Technical Evaluation: HV1000 (May 17, 2001)

• Attic Cooling

• Effective "R" Values

• Conductive Heat Loss

• Convective Losses

• Cost

• Cooling Savings

• Glossary

• References

When heat builds up in the attic, some of that heat will transfer into the house. It is similar to having an overheated blanket covering the top of the house. This will increase the temperature of the interior of the home. This increased interior temperature has a negative effect on human comfort. Additionally there is ongoing research on the negative effects of excessive heat on building materials. The traditional answer to this problem is to ventilate the attic with either passive or powered ventilation, moving air through the attic to reduce the temperature.

Passive ventilation: Many homes have passive attic ventilation in the form of ridge vents at the peak of the roof, gable vents at the ends of the roof, soffit vents in the eaves, or some combination of these vents. Turbine and/or roof vents (passive vents that penetrate the roof) are also used. A driving force, such as wind or a pressure differential, must be present for air to move. The hottest days of the year are the calmest, with little or no wind.

Powered ventilation: Attic fans are roof or gable mounted fans that draw air out of the attic, relying on passive vents to supply cooler outside air to replenish the air being exhausted. An attic fan does not exhaust the heated air from the living space.

Whole house fans are mounted in the attic floor pushing house air out through the passive attic vents and bringing cooler air into the living space through the windows. This directly cools the inside of the home, enhancing the comfort level for the occupants. The greater the air flow generated by the whole house fan the greater the area of the passive attic vents must be to relieve the attic pressure. If the relief vents are too small, the air will "squirt" out through any available hole, which can damage building materials. At the same time, excessive pressure will reduce the effectiveness of the fan, and materials in the attic such as insulation and stored items can be blown about. The relief opening should be based on: 1 square foot of opening for each 650 cfm (cubic feet per minute) of air flow. Using this formula, a 1000 cfm fan would require about a 1.5 square foot opening and a 6000 cfm fan would require a 9.25 square foot opening, equivalent to about five 12" turbine vents or 14 roof vents.

Summary: Removing the "blanket" of hot air in the attic can help to reduce the temperature in the home. Although an attic exhaust fan can effectively exhaust the hot air from the attic, it does nothing to exhaust the hot air in the home. A small whole house fan can do both jobs - reducing both the attic and house temperatures. This will only work effectively throughout the year, however, if the whole house fan is sealed when it is not in use.

An insulation system is rated by its overall "effective" value, the average of its weakest and strongest points. This is the "effective" R value. Voids or under-insulated areas in the insulation blanket drastically reduce the effectiveness of the insulation system.

The following is a comparison between an uninsulated, 36" whole house fan and Tamarack Technologies, Inc. HV 1000 installed in a 1008 square foot attic floor, insulated to R-49 (per DOE insulation fact sheet for electric heat).

Where:

R_eff = Effective R value

U₁ = Conductance of material 1

Area₁ = Area in square feet of material 1

U₂ = Conductance of material 2

Area₂ = Area in square feet of material 2

.

The installation of a traditional whole house fan carries a huge energy penalty when the homeowner fails to cover the fan. Anecdotal evidence suggest that the American homeowner will forget or neglect system maintenance - covering and uncovering the fan. The only way to ensure that the attic air sealing and insulation is not compromised is to have a well insulated, automatic cover.

Conductive Heat Loss

In terms of heat loss, reducing the effective R-value of the attic floor from R-49 to R-15 would have a dramatic effect.

Where:

H = heat loss in BTU's per unit of time

A = area of section in square feet

DD = number of heating degree days at the site (about 7000 for Madison, Wisconsin)

R = Effective R value of section

1. The attic alone with no penetrations:

   H = 24 x 1008 x 7000 = 3,456,000 Btu's

                    49

    

2. A traditional, uninsulated 36" fan:

   H = 24 x 9 x 7000 = 7,560,000 Btu's

                  .2

   (Attic with 36" fan installed total: 10,985,000 Btu's)

    

3. An HV 1000:

    H = 24 x 2.66 x 7000 = 20,312 Btu's

                              22

   (Attic with HV 1000 installed total: 3,466,880 Btu's)

These numbers indicate decisively the enormous penalty that an uninsulated 36" fan can create (increasing Btu loss by over 215%). The same attic with an HV 1000 installed reflects a less than 1% difference from an attic floor with no penetration.

The convective losses through an uncovered fan are very high. Despite the fact that the average whole house fan has a louvered cover, the number of edges to the louvers makes the total "crack" length very high. The louvers are certainly better than an open hole, but with wind pressure and the stack effect, the louvers rattle and are often lifted even when the fan is not running. At an extreme (eighty degree) temperature differential (70ºF inside and -10ºF outside) there would be about 600 pounds of lift exerted on the ceiling of a 1000 square foot house just from the stack effect.

Seasonal convective loss = 24 x c x Q x L x DD

Where c = .018 (the heat capacity of air)

Q is a constant based on the width of the crack est. 3/32"

L = length of crack in feet

DD = Degree day year

A. Convective loss through a 36" uninsulated whole house fan with gravity louvers: 12 slats x 3 feet wide

24 x .018 x 27 x 36 x 7000 = 2,939,328 BTU's

B. HV 1000 convective loss: the measured leakage through the HV 1000 in a situation when the house is under positive pressure of 5 pascals relative to the attic, is 10 cfm.

24 x .018 x 10 x 7000 = 30,240 BTU's

Cost

What does all this mean in terms of cost?

* 85% efficiency

All amounts rounded to the nearest whole cent. Fuel costs differ by region and time. It is likely that fuel costs are higher, but locating the house in a 7000 degree day climate is a very cold climate, and a 1008 square foot house is small.

It is clear that a whole house fan should be covered when it is not being used. There is almost a 300% increase in heating cost with an uncovered 36" fan. The HV 1000, however, adds an insignificant amount to the annual heating load.

Cooling Savings

To start with, a whole house fan can often be used in place of using an air conditioning system. There are many days in many parts of the country where the conditions are ideal for just drawing in fresh, cooler outside air to replace the over heated inside air. For every degree Fahrenheit that a thermostat is raised , air-conditioning costs can be reduced by 7 - 10%. Since air-circulation with a fan allows a thermostat increase of 4º F with no decrease in human comfort, a fan can provide as much as a 40% savings in cooling cost.

• Operating a properly sized, 2-ton air conditioner with a seasonal energy efficiency ratio (SEER) of 10 in Atlanta, Georgia, costs over $250 per cooling season (1,250 hours) based on 8.5 ¢/kwh or roughly 20¢ per hour of runtime.

• A large, 18,000 Btu/hour window unit air conditioner with an energy efficiency ratio (EER) of 8.8 costs more than 17¢ to operate for one hour.

• By contrast, an HV 1000 whole house fan draws only 115 watts and costs less than 1¢ per hour of use.

Glossary

CFM: Cubic feet per minute - this is a measure of the volume of air moving in one minute

FPM: Feet per minute - this is a measure of the rate of air moving in one minute

R value: The ability of a material to resist the conductive flow of heat. The higher the R value the greater the resistance.

R_eff: The average R value for an area such as an attic floor.

U value: The ability of a material to conduct heat. The inverse of the R value.

Conductance: A term referring to the U value of a material. The flow of heat energy from molecule to molecule, molecules warming or their neighbors.

DD: Degree Day - "The number of degree days in one day is the difference between the average temperature for that day and 65º F. The accumulated number of degree days during the heating season is kept by the weather bureau as an indication of the amount of fuel consumed. A similar concept is cooling degree days, the difference between the average temperature and 78º F."

BTU: British Thermal Unit: A measurement of heat energy. One BTU is about the energy in a single kitchen match.

Convection: The flow of heat energy via fluid or gas motion.

Pascal: A metric measurement of pressure in a very small increment. One thousand pascals equals 0.145 pounds per square inch.

References

"From the Walls In", Charles Wing

DOE Technology Fact Sheet "How to install and use a whole house fan"

"The Solar Home Book", Bruce Anderson

"Environmental Engineering", Burgess H. Jennings

1997 ASHRAE Handbook of Fundamentals

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