AirScape Engineer's Blog

new efficient house

Low leakage windows, better insulation, and more efficient heating and cooling systems are among the many wonderful energy saving products and techniques that allow us to build very efficient houses.

Houses built to the high standards of a passivhaus , can use as little as 10% of the energy required to run the average U.S. single family house. For so many reasons, reducing energy use is critical to our society’s future. So….. let’s build a bunch of new passiv houses with an American flair. Well…..here’s the problem.

In a typical year, the housing industry produces about 1.5 million new housing units per year. This is within a base of approximately 128,000,000 existing housing units. (source census.gov). So, it would take about 85 years (with no population growth) to replace all of our existing houses.

This leads us to the conclusion that we must devise a way to make our existing housing stock efficient users of energy. It’s not a simple task. Standards must be developed and a process must be implemented. Oh, and by the way, perhaps government (good government) should direct this. I think we’re not even going to ask the finance industry to be involved in this one

So, here is the idea proposed by one of our local green minded restauranteurs. Exhaust hoods in restaurants  run almost continuously, propelling a  great deal of warm air outside.  The particular idea was to use this stream of warm air to pre-heat incoming water.  Restaurants use a lot of hot water.  Seems like a good idea… continuous exhaust of hot air, and an almost continuous need for hot water.

So how would you do this. A device that transfers heat from one medium (typically a fluid like air or water) to another, without contaminating or mixing the fluids is what an engineer would call for.  The generic term is a heat exchanger.  A radiator is an example of a liquid to air heat exchanger.  A serpentine row of copper water pipes would work as a basic heat exchanger.

Now the engineering fine print.  If you put this heat exchanger in the exhaust air, the exhaust fan will have to run harder to overcome the additional resistance to airflow (there is now an obstruction in the airstream).  In addition, the heat exchanger will require cleaning, because restaurants are very definitely not grease free.

No piece of equipment lasts forever, so the cost to maintain or replace the heat exchanger must be taken into account.  Ah, but you say saving energy is more important than money, which is a mere human construct.  Perhaps, but since money is the indirect measure of human effort, we should apply some economic measurements to our proposed project.  In short, we would measure the financial return on our heat exchanger investment by summing up purchase costs, maintenance costs,  and on the plus side energy savings.

So, we now at least have a concept of how to determine whether this idea makes sense.  It may in fact, be a great idea.

As usual,in all human endeavours, it gets more complicated.  Most restaurants do not have an excess of capital, they rent their building, and perhaps most importantly, can not reasonably plan on being in business for 10+ years that it may take to recover such an energy investment.

OK, first the boring defintion. A CFM stands for cubic foot per minute. This term is used as a measurement of airflow rate for ventilation systems. The cubic foot refers to a (mythical) cube of air 1 foot x 1 foot x 1 foot. CFM becomes a flow rate since we measure how many cubic feet are flowing by per minute.

Now, let’s get some perspective on what a cubic foot and CFM represent:

• It takes about13.5 cubic feet of air to weigh one pound. A 2,000 square foot house will contain 16,000 cubic feet of air. The weight of all that air is only 1,185 pounds.
• Warming or cooling air is “low calorie”. To warm all that air in your house up from 50 degrees F to 70 degrees F takes about 5,688 BTU’s . The smallest house furnace puts out 40,000 BTU’s per hour. So how come it takes so long to heat up the house on a cold morning?
• An unsealed door jamb, leaking 50 CFM would over the course of 24 hours, leak out 72,000 cubic feet of air – not “low calorie”

Many writers, including us, seem to be switching back and forth between energy savings and carbon dioxide reduction. So what’s the deal? Is there a conversion factor between these measurements? Unfortunately, there are no hard and fast conversion factors. And that is simply because it depends upon what we’re burning and how we are turning that (typically fossil) energy into useful work.

Let’s take the case that we, as whole house fan manufacturers, and users of electricity face. Imagine a typical fossil fueled electric generating plant with a windmill in the front yard. In fact, hold the imagination, just look at the actual picture.

Per the U.S. Department of Energy studies, the average pounds of carbon dioxide per kilowatt-hour of electricity is 1.34. This is the US average. It is reasonable to use this figure even if your electricity is “clean hydropower” or “green”. If your household does not use that hydro electricity, the dam operators will not “throw away” your kilowatts. That electricity will be used to slow down another power plant – perhaps a “dirty” coal fired plant.

Let’s do the math. For example if you replace 10 of your 100 watt halogen bulbs with fluorescent bulbs, what kind of carbon dioxide reductions can you expect. In this example, the bulbs are on for an average of 4 hours per day, 365 days per year.

Pounds of CO2 saved per year = 10qty. x (100watts-15 watts)per bulb x 4 hours/day x 365 days/year x 1kwh/1,000 w-hours x 1.34 lbs-CO2/kwh = 1,663 lbs.

Not bad, but just put that into perspective, the US per capita carbon dioxide production is 46,860 lbs – so this small example would get you 3.5% towards your zero carbon goal.

For extra credit:

Nuclear plants should emit no carbon dioxide. Is that correct ?

Answer:

There are no “tailpipe” emissions from a nuclear power plant, but compared to energy conservation, you still have to take into account the CO2 embodied in the construction materials.

Map of Whole House Fan Sales

Where are all those fans going?

We wanted to find out where our fans from 2008 ended up.  This map has a dot for every zip code in which we have shipped at least one whole house fan.  As we expected, California is the leader, for a couple of reasons: great opportunity for night time cooling, and great, big electrical bills (thanks  Enron).

What surprised us is the amount of activity in other states.  In fact, the distribution of our whole house fans looks very similar to the US population distribution.

We think a couple of factors are at work.  People are rediscovering whole house fans as electrical prices rise (electricity seems to be recession proof BTW).  Another factor (according to our surveys) is that people are looking for fresh outside air as an alternative to air conditioning.