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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
Improving the Energy Efficiency
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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.
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