This article was originally published in the May/June 1993 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online May/June 1993
Integrated Heating and Ventilation: Double Duty for Ducts
by Mark A. Jackson
Northwest building codes require mechanical ventilation in new homes. Combining heating and ventilation can fit the bill if the builder considers the whole system carefully.
Building codes in Washington state and Bonneville Power Administration's specifications for new residential construction require mechanical ventilation systems. Since the majority of new houses built in the Northwest have forced-air heating systems, using the heating system ducts to supply fresh air as part of the ventilation system has become a popular option for home builders.
When properly designed, installed, tested, and operated, integrated heating and ventilating systems can be effective, and relatively inexpensive to install. However, there are inherent inefficiencies in these systems, and the overall cost for ventilation using an integrated system is higher than for any other ventilation system commonly used. Worse, poorly designed systems can function like a rather large uncontrolled leak in the heating system, with unacceptable energy consequences and little to offer in the way of effective ventilation.
The Basic System
Until recently, the typical approach to integrate the heating and ventilating systems required that mechanical components plus an electric control system be fabricated on site. In the last year or so, several manufacturers have responded to the need for an all-in-one mechanical and electrical control unit. There are still many possible variations using a diverse array of air inlet and exhaust strategies, but the essential elements are pretty straightforward (see Figure 1):
* A fresh air duct which brings outside air to the return side of the air handler of a forced air heating system (gas, electric resistance, or heat pump). The fresh air duct must be tight and well-sealed to the return plenum to prevent the introduction of unwanted combustion gasses from combustion appliances.
* A balancing damper or other device that limits the flow rate of fresh air into the system. The volume of fresh air required has to be calculated, and the damper needs to be set using an accurate air flow measuring instrument.
* An electrically operated damper controls the flow of air into the system to prevent fresh air flow when it is not needed or wanted.
* A clock timer is used to periodically cycle the system. A bypass or twist timer allows manual control to provide additional ventilation as needed. These controls open the electrically operated damper, and turn on the air handler and exhaust fan.
* An exhaust fan removes stale air from the house. A centrally located fan dedicated to the system or one of the bathroom fans can be used.
* The air handler blower provides the fan power to pull fresh air from outside into the system and to distribute the fresh air through the supply ducts. A two-speed blower is preferrable, so that the system can operate at low speed during ventilation mode. The fan-motor sets in residential forced air heating systems are typically inefficient, requiring 400-600W for shaded pole blower motors. Better motors are available, one of which is the permanent-split capacitor motor. New adjustable speed drives may be somewhat more efficient at low speeds, but these are not yet common in residential applications.
One of the first questions that arose when integrated systems became popular was: What kind of damper do houses need to control fresh air entry into the system--are barometric dampers acceptable, or should electrically activated dampers be required? To answer this question, several types of dampers were evaluated in 25 single-family homes as part of the Residential Standards Demonstration Project, sponsored by the Bonneville Power Administration. In addition to assessing the applicability of various dampers, we tested duct leakage, envelope tightness, and exhaust fan flows.
We found that not only is the type of damper important (see Table 1), but other aspects of the envelope, ducts, and heating and exhaust system have to be carefully addressed for an integrated system to work as planned, in particular:
* Tightness of the building envelope
* Pressure differentials within the house
* Leakage of ducts outside the heated space
* Control of and quantity of fresh air supply
* Exhaust fan flow rate
Tightness of Building Envelope
The number of air changes per hour (ACH) when a home is depressurized to 50 Pascals (50 Pa) is a common comparison for the relative tightness of houses. I would consider a house leaky if a blower door test showed more than 6 ACH at 50 Pa. Conversely, I would consider a house to be reasonably tight if it tested less than 3 ACH at 50 Pa. In a leaky house, there is no control over where or when ventilation, or more properly outside air infiltration, will occur. Keeping the envelope tight permits more control and predictability for ventilation systems in general. In the Northwest, we find that tightness is well correlated with climate. Houses built in colder areas are significantly tighter than houses built in milder areas.
Pressure differences occur naturally between crawl space and main floor, and between ceiling and attic. They are caused by temperature differences (stack pressure) and wind effects. These pressure differences drive natural infiltration through the leaks in the house envelope. Over time, stack pressures seem to be dominant and are usually 1-2 Pa between floor and ceiling in single-story houses, relative to outside ambient pressure. In multi-story houses, stack pressures can become quite large between the lowest and highest points of the structure, relative to outside ambient pressure. We have measured stack pressures as high as 11 Pa between the ground floor and the upper story of three-story multifamily structures.
Closed doors between supply and return ducts can generate large pressures. In some houses, we noted 12 Pa across closed bedroom doors. In leaky houses, this overpressurization causes increased exfiltration through the building envelope, and increases leakage from supply ducts for both heating and ventilating. And overall air flow into rooms with large pressure differentials is reduced.
Creating pathways to allow air to flow between supply ducts and return ducts when closed doors separate them may reduce pressure differentials. BPA is evaluating such passive pressure reduction measures as an 8 in. duct connecting registers in the ceilings between rooms with supplies and rooms with returns, using offset registers placed in the walls between rooms, and transoms over doors.
Ducts and air handlers of forced-air heating systems are typically exposed to large positive pressures in the supply ducts and negative pressures in the return ducts and plenum. Small leakage areas translate to large leakage volumes under pressure. Unintentional duct leakage carries a thermal penalty, and can promote indoor air quality problems if leaks are located in return plenums and air handlers are located in garages and crawl spaces.
We measured duct leakage to the outside of the air barrier by using a blower door and depressurizing the houses by 50 Pa. Flows from duct leaks were measured using a flow hood. Most of the houses we tested had significant duct leakage, in most cases exceeding the amount of fresh air intentionally supplied to the return plenum.
For an integrated system to work well, the ducts have to be within the air barrier of the home so that leaks have no thermal impact--or the ducts have to be very tight. Any house with over 30 cfm of duct leakage at 50 Pa is not a viable candidate for an integrated system.
Fresh Air Supply
Fresh air inlet flow rates in the houses we tested were 0-187 cfm. The negative static pressure, or suction, available in the return plenum to pull fresh air into the system can turn out to be very low, especially when the supply uses lots of flex duct. Keeping the outside fresh air supply duct short, making it at least 6 in. in diameter, building it out of smooth sheet metal, and tying it at or close to the air handler fan helps to ensure adequate fresh air flow. To measure flow, use a micro manometer with a pitot tube array, a pitot tube traverse, or use a hot wire anemometer. A flow hood can also be used.
A balancing damper is used to restrict air flow to a pre-determined amount, or if sufficient pressure is available, a constant air flow regulator, available from American ALDES (4539 Northgate Ct. Sarasota, FL 34234-2124. Tel:(813)351-3441; Fax:(813)351-3442), can be installed. A reasonable flow rate would be 30 cfm for the first bedroom and 15 cfm for each additional bedroom, or alternately 0.35 ACH (according to ASHRAE standard 62-1989, for Ventilation for Acceptable Indoor Air Quality ).
Dampers to Prevent Unwanted Air Flow
We evaluated several types of dampers, including barometric dampers and electrically operated or automatic dampers. To prevent unwanted or excess ventilation, an electrically operated damper is the best choice, installed in the fresh air duct. A balancing damper or air flow regulator must also be installed to control the volume of air drawn into the system.
If a simple balancing damper is used to control the flow rate of air into the system, but an electrically activated damper is not installed to control the duration of fresh air supply, the amount of ventilation will be in direct proportion to the operating time of the heating system. During the winter, when infiltration is at its greatest and the need for additional ventilation may be small, the system will have a large and probably excessive ventilation rate, and the energy costs can be large. During the spring and fall, when temperatures are mild and the driving forces for infiltration are low, the system may not provide adequate ventilation. It is essential that an electrically activated damper be used to provide uniform and adequate ventilation and minimize the energy penalties.
Exhaust Fan Flow Rate
To maintain balanced air flow, the exhaust flow rate should be equal to the fresh air supply flow rate. For instance, a three bedroom house with 60 cfm of fresh air supply, would need an exhaust fan capable of removing 60 cfm. Typically, fan flow ratings are measured at a pressure lower than that found in the exhaust duct run in most houses. To compensate for duct pressure loss, a fan rated at 80 cfm or greater is recommended. Installing the fan remotely, or use a quiet fan (1.5 sone, a measure of noise) to keep the noise to a minimum. Excessive fan noise is one of the reasons occupants disable ventilation systems.
Controlling the Integrated System
New control systems have greatly simplified the installation and operation of integrated systems. In the past, installers had to cobble together time clocks, transformers and relays to allow the pieces of the system to work together .
Probably the most sophisticated and complete pre-packaged control systems are manufactured by the Duro Dyne Corp. (130 Route 110, Farmingdale, NY 11735. Tel: (800)899-3876; Fax: (516)249-8346). For about $120, this company can supply a package which has an Air Quality Control Center (AQC) and a fresh air intake damper. The AQC is an Underwriters Laboratory-listed device that contains all the electrical components needed to integrate the systems. It has a clock timer for intermittent operation, an override switch for continuous operation, outputs for 24V or 110V dampers, and output for a 110V exhaust fan. The cost of the AQC plus the damper is less than half the cost of the individual components purchased separately.
Less advanced, but still an improvement over site-fabrication, is the Timer Make-Up Air Control by Trol-a-Temp (57 Bushes Ln., Elmwood Park, NJ 07407. Tel: (800)828--8367; Fax: (201)794-1359). This device combines a Honeywell thermostat with an electrically controlled damper, clock timer, and low voltage connections for the fans. It requires 24VAC from a separate step-down transformer.
Any Energy Savings?
Penalities of Choosing The Wrong Damper
Even in the mild climate of the Pacific Northwest, an integrated heating and ventilating system can cause excessive energy use if all the pieces are not put together properly and tested in the field for performance. For example, the annual energy cost penalty from ventilation exceeding 0.35 ACH resulting from not installing an electrically activated damper to restrict the timing of fresh air introduction into a resistance heating system can be over $60 per year for a medium size, new house in Portland. This is a somewhat extreme example where natural infiltration in the home is large (0.30 ACH) and the furnace run-time exceeds ventilation system run-time required for adequate ventilation. In colder climates the penalties can be even larger.
Simple Exhaust versus Integrated
Consider a 1,500 ft2 house in Portland, with 8 ft ceilings, three bedrooms, and an annual heating load of 6,500 kWh. ASHRAE 62-1989 suggests that this house would require 60 cfm (based on the number of bedrooms) or 70 cfm (based on 0.35 ACH). A builder would use one of the following two approaches:
* An 80 cfm fan with Fresh 80 or some other fresh air vents in bedrooms and the main living area
* An integrated system with 60 cfm exhaust and 60 cfm fresh air supply to the return duct
The homeowner will find these two systems incur different operating costs, even if they provide equivalent amounts of ventilation at an identical thermal cost. The reasoning follows.
For the simple system, whe the interaction between unbalanced exhaust fan flow and infiltration are considered, 80 cfm or measured exhaust yields 40 cfm of additional mechanical ventilation and 40 cfm of displaced infiltration. For the integrated system, since supply and exhaust rates are equivalent, the ventilation rate is 60 cfm.
Assume that the simple exhaust system runs for 8 hours per day, and the integrated system runs for 5.3 hours per day so that the thermal energy cost is the same and the average ventilation rates are equivalent. (This assumes that all ducts are tight and located within the heated space.) The primary difference in operating costs comes from the fan power required for the integrated system.
At 8 hours per day, the simple exhaust system uses 146 kWh per year in fan power. At 5.3 hours per day, the integrated system uses 97 kWh in exhaust fan energy plus 773 kWh in supply blower energy, assuming a 400W fan-motor set. Coincident heating--from operation of the fan--would occur during 433 hours. The net additional blower energy for ventilation is 340 kWh per year, with a net increase of 294 kWh when the exhaust fan difference is included. At 6.5cents per kWh, the integrated system costs an additional $19 per year to operate. If longer periods of ventilation are required, the cost difference would increase. This applies to electric resistance heaters. The cost difference is greater for other types.
Ventilation and Heating--With Reservations
Under the right conditions, integrated systems work well and have a fairly minimal energy penalty. However, the right conditions are not common in Northwest housing stock. Our houses typically are built with heating system supply ducts located in crawl spaces and return plenums in attics. Duct leakage is usually large, in excess of 100 cfm, as is the potential for conductive heat loss from the ducts. This type of heating system should not really be used for an integrated heating and ventilating system unless the ducts can be tightened up and the heating system and ducts insulated to reduce conductive heat loss.
A more appropriate home for an integrated system would have:
* The entire heating and distribution system within the air barrier of the home,
* A tight envelope,
* Adequate controls--at least a 24-hour time clock and a manual override twist timer--and
* Occupants who know how the system works.
Furthermore, to avoid unbalanced pressures within the home, it would have well-distributed return ducting, or make it possible for air to freely flow from supply to return registers to minimize the pressure imbalances.
Heating and cooling systems can be very inefficient if poorly sealed, poorly insulated, badly designed, and improperly installed. Integrating a ventilation system with an inefficient heating and cooling system compounds the problems and the results are not acceptable. Those considering an integrated system would do well to ensure that the heating distribution system is as efficient and tight as possible. Once this has been achieved, the integration of ventilation can proceed at minimal energy penalty. n
Figure 1. Integrating ventilation with a central forced-air heating system.
Table 1. Ten Houses, Ten Ventilation-Heating Performances
Leakage tests Measured (House pressurized to 50 Pascals) Fresh Exhaust Fan Infiltration Duct leakage Air Supply Air Flow Damper (air changes (cubic feet (cubic feet (cubic feet House Type per hour) per minute) per miute) per minute)
1 Electronically 2.8 66 65 45 activated
2 Electronically 2.9 0 80 58 activated
3 Electronically 3.6 66 187 94 activated
4 Electronically 2.5 130 100 84 activated
5 Barometric 6.1 115 80 80
6 Balancing 6.7 136 53 90 damper only
7 Balancing 4.9 65 70 75 damper only
8 Barometric 10.1 140 0 55
9 Electronically 4.9 154 40 75 activated
10 Electronically 2.5 35 72 114 activated
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