SMART AND COOL-THE ART OF AIR CONDITIONING
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SMART AND COOL—THE
HOME ENERGY
ART OF AIR CONDITIONING
Common misconceptions can contribute to a home environment
that is too cold, too hot, or too humid.
BY JOHN PROCTOR
I f you’re confused about how to get an
air conditioning system to work well,
you are not alone. Last October,
builders loaded with questions showed up
for a talk I gave at the Energy and Envi-
ronmental Building Association (EEBA)
conference. Most of the builders’ ques-
tions were about their own homes—the
ones they live in. Some complained that
they were blasted with cold air when the
air conditioning unit was on; others com-
plained that the house was uncomfortably
humid. We started talking about why
PROCTOR ENGINEERING
oversized air conditioners cause both of
these problems—and also why the situa-
tion is worse now than it was in 1995,
when Home Energy published “Bigger Is
Not Better.” During that discussion last How much air conditioning is enough?
October, builders vented strong frustra-
tions over the numerous problems they’ve
experienced in their attempts to get their
customers and their HVAC contractors to review of cooling terminology, see the answer is clear:The single accelera-
understand the advantages of properly “Key Cooling Terms,” p. 50.) tion onto the freeway is much better than
sized air conditioners.These frustrations the stop and start of the city driving.
and questions are not at all unique to this “My friend got a new air condi- The situation is the same for air con-
group of builders; I get these types of tioner that was smaller than the one ditioners. Larger air conditioners start and
questions at every presentation I give. she had before. It runs all the time, stop all the time; they waste your pre-
and that’s a problem.” Here is a cious money and contribute excessively
Homeowner related question: Is it more economical to to pollution. These problems all stem
hop in an eight-cylinder SUV and go 1 from the fact that whenever an air condi-
Misconceptions mile, racing each block and stopping at tioner starts up, it is very inefficient. Once
What follows are some of the every corner, or to hop in a six-cylinder you have it running, you want it to keep
misconceptions and questions that per- car and drive 1 mile on the freeway from running as long as you can.The air con-
sist about air conditioning systems, as one entrance ramp the next exit? In term ditioner’s “gas mileage” is measured in
well as my responses to them. (For a of gas mileage, pollution, and economics, 1,000 Btu per kilowatt hour. Let’s call
46 www.homeenergy.org 2005 SPECIAL ISSUE • HOME ENERGY
AIR CONDITIONER EFFICIENCY “MILEAGE"
ing system that keeps the inside
100 humidity low, distributes the
cooling to all rooms of the
90 Steady state (continuous house based on what each
running) "highway mileage"
efficiency when run continuously
Kbtu/kWh as % of steady state
room needs, is quiet, and does
HOME ENERGY
80
not produce blasts of cold air.
70
Homeowners from West
60 Texas to the California coast live
50
in a dry climate.They generally
only have to worry about how
40 much the air conditioner lowers
30 the temperature measured by
Cycling "city the thermostat (the dry bulb
20
mileage" temperature). The ability of an
10 air conditioner to lower dry
PROCTOR ENGINEERING
0
bulb temperature is known as
0 1 2 3 4 5 6 sensible capacity.
Compressor on-time (minutes) Homeowners in the rest of the
United States, and in particular western
Figure 1. The efficiency “electricity mileage” (kbtu/kWh) improves when the air conditioner stays on microclimates, have to worry about the
longer. The total energy consumption drops because there are also longer pauses between cycles when amount of moisture the outside air brings
the air conditioner does not run. into the home.When it is hot and humid
outside, the air entering the building is
cooled, but unless moisture is removed
AIR CONDITIONING MOISTURE REMOVAL from the incoming air, the indoor relative
16 humidity rises, often making a home feel
Blower on and off
14
with indoor unit damp and uncomfortable.
A standard thermostat does not
12 measure the amount of moisture in the
% of total cooling capacity
home. So setting the thermostat lower
does not ensure comfort. What we
Moisture removal
10
need in the hot-humid climates is to
8 remove moisture.
Can an air conditioner do that? Yes
6
and no. Air conditioners almost always
4 remove some moisture from the air, and
Continuous blower that moisture stays on the inside coil.
2 (The ability of an air conditioner to
remove moisture is known as latent
0
PROCTOR ENGINEERING
capacity.) But in order to remove a signif-
0 5 10 15 20 25 30 35 40 45 50 55 60
icant amount of moisture, the air condi-
Compressor on-time (minutes)
tioner must run long enough for the
condensed water to run off the coil and
down the condensate drain. For a coil
Figure 2. An air conditioner connected so that the blower turns on and off with the compressor that is starting dry, this can be as long as
provides the most moisture removal. In addition, that moisture removal improves dramatically when the 10 to 20 minutes. A short run time—
compressor runs longer. Continuous blower operation, on the other hand, reduces moisture removal to which is what larger air conditioners
zero, unless the compressor runs for more than 13 minutes. generally provide—fails to remove suffi-
cient moisture in a wet climate. An air
conditioner connected so that the blower
that figure electricity mileage.The elec- “A bigger air conditioner will turns on and off with the compressor
tricity mileage of the air conditioner gets make me more comfortable.” provides the most moisture removal. In
better the longer it runs (see Figure 1). Unfortunately, the intuitive way to addition, moisture removal improves dra-
The simple fact is that larger air condi- achieve comfort—put in a bigger air matically when the compressor runs
tioners run very short cycles, with worse conditioner—is incorrect. Instead, longer (see Figure 2). Smaller air condi-
electricity mileage, than smaller units. comfort comes from an air condition- tioners will run longer and do a better
HOME ENERGY • 2005 SPECIAL ISSUE www.homeenergy.org 47
job of removing moisture than
larger units.The effect of running
the blower all the time (an
increasing practice) is dramatic as
well—in a very different way.
Under continuous blower opera-
HOME ENERGY
tion, moisture removal is zero
until the compressor is running
over 13 minutes every time it
comes on. Most compressor runs
are less than 10 minutes.
“Are you saying that run-
ning the blower continuously
is a bad idea?”You bet! First of
all, running the blower continu-
PROCTOR ENGINEERING
ously will cost you big time. In
Wisconsin, for example, running
a standard blower year-round will
cost you an additional 3,560 kWh ($365
at 10¢/kWh) just to pay for running the
Proper installation takes time and competent technicians.
blower. On top of that, running the
blower all the time increases duct system
losses.The outside air leakage into the
house increases by a factor of four in AIR HANDLER kWh
many houses when the blower is on. • ducts that leak over 20% of
This brings hot moist air into your 8 the air they carry;
home in the summer, and cold air into • low-quality brazed joints
your home in the winter. that leak refrigerant;
6
• refrigerant contaminated
PROCTOR ENGINEERING
“A bigger air conditioner doesn’t with air, moisture, oxidation, and
kWh
cost that much more.” If your con- 4 particles; and
tractor is offering to upsize your air con- • callbacks, warranty costs,
ditioner for very little more, he or she is 2 and frustrated customers.
cutting corners that will cost you comfort It is worth paying extra to
and money. A larger air conditioner 0 have a duct system that is tested
requires a larger furnace (air handler) and 4-ton 3-ton and shown to have less than 10%
larger ductwork. Without proper ducts leakage (a new duct system
and a proper furnace, the air conditioner Figure 3. In a side-by-side test of identical homes, the 3-ton should be built and tested to leak
will be noisy and inefficient. air conditioner ran 30% more than the 4-ton unit. The 3-ton less than 6%). It’s also worth-
air handler still used 27% less energy.
Furthermore, it is more difficult to while to have the installation
install the larger ductwork to get commissioned and verified.
proper flow to every room.The larger These two steps alone will pro-
furnace has a larger blower motor that furnace (air handler).These components duce an average energy savings of
uses more energy. There is significant are shipped from the factory, and final 24%–35%, depending on the local cli-
increase in blower energy consumption assembly takes place at your home.The mate and on air conditioner usage.
when a 4-ton air handler is used outdoor unit is joined to the inside coil
instead of a 3-ton air handler in other- by a set of copper lines that carry the “If I’m not comfortable, it’s the
wise identical homes (see Figure 3). refrigerant to and from the inside coil. HVAC contractor’s fault.” A contrac-
Builders or homeowners who look only tor’s ability to make a homeowner com-
“Any new air conditioner is as to the lowest bid are pretty much guar- fortable is severely limited by the
good as any other.” Most residential anteed to get what we call a business as performance of the house and by the
air conditioners consist of an outdoor usual installation.These installations are willingness of the builder or homeowner
unit and an indoor unit. The outdoor fast and cheap, but they result in a num- to pay for a high-quality installation.
unit consists of the compressor, the con- ber of problems.Those problems include In some ways, buildings have gotten
denser coil, and the outside fan. The • too much or too little refrigerant; much better over the last 15 years. Insu-
indoor unit consists of an inside coil on a • low air flow through the inside coil; lation is more common, low solar heat
48 www.homeenergy.org 2005 SPECIAL ISSUE • HOME ENERGY
AIR FLOW WATT DRAW
1,800 800
Standard (PSC) Motor
1,600 ECM Motor 700
1,400
HOME ENERGY
600
Air flow (CFM)
1,200
Standard (PSC) Motor 500
Watts
1,000
400
800 ECM Motor
300
600
400 200
200 100
0 0
0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0
Static pressure (IW) Static pressure (IW)
Figure 4. The air flow and watt draw of a standard permanent split capacitor (PSC) motor as well as of an electrically commutated motor (ECM) in
the same furnace with the same blower are shown here. The flow and watt draw drops when a PSC motor encounters increased flow resistance. An
ECM system works to maintain a constant air flow and the watt draw rises with increased flow resistance. (Figure courtesy of Bruce Wilcox, LBNL,
and PG&E Laboratory).
gain roofs are available, and newer, high- by always installing large air condition- most exciting results we have seen
efficiency glass can block much of the ers.This means that your new air condi- involve contractors who take pride in
heat gain during the summer. In many tioner will do a poor job of removing doing a better job and are comfortable
homes, 50% or more of the heat gain at moisture, will have low air flow across with ethical third-party verification to
the hottest part of the day comes the inside coil, will be noisy, will cost their customers.These contractors refuse
through the windows. When low solar you more to purchase, and will increase to sell business as usual jobs. They are
heat gain windows are used, the air con- your utility bill for 15 to 20 years. paid for their higher value, have high
ditioner can often be downsized by as customer satisfaction, and experience
much as 1 ton. Contractor Misconceptions very few callbacks and warranty calls.
None of these improvements has
reduced the amount of moisture gener- It’s not just the homeowners who “If I use ACCA Manual J, I have
ated in the home or the amount of are confused. Here is the contractors’ to use higher outdoor temperatures
moisture entering the home with out- side of the equation. than they specify and lower indoor
side air.As the heat gain through walls, temperatures than they specify. And
roofs, and windows is reduced, moisture “Customers will not pay for I have to add a fudge factor at the
removal becomes a larger and larger part quality; they want the biggest air end just to make sure there is
of the cooling load. Simply put, if you conditioner for the least money.” enough cooling.” This is a very com-
live in a leaky house, most thermal There will always be customers—and mon misunderstanding among contrac-
improvements will make your house builders, too—who just want the lowest tors. To figure out whether Manual J
more susceptible to moisture problems. price. However, there is a growing produces load estimates that are too
Therefore reducing house air leakage is recognition that smaller air conditioners small, we monitored homes in three
a very high priority in moist climates. do a better job of dehumidification, and states. We found that, on average, the
that better installations save energy and actual sensible load was two-thirds of
“My contractor can tell what money. Installation specifications are what Manual J estimated. This means
size air conditioner I need just by being enforced in some states and are that Manual J actually overestimates a
calculating the floor area.” If you let under consideration by EPA for Energy home’s air conditioning needs. Simply
the contractor use this method, he will Star products. put, if the contractor uses Manual J
probably oversize your air conditioner by One of the biggest hurdles you will without any fudge factors and selects an
a ton or more. In order to use the floor face is finding a way to convince the air conditioner to just meet the sensible
area method, contractors have to deal customer that you are doing a better job and moisture removal loads, the air con-
with all the differences between homes than your competitors. Some of the ditioner will be properly sized. In addi-
HOME ENERGY • 2005 SPECIAL ISSUE www.homeenergy.org 49KEY COOLING TERMS
Sensible cooling load. The heat continuously. The total capacity is the SEER. The seasonal energy
gain of the home due to conduction, sum of the latent capacity (ability to efficiency ratio is a standard method of
solar radiation, infiltration of outside remove moisture from the air) and sen- rating air conditioners based on three
HOME ENERGY
air, appliances, people, and pets. Burn- sible capacity (ability to reduce the dry tests. All three tests are run at 80°F
ing a light bulb, for example, adds only bulb temperature). Each of these capac- inside and 82°F outside. The first test
sensible load to the house. This sensible ities is rated in Btu per hour (Btu/h). is run with humid indoor conditions,
load raises the dry bulb temperature. The capacity depends on the outside the second with dry indoor conditions,
Dry bulb temperature. The tempera- and inside conditions. As it gets hotter and the third with dry conditions
ture measured by a standard thermometer. outside (or cooler inside), the capacity cycling the air conditioner on for six
Latent cooling load. The net drops. The capacity at a standard set of minutes and off for 24 minutes. The
amount of moisture added to the inside conditions is often referred to as "tons published SEER may not represent the
air by plants, people, cooking, infiltration of cooling." actual seasonal energy efficiency of an
of outside air, and any other moisture Tons of cooling. Air conditioner air conditioner in your climate.
source. The amount of moisture in the air capacity is rated at 95°F outside with Manual J. A widely accepted method
can be calculated from a of calculating the sensible and
combination of dry bulb latent cooling (and heating)
and wet bulb temperature loads under design conditions.
measurements. Manual S. The ACCA method
Wet bulb tempera- of selecting air conditioning
ture. When a wet cotton equipment to meet the design
wick is placed over a loads. It ensures that both the
standard thermometer sensible capacity and the
and air is blown across latent capacity of the selected
the surface, the water equipment will be adequate to
evaporates and cools the meet the cooling load.
thermometer below the Manual D. The ACCA
dry bulb temperature. method for designing duct sys-
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This cooler temperature is tems. Contractors often find it
called the wet bulb tem- a laborious process and most
perature, and depends on duct systems are just
how much moisture there installed—not designed. The
is in the air. amount of time necessary to
Design conditions. Cooling loads design a duct system is certainly
vary with inside and outside conditions. an inside temperature of 80°F and 50% warranted in tract construction, where
A set of conditions specific to the local relative humidity. Each ton of air condi- the design is used repeatedly, and for
climate is necessary to calculate the tioning is nominally 12,000 Btu/h (this custom homes, where the total cost of
expected cooling load for a home. Inside comes from the fact that it takes the home warrants a proper design. In
conditions of 75°F and 50% relative 12,000 Btu/h to melt a ton of ice over short, designing a duct system is essen-
humidity are usually recommended. Out- a 24-hour period). While an air condi- tial for proper equipment performance
side conditions are selected for the tioner may be called a 3-ton unit, it and customer comfort.
2.5% design point. may not produce 36,000 Btu/h. There is External static pressure. The pres-
2.5% design. The outside summer a wide variety of actual capacities that sure drop external to the furnace (air
temperatures and coincident air moisture are called 3 tons. handler). The external static pressure
content that will be exceeded for only EER. The energy efficiency ratio is includes the pressure drop across the
2.5% of the hours from June to Septem- the efficiency of the air conditioner. It A/C coil, any humidifier, any high-effi-
ber. In other words, 2.5% design condi- is capacity (in Btu/h) divided by the ciency air filter, and the duct system.
tions are outdoor temperatures that are electrical input (in watts). EER Most furnaces are certified at 0.5
historically exceeded in 73 of the 2,928 changes with the inside and outside inches of water column (WC) external
hours in these summer months. conditions, falling as the temperature static pressure. However, average sys-
Capacity. The capacity of an air difference between inside and outside tems operate at 0.75–1 inch WC.
conditioner is measured by the amount increases. EER should not be confused
of cooling it can do when it runs with SEER.
50 www.homeenergy.org 2005 SPECIAL ISSUE • HOME ENERGYtion, it will provide good comfort even if the indoor humidity is to be held at than they should, robbing cus-
when the weather is scorching hot. 50%, the latent load cannot exceed tomers of the high efficiency
25% of the total cooling load. When they paid for.
“Customers want air condition- these electrically commutated motors
ers that are oversized.” Customers (ECM) are faced with restrictive ducts, Ways Contractors
depend on the expertise of contractors filters, and other paraphernalia, they
in selecting an air conditioner.Yet con- are controlled to spin faster and work Can Control Moisture
HOME ENERGY
tractors generally size air conditioners at harder in an attempt to maintain the The primary actions to con-
least 1/2 ton larger than necessary and air flow.To do this, they increase their trol moisture in homes in moist
often oversize them by 1 ton or more. power draw. When the work gets too or wet climates are as follows:
Even the most conscientious contractor hard—above 0.8 inches of water col- • Reduce air leakage into the
is driven to avoid callbacks (or even umn (IWC)—they give up and may house through the building shell.
lawsuits). If the air conditioning system burn out.The watt draw of a standard • Provide controlled and ade-
isn’t working properly (duct leaks, permanent split capacitor (PSC) motor quate ventilation through
improper flow across the coils, improper doesn’t change much in relation to mechanical means.
charge), an oversized air conditioner changing air flow, while the watt draw • Eliminate duct leaks to
can mask the problem. Unfortunately, of the ECM increases sharply as it and from outside. Duct leaks
many customers think that cause moist air to
bigger is better, so in a com- be drawn in from
petitive situation, the contrac- outside.
tor who proposes a properly • Reduce internal mois-
sized unit may lose the bid. ture sources as much as pos-
Contractors are hesitant to sible. Use vent fans in areas
adopt an unfamiliar method where moisture is gener-
of sizing when the methods ated, such as bathrooms and
they have developed over the kitchens.
years have served them • Reduce heat gains to
well—“I’ve done it this way the return ducts by placing
for 30 years and I’ve never them within the building
had a complaint.” It is no sur- envelope or (if you have to
prise, then, that air condition- put them in the attic) by
ers are oversized. However, reducing their surface area
PROCTOR ENGINEERING
the advantages of a properly and insulating them well.
sized air conditioner are so • Use the smallest air
large that these barriers need conditioner that will meet
to be overcome. Customers Manual J estimated loads.
pay a price for oversized air This technician is taking the final readings for a third-party verification of a • Reduce the cooling
conditioners and, in many cli- high-quality installation. blower speed to 300–350
mates, lose comfort as well. CFM per ton.
The secondary actions
“The new air condi- to control moisture in
tioners with two-speed compres- attempts to maintain air flow (see Fig- homes in moist or wet climates are:
sors, variable-speed blowers, and ure 4, p. 49). • Precondition (dehumidify) the ven-
ECM motors will eliminate It is important to realize that the effi- tilation air by using an energy recovery
humidity and air flow problems. ciency gains of two-speed machines ventilator or a dehumidifier on the
On top of that, they use less occur primarily on low speed. When incoming air.
power and the customer can use a the temperature is very hot, two-speed • Add a dehumidifier.
constant fan.” Multispeed units step machines may not be better (and may
down to a lower capacity so that they sometimes be worse) than single-speed Recommendations
run longer. This helps with moisture 13-SEER or higher machines. This for Customers
removal. The variable-speed blowers occurs when the two-speed machines
can also be controlled to lower air flow run on their less efficient high speed There are many contractors who
when the humidity is high—a feature (see Figure 5). It is imperative that a would like to do the job right.“I love to
that helps to remove moisture. How- two-speed air conditioner runs on its do houses but only if we can do them
ever, there are real limits on what these low speed almost all of the time. Duct properly. I try to work inexpensively, but
machines can do. Even under the most losses and poor installation can cause not cheaply.” (One contractor’s response
favorable moisture removal conditions, these units to run on high speed more in a Florida Solar Energy Center survey).
HOME ENERGY • 2005 SPECIAL ISSUE www.homeenergy.org 51If you are purchasing air condi-
tioning equipment, here is what
you should do:
• Give the contractor the
Recommendations for Contrac-
tors listed below and insist that
HOME ENERGY
the contractor follow those rec-
ommendations.
• If the contractor wants to
size by floor area, find another
contractor.
• Insist on a copy of the calcu-
lations (or computer inputs and
outputs)—even if you don’t
understand them.
• Be willing to pay for the
time the contractor must spend
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to do the job right. Don’t take
the lowest bid.
Recommendations for
Contractors
Figure 5. At peak conditions (in this case a load of 26,000 But/hr at 82ºF outside temperature),
To offer your customers the most two-speed machines may not be better (and are sometimes worse) than single-speed SEER 13 +
energy-efficient and problem-free air machines. At high temperatures, two-speed machines run on their less efficient high speed.
conditioner, observe the following:
Sizing
• Calculate the sensible and latent 1.3 nor any other) to calculate the total Air Flow
loads for each installation using ACCA load from the sensible load. • Measure air flow using the pressure-
Manual J or equivalent. • Calculate ventilation load (latent matching method, the flow grid method,
• Use design outdoor conditions and and sensible) when mechanical ventila- or a method with equivalent accuracy.
daily temperature range exactly for your tion is used. • Verify that air flow is be at least 350
exact location per Manual J or ASHRAE • Select equipment based on the CFM per ton in dry climates and at
Handbook of Fundamentals. If this isn’t manufacturer’s detailed performance least 300 CFM per ton in moist or wet
possible, use the data for the closest loca- data. Do not rely on the nominal ton- climates, when measured through a wet
tion with a similar climate. nage at 95ºF outside. coil (unless the manufacturer specifies a
• Use standard 75°F design indoor lower air flow for the local design con-
temperature. Coil Matching ditions).Air flow must meet these crite-
• Pay great attention to window Correctly match the indoor evapora- ria before refrigerant charge tests.
type, material, and interior shading.An tor coil to the outdoor coil for the sys-
error in this area can throw off the tem, according to the manufacturer’s Refrigerant Charge
window heat gain estimate by as much specifications or Air Conditioning and Verify refrigerant charge using the
as 100%. Refrigeration Institute (ARI) standards. superheat method or subcooling
• Always account for the effect of the method. When weather conditions
overhang shading. This is one of the Brazing, Leak Testing, make this impossible, the charge may
most efficient load reduction measures. and Evacuation be weighed in.The charge adjustment
• Calculate infiltration rate depend- • Purge refrigerant lines and indoor must be calculated based on the differ-
ing on the airtightness of the building, coil with inert gas during brazing, to ences between the standard coil and the
based on blower door measurements. prevent oxidation. installed coil, and between the standard
While you are at it, measure the duct • Prior to start-up, evacuate systems lineset and the installed lineset.
leakage and fix it when the air condi- to 500 microns or less.
tioner is installed. • Isolate the system from the vacuum John Proctor, P.E. is president of Proctor Engi-
• Calculate the latent load based on pump and let sit for at least five minutes. neering Group, Ltd., in San Rafael, Califor-
the number of people in the building The micron gauge should not raise nia. He is a frequent contributor to Home
and the outdoor air humidity ratio. Do more than 300 microns above the initial Energy magazine.
not use a “typical” multiplier (neither vacuum level.
52 www.homeenergy.org 2005 SPECIAL ISSUE • HOME ENERGYYou can also read
