• Return Air Pathway Study

Scope

In effect since March 2003, Section 601.4 of the Florida Building Code applies to residential and commercial buildings having interior doors and one, centrally located return air intake per heating and cooling system.

Objective Of The New Florida HVAC Code Requirement

Reduce pressure difference in closed rooms with respect to (wrt) the space where the central return is located to 0.01” water column (wc) or 2.5 pascal (Pa) or less. Pressure imbalances created by restricted return air flow from rooms isolated from the central return by closed interior doors create uncontrolled air flow patterns.

Technical Background

Figure 80 Return Air Flow Test Chamber

Ideally, forced-air heating and cooling systems circulate an equal volume of return air and supply air through the conditioning system, keeping air pressure throughout the building neutral. Each conditioned space in the building should, ideally, be at neutral air pressure at all times.

When a space is under a positive air pressure, indoor air will be pushed outward in the walls, floor and ceiling. When a space is under a negative pressure, air will be pulled inward through the walls, floor and ceiling. Negative and positive air pressures in buildings result from uncontrolled air flow patterns.

Section 601.4 of the Florida Building Code specifically
deals with the uncontrolled air flow pattern when interior doors are closed thereby reducing return air flow from the closed room, while maintaining the same supply air flow to the room. This imbalance of supply and return air has been addressed conventionally by the common practice of undercutting interior doors to allow return air to flow from the room. This research quantifies the volume of air flow provided by this and other methods of return air egress from closed rooms.

Section 601.4 limits the air pressure imbalance in closed rooms to 0.01” wc or 2.5 pascals when compared to, or with respect to (wrt), the main body of the building where the return is located. With door undercuts, researchers have regularly observed room pressures with respect to the main body of the house (wrt mainbody) of +7 pascals (pa) or more. A room with this level of air pressure (+7pa, wrt mainbody) is trapping air, starving the heating/cooling system of return air. As the heating/cooling system struggles to pull in the designed amount of air, the resulting negative pressure pulls air into the main body of the building along the path(es) of least resistance. Usually this means that air is flowing through the walls, floor and ceiling from unconditioned spaces or outside environment to makeup for the trapped air in the closed room.

In the closed room, positive pressure builds up when return air is trapped. Conversely, the space with the central return gets depressurized because extra return air is being removed to make up for the air trapped in the closed room. More air is leaving the space (return air) than is entering the space (supply air). The positive pressure in the closed rooms pushes air into unconditioned spaces, such as the attic and wall cavities. The negative pressure in the main body of the building pulls air from unconditioned spaces. In Florida, the air brings heat and moisture with it that become an extra cooling load. This air is referred to as “mechanically induced infiltration” since the negative pressure drawing infiltration air in was created by the mechanical system.

Styles of Pressure Relief

Figure 81 Installing unbaffled return air flow through wall grille

Figure 82 Installing sound baffled return air flow through wall insert made by Tamarack.

When return air flow is restricted by closed doors, it creates pressure differences between parts of the building. This can be prevented by installing a fully ducted return system, by creating a passive return air pathway such as a louvered transoms, door undercut, “jump duct”, through-wall grilles, or a baffled through-wall grill.

A “jump duct” is simply a piece of flex duct attached to a ceiling register in the closed room and another ceiling register in the main body of the house. A jumper duct provides some noise control while providing a clear air flow path.

A through-wall grille is the simplest and least expensive approach to pressure relief for closed rooms. Holes opposite each other on either side of the wall within the same stud bay are covered with a return air grilles. The
downside of this approach is a severe compromise the privacy of the closed room. An improvement on this theme would be to locate one of the grilles high on the wall and the opposing opening low on the wall. Also, such openings in interior wall cavities introduce conditioned air into what is typically an unconditioned space possibly contributing to other building problems.

However, connecting the two openings with a sleeve of rigid ducting forms an enclosed air flow path that limits introduction of conditioned air into the wall cavity but doesn’t solve the visual and sound privacy issues. To address this problem, BAIHP Industry Partner Tamarack developed a sleeve with a baffle that can reduce the transfer of light and sound but still provide adequate air flow to minimize pressure differences. The product is called a Return Air Path (RAP).

To validate the effectiveness of this product and other approaches to providing return air pathways, Tamarack and BAIHP researchers devised a test apparatus and conducted experiments in FSEC’s Building Science Laboratory.

Testing Protocol

In May of 2003, a chamber was constructed at FSEC (Figure 80) that simulated a frame construction room with an 8 foot high ceiling. A “Minneapolis Duct Blaster” was connected to one end of the room with a flexible duct connection leading out of the room to provide control over pressure in test chamber.

In the middle of the chamber, on a stool, a radio was tuned “off station” to effectively create a standardized level of “white noise” at 57 dBA inside the chamber with the “door” closed. The temperature at the start of the tests was 80°F at 40%RH. A sound meter was located outside the chamber on a stand 4 feet above the floor and 20 inches from the middle of the chamber wall surface.

The sound level in the test facility outside the chamber with the “white noise” turned off was 36.4 dBA and with the “white noise” turned on was 41.5 dBA, an average, sampled over a 30 second period. A series of tests on 31 different set-ups were performed, measuring the flow at 3 different pressure levels and recording a 30 second sound sample with the “Duct Blaster” deactivated.

Tests were made for 6” and 8” jump ducts, five different sized wall openings (Figure 81) in different configurations including straight through with and without sleeves, straight through with sleeve and privacy baffle (Figure 82), and high/low offset using the wall cavity as a duct, and three different slots simulating three different size undercut doors.

Results

Table 51 summarizes the results of these tests arranged in ascending air flow order based on the results at 2.5 Pascals (0.01” wc), the maximum allowable pressure in a closed room under new requirement in Florida Building Code, Section 601.4.

Figure 83 Return air flow path provided by jumper duct

By comparing the air flow of the slots (door undercut) to the openings with grilles, the detrimental effect of the grille becomes clear. The ratio of air flow (cfm) to the surface area of the slot (in 2) is more than 2 to 1 (for example; 30 in 2 to 61 cfm), whereas with grilles in place the ratio of air flow to area averages 0.83 to 1 (for example; 72 in 2 to 60 cfm). Similarly, the jump duct (Figure 83) assemblies’ air flow to area ratios average 1.19 to 1. In any calculation for the size of the through wall assembly, the resistance of the grille becomes the critical factor in determining the size of the opening for achieving the desired flow.

The following formulas account for the grille resistance and maybe used to size return air path openings.

• Door undercuts: Area Sq. In. = CFM/2
• Wall opening with grilles: Area Sq. In. = CFM/.83
• Flexible jumper duct with grilles: Diameter = ÖCFM

Although there does not appear to be significant flow improvement when a sleeve is used, such an assembly will reduce the possibility of inadvertent air flow from the wall cavity itself.

The high/low grilles using the wall cavity reach maximum flow at 72 cfm because of the dimensional limitations of the wall cavity itself. Increasing the opening of each grille beyond 112 square inches does not significantly increase the flow of air through the wall cavity.

The accompanying bar chart (Figure 84) can be used to select the best method at various air flows while maintaining the room-to-building pressure difference at .01” wc. The strategies are ranked by air flow allowance (cfm) on equivalent to supply air delivered to the room. For example, an 8” jumper duct could be used to maintain 0.01 wc in rooms with supply air up to 60 cfm. Note that these transfer methods are additive so that, for example, combining a 6” transfer duct with a 1” undercut a 30” door, will provide a flow of 95 cfm to be delivered at .01” wc (Figure 85) or combining a R.A.P. 12.12 with a 1” undercut would allow up to 175 cfm to be delivered (Figure 86). It should be noted that door undercuts are under builder not HVAC control and that the actual dimensions are greatly affected by the thickness of the floor coverings.

Summary

Ideally buildings with forced air heating/cooling systems are pressure neutral. The same amount of air is removed from the building (and each room) as is supplied to it. However, this balance can be disturbed in homes that have one, centrally located return intake when interior doors are closed, blocking return of air supplied to private rooms. Other factors outside the scope of this study may also result in household pressure imbalances.

These research results are relevant to homes with forced air heating and cooling systems having a single, centrally located return air inlet with no engineered path for return air to exit closed rooms. Such systems pull return air from the whole house as long as interior doors are open. When an interior door is closed, more air is supplied to the closed room than can be removed, or returned, from the room.

Positive pressure builds up in the closed room while a negative pressure occurs in the connected spaces. Positive pressure presses outward on all surfaces and may eventually reduce supply air flow into the closed room and while pushing conditioned air through small breaks in the room’s air barrier.

To overcome house pressure imbalances caused by door closure, a variety of passive return path strategies are studied including a product produced by BAIHP Industry Partner Tamarack that overcomes privacy issues associated with through-wall grills. Achievable air flows for jump ducts, through-wall grilles, sleeved through-wall grilles, and the Tamarack baffled through-wall grille are presented.

Figure 84 Maximum air flow achievable using various return air paths from closed rooms for a give supply at a room pressure of 2.5 pa or 0.1” wc with respect to the return zone. For example, an 8” jumper duct could be used to maintain 0.01 wc in rooms with supply air up to 60 cfm.

• Heat Pump Water Heater Evaluation

Figure 87 Airflow measurements using a Duct tester on heat pump cold air discharge side

BAIHP researcher tested the efficiency of a heat pump water heater manufactured by EMI, a division of ECR International. The unit features a compressor (R-134A refrigerant) with a wrap-around heat exchanger mounted on top of a 50-gallon storage tank. The latest controller board model #AK 4001 was installed during the test.

The temperature regulation of the unit is achieved by an adjustable potentiometer which sets a resistance that is measured by the controller board and translated into the corresponding temperatures. The set temperature is stored in the controller’s memory.

The controller logic is designed to operate
the heat pump when the temperature in the bottom of the tank drops below the effective dead band temperature of 30°F (20°F deadband + assumed stratification of 10°F). The heat pump shuts off when the temperature in the bottom of the tank has reached 10°F below the set point temperature. The upper element of the tank operates only when the temperature in the upper tank reaches 27°F below the set point temperature.

During laboratory testing the controller’s performance was evaluated by measuring inlet and outlet water temperatures using thermocouples mounted to the copper inlet and outlet pipes as well as a Fluke hand-held thermometer inserted into the hot water outlet stream. One minute average measurements during draws were in agreement with the 10°F stratification logic utilized by EMI.

Also, following a series of hot water draws during the efficiency test (described below), the compressed refrigerant heat was able to replenish the tank to the 130 °F temperature level. However, following the heating recovery, neither compressor or resistance element were activated during standby until three days later when bottom tank temperatures dropped below 95°F. The compressor was called into operation when the tank was submitted to a hot water draw which triggered the ON compressor event in less than a minute.

Table 52 is a summary of electrical efficiency results generated from three tests performed in the laboratory. Tank pre-heating for test #1 and #2 were performed in a similar way, by forcing the compressor to turn “ON”. The tank was allowed to loose heat on standby (1-2 days) and then purged with a draw of at least 30 gallons of new water. The purge forced the compressor to operate. Preheating for the test #3 was performed with the tank relatively hot and only twelve gallons of hot water were purged. This might explain the higher outlet temperatures read during test 3. For all three tests, we attempted to heat water so that initial hot water draws were near 130 °F (+/- 5 °F). However, we noticed that temperatures at the top of the tank (upper level) increased slightly with each purge (i.e., 10.7 gallon draw). During the third test shown in Table 52 for example, outlet temperatures during the first draw averaged 129.2 °F, but during the last draw temperatures reached an average of 143.4 °F. The values shown for test #3 shows an overall hot water delivery temperature (T outlet) of 136.6 °F. The controller never called for compressor or auxiliary energy when left on standby during the completion of the test (24-hr.).

Conclusions

The WattSaver™ heat pump water heater is rated with an energy factor (EF) of 2.45 and clearly demonstrates that heating water can be accomplished at a relative higher efficiency when compared to conventional electric water heaters. Installed in a conditioned space, and under operation with inlet water temperatures above 80 °F (e.g., Central Florida summer water mains temperatures), an average electrical (COP) efficiency of 2.0 was attained. Other measurements and performance indicators are summarized in Table 53.

Two caveats to the heat pump water heater’s performance was first the delayed recovery during standby which would present larger hot water temperature variation to the residential user. This also leads to diminished hot water capacity during long periods of no hot water use activity. Second, because the compressor’s discharge refrigerant (i.e., hottest temperatures) enter the wrap-around heat exchanger at the top of the tank, the unit demonstrated larger hot temperature variations at the tank’s upper levels when the top portion was already pre-heated. These stratified tank temperature levels differ from those obtained when heating is started with the tank filled up with mains (colder) water conditions.

• NightCool - Building Integrated Cooling System

Technical Background

Using a building’s roof to take advantage of long-wave radiation to the night sky has been long identified as a potentially productive means to reduce space cooling in buildings. This is because a typical roof at 75 F will radiate at about 55-60 W/m2 to clear night sky and about 25 W/m2 to a cloudy sky. For a typical roof (250 square meters), this represents a cooling potential of 6,000 - 14,000 Watts or about 1.5 - 4.0 tons of cooling potential each summer night. Various physical characteristics (differential approach temperature, fan power, convection and conductance) limit what can be actually achieved, however, so that perhaps half of this rate of cooling can be practically obtained. Even so, careful examination of vapor compression space cooling in many homes in Florida shows that typical homes experience cooling loads averaging 33 kWh per day from June - September with roughly 9.2 kWh (28%) of this air conditioning coming between the hours of 9 PM and 7 AM when night sky radiation could greatly reduce space cooling.

The big problem with night sky radiation cooling concepts has been that they have typically required exotic building configurations. The research literature is extensive. These have included very expensive “roof ponds” or, at the very least, movable roof insulation with massive roofs so that heat is not gained during daytime hours. The key element of this configuration is that rather than using movable insulation with a massive roof or roof ponds, the insulation is installed conventionally on the ceiling. The operation of the system is detailed in the attached schematic.

During the day, the building is de-coupled from the roof and heat gain to the attic space is minimized by the white reflective metal roof. During this time the space is conventionally cooled with a small air conditioner. However, at night as the interior surface of the metal roof in the attic space falls two degrees below the desired interior thermostat setpoint, the return air for the air conditioner is channeled through the attic space by way of electrically controlled louvers with the variable speed fan set to low. The warm air from the interior then goes to the attic and warms the interior side of the metal roof which then radiates the heat away to the night sky. As increased cooling is required, the air handler fan speed is increased. If the interior air temperature does not cool sufficiently or the relative humidity is not kept within bounds (<55% RH) the compressor is energized to supplement the sky radiation cooling. However, by midnight on clear nights, the temperature of the metal will have dropped sufficiently to begin to dehumidify the air introduced to the attic. The collected moisture on the underside of the roof will then drain to collection points at either side of the soffits so that the home can be dehumidified during evening hours by way of only the operation of the blower fan (200-300 W). The massive construction of the home interior (tile floor and concrete interior walls) will store sensible cooling to reduce space conditioning needs during the following day.

Experimental Design

BAIHP researcher Danny Parker developed an experiment to test the viability of NightCooling in Florida’s hot-humid climate. However, construction of a suitable laboratory facility to conduct this study has been delayed. BAIHP is working with UCF and local officials to develop a design allowable under current codes for a pair of free standing, room size structures to serve as a “control” and a “test” case. A schematic of the test case and a similar drawing of the concept in a real home are shown in Figures 88 and 89.

Figure 88 -Scehmatic design for NightCool test facility.

Figure 89 Schematic of NightCool concept in typical residential building.