Forced-Air Systems
Need Pressure Relief

The Residential Construction Demonstration Project, Cycle IV (RCDP IV), aimed to find ways to improve the operation of forced air distribution systems (ADS) and to get a sense of the costs. The study followed construction of 41 new homes and 25 retrofits in Idaho, Montana, Oregon and Washington. It involved 17 builders and 15 heating contractors. (For more on this study, see December 1995.)

One of the study's key findings concerned pressure balancing. The study found that many builders--and even some HVAC contractors--don't understand how air needs to flow through a home in order for an ADS to work right. Even when the duct system is well-designed and properly sealed, the house itself may contain structural flaws that restrict airflow. Airflow problems made the systems less efficient, while causing a number of air quality problems.

To understand the importance of good airflow, you need a firm grasp of how an ADS works. The ideal system is a closed loop. A furnace blower delivers air to the living space through a network of supply ducts, then draws it back through one or more return ducts. The amount of air that passes through supply registers should equal the amount that flows back into the duct system through return grilles.

Supply ducts tend to pressurize the rooms where they're located, while returns put the spaces around them under a slight suction. It's this pressure difference that circulates air through the building. The pressure difference between rooms should not exceed 2 or 3 pascals with the doors closed and the furnace blower running. In many homes, however, closing interior doors blocks the normal airflow from the supplies to the returns. According to Ted Haskell, an Oregon State University Extension Service energy agent who helped administer the project, it's not unusual to see positive pressure of 10 to 15 pascals around supply ducts in newer houses. Such excessive pressures can double or triple air leakage into wall and ceiling assemblies. Not only does this increase heat loss, but moisture riding with the escaping air can condense on cold framing and sheathing, wetting the insulation and breeding mold, mildew and rot.

Restricting airflow also starves the returns. This increases the suction in the spaces around the returns and forces them to draw air from other sources. The suction can pull soil gases, moisture and sewer gases into the home. It can also cause chimneys and combustion appliances to spill carbon monoxide and other harmful flue gases.

To avoid these problems make sure there is a direct airflow path from the supply ducts to the returns, even when the home's interior doors are closed. You can create an airflow path by running active return ducts to each room where there's a supply, or by cutting through walls that separate supplies from returns. The size of the hole you need in each room depends on its air supply. The rule of thumb developed for this project was 1 sq. in. of wall opening per cfm delivered to the room. The average bedroom will require 70 cfm, but ask the heating contractor to be sure. (If the heating system was correctly designed and sized, the numbers should be available.)

Moving air

RCDP IV participants tested the following pressure relief techniques and found them to be effective. Each technique has its own advantages and disadvantages. The technique you choose for a given job will depend on the needs of the client, the house design and the budget.

Even with an active return, you may need quite a big duct to get the required airflow. Luckily, you make your pressure relief techniques work as a team by using a combination of methods. For instance, you could get the required airflow with a small active return and a 1/2-in. door undercut in the same room.

Cost

According to Haskell, the average cost of pressure relief for new homes in the RCDP IV study was $403. However, the average was skewed by contractors using the expensive active returns. If you leave those out, then the average cost per house drops to $207. "The point," he says, "is that there are inexpensive ways of doing pressure balancing that will get you in the ballpark of $200."

Undercut door

Cut through the door or wall

These are the simplest pressure relief methods. In the first case, the door bottom has been cut higher off of the floor than normal. Unfortunately, you might need to undercut a door as much as 2 in. to get 70 sq. in. You can also put a transfer grille in the door, install a transfer grille next to or above the door, or put an open transom above it.


Transfer grille in door

Transom or transfer grille
above door

It is not surprising that many people find a 2-in. door undercut visually unacceptable. And passive grilles and louvers create a privacy issue, because they let noise flow directly to the bedroom from the home's common areas. The noise could eventually spell doom for such techniques. If the people living in the home get too uncomfortable, they might just cover up the grilles.

Send air through the closets.

If the house design permits, you can send return air through a closet or closets to an adjoining hall. For this technique to work, the closets must have louvered doors. In the drawing on the right, air is routed through a wall grille cut into the back of the hallway closet. It's a step up from a door grille, but some people might not find it quiet enough. In the two drawings to the left, the bedroom has been further isolated from the common areas without sacrificing air flow. You can carry this technique as far as the house design and the needs of the client dictate.

If the house design permits, you can send return air through a closet or closets to an adjoining hall. For this technique to work, the closets must have louvered doors. In the drawing on the right, air is routed through a wall grille cut into the back of the hallway closet. It's a step up from a door grille, but some people might not find it quiet enough. In the two drawings below, the bedroom has been further isolated from the common areas without sacrificing air flow. You can carry this technique as far as the house design and the needs of the client dictate.

Install passive ducts

If you can't use closets to move air, you can run a duct from the bedroom to the common area. A duct can cost more than a simple opening, but it can also accommodate any design and solve the privacy problem. The design of passive ducts is less critical than that of the ductwork in the forced air distribution system. Because air moves slower through these than through heating ducts, there's little worry about turbulence and friction losses.

Install active returns

The most effective--and most expensive--solution requires a ducted return to the furnace from each room with a supply duct. In the RCDP IV project they called these "active" returns. To reduce costs, you could install an active return only in the master bedroom, where you're most concerned about privacy, then use passive methods in other rooms.

Keep Ducts Out of the Bathroom

Most builders wouldn't install an active return in a bathroom, because it would pull odors from the bathroom, then redistribute them to the rest of the house.

To avoid the odor problem, consider running neither a supply duct nor a return to the bathroom. Instead, install an electric resistance heater and an exhaust fan that's connected to a dehumidistat, a timer or the light switch. The fan will draw heat from the hallway, while venting odors directly outside. When needed, the electric heater can take a chill out of the bathroom.

A fan-forced heater can be mounted in a wall or in the toe-kick area of the cabinet. A radiant electric panel could be attached to the ceiling or a radiant mat embedded in the floor. Electric resistance heaters might be more expensive to run than a heat pump, but as supplemental heat, they should be needed for only a couple of hours per day.

ęCopyright 1996 Iris Communications, Inc.


Source: Oikos

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