Visit our updated. Technicians typically measure static pressure in inches of water gauge. 2. My contractor questioned this idea. Besides holding all NCI certifications, David has held certifications as a HERS rater, BPI building analyst, and is a BPI field and written exam proctor. Subtracting the pressure drops for all the things that arent ducts or fittings from the total external static pressure yields the available static pressure. Now you can use a duct wheel to match up your desired CFM with the FR to determine the required duct size. MW, Your email address will not be published. I typically specify not to exceed 0.4 IWC TESP for conventional air handlers, but I go lower for ducted mini-splits and other low static AHUs. 2540 S Military Trail, West Palm Beach, FL 33415. During the ductwork design process, it is very importantto avoid clashes with other building systems, such as electrical and plumbing installations. Whether or not that becomes an issue depends on how well the house is insulated and sealed. specifically referred to the presumption / rule-of-thumb that supplies should be located at the perimeter, where the load is. how? In my experience, most air handlers have a higher maximum static than 0.5 IWC. If your ducts are too small, it can cause an increase in the static pressure. Static pressure creates resistance against air movement in the ducts of an HVAC system, and air handling units must overcome this pressure to provide heating and cooling. Cabinet mounted filters, especially when shipped with the unit, are typically accounted for in external static, although designers should always verify one way or the other. Please read Google Privacy & Terms for more information about how you can control adserving and the information collected. If the AHU has apecial filters then add the PD as per filters manufacturers. Low static in airways is aLow static in airways is a non-issue, although dropping from 5 tons to 3 tons would likely require re-balancing (assuming system was ever balanced in the first place!) 7: Installing a new air-handling unit is a "plug-and-play" process. 6.5.4.1 Hydronic Variable Flow Systems. David agreed that was one possible option. since fittings have non-linear friction losses. As a result, your unit will run longer which can significantly increase your energy expenditure. I have tried the methods in ASHRAE and SMACNA and find then very lengthy, had to make too many assumptions, and hard to apply. Brendan Reid is President of the Comfort Institute (CI), Bellingham, Wash. CI provides building science training and instruments for whole-house and air-distribution diagnostics and repair. AllSponsored Content is supplied by the advertising company. You generally want to design a system to operate on medium speed (tap 3 in the table above). At some point it may be worthAt some point it may be worth pointing out that like many other budgets, it is worthy not to expend every available asset if avoidable. The formula used incorporates different factors such as the square footage of your home, friction loss and the size of your unit, static pressure, and the appropriate airflow rate. Privacy Policy. The next important unit to know when determining duct sizing is the friction loss rate. The following are some useful guidelines from HVAC experts when designing air ducts: There are three types of air duct systems, classified based on their static pressure: A higher static pressure causes more noise and vibration. Thats another discussion if you need some help with that. They must also find optimal locations for diffusers, air handling units and HVAC equipment. In most cases, you only need to get the air into the room. w.g. In my experience, the duct system was likely undersized for the original system, so a smaller system will end up being significantly less oversized than the nominal tonnage would suggest, especially if the new system operates at greater than 400 CFM/ton, which is often beneficial. For example, when a 5 ton system is downgraded to a 3 ton because of a newly encapsulated attic with continuous rigid foam under the siding. Were available 24/7 and we can help you get the proper duct size as soon as possible! John was embarrassed to admit that he wasnt really sure. While Charlie prepared his cordless drill, David put John on Here's a quick review of TESP, why it's important, and how to make the measurements. The latter can be done with a TrueFlow meter from Energy Conservatory or by taking external static pressure measurements at the AHU and refer to the blower table for the unit. Duct Sizing Rules Of Thumb Rely On Friction Loss Rate. In that case, subtract the heat strip kit static spec from the SP numbers given in the blower table. @Mark, external static, by@Mark, external static, by definition, accounts for permanently affixed internal components such as the heat exchanger in the case of a furnace, or the refrigerant coil in the case of an air handler. I should have clarified that my comment (re: register placement being irrelevant in high performance homes) specifically referred to the presumption / rule-of-thumb that supplies should be located at the perimeter, where the load is. However, if youd still like to do it yourself, make sure to follow the exact steps we show you. If building leakage is the cause of your customers woes, return grille placement and supply register selection wont be enough to cure the problem. Also, I was wondering about filter returns. you wrote: For example, ifyou wrote: For example, if youre using wooden grilles, the pressure drops will be significantly higher.. The first thing youll need to learn is the size of your air conditioning unit in tons. Next you use this equation: ASP/TEL*100 (0.21 / 115 * 100 = 0.18). air noise made by the furnace fan, youve been installing systems for quite a Industry Leading Commercial Kitchen Ventilation Systems - CaptiveAire Having a little troubleHaving a little trouble understanding your example with the software you used to determine allowable SP for duct design If you have a system, lets say a HP since that is what you described above, and the AH is a one piece with a coil pre-installed. are often other issues, said David. resistance caused by pushing air through the a/c coil, as well as the supply Therefore, I would take my measurements with a probe in the blower compartment for the negative side and a probe above the coil, but below the heat pack for my positive pressure, and add them together to get the less than .5 (hopefully) SP that is max for that unit?? They will make it very difficult to get good air flow.. transitions or other fittings with uneven pressure in the duct. One thing I might add regarding supply register throws with high performing building envelopes is, in context of cooling mode, that a high performing envelope will not provide as much reheating of chilled supply air as a marginal performing envelope. Now, I'm taking a huge risk by putting this table out there and I will probably get a lot of grief for it, but here it is. as long as you are on the furnace side of the media, said David. Unfortunately, the formula to calculate duct size is not an easy one. 2. To specify these components correctly, and accurate static pressure calculation is necessary. We cant be moving the right amount Google use cookies for serving our ads and handling visitor statistics. Is Wind Washing Reducing Your Insulation R-Value? fundamental part of diagnosing how a duct system is performing. But and cookie policy to learn more about the cookies we use and how we use your 3. Absolutely, Cameron. Once you determine the appropriate design airflow, study the blower table and determine which speed yields the desired airflow at an acceptable ESP, interpolating as necessary. A basic place to start with duct air friction is that Total Pressure = Static Pressure + Velocity Pressure. If you must use a metal duct calculator and rule of thumb . Since airflow takes the path of least resistance, a poorly installed filter rack allows unfiltered air to go around the filter instead of through it. TESP is a pressure reading of the "balloon" pressure inside the ductwork. What Is The Right Time To Replace Your Air Conditioner. Or, if you can go through the filter cabinet, thats often better, Its kind of Outdoor Air Sensing to inhibit Auxiliary operation in mild weather. Thats how the manufacturers test them in the lab, he Upblast Exhaust Ventilator Energy Efficient Power Packs. w.g. For these reasons, static pressure calculation is a very important step in the HVAC design process. Now that youre familiar with the HVAC duct sizing rule of thumb, youll need to whip out your tape measure. By visiting this website, certain cookies have already been set, which you may delete and block. He has been involved in Performance-based contracting since 2001. HVAC also represents the highest energy expense for most residential and commercial buildings, and smart design decisions can reduce power and gas bills. The half-inch rule of thumb is only that, a rule of thumb. So select a pressure transmitter with a range greater than this. Lastly, you asked about positioning the relief damper It doesnt make any difference where its located as long as its upstream from all VAV boxes. In this role, he develops and teaches practical, real-world training focused on the HVAC and Home Performance industries. Now a nominal 2000 cfm capable duct system might present much lower low static pressure when driven by a 1200 cfm air handler. The diagram below shows the components of our system. To determine this measurement, youll have to know the friction loss rate first. document.getElementById( "ak_js_2" ).setAttribute( "value", ( new Date() ).getTime() ); By submitting this form, you are consenting to receive marketing emails from: Energy Vanguard, 533 West Howard Avenue, Suite E, Decatur GA, 30030, http://energyvanguard.com. Keep in mind that the filter resistance will increase over time as it collects dust. The table below provides some rules of thumb when estimating pressure gain at the . Ductwork is also important: the duct static pressure rule of thumb is to have the right size for the right air conditioning system; a one-ton air conditioning . Example For Duct Static Pressure Calculation Example For Duct Static Pressure Calculation Air Flow Air Systems Pressure and Fan Performance. Firstly, you may be aware what the total atmospheric pressure is at sea level (which is always higher than any point on earth except perhaps the dead sea in Israel). You also miss one crucial thing that HVAC professionals have years of experience. @Abiola, the pressure setting@Abiola, the pressure setting (or more typically the position of the counterweight on the relief damper) must be determined empirically by measuring the airflow at the AHU. It might be nice to think about making the duct system more appropriate, by reducing AC tonnage and therefore airflow. Multiply the rated Total External Static Pressure by the 25% rule of thumb. Finally, the air duct layout can be designed according to the space available. drill a hole through the plywood platform into the return plenum so we can We subtract them from the total external static pressure number (typically 0.5 iwc). 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