Industrial Utility Efficiency

Seven Sustainability Projects: HVAC Optimization

Seven Key Sustainability Projects

1. Metering 5. Lighting
2. Demand Control 6. Heat Recovery
3. HVAC Optimization 7. Project Implementation
4. Compressed Air


Reducing energy costs and pollution emissions involves many areas within an industrial facility. My studies have found seven (7) key (or common) areas where low cost practical projects can be implemented. Combined, these projects provide savings exceeding 10% of the annual energy spend with an average payback of less than one year.

This month’s article will focus on the heating, ventilation, and air conditioning (HVAC) component of energy management. As you read this article many of you may just be exiting this year’s long cold winter season. It may still be fresh on your mind just how much extra energy costs are because of winter heating. I will focus on the subject of winter heating but the concepts also apply to air- conditioned manufacturing facilities.

HVAC Optimization Project Objectives

We can describe our HVAC Optimization project as an effort to reduce costs and energy for heating and air conditioning by improved control of exhaust air. The symptoms to help identify the opportunities for this project include:


Take a look around your facility. Do any of these photos look familiar?

  Filtration systems sometimes called “Bag Houses” act like vacuum cleaners sucking the air out of the factory.    
  Inside the factory you can see many different types of exhaust hoods. Many times exhaust hoods are sucking the air out of an area even when there is no production at the machine.    

Step #1: Calculate Gas Costs for Winter Heating

To begin this project you will need some basic information about the costs related to winter heat and the amount of air exchange happening on the manufacturing floor. A quick check for winter heating costs is to take the monthly gas bills and make a table of the amount of gas used each month as in the table below.

Does the amount of gas used increase during the winter months? If so, estimate the amount of gas used during a typical non-heating month such as August. Multiply this amount by 12 months to get an estimate of the amount of gas used for production that is not related to the weather. Now add the total amount of gas used for the year and subtract the amount used for production. This will give you an average amount of gas that is related to the cold weather.

The next step is to estimate approximately how many hours winter heat is required per year. Adding up the hours during the winter months from November through March provides a rough estimate. A more accurate number can be derived by adding up all the days where the average temperature is < 56 F. This number can be found by using the free data available in the website Enter your zip code and choose the selection “history”. From this data you can also determine what the average temperature was during the heating season. In our sample factory from central Ohio we found there were 3,960 hour per year the temperature was below 56 F and the average temperature was 42 F. The differential temperature is 56 – 42 = 14 degrees.


Step #2: Calculate Amount of Exhaust Air it’s Heating Costs

Another piece of data is to know the amount of exhaust air that is being removed from the factory by dust collectors, furnace exhausts, roof and wall exhausts, etc. Sometimes you can find this data in the environmental survey reports. Another way is to measure the air flow using a Velometer. System name-plate data can be used but is often far different from the actual exhaust flow. HVAC service personnel often have a small probe meter than can be inserted into a duct and determine the flow rate. In my sample factory the volume of exhaust air is 100,000 cubic feet per minute (cfm).


With this data we can roughly calculate the cost of heating the make-up air and then rejecting it back into the countryside. The formula is: Differential Temperature x Flow rate (cfm) x heating hours/yr x cost of gas x 0.00000182 = annual cost

1. 14 degrees x 100,000 cfm x 3,960 hours x \$9.80/mmbtu x 0.00000182 = \$98,000 per year.

Think about this: Every 1,000 cfm of air that is being exhausted costs \$1,000 per year or \$1.00 per cfm. A small squirrel cage fan of 1 horse power can eject 3,000 cfm or \$3,000 of heated air.

What are some of the opportunities to reduce cost in this sample model?

Reduce the roof exhaust air by reversing the flow of the roof exhaust units and distributing the air into the roof area of the factory using flexible distribution ducting as shown in the photo below. Our model factory was under negative pressure and drawing in cold air from the truck bays and other doors, windows, and cracks. This caused the need for local heaters while hot air was going out the roof. Reducing the exhaust air by 30,000 cfm at \$1.00/cfm will save \$30,000 per year. A plant in central Ohio recently applied these principals and even though the 2008/2009 winter has been extra cold they have not run the large steam boiler system yet this year to heat the manufacturing areas.

  Fresh air make-up tubes and fans can be used to distribute the cold air from outside and mix it with the warm ceiling air. These systems help reduce negative pressure problems and can significantly reduce the winter heating costs.    

Shutting down excess exhaust hoods during periods of reduced production can also help reduce the heating costs. Twenty (20) exhausters could be shut down for 80 hours per week (50% of the time)
20 exhausters x 1,500 cfm/exhauster x \$1.00/cfm x 50% of the time = \$15,000 of savings per year.

Look for any areas where the exhaust flow can be reduced during periods of reduced production.

  1. The facility has a large amount of air exchange (air changes per hour)
  2. The facility requires heating during the winter months.
  3. The existing ventilation system does not have optimization controls.
    1. Gas used in August = 4,657 mmbtus. X 12 months = 55,884 mmbtu (gas for production)
    2. Total gas used in 1 year = 67,632 mmbtus
    3. Total gas used – gas used for production = 67,632 – 55,884 = 11,748 mmbtus (gas used for winter heat)
    4. At an average cost for gas of \$9.80/mmbtu the cost for winter heating is \$115,130 per year.


Exhausting air is expensive. Every cubic foot of air that is exhausted means an equal cubic foot of air must be returned. If it is cold outside and you have to heat this return air it costs \$\$\$. Challenge everyone in your facility to look for excess exhaust air, opportunities to supply fresh air to hot areas, and to find way to shut down or slow down roof exhausts during winter months.


Thomas Mort is a Senior Auditor with Thomas Mort Consulting.


To read the next six Sustainability Projects from this series please make your selection below:


Heat Recovery

Project Implementation

Lighting Optimization

Air Compressors

Demand Control