HVAC SYSTEM PRESSURE RELIEF Correcting Pressure Imbalances in Your HVAC System Can Result in a Healthier, More Efficient Home

Home , Grille

HVAC SYSTEM PRESSURE RELIEF Correcting pressure imbalances in your HVAC system can result in a healthier, more efficient home.

BY PAUL H. RAYMER AND NEIL MOYER grows mold, which may not be HVAC noticed for a long time. The Florida Solar Energy Cen- ithout central ter (FSEC) and my company, air condi- Tamarack Technologies, Incorpo- Wtioning, the rated, decided to test a variety of South wouldn’t be what it is pressure relief solutions to see today. Central air conditioning which worked best to solve these has made living in the South pressure problems. year-round a real pleasure, but it has also created its own set of Equalizing problems—including the subtle Circulation but critical problem of pressures that differ from room to room. Ideally, forced-air heating and To keep the installed costs of cooling systems circulate an equal air conditioning down, it became volume of return air and supply common practice to put supplies air through the conditioning sys- into each room and use a central tem, keeping air pressure in the return, eliminating individual house neutral. Each conditioned return runs. Rooms can serve as space in the building should, ide- ducts as long as all the doors in ally,be at neutral air pressure at all the house stay open. As soon as times.When the building is under doors, working as dampers, start a positive air pressure, indoor air to close, the system changes. PAUL RAYMER will be pushed outward to uncon- Uneven pressures are created, and ditioned spaces and beyond to system performance and comfort outside. When the building is are compromised causing the under a negative pressure, outside occupants to compensate by run- air will be pulled inward, toward ning the system longer or at a Neil Moyer installs a privacy insert, which enhances privacy and into conditioned spaces. lower set point. between rooms while correcting pressure imbalances. Pressure imbalances are also cre- Complicating this problem is the ated when interior doors are closed fact that some rooms are pressurized and small, constant drip, they can cause seri- in buildings with heating and cooling sys- some rooms are depressurized. Air will ous damage over time. tems that have only a central return air seek to leak out of a pressurized room When a home’s walls have been intake. Positive pressure in a closed room and leak into a depressurized room. chilled by the air conditioning, the results when return air ﬂow does not Even though air has always leaked into warm, moist outside air that leaks into equal supply air ﬂow.Conversely,negative and out of houses, this poses a greater the rooms under negative pressure slith- pressure results when air leaving the space problem now because of air condition- ers its way down from the attic or from (return air) exceeds air entering the space ing. The leaks may be very small and the the outside through the wall system (supply air).The resultant positive pressure pressures tiny,but even a slow leak over until it strikes something that is below in a closed room pushes air into uncondi- the long term can cause serious damage. the dew point, where it gives up its tioned spaces, such as the attic and the The pressures are as small as a couple of moisture. This commonly happens interior and exterior walls.The negative carbonated bubbles popping out of a soft behind vinyl wallpaper, which acts as a pressure in the main body of the building drink. But in a house, those tiny pressure vapor barrier that moisture can’t get pulls air from unconditioned spaces into differences just don’t go away. Like a through. So instead, the moisture slowly conditioned spaces. This is known as

42 www.homeenergy.org JULY/AUGUST 2006 • HOME ENERGY “mechanically induced infiltration,”since imbalance, opening a hole to the wall Adding a very short piece of the negative pressure is created by the cavity invites unwanted air flow into rigid duct to the assembly pro- mechanical system. the wall cavity itself, which may be vides a sleeve that effectively If the system is balanced, there will connected to other spaces. restricts the passage of air to be no pressure variations. This can moving from one side of the be accomplished by installing dedi- wall to the other. This cated returns; by the use of tran- Table 1. Test Results for Pressure Relief reduces the unwanted flow soms; by undercutting the doors by Devices (at 2.5 Pa pressure difference) problem, but it doesn’t help approximately 3 inches; or by using Dimension Area much from the point of one of a number of other alterna- CFM (in) (in2) Type* view of privacy. Adding a tives. These include installing a 36 6 in dia. 28 J baffle (such as the R.A.P.) to jumper duct (a piece of duct that 41 4 x 12 48 O the sleeve can reduce the “jumps” over the partition); wall- 42 4 x 12 48 TW, S, RAP transfer of light and sound HVAC to-wall grilles; or a baffled return air 45 4 x 12 48 TW and, if it is properly designed, pathway (R.A.P.).The R.A.P. is a 46 4 x 12 48 TW, S will have little effect on the passive pressure-balancing system 49 8 x 8 64 O movement of air. for use with ventilation or forced- 52 12 x 6 72 O Another approach is to air heating or cooling systems, 56 12 x 6 72 TW, S, RAP offset the holes on either side where it is often impossible to pro- 57 8 x 8 64 TW of the wall, cutting one high vide both supply and return ducts 58 8 x 8 64 TW, S, RAP and one low. Like the to every room. 59 8 x 8 64 TW, S jumper duct or the assembly with the 60 12 x 6 72 TW 60 12 x 6 72 TW, S light and sound baffle, this arrange- Styles of 61 1 x 30 30 U ment can enhance the privacy Pressure Relief 62 8 in dia. 50 J between the rooms. But the passage 65 1 x 32 32 U of air is limited to the dimensions of A jumper duct is created when a 67 8 x 8 64 O the wall cavity and, like the simple grille and collector box are installed 70 8 x 14 112 O hole approach, a potential path is cre- in the ceiling on each side of the 72 12 x 12 144 O ated for unwanted air flow into the wall, and they are connected by a 73 1 x 36 36 U wall cavity. short section of ductwork that 101 8 x 14 112 TW, S, RAP “jumps” over the top of the partition. 107 8 x 14 112 TW Testing The duct commonly has a 6-inch, 8- 110 8 x 14 112 TW, S inch, or 10-inch diameter. The 119 12 x 12 144 TW FSEC constructed a chamber grilles are normally standard return 120 12 x 12 144 TW, S that imitated the conditions of a air grilles. A jumper duct limits the 120 12 x 12 144 TW, S, RAP room with an 8-ft high ceiling in physical connection between the * J—Jumper duct S—Sleeve order to test the different arrange- O—High/low offset RAP—Baffled return air pathway rooms, providing a reasonable TW—Through-the-wall U—Door undercut ments for pressure relief; testing amount of privacy. But the common began in May 2003. A Duct Blaster use of flexible ducting causes a sub- was connected to one end of the stantial amount of back room with a flexi- pressure, limiting the ble duct connec- amount of air that can Maximum CFM for Pressure Relief Devices tion leading out of be supplied to the (at 2.5 Pa pressure difference) the room. room. Tests were run 1 in x 32 in (U) The simplest 1 in x 30 in (U) for the 6-inch and 12 in x 4 in (O) approach to pressure 8 in x 8 in (O) 8-inch jumper 12 in x 12 in (O) relief is to cut oppos- 12 in x 4 in (TW, S, RAP) ducts; four different 10 in x 6 in (TW, S, RAP) ing holes on either 8 in x 8 in (TW, S, RAP) configurations with side of the wall and 12 in x 6 in (TW, S, RAP) various sizes of wall 14 in x 8 in (TW, S, RAP) cover the openings 12 in x 12 in (TW, S, RAP) openings (straight 14 in x 8 in (TW) with a return air 12 in x 12 in (TW) through with and 6 in (J) grille. This is the least 8 in (J) without sleeves, expensive approach, 020406080 100 120 140 straight through but the opening pro- Legend: CFM with sleeve and vides hardly more * J—Jumper duct S—Sleeve privacy insert, and privacy than a hole in O—High/low offset RAP—Baffled return air pathway high/low offset TW—Through-the-wall U—Door undercut the wall. At the same using the wall cav- time, if there contin- ity as a duct); and ues to be a pressure Figure 1. Using a sleeve assembly will reduce the possibility of inadvertent air flow into the wall cavity. three different slots

HOME ENERGY • JULY/AUGUST 2006 www.homeenergy.org 43 simulating three different-sized reduce the possibility of inadvertent air Since these transfer methods are addi- undercut doors (see Table 1 and flow to and from the wall cavity itself. tive, combining a 6-inch jumper duct Figure 1).The results in Table 1 The high/low grille assembly using the with a 1-inch crack under a 30-inch door are arranged in ascending maxi- wall cavity as a duct reaches its maximum will allow a flow of 95 CFM to be deliv- mum air flow needed to maintain flow at 72 CFM flow because it is lim- ered at 2.5 Pa, or combining a 12 inch x 1 the pressure differential at 2.5 Pa ited by the cavity itself. Assuming a 3 /2 12 inch R.A.P. with a 1-inch undercut 1 (0.01 inches WC). Figure 1 gives inch x 14 /2 inch wall cavity,increasing would allow up to 181 CFM to be deliv- the same information in a ered. It should be noted that different format. door undercuts are under When the perfor- builder, not HVAC, control, mance of the slots under and that the actual dimen- the door is compared to sions of the cut are greatly the performance of the affected by the thickness of openings with grilles, the floor coverings. the deleterious effect of If the designed flow of air

HVAC the grille becomes clear. to the room is unknown, an The ratio of the CFM approximation can be made. ﬂow necessary to main- If the grilles are rectangular tain a pressure difference or square, the CFM delivered of 2.5 Pa and the area of the to the room will be approxi- opening of the slot is more mately twice the area of the

than 2 to 1 (61 CFM through PAUL RAYMER grilles. (A 4 inch x 12 inch 30 in2, for example), whereas grille, for example, is likely to with the openings with grilles be designed to deliver about it averages 0.83 to 1 (60 CFM 100 CFM.) If the grille is through 72 in2, for example). round, square the diameter The jumper duct assemblies Pressure measurements were made using an inexpensive digital manometer. and double it. (An 8-inch average 1.19 to 1. round register, for exam- In any calculation for ple, is likely to have been Sound Test the size of the through- 43.5 designed to deliver the-wall assembly, the approximately 130 CFM.) resistance of the grille 43 becomes the critical factor 42.5 No Opening Sound Solutions in determining the size of High/low offset the opening needed to A 42 Through-the-wall For sound testing, we optimize the flow. If a dB Through-the-wall tuned a radio inside the through-the-wall opening 41.5 sleeve test chamber to effectively Through-the-wall, is to be used, to be sure 41 sleeve, R.A.P create a standardized level that the opening is ade- PAUL RAYMER of white noise of 57 dBA quate to maintain no 40.5 with the “door”closed. A more than a 2.5 Pa pres- 13579 11 13 15 17 19 21 23 25 27 29 31 sound meter was located sure difference, the open- Reading # outside the chamber on a ing should be equal to or stand 4 ft above the floor greater than the total air Figure 2. This graph shows the comparative sound attenuating performance of four pres- and 20 inches from the flow delivered divided by sure approaches to a baseline noise level with no openings in the walls of the test room. middle of the chamber 0.83. (The following are wall surface. meant as useful rules of thumb.) the opening of each grille beyond 112 Interior wall structures and hollow- in2 does not significantly increase the core doors are marginally effective in Wall opening with grilles: CFM = flow of air through the assembly. masking average sound levels in a room. 0.83 x area (in2) The best method to use at various Consequently,none of these approaches Slot (no grilles): CFM = 2 x area air flows can be determined by calcu- could dramatically reduce the sound (in2) lating various air flows while maintain- through the walls of the room. For Flexible jumper duct with grilles: ing the pressure difference at 2.5 Pa. example, the 30-second average white- CFM = diameter2 Knowing how much air is delivered to noise sound through the walls of the the room tells us which method would room with no opening was 41.5 dBA. Although there appears to be no sig- be most suitable. For example, an 8- The 30-second average white noise nificant improvement in flow when a inch jumper duct could be used at air sound through an 8 inch x 14 inch sleeve is used, a sleeve assembly will flows up to 60 CFM. opening with a wall sleeve, grilles, and a

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The Florida Neil Moyer installs the grille to test its performance. code specifies a pressure difference sound-attenuating (baffled) insert was of less than 2.5 Pa. There are many 41.9 dBA (see Figure 2). devices currently available (such as Overall, slots under the door proved Bachrach draft-rite, Magnehelic, and to be the worst for sound transfer and Dwyer 460 Air Meter) that measure pres- the through-the-wall installations with a sure differences at this level, and several of privacy insert the best—better than the them can be purchased for less than $20. grilles offset in the wall (wall cavity) or Equipped with such basic gear, an inspec- the jumper duct approach. tor should be able to determine if an Privacy is also affected by light trans- acceptable level of pressure relief has been fer, even just enough transfer to indicate achieved. whether the light in the room is on or off. The wall cavity, jumper duct, and Paul Raymer is the president of Tamarack through-the-wall with baffled insert Technologies, Incorporated, which is a principal offer the most effective approaches to team member of DOE’s Building America light attenuation. Again, the slot under Program and a member of the Home the door is the worst. Ventilating Institute (HVI).The R.A.P.is a product manufactured by Tamarack Simple Solutions Technologies, Incorporated. Although the problem may seem Neil Moyer is a principal research engineer at complex, these pressure relief solutions the Florida Solar Energy Center (FSEC). are reasonably simple. And once the FSEC is part of the University of Central correct solution is installed, the HVAC Florida and the leader of the Building system will perform better, the occu- America Industrialized Housing Partnership. pants will be more comfortable, and the risk of mold or deterioration in the walls of the house will be reduced. It’s a FOR MORE INFORMATION: small cost for a lot of benefit. Both occupant comfort and building To learn more about the R.A.P. go to durability should be considered in deter- www.tamtech.com. mining the best approach. The simplest approach—cutting a hole and installing To learn more about FSEC and its grilles on both sides of the wall—may buildings research, go to result in occupant complaints and long- www.fsec.ucf.edu/bldg/BAIHP/ term building deterioration problems. index.htm.

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