ECOVENT WHEN BIGGER ISN’T BETTER - Ecovent … · 8% 10% 12% 14% table 2: effect of a/c unit...

ECOVENT

PUBLISHED AUGUST 2014

HOW ECOVENT HELPS RIGHT-SIZE HOME HVAC SYSTEMS

A WHITEPAPER

WHEN BIGGERISN’T BETTER

TABLE OF CONTENTS

01 INTRODUCTION 03

02 WHY RIGHT-SIZING IS SO DIFFICULT

06

03 THE PROBLEMS WITH OVERSIZING

09

04 GETTING BEYOND ‘RULES OF THUMB’

12

05 ROOM LEVEL COMFORT: THE NEXT FRONTIER

15

06 REAL DATA, NOT JUST ESTIMATES

18

07 CONCLUSION 19

08 SOURCES 20

03

Imagine the hottest day of summer. The sun is beating

down, the air is still, and the humidity is so bad that it’s

nearly impossible to go outside.

Now imagine that your air conditioner breaks down.

If you are a heating, ventilation, and air conditioning

(HVAC) technician, you have probably received dozens

of calls from homeowners at just this moment of frus-

tration and panic. And since everyone’s A/C unit works

hardest during a heat wave, you have probably received

more than one of these calls on the same day.

It’s no wonder then that most contractors try to avoid

these distress calls by installing HVAC equipment that is

powerful enough to withstand the very hottest days of

summer—or the coldest days of winter. However, these

“rule of thumb” methods for sizing heating and cooling

equipment can be far from ideal.

01 INTRODUCTION

WHEN BIGGER ISN’T BETTER

04

Study after study has found that residential HVAC systems are routinely oversized by as

much as 200 percent. A survey of homes built between 1994 and 1999 in Fort Collins, Col-

orado found that cooling systems were significantly oversized, ranging from 143 percent

to 322 percent of the design requirements.1 And according to a large-scale survey cited by

the National Renewable Energy Lab (NREL), nearly 40 percent of HVAC contractors report

purposefully oversizing equipment to reduce “call backs” or due to customer demand.2 That

may translate into fewer distress calls for HVAC contractors, but it is costly for homeowners.

For example, Boston-area HVAC contractors report that a fully-installed 2.5 ton air condition-

ing system, including the air handling unit, condenser, refrigerant lineset, ductwork, thermo-

stat, and wiring, costs the homeowner approximately $12,000, while the same system in a 5

ton size will be about $20,000. In other words, a double-sized air conditioning system costs

the homeowner approximately 66 percent more—a high price for comfort.

With better planning, design, information, and technology, it is possible to right-size home

heating and cooling equipment. This process can reduce homeowners’ upfront costs and

save money on long-term operations. It can also help HVAC technicians reduce the peak

season blitz while retaining happy, loyal customers.

FIGURE 1 : AVERAGE HVAC SYSTEM COSTS BASED ON SIZE

0

5000

10000

15000

20000

SY

ST

EM

CO

ST

IN $

US

D

2.5 5.0

SYSTEM WEIGHT IN TONS

“A double-sized A/C system is a high price for comfort.”

“A double-sized A/C system is a high price for comfort.”

06

Equipment sizing is a key factor in designing residential

climate control systems. An appropriately sized system

can deliver energy-efficient comfort for years and years.

However, in most cases, equipment sizing receives min-

imal attention in the design process. This is partly be-

cause it is so difficult to do well.

In order to appropriately size an HVAC system, a tech-

nician must develop a clear model of heat loss and heat

gain throughout the home. This requires the technician

to account for a variety of variables, including building

characteristics, construction materials, air sealing tech-

niques, window materials, installation quality, and many

other factors.

02 WHY RIGHT-SIZINGIS SO DIFFICULT

07

The HVAC industry has developed manuals

and software to assist contractors in man-

aging these variables for the sizing of heat-

ing and cooling systems. For example, the

Air Conditioning Contractor’s Association

of America’s (ACCA) Manual J offers proce-

dures for determining heating and cooling

loads based on the amount of wall, floor, ceil-

ing, and window area, as well as the relative

insulation value of these materials, the ob-

served duct leakage, and other characteris-

tics of the building envelope. Once this pro-

cess is complete, ACCA’s Manual S is used

to select the appropriate equipment.

Unfortunately, due to the competitive na-

ture of the industry, many HVAC contractors

are unwilling to spend the time necessary to

measure and investigate the home, input the

data into a software program, and adapt

their designs accordingly. In a widely-cited

survey of Florida air conditioning contrac-

tors, researchers revealed that only 33 per-

cent of contractors size air conditioning sys-

tems using a Manual J calculation.3

And even the authors of the survey admit

that estimate may be high. They wrote:

In other words, the contractors who avoid

entering data into Manual J software weren’t

likely to fill out a survey, either.

“It seems likely that any bias in the sample is probably toward those with a greater interest in the subject, and possibly in the profession, hence the answers may be skewed towards more detailed methods of sizing.”


“IN A COMPETITIVE INDUSTRY, TIME IS AT

A PREMIUM.”

09

Although over-sized equipment is often installed as a

preventative measure, it can create a variety of prob-

lems on its own.

03 THE PROBLEMSWITH OVERSIZING

01

SHORT CYCLING

10

For one, over-sized equipment can lead to

inefficient “short cycling.” If a furnace is too

big for the home it is heating, it only needs

to run for a few minutes before it reaches the

target temperature. At that point, the system

shuts off, only to turn back on moments lat-

er when the heat dissipates. The same issue

occurs when an over-sized air conditioner is

installed. The system runs long enough to

achieve the target temperature, but not long

enough to achieve optimum dehumidifica-

tion. That leaves homeowners running to the

thermostat to reduce the temperature even

further, believing that that will improve their

comfort, when the real culprit is humidity.

Short cycling also reduces the efficiency of

A/C operations. The effect is similar to driv-

ing a car in stop-and-go traffic. It’s inefficient

to slam on the gas, slam on the brakes, then

slam on the gas again. Your car is much more

efficient when it is cruising along on the

highway at a steady 60 miles per hour. HVAC

equipment is similar: It works best when it’s

operating consistently. Short cycling puts

Oversized HVAC equipment also requires

more energy to operate. NREL cites a Flor-

ida study that showed an average increase

of 9 percent of annual space cooling elec-

tricity usage for units that were oversized by

50 percent or more. A 2005 study by Proc-

tor Engineering yielded similar results, not-

ing that reducing a system’s size by approx-

imately 50 percent saved nearly 9 percent

of the energy consumed by the A/C unit.

The following table is reproduced from that

study. Given these drawbacks, it’s no wonder

that the HVAC industry advises that over-

sized air conditioners force the consumer to

pay more for equipment and maintenance

while providing lower levels of comfort and

wasting more energy.

INCREASED ENERGY USE

unnecessary wear on the system, and many

people also find it irritating to hear their sys-

tem constantly turning on and off through-

out the day.

SIZING REDUCTION (% OF ORIGINAL SIZE)

ENERGY SAVINGS (INDIVIDUAL UNIT)

13% 23% 31% 37% 43% 47%

1.6% 3.1% 4.6% 6.0% 7.4% 8.7%

TABLE 1: EFFECT OF A/C UNIT SIZING REDUCTIONS ON INDIVIDUAL UNIT ENERGY CONSUMPTION 4


“Short cycling is like driving a car in

stop-and-go traffic.”

“Short cycling is like driving a car in

stop-and-go traffic.”

12

Many HVAC professionals have already moved beyond

“rules of thumb” and are using load modeling to gener-

ate more accurate equipment sizing calculations. New

code amendments may soon require that others follow

in their footsteps.

04 GETTING BEYOND‘RULES OF THUMB’

01

BUILDING CODEIMPROVEMENTS

13

Local and state governments are beginning

to mandate Manual J software calculations

on all new buildings or extensive remodeling

projects through new building codes. These

local code changes may take years to be im-

plemented nationwide, but they tend to be a

bellwether for the industry as a whole.

Beyond regulatory requirements, there are

other reasons why the “rule of thumb” system

sizing method may no longer cut it. Tech-

nological advances now allow for improved

comfort with greater energy savings. Today’s

heating and cooling equipment is capable of

modulating or operating in multiple stages in

order to manage efficient system operation

during partial load conditions. These adap-

tive systems can enhance life expectancy of

equipment while reducing energy use.

Most multi-stage equipment has two stages:

low and high. Low stage capacity can be set

as close to the load at an average summer

temperature for cooling, while heating levels

should be set for average winter conditions.

This ensures efficient operation during the

majority of the year. Under peak load con-

ditions, the system would operate on high

stage capacity to meet the increased de-

mand.

TECHNOLOGY ADVANCES


Indeed, peak demand is becoming a bigger

and bigger concern for homeowners and

the electric power utilities that serve them. In

order to offset the cost of high-priced peak

power, many utilities are beginning to offer

incentives to customers who can cut their

peak power consumption.

In 2012, approximately 24 percent of U.S. util-

ities offered such price-based incentives to

residential customers.5 For example, in 2014,

Baltimore Gas and Electric offered an op-

tional, variable “Time of Use” rate plan that

charges nearly 12 cents per kilowatt-hour

(kWh) during summertime peaks, and as

little as 7 cents per kWh during off-peak

hours.6

Proctor and Pira’s study indicates that

right-sizing an HVAC system also has an im-

pact on the homeowner’s peak demand, as

illustrated in Table 2.

For customers who have opted into variable

electricity rate programs, peak demand for

power will have an even bigger impact on

their electricity bills. Therefore, these cus-

tomers will have an even stronger desire to

choose a smaller, right-sized A/C unit.

VARIABLE ELECTRICITYRATES

SIZING REDUCTION (% OF ORIGINAL SIZE)

PEAK ENERGY SAVINGS (INDIVIDUAL UNIT)

13% 23% 31% 37%

8% 10% 12% 14%

TABLE 2: EFFECT OF A/C UNIT SIZING REDUCTIONS ON DIVERSIFIED PEAK LOADS 7

14


“RIGHT-SIZING HVAC EQUIPMENT REDUCES PEAK DEMAND FOR

ENERGY.”

15

In addition to these advances in codes, technology, and

electricity rates, the delivery system for conditioned air

has also evolved. Room-by-room zoning is now possi-

ble via interventions within the ductwork or at the vent

register. The propagation of this technology will further

enhance comfort and energy savings for homeowners

with forced air systems.

05ROOM-LEVEL COMFORT: THE NEXT FRONTIER

0116

Many homes have been equipped with two-stage equipment but do not have the room-by-

room control that would optimize this design. Operating two-stage equipment on low stage

capacity without modifying the duct volume with room-by-room zoning creates a whole

different set of issues.

Room-by-room zoning solutions, like ecovent’s, can modulate airflow in response to user

input, occupancy, and the dynamic environmental conditions of each room as they change

throughout the day or over the course of the year.

ecovent addresses airflow management at the vent register level. Other room-by-room

zoning solutions use mechanical or inflatable baffles to achieve the same goal. In effect,

room-level zoning reduces the amount of space that is heated or cooled, which reduces run-

times, improves the overall efficiency of HVAC operations, and enables the HVAC technician

to select a smaller air conditioner or furnace for a customer’s home.

ecovent is in the process of validating its technology through a beta program involving doz-

ens of homes across the United States. These homes are distributed across different climate

zones, feature distinct design characteristics, and are experiencing different issues with their

heating and cooling equipment. We plan to publish case studies based on the results of this

program, including information on the energy savings and comfort improvements facilitated

by room-level zoning.


“Room-level zoning reduces run-times

and increasesoverall efficiency.”

18

Another benefit of advanced zoning is the installation of

environmental sensing equipment across multiple points

in a home. For example, ecovent’s system includes a

sensor for each room in the home. This sensor constant-

ly measures the temperature and humidity conditions

at the room level. Not only does this ensure occupant

comfort, but also, it represents a new opportunity for

HVAC technicians to right-size equipment.

If a homeowner has ecovent installed and needs to re-

place the home’s heating or cooling equipment, the

technician may utilize the system’s sensor data to tailor

the equipment purchase to the actual heat loading con-

ditions in the home. Even the best Manual J calculation

is an approximation. But with room-level sensor data,

ecovent can report on actual heat loss and heat gain

conditions throughout the home under real-life condi-

tions. This new data source is a potential boon to con-

tractors and homeowners alike.

06 REAL DATA, NOT JUST ESTIMATES

19

Today, the process of HVAC system sizing is an educated guess at best. Most

heating and cooling takes place during average weather conditions, but most

HVAC systems are sized for the hottest and coldest days of the year. In that

light, most systems are significantly over-sized in relation to their average

load. With better design tools and technology, there is plenty of room for

improvement.

Proper system sizing technique can enable HVAC technicians to design sys-

tems that meet the demands of rare peak load days while operating efficient-

ly on average load days. These optimized systems would include room-by-

room control and two-stage equipment that can adapt to heating and cooling

based on the demands of that zone of the house and particular weather con-

ditions.

As sensing equipment becomes more widespread, comfort control systems

will be able to monitor operating conditions both indoors and outdoors at

multiple points. These smarter systems could eventually calculate the exact

capacity required at both the seasonal average days as well as the coldest

and hottest days.

For now, there are tools readily available for HVAC technicians to improve the

comfort and efficiency of their customers’ homes. And by doing so, they can

ensure that when the phone rings, it’s a happy customer on the end of the line.

07 CONCLUSION

SECTION

20

1. As cited in Dagostino, Frank and Joseph B. Wujek, Mechanical and Electrical Systems in

Architecture, Engineering and Construction, Prentice Hall, 2009

2. http://www.energy.wsu.edu/documents/AHT_Energy%20Efficient%20Home%20Cooling.

pdf

3. Vieira, Robin K., Danny S Parker, Jon F. Klongerbo, Jeffrey K. Sonne, Jo Ellen Cummings.

“How Contractors Really Size Air Conditioning Systems.” Presented at the 1996 ACEEE

Summer Study on Energy Efficiency in Buildings. American Council for an Energy-Efficient

Economy, 1001 Connecticut Avenue, Washington, DC

http://www.fsec.ucf.edu/en/publications/html/FSEC-PF-289-95/

4. Proctor, John, Joe Pira. “System Optimization of Residential Ventilation, Space Condition-

ing and Thermal Distribution” Proctor Engineering Group. San Rafael, CA. October 2005.

http://www.proctoreng.com/dnld/SystemOptimizationofResidentialVentilation.pdf

5. U.S. Energy Information Administration. “2012 Electric power sales, revenue, and energy

efficiency Form EIA-861 detailed data files” October 29, 2013. http://www.eia.gov/electricity/

data/eia861/index.html

6. Baltimore Gas & Electric Company. “Standard Offer Service (SOS) Rates/Miscellaneous

Charges” http://www.bge.com/myaccount/billsrates/ratestariffs/electricservice/Electric%20

Rates%20Information%20Documents/POLR_Rates_PTC_MiscCharges.pdf

7. Proctor, John, Joe Pira. “System Optimization of Residential Ventilation, Space Condition-

ing and Thermal Distribution” Proctor Engineering Group. San Rafael, CA. October 2005.

http://www.proctoreng.com/dnld/SystemOptimizationofResidentialVentilation.pdf

08 SOURCES

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