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Proper Ventilation

I read that for each occupant in a properly ventilated home, 10 to 40 cubic feet per minute (cfm) should be exchanged with fresh outside air. Sounds like a lot of air exchanges to happen 24 hours a day, especially if it is really hot or cold outside.

What is the normal rate for a typical, well-insulated, stickframe house that does not have special systems like heat recovery ventilators (HRV) or powered venting? How much exchange occurs from leakage and typical door opening in seasons where all the windows are normally shut (i.e., during very hot or cold conditions)? And why is ventilation so important?

Julian Weckner • Vail, Colorado

According to the U.S. Environmental Protection Agency and the U.S. Green Building Council, the average American spends 90% of his or her time indoors (mostly sleeping). Unfortunately, all of this indoor living may not be so good for us. Indoor air pollutants from off-gassing furniture, cooking, mold, pets, and many other sources can make indoor air two to five times more polluted than outdoor air. In response to these statistics, the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) created Standard 62.2, "Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings." This standard has become the construction industry's guideline for mechanically ventilating the interior of houses.

To figure out how much ventilation in cubic feet per minute (cfm) is recommended for your home, multiply the square footage of the home by 0.01. Then, multiply the number of bedrooms in your home plus 1 by 7.5 cfm (the air exchange required per assumed occupant). Add these results together. For example, the calculation for a 2,400-square-foot house with three bedrooms would be 54 cfm [(2,400 x 0.01) + (4 x 7.5)] of constantly supplied fresh air. There are exceptions to the rule, but this is the basis of the standard.

A leaky home can cause 40% to 60% of the home's interior air volume to be exchanged each hour. Although this may satisfy the air-exchange rate for the standard, this abundance of "exchanged" air must be heated or cooled, which can lead to increased energy use. And the uncontrolled source of the air means that contaminated air can enter the house. Most of the natural infiltration in a home is caused when hotter air rises and exits through the top of the house (chimneys and attics) and cooler air enters through gaps and cracks in floors, crawl spaces, and foundations.

Let's use our 2,400-square-foot house to compare natural air exchange to ventilation recommendations. Let's say that this home

Fresh Air Port:

Brings fresh, outside air into ventilator core

Stale Air Port:

Draws stale, indoor air and delivers to ventilator core

Fresh Air Port:

Brings fresh, outside air into ventilator core

Stale Air Port:

Draws stale, indoor air and delivers to ventilator core

Heat Recovery Ventilator (HRV)

Supply Port:

Delivers fresh, conditioned air to home

Exhaust Port:

Stale air is exhausted after heating or cooling energy has been transferred to incoming air

High Efficiency Exchanger Core:

Transfers heat or coolness from exiting stale indoor air to incoming outdoor air, without mixing airstreams has 20,000 cubic feet of air space. If 40% of this air leaks to the exterior each hour, that's a natural exchange of 8,000 cubic feet per hour, or 133 cfm—more than twice ASHRAE's recommended value of 54 cfm. Compare this to a well-sealed house with a natural air exchange rate of 5%, which would have a natural exchange rate of 17 cfm. Adding the mechanical ventilation requirement of 54 cfm to this would only amount to about half of the natural exchange rate of the leaky house.

Interestingly enough, the standard doesn't set guidelines for how the specified ventilation is achieved. In some homes, a bathroom exhaust fan running constantly would satisfy the standard. In most cases, balanced ventilation—such as heat recovery ventilators and energy recovery ventilators—are the best option. These ventilation systems pull in fresh outdoor air, while exhausting indoor air. An air-to-air heat exchanger transfers some of the energy in the outgoing air to warm or cool the incoming air.

Building scientists might say, "It is better to build it tight, provide source control, and ventilate it right." We know how to stop air infiltration by weatherization techniques and using effective air barriers like house wraps. We can control the sources that contribute to poor indoor air quality by choosing building materials, furniture, and other products that do not off-gas harmful chemicals, and by removing moisture and combustion by-products.

Bart Laemmel • B2 Building Science

"Indoor air pollutants—can make indoor air two to five times more polluted than outdoor air."

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Generator Grid-Tie

I have an old 6 hp, 600 rpm Lister diesel engine that's been running on waste veggie oil for about six months. It drives a 3-kilowatt, 240 VAC, 60 Hz generator, which powers various equipment in my shop.

I'm now wondering if I might convert it to back-feed the grid to take advantage of net metering. Aside from any legal issues, what would I need technically? Can the AC generator work with some kind of grid-tie inverter to back-feed the grid, or would I need a DC generator? If this could work, what type, voltage, and brand of grid-tie inverter would work best? I would like to run the generator for eight to 10 hours a day to offset some of our electrical loads. Though it will not be enough to run the whole farm, it could offset a good portion of our electricity use.

George Berz • Fresno, California

Congratulations, George. You've made an important piece of internal combustion history come alive! Diesel-fueled stationary engines from the U.K.-based R.A. Lister Company were first produced in 1929. They are legendary for their low fuel consumption and tolerance of varied fuels, plus quiet, reliable, and low-rpm operation. Lister clones are still produced in India, and are widely used there for pumping water and generating electricity in remote areas.

However, using this engine and generator to offset a portion of your electricity use may not be cost-effective here in the United States. To determine if it's a wise idea, try connecting a steady load of about 1 kW to your generator through a kWh meter (like the inexpensive Kill A Watt unit) and measure exactly how much fuel the generator consumes to produce 1 kWh. Compare the utility's retail electricity rate that you pay per kWh, the wholesale rate that they will pay you for electricity you generate, and your cost per kWh for bringing the fuel to your Lister. Even with your "free" fuel source, you'll still have to consider the costs of procuring, transporting, and processing the vegetable oil, as well as engine wear, maintenance, and your time costs. The profit margin will be slim at best, and most likely negative.

If you somehow still find the math favorable, consult with a renewable energy dealer for advice on which battery-based grid-tie inverters to consider and the cost of the balance-of-system components, wiring, permits, and inspections you'll need. It's unlikely that any "direct" batteryless grid-tie inverter would be guaranteed to work properly with the output of your generator, even if you were to convert it to produce DC directly. These inverters are intended for either the DC output of a photovoltaic system or the wild three-phase AC output of a small wind or hydro turbine rectified to DC.

Once you have a grid-tie system cost estimate from a dealer, you can predict how long it would take you to pay off the investment—I would guess many years, if ever. Instead, consider adding a solar-electric array with battery backup to your grid-tie system instead. Then, in the dire case of a grid blackout with no solar input, you can listen to the gentle "putt-putt-putt" of your veggie-oil-powered Lister while your inverter powers your home from the battery bank.

Dan Fink • www.otherpower.com

"Using this engine and generator to offset a portion of your electricity use may not be cost-effective here in the United States."

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Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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