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

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