"Accurate" Is an Eight-Letter Word

The extreme readings are particularly suspect. How did he measure a 13-watt load with a 1,000 ohm shunt? The measurement equipment itself would affect the measurement. A high resistance shunt gives you high gain, but it introduces a significant voltage drop when the current is high. The tester should actually use shunts of different values with different loads-a high resistance shunt for small loads, and a lower resistance shunt for high loads.

It's true that a rig like this would be useful for telling if an appliance leaks significantly or not. But it shouldn't be used for making detailed measurements.

Leo Rainer
Davis Energy Group
Davis, CA

Editor's reply: You're right, the significant figures are excessive. The accuracy of the meter warrants two to three figures, but the accuracy of the technique (using current as a proxy for power) is good to only one digit, given that the power factor can vary by a factor of four or more with these loads. And yes, the loads of more than a few watts have extra uncertainty without using a lower-resistance shunt. (Also, note editing errors for the personal computers AST 286 and generic 386 CPU, and the TI 865 printer. All should have negligible yearly kWh.)
 
 

Who's Using All the Hot Water?

Jonathan Beers
Madison Gas & Electric
Madison, WI

Author Fred Goldner responds: My numbers come from field monitoring of multifamily buildings around the country. As much as I might wish the results were lower, they are based on real time measurements. Multifamily tenants might use more water than single-family homes because tenants often do not directly pay for water, or the fuel to heat the water, based on how much they use. When apartment buildings are submetered for electricity, usage typically drops by 20%-30%; the effect of submetering water use would probably be similar. Another major contributor to the high usage in apartments is leaks. Leaks are much more likely to be fixed in single-family homes, where the homeowner pays the water bills and is responsible for the maintenance. In multifamily buildings, leaks can go for long periods of time undetected or unreported (including leaks into the basement or the boiler, as well as potentially large numbers of dripping or running faucets). Remember that a 2 1/2 drop per second drip will use about 500 gallons per month.

Of course there is tremendous variability in usage even within categories like single family or multifamily. This is what prompted us to develop a range of usage factors (whose selection is based on demographic and building characteristics) for the ASHRAE sizing guidelines, instead of using one standard average value for every case.

Fiberglass Ups Its R-Value

Thomas A. Newton
CertainTeed
Valley Forge, PA
 
 

Only Soggy Ceilings Sag

On page 26 of the "Consumer Guide to Insulation," you discuss weight and density with regard to the potential for ceiling sags. You correctly say that: (1) 1/2-inch drywall 24 inches on-center contributes to the potential, (2) such is a rare combination, and (3) if using either 5/8-inch drywall or framing 16-inches on center, there should be no problem.

United States Gypsum Company (USG) identifies excess moisture as the major cause of objectionable ceiling sags. Fiberglass manufacturers have taken portions of USG's Gypsum Construction Handbook out of context when promoting their lighter density loose-fill insulations. You were correct as far as you wrote. Perhaps next time you can go a little further to dispel the myth that rock wool and cellulose at R-38 and above causes ceilings to sag.

Bob Sullivan
Rock Wool Manufacturing
Leeds, AL

Editor's note: The Gypsum Construction Handbook is available from USG, Department 147-4 Handbook, P.O. Box 806258, Chicago, IL 60680-4124.
 

Forgotten Films

Virginia L. Kubler
Courtaulds Performance Films
Martinsville, VA

Editor's reply: We didn't mean to give window films short shrift. Window films are indeed another good way to reduce solar heat gain, and are much less expensive than replacing the window. New high quality window films-and even a low-e window film-are much more permanent and effective than the older products that tended to peel off and significantly reduced visible light transmission.
 
 

Progress?

The 1977 Leger House in Pepperell, Massachusetts, demonstrated that a well-built house performed well with a Japanese Paloma natural gas instantaneous hot water heater, and fourteen feet of baseboard. Annual heating bill: $38.50.

The 4,000 square foot New Ipswich, Massachusetts, Leger House also demonstrated that a single point source of heat, an Austrian tiled wood stove, keeps the entire structure at a constant temperature, room to room, ceiling to floor, without duct work or fans. I often wonder if we have made any progress since 1977.

Gene Leger
Leger Designs
New Boston, NH
 
 

Ventilate Right, 
Then Build Tight

In 1978, the clever Swede Thomas Lindvall said, "Ventilate Right, then Build Tight." He was right then, he is right now, and he will be right in the future. There is some room for variation in what is "Right," but the sequence of "ventilation then air tightening" is vital. We should install balanced ventilation systems, or at least ventilation that cannot cause significant depressurization, and then tighten every leak we can find. This will give us better indoor climates at lower energy costs, and better comfort as well.

As long as we recommend tightening "just enough" to avoid problems, we are setting ourselves up for serious failures over time. The lawyers will have a heyday, Americans will get sick and lose money, and energy conservation will get another black eye.

Jim H. White
Senior Advisor, Building Science
Canada Mortgage and Housing Corporation
Ottawa, ON
 
 

Built Tight, Now Ventilating Right

I recently purchased a vent fan with a heat exchanger. I intend to create positive indoor pressure by using this fan to force fresh air into the house. When it is cold enough outside, the heat exchanger would be fed by a heat storage tank. (A heat pump-heat storage system is part of the HVAC system.) I plan to exhaust through existing vents in the bathroom and kitchen and through new ones in some bedrooms.

The HVAC system was well designed and built. The return ducts and vents are twice the size of the supply, all ductwork is formed fiberglass with foil backing, and all is sealed very well with caulk.

I am concerned about humidity (or lack thereof) in the winter. Is my plan sound? How can I test?

Jon Geissinger
Elliottsburg, PA

Editor's Reply: According to ventilation consultant Don Stevens, your idea of using a small central vent fan to help ventilate your house is a good one. If you lived in a hot climate and used air conditioning, positive pressure might help minimize condensation in the walls of your home. However, since you live in a cold climate, positive pressure may cause condensation as warm, humid household air is forced out through small cracks. (Yes, virtually all houses leak somewhere!) To find out whether your space is pressurized, hold a smoke stick or a tissue over the crack of a barely open door and see if air is flowing in or out of the space.

Stevens suggests that you wire one or more exhaust fans to go on whenever your central vent fan operates. So long as the two are on the same circuit, you can wire the central vent fan in parallel to a bath fan switch. That way, the bath fan can be turned on when needed, and will also go on whenever the central fan goes on. If you discharge the central vent fan into the furnace ductwork, you can use the furnace ducts to distribute the fresh air.

Pick the exhaust fans by combining the rated flows to approximately match the flow of the central vent fan. A reasonably balanced flow of outdoor air and exhausted stale air will minimize the potential for condensation and eventual structural damage. Total ventilation flow should be about 15 cubic feet per minute (CFM) plus an additional 15 CFM per bedroom.

Stevens suggests that you operate your ventilation system at least eight hours a day. ASHRAE recommends supplying outdoor air and exhausting stale air whenever the house is occupied, just as is done for commercial buildings. If your bath fans are too noisy to operate this long, you might look at the quiet fan list in Stevens' article "Mechanical Ventilation for the Home" (HE March/April '96, p. 13). The effect on humidity depends on the climate. If the winter air is cold and dry, the air you bring in will absorb a tremendous amount of household moisture, and overventilation will cause household humidity levels to be too low.
 

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