What does a Vermonter know about air conditioning? I live so close to the Canadian border that half of the radio stations are in French. If my house needs cooling, I just let the fire in the wood stove die down.
When I first began reporting on air conditioning topics over a decade ago, I felt out of my element. Impelled by the certainty that there’s no such thing as a dumb question, I’ve managed over the years to badger a few air-conditioner experts, all of whom contributed to my education. So now I finally know the difference between an evaporator coil and a condenser coil.
In no particular order, here are the answers to a few air-conditioning questions.
Q. What’s a “split” air conditioner?
A. A split air conditioner is the typical central air conditioner found in many U.S. homes. It consists of an outdoor unit that sits on a small concrete pad and an indoor coil located in the furnace plenum or the air handler. The term “split air conditioner” distinguishes this type of cooling system from other types of air conditioners, including window units.
Two thousand pounds of … I dunno
Q. What’s a ton of cooling?
A. In the old days, people used to buy ice to keep cool. A “ton” of cooling capacity is based on the amount of heat absorbed by one ton of ice melting over 24 hours. One ton of cooling capacity is equal to 12,000 Btu/h.
Maybe “Dental X-Rays”?
Q. What does “DX” stand for?
A. DX stands for Direct eXpansion — the standard refrigeration cycle used in most American air conditioners. (There are other refrigeration cycles — for example, the ammonia absorption cycle used in propane-fueled refrigerators.)
Q. What’s a SEER rating?
A. SEER stands for Seasonal Energy Efficiency Ratio. An air conditioner’s SEER rating is its total cooling output in BTU during a “normal” annual cooling season divided by the unit’s total energy input in watt-hours.
The EER (energy efficiency ratio) is a similar rating that is based on the unit’s performance during peak conditions (typically 95°F).
The higher a unit’s SEER or EER, the more efficient the unit. The minimum permissible SEER for residential split-system air conditioners is 13.0.
Q. What’s the difference between the condenser and the compressor?
A. Most people use the term “condenser” to refer to the outdoor unit sitting on the concrete pad. The condenser has several components, including the compressor (a pump used to compress the refrigerant), the condenser coil (also known as the outdoor coil), and the condenser fan. The condenser fan blows outdoor air over the condenser coil to cool the refrigerant in the coil.
Q. What’s a two-speed condenser?
A. A two-speed condenser operates at a low setting (for energy efficiency) in moderate weather and a high setting in very hot weather. A two-speed condenser usually has a two-speed compressor as well as a two-speed condenser fan motor.
Nuts and bolts
Q. How does an air conditioner work?
A. The refrigerant cycle depends on five components: the compressor, the refrigerant, the condenser, the expansion valve, and the evaporator coil. The compressor is the pump that moves the refrigerant. When the refrigerant passes through the condenser, it starts out as a vapor. As the refrigerant’s heat is extracted by the air flowing over the outdoor coil, the refrigerant cools and condenses back into a liquid.
After the refrigerant leaves the condensing coil, it passes through the expansion valve, which is a valve with a very small opening. While the entry side of the expansion valve is at a high pressure, the far side of the valve is at a much lower pressure. Gases cool as they expand; the refrigerant expands suddenly as it finds itself in a low-pressure environment and therefore cools. The valve turns the refrigerant into a cold, fast-moving mist.
The cold refrigerant circulates through the indoor evaporator coil located in the furnace plenum, where the furnace fan blows indoor air over the coil. The evaporation of the refrigerant in the indoor coil occurs at a very low temperature — this low boiling point is a characteristic of the fluid chosen as a refrigerant — and (as you may remember from high school) the evaporation process draws heat to the boiling refrigerant from its surroundings. (As a liquid evaporates, it absorbs heat.) The warm air blowing over the coil is cooled as the refrigerant warms up.
The refrigerant leaves the coil as a warm vapor; it then cycles back to the outdoor condenser.
Q. What’s a heat pump?
A. An air conditioner removes heat from a house and dumps it in the back yard. If you reverse its operation during the winter, the same equipment can be used to remove heat from the back yard and dump it indoors. Such a reversible air conditioner is called a heat pump.
Some people wonder how a heat pump can scavenge any useful heat from the outdoor air when it’s only 35°F out. The answer is that, through the magic (well, the physics) of the refrigeration cycle, as long as you can lower the temperature of the outdoor air, you can extract useful heat. So, if you lower the temperature of a volume of outdoor air from 35°F to 27°F, you can extract heat from the outdoor air and dump the heat indoors — raising the indoor temperature from, say, 69°F to 71°F.
Wringing the air dry
Q. How does an air conditioner help to dehumidify a house?
A. When warm, humid indoor air blows over the indoor evaporator coil, some of the moisture in the air condenses on the cold coil. After a while, the coil begins to drip. A pan under the coil collects the dripping condensate; after several minutes, the water begins to accumulate in the pan and eventually goes down the drain.
If the air conditioner short cycles — that is, if the home’s thermostat is quickly satisfied because the air conditioner is too big — then the air conditioner may shut off before the condensed moisture has time to find its way down the drain. If that happens, all of the moisture on the cold coil and in the pan re-evaporates, and the moisture stays in the house.
The moral of the story: a small air conditioner with long duty cycles can do a better job of dehumidification than a large air conditioner with short duty cycles.
Not overt but latent
Q. What is latent cooling?
A. An air conditioner provides both sensible cooling — that is, it lowers the temperature of the air flowing through the air handler — and latent cooling — that is, it dehumidifies the air flowing through the air handler.
Dehumidification is a type of cooling because the process of dehumidification adds heat to the air. Water has a latent heat capacity of about 1,000 BTU per pound. If you’re dehumidifying indoor air, you need to use some of your equipment’s cooling capacity — even if don’t end up lowering the indoor air temperature at all.
The ratio between a building’s sensible heat load and its total heat load (including the latent load) is called the sensible heat ratio. The sum of sensible energy and latent energy is called enthalpy. (“ERV” stands for “enthalpy-recovery ventilator.” Since few people understand the meaning of the word “enthalpy,” ERV manufacturers decided to start telling their customers that ERV stands for “energy-recovery ventilator.” They figured that lying about the acronym’s origin was easier than explaining what “enthalpy” means.)
Under peak conditions, the typical residential air conditioner provides about 70% of its cooling capacity as sensible cooling and 30% as latent cooling. In a hot, humid climate, however, it’s possible for 40% or 50% of a home’s cooling load to be a latent load. That’s why many energy-efficient houses in Houston require supplemental dehumidification — often provided by a stand-alone dehumidifier.
Air flow adjustments
Q. If I want more moisture removal — more latent cooling — do I want more air flow or less air flow over the indoor coil?
A. As a rule of thumb, air conditioner manufacturers recommend that a residential cooling system should provide 400 cfm of air flow per ton of cooling capacity. However, lower air flows (less than 350 cfm per ton) will provide more latent cooling.
Reducing the air flow to improve a unit’s latent capacity only works up to a point, however. If you slow the air flow too much, the coil can freeze.
Last week’s blog: “Duct Leakage Testing.”