Recovering Heat from Restaurant Exhaust Air

kitchen_hoodSo, 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.

What is a cubic foot and a CFM?

balloonOK, 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”

How to calculate CO2 reductions.

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.coal-wind-power-plant

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 ?


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.

Who buys these fans anyway ?

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.

Can we grow our own energy ?

farmer-plowingSo you think we can grow our own energy?  My first thought was “why not”, but the inner engineer said “do the math”.  OK, inner engineers always win.
Now what facts and numbers are we interested in.  I thought that it would be interesting to take the average American, put him and his average family on a piece of land.  This piece of land would have to provide him with all the direct energy the family uses.  I’ll define direct energy as the energy used for heating, cooling, electricity, and automobile transportation.  For this scenario, we’ll assume that each factory, office, etc. has its own “energy farm”.

The best source for energy statistics is the US department of energy website (

I thought this was going to be hard. In fact the data is all right on this one table:

It seems that the average US household consumes 95,000,000 BTU (equivalent) for electricity, natural gas, and heating oil.The average daily consumption of gasoline for the U.S. is about 9,000,000 barrels (42 gallons.)  I’ll assume that half of that consumption is for individuals (tell me if I’m wrong). There are 127,000,000  dwelling units.

The total energy growing requirements calculate out to about 157,000,000 BTU per year.

On the supply side…How much solar energy do plants absorb ?  According to it’s possible to obtain 10 tons of dry plant material per acre per year.  At an average of 19GigaJoules per ton, this would yield (assuming 50% conversion to useful energy) 45,000,000 BTU per acre.  Doing the division 157,000,000/45,000,000 gives us 3.5 acres.

In reality, you’ll need much more area. Because you have to eat, the horses (or tractors) have to eat, you may have to irrigate your energy crops, and you need manufactured products.

Assuming very little “home grown energy”, we would need: 127,000,000 households x 3.5 acres = 445,000,000 acres of additional cultivation. The US has about 407,000,000 acres of arable land.


Well, maybe this efficiency thing isn’t such a bad idea after all..

Other sources:

  • has conversions from different energy sources and units.

“I have been so happy as by accident to hit upon a method of restoring air which has been injured by the burning of candles and to have discovered at least one of the restoratives which Nature employs for this purpose. It is vegetation.”
Joseph Priestley 1771

We’re keeping plastic out of the dump

landfillThe vast majority of our whole house fans are shipped to the customer by UPS. Now they are a wonderful company, full of energetic 🙂 employees, who find that the best exercise is at work. Translated out of politically correct speak, it means that they throw our stuff as far as they can.

Faced with this athletic problem, we have developed and tested some great packing solutions. What drives us nuts, is that this typically means packaging our WHF’s in wonderful, resilient, polyethylene foam. Not only is the polyethylene foam expensive (it’s worth it), but more importantly it either goes straight to the landfill or at best is ground up and recycled.

Our goal was to go one step better and re-use the packaging foam. Each time that we re-used the plastic foam, that would be less plasic in the landfill. If we were lucky, maybe we could save some money along the way :-).

Since we ship our whole house fans throughtout North America, our first concern was the cost and inconvenience involved in shipping the foam back to us. packaging-reuse

We decided that if we had to provide another box to ship the foam back in that would be counter-productive. However the foam has to be assembled into something resembling a package.  What we came up with is to send a small roll of packaging tape. There is some inconvenience for the customer. They have to tape the foam together, attach the return label, and drop the “package” at a UPS store or truck.  It will be interesting to see how customers react to this.  This is what the returned polyethylene foam looks like after being returned to us.