has nowhere to go but out through the holes and leaks). We use the digital micro manometer to measure the difference in air pressure between the interior of the duct and the surrounding air. In current building science practice, we measure something called CFM25. We pressurize the ductwork to 25 Pascals relative to the surrounding air, which we measure with the manometer, and then read how many CFM are being pushed with the calibrated fan in order to achieve this pressure difference. So if the ductwork is very tight, we would need very little air (CFM) to achieve a pressure equal to 25 Pascals. If the ductwork is extremely leaky, we would need a lot of forced air (high CFM). (If the ductwork were completely open, the fan would never be able to push enough air to achieve a 25 Pascal pressure difference because it would be trying to pressurize the completely open duct relative to the atmosphere). We then compare the CFM reading to the rated CFM for your HVAC system’s blower fan. This is how we calculate the percent leakage in your ductwork. We do this test on every home we build.
This article barely touched on a very important topic, but at least you now know two very important facts. One, it is important to know how leaky (not if. That’s a given) your HVAC system is. Two, you now know that the leakiness can be measured and quantified. Of course, we can also do this test in your present home. More importantly, leaky ducts can be corrected. (Just don’t use common gray duct tape to do it, but that’s another article). We can also test the leakage of an entire home’s enclosure to quantify how leaky an entire home is. We can also test for pressure differences between different rooms in a home. You care about this test if you have hot or cold areas or rooms in your home. These topics will be subjects for future articles.
So if your home is relatively new, or if you are currently having a new home built, ask your builder what your CFM25 reading was, or what standard he is building to. It is in your family’s best comfort and health interests to do this. If you are thinking about building a new home, and you care about air quality and utility bills, give us a call.
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