The goal for a HVAC system is to provide proper air flow, heating, and cooling
to each room. This sets out key criteria that describe a quality system, and key
design and installation
considerations that should be met to achieve this goal. The following contains
detailed information on design, fabrication, installation, and performance testing.
Criteria for a Quality HVAC System
An HVAC system should:
Be properly sized to provide correct air flow, and meet room-by-room calculated
heating and cooling loads.
Be installed so that the static air pressure drop across the handler is within
manufacturer and design specifications to have the capacity to meet the calculated loads.
Have sealed supply ductwork that will provide proper air flow.
Be installed with a return system sized to provide correct return air flow.
Have sealed return ductwork that will provide proper air flow to the fan, and
avoid air entering the HVAC system from polluted zones (e.g., fumes from autos and stored chemicals,
and attic particulates).
Have balanced air flows between supply and return systems to maintain neutral
pressure in the home.
Minimize duct air temperature gain or loss between the air handler and room registers,
and between return registers and the air handler.
Be properly charged with refrigerant.
Have proper burner operation and proper draft.
Procedures to Design and Install an Air Distribution System
The following steps should be followed in the design and installation of the
HVAC system to ensure efficiency and comfort:
Determine room-by-room loads and air-flows using ACCA Manual J calculation procedures
(or substantially equivalent).
Layout duct system on floor plan, accounting for the direction of joists, roof
hips, fire-walls, and other potential obstructions. Determine register locations and types, duct lengths,
and connections required to produce layout given construction constraints.
Size duct system according to ACCA Manual D calculation procedures (or substantially equivalent).
Size HVAC equipment to sensible load using ACCA Manual S procedures (or substantially equivalent).
Install equipment and ducts according to design specifications, using installation
requirements and procedures from the manufacturers' specifications. The duct system should be substantially air tight.
Charge the system appropriately, and verify charge with the evaporator superheat
method or subcooling method (or substantially equivalent).
Check for proper furnace burner operation and fire-box drafting.
Test the system to ensure that it performs properly by determining that the system
is properly sized,does not leak, and either has proper plenum static pressures
or proper room and return airflows, and proper plenum static pressures.
Recommended Details for an HVAC System:
Materials, Fabrication, Design, Installation, and Performance Testing
MINIMUM MATERIALS SPECIFICATIONS
The following are minimum materials specifications recommended to achieve a substantially
tight
installation that will last:
All Materials
· Shall have a minimum performance temperature ratings per UL181 (ducts), UL181A
(closure
systems for rigid ducts), UL181B (closure systems for flexible ducts) and/or
UL 181BM (mastic);
· Shall have a flame spread rating of no more than 25 and a maximum smoke developed
rating of 50
(ASTM E 84);
Factory-Fabricated Duct Systems
· All factory-fabricated duct systems shall include UL 181 listed ducts with
approved closure
systems including collars, connections and splices;
· All pressure-sensitive and heat-activated tapes used in the manufacture of
rigid fiberglass ducts
shall be UL 181A listed;
· All pressure-sensitive tapes and mastics used in the manufacture of flexible
ducts shall be UL 181B
(tape) or UL 181BM (mastic) listed.
Field-Fabricated Duct Systems
· Ducts:
- Factory-made ducts for field-fabricated duct systems shall be UL 181 listed.
· Mastic sealants and mesh:
- Sealants shall be UL 181BM listed, non-toxic, and water resistant;
- Sealants for interior applications shall pass ASTM tests C 731 (extrudability
after aging) and D
2202 (slump test on vertical surfaces);
- Sealants and meshes shall be rated for exterior use;
- Sealants for exterior applications shall pass ASTM tests C 731, C 732 (artificial
weathering
test), and D 2202.
· Pressure-sensitive tapes:
- Pressure sensitive tape shall be that recommended by and meet the requirements
of the flexduct
manufacturer;
- Tape used for duct board shall be UL 181A listed and so indicated with a UL
181A mark or
aluminum-backed butyl adhesive tape (15 mil. minimum).
· Drawbands:
- shall be either stainless-steel worm-drive hose clamps or uv-resistant nylon
duct ties;
- shall have a minimum performance temperature rating of 165 degrees Fahrenheit
(continuous,
per UL 181A-type test) and a minimum tensile strength rating of 50 pounds;
- shall be tightened as recommended by the manufacturer with an adjustable tensioning
tool.
DESIGN, FABRICATION, AND INSTALLATION
The following are design, fabrication, and installation guidelines, that, if
carefully followed, will
provide a duct installation that is substantially airtight:
General Issues
· Ducts, plenums, and fittings should be constructed of galvanized metal, duct
board, or flexible
duct. Building cavities may not be used as a duct or plenum without a sealed
duct board or metal
liner.
· The air handler box should be air-tight;
· Air filters should be easily accessible for replacement, and evaporator coils
should be easily
accessible for cleaning;
· Ducts should be configured and supported so as to prevent use of excess material,
prevent
dislocation or damage, and prevent constriction of ducts below their rated diameter;
· Flexible duct bends should not be made across sharp corners or have incidental
contact with metal
fixtures, pipes, or conduits that can compress or damage the ductwork;
· Sheet metal collars and sleeves should be beaded to hold drawbands.
DESIGN HVAC SYSTEM
Loads and CFM Calculation
· ACCA Manual J Load Calculation or equivalent required;
· Calculate heat loss and heat gain for each room;
· Total room loads to determine system requirements.
Lay Out Air Distribution System
· Lay out duct system on floor plan and determine register positions and duct
paths to optimize room
air circulation and minimize duct length;
· Duct paths must account for locations and directions of joists, roof hips,
fire walls, and other
potential obstructions;
· Duct paths must be planned to avoid sharp turns of flexduct that will kink
the duct.
Size Air Distribution System
· ACCA Manual D Duct Design or equivalent required;
· Calculate correct cfm for each room and total for building for both supply
and return;
· Size ducts according to Manual J loads, Manual D air flows, and final layout
on plans;
· Choose registers to optimize air distribution and duct static pressure;
· Size and locate returns to optimize air flow per Manual D;
· For return-filter grills, calculate minimum return filter area per Manual D.
Select System
· ACCA Manual S Residential Equipment Selection or equivalent required. ACCA,
1515 16th St.,
NW, Washington, DC 20036, (202)483-9370;
· From Manual J loads and Manual D cfm, determine appropriate equipment
· Equipment should be sized to sensible loads;
· Equipment sensible capacity should not be more than 15% larger than the total
sensible design load
(as specified in Manual S).
FABRICATE AND INSTALL AN AIRTIGHT DUCT SYSTEM
All Duct Types
· All joints and seams of duct systems and their components should be sealed
with mastic, mastic
and embedded mesh, or pressure-sensitive tape approved for use by the duct manufacturer
and
meeting UL181 specifications ("approved tape"); this includes around junctions
of collars to
distribution boxes and plenums;
· All sealants should be used in strict accordance with manufacturer's installation
instructions and
within sealants moisture and temperature limitations;
· All tapes used as part of duct system installation should be applied to clean,
dry surfaces and sealed
with manufacturer's recommended amount of pressure or heat. If oil is present,
taped surfaces
should be prepared with a cleaner / degreaser prior to application;
· It is recommended that all register boxes should be sealed to the drywall or
floor with caulking or
mastic.
Flexible Ducts
· Flexible ducts should be joined by a metal sleeve, collar, coupling, or coupling
system. At least 2
inches of the beaded sleeve, collar, or coupling must extend into the inner core
while allowing a 1
inch attachment area on the sleeve, collar, or coupling for the application of
tape;
· The inner core should be mechanically fastened to all fittings, preferably
using drawbands installed
directly over the inner core and beaded fitting. If beaded sleeves and collars
are not used, then the
inner core should be fastened to the fitting using #8 screws equally spaced around
the diameter of
the duct, and installed to capture the wire coil of the inner liner (3 screws
for ducts up to 12"
diameter, and 5 screws for ducts over 12" diameter);
· The inner core should be sealed to the fitting with mastic or approved tape;
· Tape used for sealing the inner core should be applied with at least 1 inch
of tape on the duct
lining, 1 inch of tape on the fitting of flange, and wrapped at least three times;
· The outer sleeve (vapor barrier) should be sealed at connections with a drawband
and/or three
wraps of approved tape;
· The vapor barrier should be complete. All holes, rips, and seams must be sealed
with mastic or
approved tape.
Metal Ducts and Plenums
· Metal-to-metal connections should be cleaned and sealed in accordance with
manufacturer's
specifications;
· Openings greater than 1/16 inch should be sealed with mastic and mesh, or butyl
adhesive tape;
· Openings less than 1/16 inch should be sealed with mastic or UL-181A listed
tape;
· Special attention should be paid to collar connections to duct-board and/or
sheet metal; seal around
the connection with mastic;
· Connections between collars and distribution boxes should be sealed with mastic
or approved tape;
· At least three equally-spaced #8 screws should be used to mechanically fasten
round ducts (3
screws for ducts up to 12" diameter, and 5 screws for ducts over 12" diameter);
· Crimp joints should have a contact lap of at least 1_ inches;
· Square or rectangular ducts should be mechanically fastened with at least one
screw per side.
Duct Board
· Duct board connections should be sealed with adhesive, mastic, or UL 181A listed
pressure sensitive
or heat-activated tape in accordance with manufacturer's specifications.
Duct Support
· Supports should be installed per manufacturer's specifications or per UMC requirements;
· Supports for flexible ducts should be spaced at no more than 4 foot intervals;
· Flexible ducts should be supported by strapping having a minimum width of 1_
inches at all
contact points with the duct;
· Supports should not constrict the inner liner of the duct;
· Flexible ducts should have maximum of _ inch sag per foot between supports;
· Flexible ducts may rest on ceiling joists or truss supports as long as they
lie flat and are supported
at no more that 4 foot intervals.
Boots
· After mechanically attaching the register boot to floor, wall, or ceiling,
all openings between the
boot and floor, wall, or ceiling should be sealed with caulk or mastic.
Seal Air Handler
· Openings greater than 1/16 inch should be sealed with mastic and mesh, or butyl
adhesive tape;
· Openings less than 1/16 inch should be sealed with mastic or UL 181A listed
tape;
· Unsealed access doors should be sealed with UL 181A listed tape.
CHECK REFRIGERANT CHARGE
· For systems with fixed metering devices use evaporator superheat method:
- indoor coil airflow must be greater than 350 cfm/ton;
- refrigerant system evacuation must be complete (all non-condensables must be
removed from
the system;
- in hot, dry climates be cautious to be within range of superheat charging chart
or use a different
method.
· For systems with thermostatic expansion valves use the subcooling method.
CHECK COMBUSTION PERFORMANCE
· Check each chamber for correct flame;
· Check for proper drafting.
TEST SYSTEM PERFORMANCE
The following are testing requirements and procedures that must be followed to
ensure that the HVAC
system has been properly installed. The tests are designed to determine whether:
1. Room-by-room air flows are correct;
2. Total supply is as designed;
3. Total return = total supply;
4. Ducts, plenum, and air handler are tight;
5. Static pressure is correct.
· Test the system to ensure that it performs properly, by (1) verifying HVAC
equipment sizes
installed are those specified, (2) measuring duct leakage, and measuring either
(3a) fan flow or (3b)
supply and return flows and plenum static pressures:
1. Air conditioner sensible capacity must be no more than 15% greater than the
calculated sensible
load; fan flow must be greater than 350 cfm/ton; check that the correct size
air handler is
installed.
2. Ensure that the duct system does not leak substantially:
a. A rough system, including both supply and return but without the air handler,
should not
leak more than 0.03*conditioned floor area (ft_) per system measured in cfm @
50 Pa;
b. The finished installation, including supply, return, the air handler and finished
registers,
must not leak more than 0.07*conditioned floor area (ft_) per system measured
in cfm @ 50
Pa;
3a. Measure air handler air flow and static pressure across fan; ensure that
total air handler output
is within 5% of design and manufacturer specifications at a static pressure within
0.1 in wg of
design.
3b. Supply and return air flow, and static pressure requirements: Ensure that
supply and return
flows are correct, and that the static pressure across the fan is correct:
a. Measure room-by-room air flows to ensure that each register is within 15%
of Manual D
design air flow, and that the entire supply is within 5% of design;
b. Measure return air flow to ensure that it is within 5% of the total supply
air flow;
c. Test static pressure drop across the blower to ensure that it is within 0.1
in wg of design and
manufacturer specifications.
· Duct leakage can be determined using a pressurization or depressurization technique;
for details,
see Minneapolis Duct Blaster™ manual, or other commercially available duct pressurization
or
depressurization devices;
· Duct leakage to unconditioned space can be determined with the house pressurization
or LBL
simplified technique; for details see CEC report P400-91-031CN, Section Six;
· Fan flow, supply flow and return flow measurements, see Minneapolis Duct Blaster™
manual (or
equivalent); alternatively for supply and return flows, use a calibrated flow
hood. Do not use a
pitot tube, or any type of anemometer to determine these air flows;
· Static pressure drop across the fan is measured using a small probe in the
return plenum and in the
supply plenum.
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PROBLEMS WITH ACCEPTED PRACTICE SIZING METHODS:
Relationship Between Duct System Performance,
ACCA Design Procedures, and Installed-System Quality
Background
The Air Conditioning Contractors of America (ACCA) association publishes four
manuals related to residential heating and air conditioning that address many
of the issues
associated with residential duct systems. ACCA Manual J (Load Calculation for
Residential Winter and Summer Air Conditioning, Copyright 1986) is the industry
standard
design-load calculation procedure for residences. ACCA Manual S (Residential
Equipment Selection, 2/92) provides procedures for choosing residential heating
and
cooling equipment based on the loads calculated with Manual J. ACCA Manual D
(Residential Duct Systems, Copyright 1995, 2nd Printing) provides design procedures
for
residential duct systems, focusing on how to produce the desired air delivery
at each
register, as well as discussions of the magnitudes and impacts of duct-system
inefficiencies. ACCA Manual T (Air Distribution Basics for Residential and Small
Commercial Buildings, UPB592-10M) addresses room air motion issues, focusing
on the
impacts of register/grille location and diffuser performance.
Treatment of Duct Performance in ACCA Manual J
ACCA Manual J addresses residential duct system performance in three ways: 1)
it
provides room-by-room loads, which are intended to be used to calculate the energy
that
needs to be transported by the ducts to each room, 2) it provides a table of
duct-loss
multipliers that are used to calculate the extra design load associated with
conduction
losses from the ducts, and 3) it provides a table of recommended levels of duct
insulation,
and states that “All ducts should have their seams sealed with tape”.
In calculating the energy load impacts of ducts and room-by-room loads, Manual
J makes
two fundamental assumptions: 1) that there is no duct leakage, and 2) that the
load due
duct conduction is independent of the length and design of the ducts. The implication
of
the first assumption is that the actual load associated with duct losses is in
general
significantly higher than that assumed in Manual J. The second assumption implies
that
even if the average conduction losses in the duct-loss multipliers are correct,
the
calculated room-by-room loads are incorrect due to non-uniform conduction losses.
A significant body of research performed over the past five years in California
and other
states that install ductwork in attics and crawlspaces demonstrates that duct
leakage
increases space-conditioning energy use by 15-20% on average, even in new
construction. This loss needs either to be eliminated, or to be added to the
losses
associated with conduction gains to obtain correct loads seen by the equipment.
Field
research has also demonstrated the effective increase in heating and cooling
system
capacity associated with improving duct performance (Modera and Jump, 1995).
Those
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studies show reduced fractional on-times and increased cycling under the same
weather
conditions after duct retrofit.
A logical question that arises with respect to these duct leakage losses is why
Manual J is
not resulting in significantly undersized systems because of the fact that it
does not
include these duct leakage losses. The reasons for why this is not the case seem
to stem
principally from the application of Manual J, rather than the manual itself.
In general,
Manual J leaves quite a bit to the discretion of the user, leaving numerous opportunities
for increasing the size of the unit. Some of the common points at which safety
margins
seem to creep in are:
· The use of the worst house orientation for load calculations,
· The choice of the next size up in the piece of heating/cooling equipment,
· The assumption of 50% RH indoor conditions in most manufacturer’s capacity
data, which is higher than what is found in much of California, and which results
in a lower estimated sensible capacity for a piece of equipment as compared to
the
sensible capacity the equipment would have at a lower indoor humidity level,
· Using a somewhat lower indoor design temperature,
· Using a higher outdoor design condition, such as 1%, or utility-peak outdoor
design temperature rather than the 2.5% values recommended in Manual J.
· Using the next-highest outdoor-temperature rating point, rather than interpolating
manufacturer’s capacity data.
· The recommendation of 0-15% over sizing of sensible capacity in Manual S.
To be fair, it should also be noted that there are some factors that tend to
decrease the size
of the equipment chosen with the ACCA procedures, including:
· ARI capacities are normally quoted at 80oF, whereas Manual J requires capacities
at
75oF, which will be smaller.
It is very difficult to quantify exactly how much the above trends influence
equipment
sizing. A contractor survey performed in Florida indicated that there is a large
variability
in the equipment-sizing practices used by contractors (Home Energy 1995). It
is safe to
say that there are numerous opportunities for a contractor to increase equipment
size
within the ACCA procedures so as to maintain the sizing with which they are
comfortable. A related study of equipment sizing and ACCA manuals is published
in
Home Energy magazine (Proctor et al. 1995).
The assumption of constant duct-loss multipliers for all duct sections (or in
other words,
that duct loads scale with room load, and not with duct design or length) is
more of a
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design-flaw and comfort problem, rather than an energy-use or equipment-sizing
problem. Namely, after calculating room-by-room loads including constant duct-loss
multipliers, the air flow required for each room is calculated from the loads,
the duct
system is laid out, and the cross-sectional area of the ductwork is calculated
and checked
with Manual D based upon the ability of the system to supply the required air
flow. This
implies that the percentage energy loss from the longest duct run is the same
as that from
the shortest run. It seems clear that this is not a realistic assumption, however
the
magnitude of the resulting disparity, based upon field measurements, is striking.
Namely,
the bedroom closest to the furnace for an R-4 duct system in a Sacramento attic
was
measured to have 12% of the duct energy lost by conduction on the way to the
register.
The equivalent losses for the master bedroom at the end of the duct run were
more than
40% (Modera and Jump, 1995). The 12% loss is line with the losses that are calculated
from the Manual J duct loss multipliers, and the 40% loss clearly indicates that
the master
bedroom duct is most likely undersized. Sure enough, the homeowner commented
on the
improvement in master-bedroom conditions after the retrofit. The end result of
this
disparity is that the entire duct-design process is skewed so as to provide far
less than
optimal distribution of heating and cooling.
There is another assumption within Manual J that is likely to result in inaccurate
estimates of room-by-room loads. Namely, it is assumed that the infiltration
load is split
between rooms based on the estimated relative external leakage area of that room.
The
problem with this assumption is that it ignores the fact that a significant fraction
of
residential air infiltration is driven by the stack effect. The implication of
ignoring the
stack effect in two-story houses is that in general the upstairs flows will be
oversized for
heating, resulting in unnecessary stratification and discomfort in the winter.
This upstairs duct
over sizing should actually help reduce stratification in the summer.
In addition, it is also worth noting that the duct loss multipliers for an attic
and a
crawlspace are the same, which is clearly inconsistent with intuition and field
experiments. The result is that cooling equipment with attic ductwork is likely
to be
relatively undersized as compared to cooling equipment with crawlspace ductwork.
Treatment of Duct Performance in ACCA Manual D
As noted above, the principal function of Manual D is to assure that a given
duct layout
delivers the appropriate air flows to each room, based upon the room-by-room
loads
calculated with Manual J. Thus, if the total load seen by the duct run to a given
room is
not correct, the size of the ductwork leading to that zone will not be correct,
resulting in
poorly designed system (i.e., one that does not provide uniform heating or cooling,
and
which is difficult or impossible to balance).
There is however a disconnect within Manual D. Namely, Manual D contains an entire,
fairly complete section on duct-system energy efficiency, however this section
is not
connected to the load calculation procedures used to size the equipment and ductwork.
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Treatment of Duct Performance in ACCA Manual T
As noted above, Manual T focuses on the room-air motion aspects of air distribution
systems. The way that this relates to duct performance and quality HVAC installations
is
through the performance of the diffusers. In particular, if a diffuser is designed
to provide
a given throw at a specific air flow rate, that throw will be reduced (potentially
significantly) by supply-duct leakage or by flow restrictions within the ductwork
(e.g.,
flexduct that is not fully extended, that is bent at hangers, or that is bent
at too sharp of a
radius).
Recommended Strategy for California
Based upon the discussion above, a two-phase strategy for improving the quality
of
HVAC installations is recommended. The first phase of the strategy simply addresses
the
issue of duct leakage, focusing on the interaction between duct leakage and equipment
sizing with Manual J and Manual S. The second phase addresses the quality of
the
design, focusing on a methodology for accurately laying out and sizing ductwork
so as to
provide better occupant comfort.
The essence of the Phase-I strategy is to develop a modification to Manual J
duct
loss/gain multipliers that takes into account duct leakage losses, and to combine
this with
an appropriate training course designed to help contractors take some of the
over sizing
trends out of their Manual-J calculations.
The essence of the Phase-II strategy is to address the duct-design problems in
the
combination of Manual J and Manual D. This can be accomplished by inserting an
overall duct-loss calculation procedure for each register in the house into the
process.
This may require some iteration between the duct-sizing procedure and the duct-loss
calculation procedure, however one or two iterations will most likely be adequate,
and
the final design will not only provide better comfort, but should ultimately
result in better
energy efficiency. This overall duct-loss calculation procedure should most-likely
be
based on the simplified procedure developed by Palmiter (1995) that is likely
to be
adopted into the proposed ASHRAE Standard 152P. This procedure should be used
separately for heating and cooling operation.
