Lighting’s Impact on Heating, Ventilation and Air Conditioning (HVAC) Systems

INTRODUCTION All lighting systems convert only a fraction of their electrical input into useful light output. A standard incandescent lamp, for example, converts only 11 percent of its electrical input into visible light, while the rest is dissipated directly as heat. This is why incandescent lamps are too hot to touch during operation. Even the light emitted by a light source is dissipated as heat in the workspace.

This heat must be removed by the building’s cooling system to ensure the comfort of its occupants. In large buildings in some regions, non-perimeter spaces must be cooled all year due to heat generated by electrical devices and environmental factors.

Since lighting is a major contributor to heat build-up, it is understandable that if we upgrade a lighting system so that it consumes less energy during operation, we will be able to reduce the amount of heat that must be removed. This generates air conditioning energy savings that can supplement the energy savings derived from the upgrade. Engineering studies on buildings have confirmed this positive relationship.

The question is, “How much?”

The second question is, “What about the extra cost of heat?”

In this article, we will explore the rule of thumb popularly applied to determining air cooling savings resulting from an upgrade, the problems with taking this rule of thumb too seriously, and a possible compromise solution that helps render a more accurate picture. We will conclude by walking step by step through formulas that can be used to roughly capture air cooling savings, cost of heat gains, and reduced HVAC capacity.

THE RULE OF THUMB A rule of thumb is that about one watt of air cooling energy savings result from every three watts of lighting energy savings. If we reduce the total lighting load, in other words, we can expect additional net energy savings of 30-40%. This amount of cost savings can be significant when conducing an economic analysis of an upgrade, as it makes the payback appear to be more attractive. Most corporations require a maximum three-year payback on capital investments to qualify for funding.

Unfortunately, there are problems with this rule once we examine the heating, ventilation and air conditioning (HVAC) system, building type and local climate for an individual building.

During the winter, heating requirements will conversely increase if the lighting system is no longer contributing to the heating, presenting a counteractive cost that must be accounted for. Different regions of the country experience different lengths of cooling seasons, result in varying energy savings. Netting an additional 30% energy savings is more typical in southern U.S. states such as Florida and Louisiana, where the summers are much longer and hotter.

In Chicago, for example, an energy simulation of an office building in this relatively cold city shows that the rule of thumb does not apply. In the studied office building, an upgrade from a traditional fluorescent system—fluorescent cool white lamps powered by magnetic ballasts—to an energy-efficient system—T8 lamps powered by electronic ballasts—reduced the lighting load by 4W per square meter. However, the cost of additional heating requirements resulted in a net loss of 3% to lighting energy savings. Not much. But no net HVAC savings. On the other hand, the building shaved its peak demand by 1.1W per square meter, resulting in utility demand-charge cost savings. Compare this an office building studied in Bangkok, Thailand, where an upgrade to energy-efficient lighting increased total energy savings by 23%, or to a hotel operating 24 hours a day, where an upgrade increased energy savings by 56%.

Therefore, to generate a more realistic figure for net HVAC savings resulting from a lighting upgrade, we must address:

Environmental variables. The most important variable of all is the length of the cooling season. The longer the cooling season, the greater the cooling energy savings. Regional weather conditions and the building type and envelope efficiency should also be considered to generate an accurate picture.

Building variables. These include the efficiency of the building envelope, the size of the building, and the size of the non-perimeter space, where air cooling savings will be greatest.

System variables. These include the efficiencies of the cooling and heating systems and thermostat setpoints during the summer. For example, cooling systems can be three times more efficient than direct-resistance heating systems, reducing the value of cooling savings. And during the cooling season, some buildings allow the interior temperature to increase incrementally, also offsetting cooling savings, albeit a little.

Economic variables. These include the cost of electricity (cooling) versus the cost of fuel (heating) from the building’s energy provider, and whether this utility imposes higher charges for electricity use during each day’s peak period and a “ratchet” clause, locking in the demand charge at the highest electrical energy cost during previous months. Typically, electricity is more costly than fuel, enhancing the value of HVAC energy savings resulting from a lighting upgrade. And with peak charges or a ratchet clause, reducing electrical energy consumption appears all the more attractive.

That’s a lot of variables. To precisely determine the savings for our building, we would have to use a whole-building energy simulation model where factors change by the hour, day and month of the year. If maximum accuracy is desired, then that is the path that must be taken. But there is a “compromise” that can be used by injecting an important variable, the cooling season, into the rule of thumb formula based on reasonable assumptions about the building’s HVAC system.

A MORE ACCURATE PICTURE Robert Rundquist, PE, offers a formula with supporting data and estimates to assess air cooling savings. Rundquist, president of R.A. Rundquist Associates of Northampton, MA, is a professional engineer with more than 25 years’ experience in HVAC design, energy analysis and energy calculation research. His formula was derived from independent research and research conducted by the American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE), and has been validated by DOE-2 computer runs and other methods.

Determining Air Cooling Savings Lighting energy consumption must be reduced by a specific amount that remains constant throughout the year. This is most predictable in a fixture retrofit, but can work for the use of advanced controls if the reduced hours of operation are predictable.

Determine the fraction of the year that represents the cooling season. In Table 1, we provide a list of 153 U.S. cities with their cooling season expressed in weeks. Divide by 52 to convert to a fraction of the year.

Click here to download Table 1 (Adobe Acrobat PDF).

Determine the fraction of the daily load met by mechanical cooling. Basically, how much of the lighting system’s heat must be removed by the cooling system versus dissipating through windows and other means? Rundquist estimates the fraction of the daily load met by mechanical cooling to be 90%.

Determine the air cooling system’s coefficient of performance. Tests on cooling systems have shown that for every watt put into the system, 2.7 watts of cool air is produced. The actual figure can vary due to a range of factors. While it is best to use the actual figure for the given system in use, an estimate of 2.7 can be used.

Calculate using the below formula:

Fraction of Lighting Savings as Air Cooling Savings = A x B ÷ C

A = Fraction of the Year of the Cooling Season B = Fraction of the Lighting Load Met by Mechanical Cooling C = System’s Coefficient of Performance

Example: Upgrade of office building in Spokane, WA, resulting in annual energy savings of 50,000kWh. The average cooling season for Spokane, according to Table 1, is 15.6 weeks. The average commercial cost per kWh, including all charges, is estimated at $0.06.

Fraction of Lighting Savings as Air Cooling Savings = 0.3 x 0.9 ÷ 2.7 = 0.10

This means that we have saved an additional average of 10%, or 5,000kWh, in air cooling savings as a result of our upgrade. We have saved $3,000 per year from the lighting upgrade and an additional $300 per year in air cooling savings.

Example: Upgrade of office building in Medford, OR, resulting in annual energy savings of 50,000kWh. The average cooling season for Medford, according to Table 1, is 21.2 weeks. The average commercial cost per kWh, including all charges, is estimated at $0.06.

Fraction of Lighting Savings as Air Cooling Savings = 0.41 x 0.9 ÷ 2.7 = 0.14

This means that we have saved an additional average of 14%, or 7,000kWh, in air cooling savings as a result of our upgrade. We have saved $3,000 per year from the lighting upgrade and an additional $420 per year in air cooling savings.

Determining The Cost of Heating Gains During the heating season in colder climates, the removal of the heat byproduct of lighting system operation can result in higher heating costs. In most large buildings, the additional heat would only be needed in the perimeter zone, because the interior spaces must be cooled all year. This cooling is produced by an economizer in most buildings.

Rundquist proposes the below formula to determine additional heating costs:

Extra Heat Required (BTU) = A x B x C ÷ D

A = Heating Season = 1 – Fraction of the Year Representing the Cooling Season Liberal estimate of the heating season, as there are times during the year when the building is neither heated nor cooled. B = Fraction of the Lighting Reduction that Has to Be Made Up by Heating A portion of the lighting heat is released at night. This figure is estimated at 20%. C = Annual BTU Equivalent of Lighting Saved Lighting reduction in kWh multiplied by 3,414 British Thermal Units (BTU). D = Seasonal Heating Efficiency Estimate of basic efficiency of heating system. Heating system efficiency can vary from about 65-95%, depending on the type, use and technology. We will estimate 80% for our heating system.

Example: Upgrade of office building in Spokane, WA, resulting in annual energy savings of 50,000kWh. The average cooling season for Spokane, according to Table 1, is 15.6 weeks. The heating season is liberally estimated at 36.4 weeks. The air cooling energy savings comes to approximately 5,000kWh, or $205 per year.

Extra Heat Required (BTU) = 0.7 x 0.2 x (50,000kWh x 3,414 BTU/kWh) ÷ 0.8

Extra Heat Required (BTU) = 29,872,500

Extra Heat Required (Therms) = 29,872,500 ÷ 100,000 = 299

Assuming a cost of $0.9 per therm = $269/year additional heating cost

Net HVAC Savings = $300/year - $269/year = $31/year

Example: Upgrade of office building in Medford, OR, resulting in annual energy savings of 50,000kWh. The average cooling season for Medford, according to Table 1, is 21.2 weeks. The heating season is liberally estimated at 30.8 weeks. The air cooling energy savings comes to approximately 7,000kWh, or $420 per year.

Extra Heat Required (BTU) = 0.6 x 0.2 x (50,000kWh x 3,414 BTU/kWh) ÷ 0.8

Extra Heat Required (BTU) = 25,605,000

Extra Heat Required (Therms) = 25,605,000 ÷ 100,000 = 256

Assuming a cost of $0.9 per therm = $230/year additional heating cost

Net HVAC Savings = $420/year - $230/year = $190/year

Conclusions In a building in Medford, OR, a lighting upgrade generated $3,000 per year along with $190 per year in net HVAC savings, or a 6% increase. Not bad. The same upgrade in a building in Spokane, WA, only increased total energy savings by 1%, or $31 per year in net HVAC savings added to $3,000 in lighting energy savings. In warmer climates with longer cooling seasons, and in regions such as the east coast and California where energy prices are much higher, we can see more substantial benefits. This same upgrade in Houston, TX or Jacksonville, FL, would have generated a 26% increase in air cooling energy savings with fewer losses for heat gains. However, when justifying the economic returns of investing in the lighting system, every little bit helps. Capture as many variables as accurately as possible for the given application to determine net HVAC savings for your lighting upgrade project.