4
NOTE: DIAGRAMS & ILLUSTRATIONS ARE NOT TO SCALE.
LENNOX HEARTH PRODUCTS • MERIT PLUS
®
DIRECT VENT GAS FIREPLACES (MPDP35/40) • INSTALLATION INSTRUCTIONS
Test gauge connections are provided on the
front of the electronic gas control valve (identi-
fied IN for the inlet and OUT for the manifold
side). The control valves have a 3/8" (10mm)
NPT thread inlet and outlet side of the valve
(refer to Figure 1).
Propane tanks are at pressures that will cause
damage to valve components. Verify that the
tanks have step down regulators to reduce the
pressure to safe levels.
Orifice Sizes - Sea Level to High Altitude
(All Models)
These appliances are tested and approved for
installation at elevations of 0-4500 feet (0-1372
meters) above sea level using the standard burner
orifice sizes (marked with an "*" in Table 4).
For elevations above 4500 feet, contact your
gas supplier or qualified service technician.
Manifold Gas Supply Pressure
(all models)
Fuel # Low High
Natural
Gas
(Lo)
1.6" WC
(0.40 kPa)
(Hi) 3.5" WC
(0.87 kPa)
Propane
(Lo)
6.3" WC
(1.57 kPa)
(Hi) 10.0" WC
(2.49 kPa)
Table 3
The appliance and its appliance main gas
valve must be disconnected from the gas
supply piping system during any pressure
testing of that system at test pressures in
excess of 1/2 psi (3.5 kPa).
The appliance must be isolated from the
gas supply piping system by closing its
equipment shutoff valve during any pressure
testing of the gas supply piping system at
test pressures equal to or less than 1/2 psi
(3.5 kPa).
Deration - At higher elevations, the amount
of BTU fuel value delivered must be reduced
by either:
• Using gas that has been derated by the gas
company.
• Changing the burner orifice to a smaller size
as regulated by the local authorities having
jurisdiction and by the (USA) National Fuel Gas
Code NFPA 54/ANSI Z223.1 - latest edition or,
in Canada, the CAN/CSA-B149.1 codes - latest
edition.
Flame breadth, height and width will diminish
4% for every 1,000 feet of altitude.
REQUIREMENTS FOR THE COMMON-
WEALTH OF MASSACHUSETTS
These fireplaces are approved for installation in
the US state of Massachusetts if the following
additional requirements are met:
• Install this appliance in accordance with
Massachusetts Rules and Regulations 248
C.M.R.
• Installation and repair must be done by a
plumber or gas fitter licensed in the Com-
monwealth of Massachusetts.
• The flexible gas line connector used shall
not exceed 36 inches (92 centimeters) in
length.
• The individual manual shut-off must be a
T-handle type valve.
Massachusetts Horizontal Vent Requirements
In the Commonwealth of Massachusetts, hori-
zontal terminations installed less than seven
(7) feet above the finished grade must comply
with the following additional requirements:
• A hard wired carbon monoxide detector
with an alarm and battery back-up must be
installed on the floor level where the gas
fireplace is installed. The carbon monoxide
detector must comply with NFPA 720, be
ANSI/UL 2034 listed and be ISA certified.
• A metal or plastic identification plate must
be permanently mounted to the exterior of
the building at a minimum height of eight (8)
feet above grade and be directly in line with
the horizontal termination. The sign must
read, in print size no less than one-half (1/2)
inch in size, GAS VENT DIRECTLY BELOW.
KEEP CLEAR OF ALL OBSTRUCTIONS.
Inlet Gas Supply Pressure
(all models)
Fuel # Minimum Maximum
Natural Gas
4.5" WC
(1.12 kPa)
10.5" WC
(2.61 kPa)
Propane
11.0" WC
(2.74 kPa)
13.0" WC
(3.23 kPa)
Table 2
Gas Pressure - All Models
Tables 2 and 3 show the appliances' inlet and
manifold gas pressure requirements:
Burner Orifice Sizes
Elevation 0-4500 feet ( 0-1372 meters)
Model Nat.Gas
drill size (inches)
Propane
drill size (inches)
MPDP35
#42 (.093")*
H3721•
#54 (.055")*
99K79•
MPDP40
#36 (.1065")*
18L40•
#52 (.063")*
21L01•
Table 4
* Standard size installed at factory
• Part /Cat. Number
Orange Wire
(From DFC Wire
Harness)
Pressure
Regulator
Tower
Yellow Ground Wire
(From DFC Wire
Harness)
Green Wire
(From DFC Wire
Harness)
Inlet (In) Test Port
Main Gas Inlet
3/8" NPT
Manifold (Out)
Test Port
Connect GTMS
Wire Harness
In Canada - CAN/CGA-2.17-M91 (R2009)
(high altitude):
THE CONVERSION SHALL BE CARRIED
OUT BY A MANUFACTURER’S AUTHO-
RIZED REPRESENTATIVE, IN ACCOR-
DANCE WITH THE REQUIREMENTS OF
THE MANUFACTURER, PROVINCIAL OR
TERRITORIAL AUTHORITIES HAVING
JURISDICTION AND IN ACCORDANCE
WITH THE REQUIREMENTS OF THE
CAN/CGA-B149.1 OR CAN/CGA-B149.2
INSTALLATION CODES.
Figure 1: SIT Proflame Valve
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