HVAC Equipment Efficiency
So you’re in the market for some new HVAC equipment, a heat pump, maybe a boiler, or a rooftop unit, and you want to know which one is better. Comparing pricing, features, and…efficiencies…what are all of these acronyms? Isn’t efficiency just a percentage? And why are there different acronyms?
Let’s take some time to talk about equipment efficiency. What is it? and How is it measured?
What is efficiency?
In simple terms, efficiency is the ratio of useful work the system does to the amount of energy it takes in. When we are talking about HVAC equipment, they take in a certain amount of energy in order to heat or cool a space. The work done by the system is in the cooling or heating process. A majority of the lost energy is in unwanted heat generated by the system. If you have ever felt the back of your fridge, felt the outside AC compressor, or even your laptop while it’s running, you’ll notice the heat from it. This heat is the lost energy. By increasing the efficiency of a system, you are reducing the amount of heat lost on the system, or increasing the ratio of work done to the amount of energy put in.
How is efficiency measured?
In an Air Conditioner, there are multiple components. Depending on the type of unit, there is a compressor, fan, pump, evaporator, expansion valve, and a condenser. Each of these parts have their own efficiency. Efficiency can be measured for each of the individual components or of the unit as a whole. When you see an efficiency rating for an entire AC unit, it is taking in all or some of these components into consideration.
When the efficiency is measured, electricity drawn in is measured with an electrical meter. Heat is measured by the change in temperature in a closed and controlled environment. When measuring a unit for a rated efficiency there are certain space conditions and requirements to make sure performance is comparable from unit to unit.
Now, let’s take a look at the different acronyms:
Coefficient of Performance (COP)
This efficiency measurement can be used for any system, not just HVAC equipment. It has the basic formula of Power Out divided by Power In. If you recall, “efficiency is the ratio of useful work the system does to the amount of energy it takes in”, in other words, energy is the power you put in and work is the power you get out. COP is a unit-less number and can be used for any piece of equipment.
$ Q = Power Out $
$ W = Power In $
$ COP = Q/W $
Energy Efficiency Ratio (EER)
EER is used for Air Conditioners specifically and are measured under conditions of outside temperature 95°F, indoor temperature of 80°F with 50% humidity, and at 100% capacity.
EER has a similar equation to COP but the units are different. EER is comparing cooling capacity in British Thermal Units (BTU) and Watt-hour, so yes, EER is not unit-less, it’s unit is BTU/W/h.
$EER = \frac{Output Cooling Energy in BTU}{Input Electrical Energy in Wh} = \frac{Q}{W}$
Seasonal Energy Efficiency Ratio (SEER)
While EER is measuring the unit at one set of conditions, SEER simply means it measured the unit over the entire cooling season. It is measured by varying the outside and indoor air temperatures.
SEER is typically higher than EER. This is due to the fact that the efficiency of a unit is typically higher when the outside air is cooler, because it doesn’t have to work as hard. This higher efficiency during the colder days increases the overall efficiency.
$SEER = \frac{Seasonal Output Cooling Energy In BTU}{Seasonal Electrical Energy In Wh}$
Annual Fuel Utilization Efficiency (AFUE)
Alternatively, AFUE is used to measure equipment for heating. A heating equipment, like a furnace, intakes fuel instead of electricity. Combustible fuels have a known amount of stored energy per volume. For example 100 cubic feet of natural gas is equal to 103,700 BTU. This fuel density is used for the input energy. The equation is multiplied by 100 to make the result a percentage.
Heating Seasonal Performance Factor (HSPF)
HSPF is used for heat pumps specifically during the heating season. Heat pumps are a device that can heat or cool a space depending on which direction it is running in. There is a separate equation for heat pump heating because it uses electricity instead of fuel, hence another efficiency ratio is needed.
$HSPF = \frac{Output Heat In BTUs}{Input Electrical Energy In Wh} $
Integrated Part Load Value (IPLV)
While EER is measuring the unit at 100% capacity, IPLV measures the unit at what it says, part load. IPLV is typically used for larger, non-residential units like chillers. ARI Standard 550/590–1998 gives conditions for what the chillers should be measured at.
Hopefully, that gives you some further background when looking at the efficiency values on your new unit.
Let me know in the comments if you have any questions!
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“If you have ever felt the back of your fridge, felt the outside AC compressor, or even your laptop while it’s running, you’ll notice the heat from it. This heat is the lost energy.” Lightbulb moment, totally makes sense. Love the imagery. Math formulas scare me in general but you break everything down, definitely learned a lot!
Thanks for reading Taylor! I am so happy you found this useful. Math doesn’t have to be scary 🤓