Thermo Pride MHA1-50N Manual

P/N: MG-1008 (Printed Instruction Manual)
Note: This page is not to be included as part of printed hard copy.
Overall Specifications:
Material: white plain paper
Printed text: black
Binder: staple upper left corner
Individual page specifications:
Cover page: printed single-side up
Table of contents page: printed single-side up
Warranty page: printed single-side down (facing out as last page of manual)
The balance of instruction manual pages to be printed utilizing both front and back side of paper.

INDUCED DRAFT HIGHBOY AND
COUNTERFLOW/HORIZONTAL
GAS FURNACE
INSTALLATION AND SERVICE MANUAL
MODELS
MHA1-50N MDA1-50N
MHA1-75N MDA1-75N
MHA1-100N MDA1-100N
MHA1-125N MDA1-125N
: IF YOU DO NOT FOLLOW THE SAFETY PRECAUTIONS BELOW AND IN THIS MANUAL,
A FIRE OR EXPLOSION MAY RESULT CAUSING PROPERTY DAMAGE, PERSONAL INJURY, OR
LOSS OF LIFE.
DO NOT STORE OR USE GASOLINE OR OTHER FLAMMABLE VAPORS AND LIQUIDS IN THE
VICINITY OF THIS OR ANY OTHER APPLIANCE.
WHAT TO DO IF YOU SMELL GAS:
• DO NOT TRY TO LIGHT ANY APPLIANCE.
• DO NOT TOUCH ANY ELECTRICAL SWITCH; DO NOT USE ANY PHONE IN YOUR BUILDING.
• LEAVE THE BUILDING IMMEDIATELY.
• IMMEDIATELY CALL YOUR GAS SUPPLIER FROM A NEIGHBOR’S PHONE. FOLLOW THE GAS
SUPPLIER’S INSTRUCTIONS.
• IF YOU CANNOT REACH YOUR GAS SUPPLIER; CALL THE FIRE DEPARTMENT.
INSTALLATION AND SERVICE MUST BE PERFORMED BY A QUALIFIED INSTALLER, SERVICE
AGENCY OR THE GAS SUPPLIER. (REFERRED TO IN THESE INSTRUCTIONS AS A QUALIFIED
HEATING CONTRACTOR).
PLEASE READ THESE INSTRUCTIONS PRIOR TO INSTALLATION, INITIAL FIRING, AND BEFORE
PERFORMING ANY SERVICE OR MAINTENANCE. THESE INSTRUCTIONS MUST BE LEFT WITH
THE HOMEOWNER AND SHOULD BE RETAINED FOR FUTURE REFERENCE BY
QUALIFIED SERVICE PERSONNEL.
THERMO PRODUCTS, LLC.
BOX 217
MG-1008 NORTH JUDSON, IN 46366
ECN 4283-MA PHONE: [phone]
MADE IN USA

Contents
SECTION PAGE
Notice to the Installer ..............................................................................................1
I. SAFETY ..............................................................................................................2
II. INSTALLATION GUIDELINES .............................................................................3
A. Codes .........................................................................................................3
B. Installation Location ..................................................................................4
C. Closet and Alcove Installation ..................................................................4
D. Standard Clearances....................................................................................5
E. Air for Combustion and Ventilation...............................................................6
F. MDA1 Horizontal Installation ........................................................................7
G. Duct Work and Air Conditioning ...................................................................9
H. Filter/Filter Installation ..................................................................................9
I. Use of Non Thermo Pride Filter Retention Means........................................10
J. Venting .........................................................................................................12
1. Vertical Venting..................................................................................12
2. Replacing an Existing Furnace from a Common................................13
3. Condensing in Chimney .....................................................................14
4. Vent Flue/Vent Connector..................................................................16
K. Sidewall Venting ...........................................................................................19
1. Horizontal Size and Lengths for Horizontal Vents..............................20
L. Gas/Gas Piping ............................................................................................21
1. Installation of Natural Gas Piping.......................................................23
2. Installation of Propane Gas Piping.....................................................24
M. Electrical.......................................................................................................26
1. Wiring.................................................................................................26
2. Field Wiring and Replacing Wiring.....................................................26
3. Electronic Air Cleaner and Humidifier Connections ...........................27
4. Room Thermostat ..............................................................................27
5. Blower Motor Speed Wiring ...............................................................28
III. INITIAL OPERATION OF THE FURNACE .......................................................29
A. Initial Startup ................................................................................................29
B. Adjustment of Heat Input Rate .....................................................................32
C. Burner Adjustment .......................................................................................34
D. Setting Supply Air Temperature Rise ...........................................................35
E. Checkout Procedure.....................................................................................36
IV. TROUBLESHOOTING......................................................................................37
V. DEALER MAINTENANCE .................................................................................38
A. Electrical.......................................................................................................38
B. General Inspection .......................................................................................38
C. House Air Blower..........................................................................................39
D. Return Air Filter ............................................................................................40

All installations and services must be performed by qualified service personnel.
VI. INSTALLER’S INSTRUCTIONS TO USER ......................................................41
APPENDIX A: SOURCES FOR REFERENCED STANDARDS .......................42
APPENDIX B: SEQUENCE OF OPERATION ..................................................43
APPENDIX C: TROUBLESHOOTING FLOWCHART ......................................45
APPENDIX D: PARTS PACKAGE COMPONENTS .........................................49
APPENDIX E: REPLACEMENT PARTS LIST..................................................50
APPENDIX F: ELECTRICAL DIAGRAMS ........................................................54
WARRANTY ......................................................................................................59

All installations and services must be performed by qualified service personnel.
1
Notice to the Installer
Installation of this gas-fired furnace must be performed by a qualified installer in
accordance with all local codes and authorities having jurisdiction. In the absence of
local governing codes, installation shall conform to these instructions and to the
regulations of the National Fuel Gas Code, ANSI Z223.1/NFPA 54-2002 and/or,
National Gas and Propane Installation Codes CSA B149.1, or the latest editions
thereof.
A qualified installer, also referred to in this instruction manual as a “qualified heating
contractor”, is an individual, or agency, that is responsible for the installation and
adjustment of the equipment. A qualified installer is properly licensed and
experienced to install gas-burning equipment in accordance with all local codes and
ordinances.
Any reference to L.P. or propane gas in this manual, any other label, or markings
on the furnace is to be construed to be propane HD-5, a commercial grade of
liquefied petroleum gases composed of a minimum of 90 percent liquid propane
(C3H8).
Material and Workmanship
This furnace was built with the highest quality materials and attention to
workmanship. However, omissions and defects occasionally occur. Before
installing the furnace, inspect the furnace thoroughly. If missing parts, defective
material, or poor workmanship are evident, report the model and serial
numbers imprinted on the furnace rating label to the seller.
Packaging
This is a complete, factory-assembled furnace contained within one package.
However, certain parts, including, air filter(s), air filter rack(s), the venting system
must be assembled to the furnace, in the field.
Shipping Damage
If this furnace was damaged during transit, please immediately request the
transportation company inspect the furnace and issue a concealed damage
report. The party receiving the furnace should file the claim for shipping
damage. Report any shipping damage immediately.

All installations and services must be performed by qualified service personnel.
2
It is absolutely essential that a damage report be obtained. If a concealed
damage report is not obtained, we cannot provide assistance in recovering your
claim against the transportation company.
Warranties
:
The manufacturer of this equipment assumes no liability for any
damages resulting from unauthorized modifications made to the
furnace, or components thereof, or improper installation of the furnace
in the field. Furthermore, any such field modifications VOID THE
WARRANTY and place the responsibility for safe and reliable operation
of the furnace on those who performed the modification(s).
Make certain the warranty cards are returned. These must be on file to verify
installation dates for replacement of any warranted part(s).
I. Safety
:
The following guidelines and warnings should be read and understood
before continuing with the installation of this furnace. Failure to follow
these guidelines can result in improper and unsafe operation of this
appliance, which can cause substantial property damage, severe
personal injury, or death.
These instructions must be read in their entirety before installing the furnace. It is
the installer's responsibility to do the following:
1. Use only with the type of gas approved for this furnace. Refer to the furnace
marking plate.
2. Install this furnace only in a location and position as specified in Section II, B
of this manual.
3. Provide adequate combustion and ventilation air to the furnace space. Refer
to Sections II, E of this manual.
4. Connect this furnace to an approved vent system only. Combustion products
must be vented outdoors. Refer to Section II, J of this manual for vertical
vented units.

All installations and services must be performed by qualified service personnel.
3
5. Never test for gas leaks with an open flame. Use a commercially available
soap solution made specifically for the detection of leaks to check all
connections. Refer to Section II, L of this manual.
6. Always adjust the furnace to operate within the furnace’s temperature-rise
range with a duct system, which has an external static pressure within the
allowable range. Refer to Section III, D of this manual.
7. Make sure supply and return air ducts are completely sealed to the furnace
casing. Ductwork must run to an area outside the air space where the
furnace is located and be sealed where it runs through walls, ceilings, and
floors. Refer to Section II, G of this manual.
8. This furnace is not to be used for temporary heating of buildings or structures
under construction
9. Refer to MDA1 horizontal application Section II, F of this manual for safety
switch relocation associated with horizontal installation of the MDA1.
10.Inform and demonstrate to the user, the correct operation and maintenance
of the appliance as explained in the Users Information Manual.
11.Inform the user of the hazards of flammable liquids and vapors and to
remove such liquids and vapors from the vicinity of the appliance.
12.Inform the user of warnings and cautions as well as user information in the
sidewall venting manufacturer’s manual when sidewall venting is applied.
II. Installation Guidelines
A. Codes
All local codes and/or regulations take precedence over the instructions in this
manual and should be followed accordingly. In the absence of local codes,
installation must conform to the instructions and the regulations of the National
Fire Protection Association National Electrical Code, ANSI/NFPA70-2002, and
the National Fuel Gas Code, NFPA 54/ANSI Z223.1-2002, or the latest editions
thereof.
The heating capacity of the furnace proposed for installation should be based on
a rate of heat loss calculation made according to the manuals provided by the Air
Conditioning Contractors of America (ACCA) or ASHRAE.

All installations and services must be performed by qualified service personnel.
4
B. Installation Location
:
• These furnaces are designed for indoor installation only.
• These furnaces are NOT to be used as construction heaters.
• The return air duct of the furnace must be sealed air tight to
prevent starvation of the combustion air, especially if the furnace
is located in a closet or confined area.
For best performance locate the furnace so that it is centralized with respect to
the duct system.
Place the unit so that proper venting can be achieved, with a minimum number
of elbows, in accord with the instructions in this manual. The furnace should be
located as close to the chimney (vertical venting) or to the outside vent wall
(horizontal venting) as practical.
The furnace must be located on a level, dry surface in an area, which is free
from and protected from excessive drafts. The furnace must be installed so that
the electrical components are protected from water. If the area becomes wet or
damp at times, the furnace should be raised above the floor using a concrete
base, bricks, patio blocks, etc.
When installing this furnace, provisions must be made to insure the supply of
adequate combustion and ventilation air in accordance with the air for
combustion and ventilation section of the National Fuel Gas Code, NFPA
54/ANSI Z223.1-2002, or latest edition, or applicable provisions of the local
building code. See Section II, E, of this manual
A furnace installed in a residential garage must be installed so the burners and
ignition source are located 18 inches or higher above the floor. Also, the furnace
must be located or protected to avoid physical damage by vehicles.
If this furnace is to be installed horizontally, see Section II, F, of this manual.
C. Closet and Alcove Installation
All furnace models may be installed in a closet or alcove on combustible flooring
with specified (standard) clearances to combustible construction. The furnace
shall not be installed directly on carpeting, tile or other combustible material other
than wood flooring. The MDA1-50, 75, 100, and 125, horizontal / counterflow
furnace, models requires an optional combustible surface-mounting base
for counterflow installation directly on combustible materials. The
combustible floor base model numbers are as follows: 50DA-BASE for MDA1-50

All installations and services must be performed by qualified service personnel.
5
and MDA1-75; 100CA-BASE for MDA1-100; and 125CA-BASE for MDA1-125.
refer to Figure 1.
Figure 1: Counter flow Furnace Combustible Surface-Mounting Base
D. Standard Clearances
Definitions of combustible material and non-combustible as issued by the
National Fuel Gas Code are as follows:
Combustible Material:
Materials made of or surfaced with wood, compressed paper, plant fibers,
plastics or other material that will ignite and burn whether flameproof or not or
whether plastered or not plastered.
Non-combustible Material:
Material which will not ignite and burn; such materials consisting entirely of steel,
iron, brick, concrete, slate, glass, plaster or combination thereof.
Table 1: Minimum Clearances to Combustible Materials
Model No.
Sides & Rear
of Furnace
(in.)
Front of
Furnace
(in.)
Top of
Plenum
(in.)
Flue or Vent
(in.)
Sides of
Plenum
(in.)
All Models
Covered by
This Manual
0 6 1 *6 1
*6 in. clearance may be reduced to 1 in. with UL Recognized type B1 vent.
The minimum clearances are listed for fire protection. Clearances for servicing
and proper operation, the front of the furnaces and to all points on the furnace
requiring access must be 24 in.

All installations and services must be performed by qualified service personnel.
6
E. Air for Combustion and Ventilation
:
Relief openings in the front or top of the furnace casing should not
be obstructed or blocked. These openings supply combustion and
ventilation air to the furnace.
Chloride, fluoride, iodide, and bromide bearing compounds when present, even
in low concentrations, in air supplied for combustion to the furnace, can result in
accelerated and severe corrosion of the heat exchanger and/or the venting
systems. This condition can occur with less efficient furnaces, but is accelerated
on furnaces with efficiencies of 80% or greater.
Many, household chemicals contain chloride-bearing compounds. There are
many compounds representative of this classification of chemicals. A few
common examples are listed below.
• Cleaning solvents
• Varnish and paint removers
• Bleaches
• Fabric softeners
• Water softener salt
• Tile adhesives
Avoid installing the furnace in areas where it is evident these chemicals are
being stored, or used, within close proximity to the furnace. In addition, avoid
storing or using any chemicals of an unknown nature in close proximity to the
furnace. If it is necessary to store or use these chemicals in the same space as
the furnace, all containers should be sealed when not in use. If possible, keep
these chemicals in a separate, well-ventilated, room that is closed off, from the
furnace, with a door.
The furnace shall be installed in a location within the building that permits a
satisfactory supply of air for combustion, ventilation, and proper operation of the
venting system. While all forms of building construction cannot be covered in
detail in this manual, this requirement may usually be met by application of one
of the following methods in ordinary building construction. Consult the National
Fuel Gas Code, NFPA 54/ANSI Z223.1 – 2002, or the latest edition, for special
cases and further details. However, local installation codes always take
precedence and must be followed, if applicable.

All installations and services must be performed by qualified service personnel.
7
1. Utility Room (example of a confined space)
In buildings of unusually tight construction, such as those wherein weather
stripping and storm sash windows are used, two (2) permanent openings
connecting to a well-ventilated crawl space, attic, or with the outdoors
shall be provided. Each opening shall have a minimum free area of one
(1) square inch per 1000 BTUH of total input rating of all appliances to be
installed in the utility room. One opening should be located near, or in, the
ceiling of the room and the other should be located near, or in, the floor.
2. Full Basement (example of an unconfined space)
Where a furnace is installed in a full basement, infiltration is normally
adequate to provide air for combustion and ventilation.
When an opening in the outside wall must be provided, it should be furnished
with properly screened metal sleeves.
If an exhaust fan, or additional fuel burning appliances, are present in the
furnace room, there should be increased concern about providing adequate
airflow to the furnace. Additional efforts may be required to assure an adequate
supply of combustion and ventilation air is available to the furnace under all
conditions.
F. MDA1 Horizontal Installation
:
The auxiliary limit switch must be relocated in accordance with the
following instructions when installing this furnace horizontally.
:
DO NOT hang the horizontal / counterflow furnace from a structure
or surface by any integral part or fastener of the furnace. The furnace
was not designed to support itself in this manner.
The MDA1-50, 75, 100, and 125 furnaces may be installed in a horizontal
position by placing the furnace on the left or right side (as viewed from the front
in the upright position).
If the unit is placed on its right side the auxiliary limit switch located on the right
side of the house air blower must be moved to the opposite (left) side of the
blower (refer to Figure 2).

All installations and services must be performed by qualified service personnel.
8
A plenum angles kit, (AOPS4942) is available for connecting the supply plenum
to the furnace. The kit consists of four angles that are to be screwed to into the
predrilled holes in the base of the furnace using # 8 screws.
The horizontal furnace installation should be on a service platform large enough
to allow for proper clearances on all sides and service access to the front of the
furnace (refer to Table 1). If the furnace is suspended, it must be supported at
both ends and in the middle with clearance allowed for removal of both access
doors. Line contact is only permissible between lines formed by the intersection
of the top and two sides of the furnaces casing and the building joists, studs, or
framing.
Equipment must be
installed in
accordance with
regulations of the
National Board of Fire
Underwriters and the
National Fuel Gas
Code. Authorities
having jurisdiction
should be consulted
before installations
are made.
Figure 2: MDA1 Installed Horizontal

All installations and services must be performed by qualified service personnel.
9
G. Duct Work and Air Conditioning
Design and installation of the duct system should follow the current guidelines of
the Air Conditioning Contractors of America (ACCA) or the American Society of
Heating, Refrigeration and Air Conditioning Engineers, Inc. (ASHRAE).
All furnaces are tested over a range of external static pressure that simulates the
airflow resistance of the ductwork, fittings, and diffusers connected to the furnace
for a typical (average) duct system. The furnace blower and blower motor have
been selected to work successfully against the following range of duct system
resistance.
Recommended range of duct system resistance for all models: 0.2 to 0.5 in.
W.G. external static pressure.
When the furnace is installed in a small room, and no return air ducts are used,
the return openings to the unit should extend full size to a location outside the
furnace room.
If the furnace is used in connection with summer air conditioning (cooling), the air
conditioner’s evaporator coil must be installed on the air outlet side of the
furnace to avoid water vapor condensation in the furnace heat exchanger.
NOTICE: Return air grilles and supply registers in the air distribution system
should not be obstructed.
H. Filters/ Filter Installation
Filters must be installed external to the furnace casing.
It is necessary to cut the return air opening in one or both side(s) of the MHA1
models depending upon the needs of the specific installation.
This MHA furnace has been factory supplied with a high quality re-usable filter
rated for air velocities up to 600 ft/min. An optional Thermo Products filter rack
assembly (part no. AOPS7547 for MHA50, 75, 100 and AOPS7375 for the
MHA125) is available which, is sized for the filter provided.
If an optional Thermo Pride filter rack (Figure 3) is used with the furnace, square knock
outs have been provided on each side casing to act as a template for the cut out.
Scribe lines connecting the outside corners of each knockout, cut side casing along
lines. Position the open end of the filter rack so that it is accessible for filter
replacement. Attach the filter rack to the furnace with screws or pop-rivets along the
securing flange. Connect the return plenum to the filter rack and slide the filter into
place.

All installations and services must be performed by qualified service personnel.
10
w
H
OPEN END INTO WHICH
FILTER SLIDES
SECURING
FLANGE
(FURNACE SIDE)
15" 24-1/2"
FLANGE DIMENSIONS
(DUCT SIDE)
HW
19" 24"MHA1-125
MHA1-50,75,100
Figure 3: Optional Thermo Pride Filter Rack
I. Use of Non-Thermo Pride Filter Retention Means
:
Failure to comply with minimum filter installation requirements may
affect the performance and/or void the warranty on this furnace.
If a method other than a Thermo Pride filter rack is selected for retention of the
filter and/or use of a different filter type is desired, see Table 2 for minimum size
guidelines for selecting a filter system for the MHA1/MDA1 furnaces.

All installations and services must be performed by qualified service personnel.
11
Table 2: Minimum Filter Area Required (Sq. in.)
Furnace Model
Filter Type
Maximum
Rate of
Velocity
(FPM)
MHA1-50
MDA1-50
MHA1-75
MDA1-75
MHA1-100
MDA1-100
MHA1-125
MDA1-125
*THERMO
PRODUCTS
SUPPLIED
PERMANENT
FILTER
600 172 254 328 402
STANDARD
PERMANENT
FILTER
500 208 304 394 484
DISPOSABLE
TYPE
FILTER
300 344 506 656 804
* The Thermo Products supplied filter can be cut to size to fit other filter retention
systems as long as the minimum size requirement is met.
NOTICE: The filter areas in Table 2 are the minimum areas required based on
the flowrate generated by the furnace for standard heating speeds only. The
following formula can be used to determine the minimum filter area required for
cooling, if the unit is equipped with cooling. This value should then be compared
to the value shown in table 2 and the larger of the two should then be used for
determining the minimum filter area required for that installation.
FORMULA:
(tons of cooling)(400 CFM per ton)(144 sq. in. per foot) = filter area sq. inches
(max. air velocity of filter from table 5 for the filter type)
EXAMPLE: If you had an MHA1/MDA1-100 furnace and 4 tons of cooling and a
standard reusable filter.
4 tons x 400 CFM x 144 = 460 square inches for cooling
500
For heating an MHA/MDA-100 needs 328 square inches of standard reusable
filter. The filter system must be designed for the larger flow requirement
determined for cooling of 460 square inches. A filter would have to be sized so
that the area (length x width) was at least 460 sq. in.

All installations and services must be performed by qualified service personnel.
12
J. Venting
1. Vertical Venting
• The vent installation of this Category I furnace must be in
accordance with National Fuel Gas Code, NFPA 54/ANSI Z223.1-2002
or latest edition, and/or local building codes and these instructions.
• A furnace or any other gas utilization equipment must not be
connected to a chimney flue serving a separate appliance designed
to burn solid fuel.
• When venting two or more appliances into a common vent, the
smallest appliance must be vented into the top opening.
• The maximum gross stack temperature must not exceed 480°F
(249°C) under any circumstances
NOTICE: This section does not apply to units, which are sidewall vented. For
sidewall venting, see Section II, K, of this manual.
The following requirements must be followed when connecting this furnace to a
vent system.
The MHA1/MDA1 (induced draft gas highboy and counterflow/horizontal) series
furnace must be vented in accordance with these instructions into a factory built
chimney or vent complying with a recognized standard. Type B1 or double wall
vent as well as masonry chimneys lined with a lining material acceptable to the
authority having jurisdiction are acceptable as described in the venting tables
included in the appendix of the National Fuel Gas Code, NFPA 54/ANSI Z223.1-
2002, or latest edition.
A good vent is one that is sealed and has the capability of producing a -0.04 in.
W.G. draft with the capacity of handling the amount of flue gas that is introduced
to it.
The vent must be vertical or nearly vertical in design, unless equipped with a UL
listed mechanical venter.
This furnace must not be connected into any portion of a mechanical draft
system operating under a positive pressure.
This furnace shall not be vented into an unlined masonry or concrete chimney.
PVC piping is not an acceptable substitute for a chimney or a flue connector. A
chimney with an internal construction of stainless steel or some other material
that will withstand a gross flue gas temperature of 480ºF is required.

All installations and services must be performed by qualified service personnel.
13
These venting tables also include venting combinations, which apply to common
venting arrangements of two or more appliances. These requirements as well as
all the installation requirements outlined in this manual for an MHA1/MDA1
series furnace must be followed when an MHA1/MDA1 furnace is common
vented with another Category I gas appliance.
See also Section II, J, 2, of this manual, “Replacing an Existing Furnace From a
Common Vent” for information on testing the proper operation of appliances in a
common vent.
The minimum allowable chimney height is determined by the height of the
surrounding trees, roof, buildings and terrain. The chimney should extend 24"
above any surrounding obstructions. In cases where the chimney flue extends to
the basement floor, the draft can usually be improved by filling the base of the
chimney with sand to within 12" of the flue pipe after relocating the cleanout
cover.
All joints of the vent must be tightly sealed. The inside of the vent should be free
of all obstructions.
2. Replacing An Existing Furnace From a Common Vent
:
CARBON MONOXIDE POISONING HAZARD
Failure to follow the steps outlined below for each appliance connected to
the venting system being placed into operation could result in carbon
monoxide poisoning or death.
When an existing furnace is replaced or removed from a common vent serving
other appliances, the vent system may now be too large to properly vent the
remaining appliances only and must be modified accordingly. Failure to correct
an oversized vent can cause the formation of condensate, leakage or spillage of
combustion byproducts, which may cause property damage, personal injury, or
loss of life.
The steps below must be followed with each appliance, which is still connected,
to the common venting system. Each appliance should be placed in operation,
while the other appliances remaining connected to the common venting system
are not in operation.
1. Seal any unused opening(s) in the common venting system.
2. Visually inspect the venting system for proper size and horizontal pitch,
as required in the National Fuel Gas Code, NFPA 54/ANSI Z223.1 –
2002, or the latest edition. Determine there is no blockage or

All installations and services must be performed by qualified service personnel.
14
restriction, leakage, corrosion or other deficiencies which could cause
an unsafe condition.
3. Insofar as is practical, close all building doors and windows and all
doors between the space in which the appliances remaining connected
to the common venting system are located and other spaces of the
building.
4. Turn on clothes dryers and any appliance not connected to the
common venting system. Turn on any exhaust fans, such as range
hoods and bathroom exhausts, so they will operate at maximum
speed. Do not operate a summer exhaust fan.
5. Close fireplace dampers.
6. Follow the lighting instructions. Place the appliance being inspected in
operation. Adjust thermostat so appliance will operate continuously.
7. Test draft hood equipped appliances for spillage of flue gases at the
draft hood relief opening after 5 minutes of main burner operation. Use
the flame of a match or use a draft gauge.
8. If the improper venting is observed on any of the appliances during the
above testing, the common vent system must be corrected. Follow the
steps outlined in the National Fuel Gas Code, NFPA 54/ANSI Z223.1 -
2002 or the latest edition thereof, to resize the vent system to
approach the minimum size using the appropriate tables in the
appendix of the Code.
9. Return doors, windows, exhaust fans, fireplace dampers and the
remaining appliances to their previous conditions of use after
determining that each appliance remaining connected to the common
venting system is venting properly when tested as outlined above and
the vent system is properly sized.
NOTICE: Common vent sizing of two Category I appliances may be done as
referenced in the National Fuel Gas Code, NFPA 54/ANSI Z223.1 - 2002 or the
latest edition thereof.
3. Condensing in the Chimney
Due to the improved designs, resulting in increasing thermal efficiencies of our
furnaces, more heat is being placed into the home instead of up the chimney.
This means the stack temperature will be lowered, which may result in

All installations and services must be performed by qualified service personnel.
15
condensing within the chimney under certain conditions. The following is an
explanation on why and what to do if condensing occurs in a chimney or vent.
Moisture (in the form of water vapor) occurs in flue gases when hydrogen is
combined with oxygen during the chemical reaction produced by combustion.
The flue gas water vapor will do one of two things as it escapes up the chimney:
1. Remain in a vapor state, if the internal chimney wall temperature is above
the dew point, or
2. Condense on the chimney walls, if they are chilled below the dew point.
Condensing will always occur on chimney walls whose temperatures are below
the dew point of the flue gas. However, the water may evaporate once the walls
are warmer than the dew point. If the chimney walls do not reach the dew point
during the cycle of the furnace, the moisture may accumulate in large enough
quantities to cause problems such as corrosion of the chimney or corrosion of
the furnace heat exchanger.
This condensation most likely will not occur at the bottom of the chimney,
because the flue gas is heating the chimney walls as it rises and the bottom will
be heated first. This heating of the walls will cause the flue gas temperature to
drop, which in turn may reduce the wall temperature below the dew point causing
condensation to appear on the upper part of the chimney first. This condensation
may drip back as far as the flue pipe and heat exchanger, where corrosion may
occur if not corrected.
To prevent condensation, it is necessary that the internal chimney wall
temperature always be kept above the dew point. The chimney may have to be
lined with a flue liner if the temperature loss is too great for the furnace. A liner
will act as an insulator and reduce the flue gas temperature loss. Insulation may
be added around the liner for further temperature stability. If the chimney is on
the exterior of the home and condensing occurs, the chimney may be insulated
around its exterior to help the flue hold it's temperature. Also, check to see if the
chimney is too large for the flue gases to heat, if so reduce to proper size by
lining. Be sure to use steel liners such as stainless types 430, 304, 316, or for
the toughest against corrosion type 29-4C.
More detailed information on condensing may be obtained from the 2000
ASHRAE systems and Equipment Handbook Chapter 30. Refer to National Fuel
Gas Code, NFPA 54/ANSI Z223.1-2002, or latest edition, Category I venting
tables, for proper vent sizing.

All installations and services must be performed by qualified service personnel.
16
4. Flue/Vent Connector
:
• Either drill a starter hole in the inducer or use a self-tapping screw to
mount the flue pipe to the inducer. Failure to follow these instructions
may damage the inducer.
• All vents and vent connectors must fit tightly to avoid air leaks.
• The MHA1/MDA1 series furnaces must not be installed with vent
damper.
The connection of this furnace to the vent system, shall be in accordance with
the local building codes, the vent manufacturers instructions and part 10,”
Venting of Equipment”, of the National Fuel Gas Code, NFPA 54/ANSI Z223.1-
2002, or latest edition.
These venting tables also include venting combinations, which apply to common
venting arrangements of two or more appliances. These requirements as well as
all the installation requirements outlined in this manual for an MHA1/MDA1
series furnace must be followed when an MHA1/MDA1 furnace is common
vented with another Category I gas appliance.
See also Section II, J, 2, of this manual “Replacing an Existing Furnace from a
Common Vent” for information on testing the proper operation of appliances in a
common vent.
The vent connector must be galvanized or stainless steel metal pipe and must be
a 3 in. in diameter for the MHA1/MDA1-50 and 4 in. in diameter for the
MHA1/MDA1-75, MHA1/MDA1-100 and MHA1/MDA1-125. The MHA1/MDA1-75,
MHA1/MDA1-100 and MHA1/MDA1-125 are supplied with a 3 in. to 4 in. adapter
as a transition from the inducer to the 4 in. vent size (see figure 4). No size
reduction is permissible. For minimum and maximum vent lengths, see Venting
Tables for Category I appliances in the National Fuel Gas Code, NFPA 54/ANSI
Z223.1-2002, or latest edition.