7.2 Cooling and Heating Equipment: Chillers (COP, kW/ton, EER, SEER), Heat Pumps, and Boilers

Cooling and Heating Equipment

The central plant is the beating heart of a commercial facility's HVAC system. It typically houses massive equipment responsible for generating chilled water, hot water, and steam. Because this equipment consumes so much energy, even minor improvements in efficiency yield massive financial savings. The CEM must deeply understand how chillers, heat pumps, and boilers operate, as well as the standard metrics used to benchmark their performance.

Chiller Systems and Efficiency Metrics

Chillers use a vapor-compression refrigeration cycle (or an absorption cycle) to remove heat from a liquid, which is then circulated through the building to cool the air. The vast majority of large commercial buildings use water-cooled centrifugal chillers because they offer superior efficiency at large capacities. Air-cooled chillers are common in smaller facilities or locations where water conservation is paramount.

Chiller efficiency is measured in several different ways, and you must be able to convert between them flawlessly.

1. kW/ton

The most common metric for large commercial chillers is kW/ton, which expresses how many kilowatts of electrical power are required to produce one ton of cooling (12,000 Btu/hr). Lower is better. A highly efficient modern water-cooled centrifugal chiller might operate at 0.50 kW/ton or lower at design conditions.

2. Energy Efficiency Ratio (EER)

EER is the ratio of the cooling capacity (in Btu/hr) to the electrical power input (in Watts) at a specific set of standard operating conditions. Higher is better. It is widely used for packaged rooftop units and air-cooled equipment.

3. Seasonal Energy Efficiency Ratio (SEER)

SEER is similar to EER but evaluates the equipment's performance over an entire cooling season, factoring in varying outdoor temperatures and part-load conditions. Higher is better. It is predominantly used for smaller residential and light-commercial split systems.

4. Coefficient of Performance (COP)

COP is a dimensionless ratio of useful cooling (or heating) output to the energy input, both measured in the same units (e.g., Btu out divided by Btu in, or kW out divided by kW in). Higher is better. A COP of 4.0 means the chiller provides four units of cooling energy for every one unit of electrical energy consumed.

Essential Conversion Formulas

Because energy inputs and outputs are measured in different units (kW, Tons, Btu/hr), we rely on conversion constants (1 Ton = 12,000 Btu/hr; 1 kW = 3,412 Btu/hr). The critical conversion formulas are:

• EER = 12 / (kW/ton)
• kW/ton = 12 / EER
• COP = EER / 3.412
• COP = 3.517 / (kW/ton)
• kW/ton = 3.517 / COP

Worked Example: An existing air-cooled chiller has an efficiency rating of 1.2 kW/ton. A vendor is proposing a replacement unit with an EER of 14. What is the efficiency of the new unit in kW/ton, and what is its COP?

First, find the new unit's kW/ton: kW/ton = 12 / EER kW/ton = 12 / 14 = 0.857 kW/ton

Next, find the COP of the new unit: COP = EER / 3.412 COP = 14 / 3.412 = 4.10

The new chiller (0.857 kW/ton) is significantly more efficient than the existing one (1.2 kW/ton).

Heat Pumps

Heat pumps operate using the exact same vapor-compression refrigeration cycle as chillers, but they include a reversing valve that allows the cycle to run backward. Instead of extracting heat from the building and rejecting it outdoors, a heat pump in heating mode extracts heat from the outdoor air (or ground) and "pumps" it indoors.

Because they move heat rather than generating it through combustion or electrical resistance, heat pumps are incredibly efficient. Electric resistance heating (like a space heater) has a maximum theoretical COP of 1.0. An air-source heat pump typically operates with a heating COP between 2.5 and 4.0, meaning it delivers 2.5 to 4 times more heating energy than it consumes in electricity. Ground-source (geothermal) heat pumps can achieve even higher COPs because the ground temperature remains stable year-round.

When evaluating heat pumps for heating, we often use the Heating Seasonal Performance Factor (HSPF), which is the heating equivalent of SEER (total space heating required in Btus divided by the total electrical energy consumed in watt-hours over the season).

Boilers and Combustion Efficiency

Boilers provide hot water or steam for space heating, domestic water, or industrial processes. They burn fossil fuels (natural gas, fuel oil) or use electricity. For combustion boilers, optimizing the combustion process is the primary pathway to energy savings.

Combustion Basics and Excess Air

Perfect (stoichiometric) combustion requires an exact ratio of fuel and oxygen. If there is too little oxygen, incomplete combustion occurs, producing dangerous carbon monoxide (CO) and wasting fuel. To prevent this, boilers are operated with "excess air." However, if there is too much excess air, the extra nitrogen and oxygen absorb heat and carry it straight up the exhaust stack, drastically reducing efficiency.

The energy manager's goal is to tune the boiler to the "sweet spot"—just enough excess air to ensure complete, safe combustion, but no more. This is typically monitored using a flue gas analyzer that measures O2, CO2, and stack temperature.

Stack Temperature

The temperature of the exhaust gases leaving the boiler (the stack temperature) is a primary indicator of heat transfer efficiency. If the stack temperature is too high, it means heat is escaping up the chimney instead of transferring to the water. High stack temperatures can be caused by soot buildup on the fireside of the tubes or scale buildup on the waterside of the tubes. Regular tube cleaning and robust water treatment programs are essential.

Boiler Efficiency Types

• Combustion Efficiency: Simply a measure of how well the burner completely burns the fuel without wasting heat to excess air.
• Thermal Efficiency: Includes combustion efficiency plus the effectiveness of the heat exchanger in transferring heat to the water.
• Fuel-to-Steam (Overall) Efficiency: The true operational efficiency, accounting for combustion, thermal transfer, and radiant casing losses from the boiler jacket.

Modern condensing boilers achieve extremely high efficiencies (up to 95%+) by recovering the latent heat from the water vapor in the exhaust gases. They do this by lowering the return water temperature enough to condense the exhaust gases, capturing energy that would otherwise be lost up the stack.