Find Required Heating Airflow at High Altitudes

When servicing an HVAC system at higher altitudes, your job is to determine if the published airflow values on the plans have been adjusted to compensate for air density or not.

Thanks for all the responses from the last Hotmail article about heating airflow. Many of you wanted more information about finding heating airflow at higher altitudes. Let’s take a look at how much heating airflow will increase as altitude increases.

Sea Level Airflow
If you missed the last Hotmail Article, here’s a summary:

To calculate required airflow for gas furnaces, begin with the rated Btu input of the furnace. Divide the rated Btu by 10,000. Then multiply this number times the following cfm per 10, 000 Btu factors:

•  Natural Draft Furnaces (70%)       100 cfm per 10,000 BTU of rated Btu input

•  Induced Draft Furnaces (80%)      130 cfm per 10,000 BTU of rated Btu input

•  Condensing Furnaces (90%)        150 cfm per 10,000 BTU of rated Btu input

Example: You check the furnace nameplate and find the furnace rated output is 80,000 Btu. Divide 80,000 Btu by 10,000. The answer is eight. Say you’re working on a condensing furnace that requires 150 cfm per 10,000 BTU of rated Btu input. 150 cfm times 8 is 1200 cfm of required heating airflow.

Remember, these required heating cfm factors are for systems at or near sea level. As you climb the hill, these airflow factors increase.

Higher Up, the Air Gets Thinner

When you go hiking at higher altitudes, you’ll find yourself winded much sooner than you will hiking a similar path at sea level. That’s because as altitude increases, the density of air decreases.

At sea level a pound of air measures about 32 in. x 32in., while at an elevation of 6000 feet above sea level, a pound of air measures 40 in. x 40 in.

The air expands at the higher elevation as it becomes thinner. When you breathe in a lung full of air, it contains less oxygen than a breath of air at sea level. You need to breathe more often to supply the amount of air your body needs.

The same goes for a furnace or heat pump in heating mode. Each cubic foot of air passing through the heat exchanger contains less oxygen at the higher elevation.

At sea level 1000 cfm per hour weighs 4500 pounds. (1000 cfm x .075 pounds per cubic foot of air x 60 minutes in an hour = 4500 pounds. At 6000 feet above sea level, the same volume of air only weighs 3735 pounds. So, more airflow is required by the same furnace at 6000 feet above sea level.

The warmer air becomes, the more its air density increases. Air at 120 degrees is seven times lighter than air at sea level. If this is getting confusing, we have a chart for that.

Check Design Conditions

The challenge to air balancing and diagnosing system performance at higher elevations is understanding whether the system designer specified required airflow based on sea level conditions, or the high-altitude conditions you’re testing under.

Your job is to determine if the published airflow values on the plans have been adjusted to compensate for air density or not. To be effective, review the plans for a reference to air density correction. If you don’t find such a reference, you should ask the question directly to the designer. To adjust the system to optimize performance requires you know if the designer adjusted for air density.

Air Density Correction Factors

This Air Density Correction Factors table (SEE DOWNLOAD BELOW) enables you to choose the elevation and air temperature, then plot to find a factor for calculating required airflow to match your test conditions. This makes a big difference when troubleshooting, improving system performance, or air balancing.

How to Use this Air Density Correction Table

1.    This table can be used for gas heating, heat pump heating, or cooling modes.

2.    Identify the altitude at which you are testing. (The best way is to use an Altimeter, the next best is to Google the town name and altitude)

3.    Measure the air temperature.

4.    Plot on the table where the appropriate altitude column and the temperature row intersect.

5.    Use this correction factor as described below.

How Much Heating Airflow Example
Normally, 1500 CFM of 70-degree, sea level air (also called standard air) is needed for the system to operate properly. But, field conditions are air at 100˚F at an elevation of 5000 feet above sea level. How much air would the system need to operate properly?                                                                                                                       

Calculation

1500 CFM of Standard Air / 0.78 (The appropriate factor from the table) = 1923 CFM 5000 foot, 100º air.                                                                                                                                             

By adapting this example, you can adjust for air density changes as you test and diagnose the performance of the heating systems your design, sell, install and service.

Depending on how critical your testing is, you will be the one who determines when to apply these correction factors. Typically air density correction is applied only above 1500-2000 feet elevations or when air temperature exceeds 100 Degrees F in most situations.

But when accuracy is critical or when the conditions demand increased accuracy, air density correction factors will often provide the solution you need to assure your testing, diagnostic and reporting are the most accurate they can be.

Rob “Doc” Falke serves the industry as president of National Comfort Institute, an HVAC-based training company and membership organization. If you're an HVAC contractor or technician interested in a free expanded air density table, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.

 

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