In this article, I will explain four performance metrics of HVAC equipment:
- Coefficient of Performance (COP)
- Energy Efficiency Ratio (EER)
- Seasonal EER (SEER)
- Heating Seasonal Performance Factor (HSPF)
These metrics are indicators of performance in much the same way that miles per gallon is an indicator of car performance. We will focus primarily on the heat pump portion of HVAC equipment. Understanding the HVAC metrics will help you to evaluate equipment and qualify for utility rebates as well as federal tax credits under the EPAct legislation, which expires at the end of 2016.
I cannot stress enough the opportunity that the expiring EPAct credits offer. You owe it to yourself to consider them for energy conservation projects this year, including lighting. Additional resources listed below will help you to navigate the credit.
Fall is always a good time of year for an HVAC efficiency upgrade but this fall is special because it is the last year for federal tax credits of up to $1.80/square foot for energy conservation projects.
COP and Heat Transfer
An example of HVAC equipment transferring heat is the simple window air conditioner, which moves heat from inside a room to outside. In the heating season, the same equipment (if a model with a heat pump) can reverse the transfer and move heat from the outside to the inside.
The performance of such equipment can be expressed as the COP:
COP = (Energy moved) / (Energy input)
A heat pump can have a COP greater than 1 because it can move more energy than it takes to operate. You may think this fact violates a law of physics, but it doesn’t. The heat pump is not transforming energy, like burning methane to liberate heat and CO2. It only moves energy, similar to how a bicycle moves you farther than walking for the same energy input.
Let's say you have a room with three incandescent lights. To keep the room at constant temperature, you must move the heat from these inefficient lights out of the room. If your heat pump has a COP of 3, then you can move 3 units of heat for every one unit of input energy. If the amount of waste heat you want to move is 3 kWh, then you would spend only 1 kWh on fuel. This efficiency is why heat pumps are used for many air conditioning processes. They can also be used to heat buildings as long as outside temperatures are not so low that there is insufficient heat in the air.
Comparing Efficiencies with the COP
Using the previous example, if the three incandescent lights are replaced with three LEDs, less waste heat will need to be moved from the room. For simplicity’s sake, let’s say that post-retrofit, the heat pump doesn’t need to move three units of heat, but the reduction in energy on the downstream side saves only one unit of fuel because Einput= Emoved/COP. In most cases, when you are saving energy downstream, you should divide by the COP to determine the fuel savings (kWh or MMBTU).
COP is elegant and has no units. However, in the U.S. we express energy moved in units – BTUs, tons and ton-hours of cooling (1 ton is a rate of energy flow and equals 12,000 BTUs per hour). This measure is different than the units we use for electric energy input (kWh). Because the units are different (BTUs divided by kWh), the EER was invented, which is the common U.S. performance metric. Because we are only converting kWhs into BTUs (1 kW = 3,412 BTUs), EER has a linear relationship with COP:
EER = (COP) x (3.412)
So if you are ever given COP, you can always find EER and vice versa. In addition, you can use the equations below to find other performance metrics that may be useful to you:
COP = (Tonscooling) / (kWinput) = (EER)/(3.412)
Note that these performance metrics are dependent on some variables that include outside and indoor temperatures. For example, it is hard to reject heat to outside air already at a high temperature, which is why ground source heat pumps are a great solution. It is easier to reject heat to ground at 60 degrees F. ground than to outside air at 90 degrees F. During winter, it is also easier to extract heat from ground at 60 degrees than air at much colder temperatures.
Because EER and COP are based on instantaneous measurements, the industry developed the SEER or seasonal EER, which is an average performance metric for a heat pump over an entire summer season and a range of temperatures. Heating System Performance Factor (HSPF) is a similar metric determined by measuring heat transfer rates during the heating season.
The performance metrics are useful to understand when retrofitting HVAC systems. If you install a system with a high COP, you will save energy, usually more than enough to justify the extra equipment expense. Many utilities also give rebates for installing high EER/SEER/COP equipment because it helps them at peak periods when they are most constrained by demand.
In my area, I get a $25 per ton rebate if I install a system with an EER greater than 12.5. My commercial building has about 20 tons of cooling equipment, which means I can spend an extra $500 to buy more efficient systems. I will also save energy – and dollars – worth far more than that rebate over the equipment’s life.
Take the Tax Gift
This is the year to upgrade your HVAC and lighting equipment. If you get a qualifying retrofit done by Dec. 31, 2016, you can get up to $1.80 in tax credit per square foot. This extra benefit can help make a 50% ROI become a 100% ROI.
In a free webcast, Dr. Woodroof provides his 2016 update on the environment and energy management solutions. This webinar's information will impact most areas of energy management, including tax benefits and utility adaptation. It will help you get approval for your energy conservation project. To view the webinar, click here.