Lennox Merit Series Gas Furnace - Manuals

Page 5 Installed in Combination with a Cooling Coil When this furnace is used with cooling units, it shall beinstalled in parallel with, or on the upstream side of, coolingunits to avoid condensation in the heating compartment.See figure 2. With a parallel flow arrangement, a damper (orother means t...

Page 6 Do not install the furnace where drafts might blow direct-ly into it. This could cause improper combustion and un-safe operation. Do not block the furnace combustion air openings withclothing, boxes, doors, etc. Air is needed for propercombustion and safe unit operation. When the furnace is i...

Page 7 Unconfined Space An unconfined space is an area such as a basement orlarge equipment room with a volume greater than 50 cubicfeet (1.42 m 3 ) per 1,000 Btu (.29 kW) per hour of the com- bined input rating of all appliances installed in that space.This space also includes adjacent rooms which ...

Page 8 EQUIPMENT IN CONFINED SPACE ALL AIR FROM OUTSIDE (Inlet Air from Crawlspace and Outlet Air to Ventilated Attic) NOTE−The inlet and outlet air openings shall each have a free area of at least one square inch (645 mm 2 ) per 4,000 Btu (1.17 kW) per hour of the total input rating of all equipmen...

Page 9 Setting Equipment WARNING Do not install the furnace on its front or its back. Donot connect the return air ducts to the back of the fur-nace. Doing so will adversely affect the operation ofthe safety control devices, which could result in per-sonal injury or death. The ML180UH gas furnace ca...

Page 10 Return Air −− Upflow Applications Return air can be brought in through the bottom or eitherside of the furnace installed in an upflow application. If thefurnace is installed on a platform with bottom return, makean airtight seal between the bottom of the furnace and theplatform to ensure tha...

Page 11 Removing the Bottom Panel Remove the two screws that secure the bottom cap to thefurnace. Pivot the bottom cap down to release the bottompanel. Once the bottom panel has been removed, reinstallthe bottom cap. See figure 11. Removing the Bottom Panel FIGURE 11 Screw Bottom Panel Bottom Cap Ho...

Page 12 Horizontal Application Unit Installed on Platform FIGURE 14 clearances. GAS ENTRY VENT PIPE See the unit nameplate for Line contact is permissible but not preferred SERVICE PLATFORM WARNING Improper installation of the furnace can result in per-sonal injury or death. Combustion and flue prod...

Page 13 Venting A 4−inch diameter flue transition is factory-installed on thecombustion air inducer outlet of all models. Figure 16 shows the combustion air inducer as shipped from thefactory. FIGURE 15 Mounting Screws Location Mounting Screws Flue Transition Upflow Position FIGURE 16 UPFLOW POSITIO...

Page 14 HORIZONTAL LEFT POSITION Top Vent Discharge FIGURE 19 Disconnect pressure switch hose from barbed fitting on the pressure switch assembly. Remove pressure switch assembly(1 screw) and cut wire tie to free pressure switch wires. Re−install pressure switch on the other side of orifice plate an...

Page 15 The ML180UH series units are classified as fan−assistedCategory I furnaces when vertically vented according tothe latest edition of National Fuel Gas Code (NFPA 54 /ANSI Z223.1) in the USA. A fan−assisted Category I fur-nace is an appliance equipped with an integral mechanicalmeans to either...

Page 16 FIGURE 24 Common Venting Using Metal−Lined Masonry Chimney 4 in. (102 mm) minimum MIN. LENGTH −− AS SHORT AS PRACTICAL MAX. LENGTH −− SEE NOTE 1 BELOW. SEALED PERMANENTLY SEALED FIREPLACE OPENING EXTERIOR CHIMNEY WITH METAL LINER VENT CONNECTOR NOTE 1 − Refer to the provided venting tables f...

Page 17 FIGURE 25 Common Venting Using Tile−Lined Interior Masonry Chimney and Combined Vent Connector MINIMUM LENGTH = AS SHORT AS PRACTICAL. FOR MAXIMUM LENGTH SEE NOTE TO LEFT INTERIOR TILE−LINEDMASONRY CHIMNEY NOTE − the chimney must be properlysized per provided venting tables orlined with list...

Page 23 Gas Piping Gas supply piping should not allow more than 0.5"W.C. dropin pressure between gas meter and unit. Supply gas pipemust not be smaller than unit gas connection. CAUTION If a flexible gas connector is required or allowed bythe authority that has jurisdiction, black iron pipeshall...

Page 24 GROUND JOINT UNION AUTOMATIC GAS VALVE FIELD PROVIDED AND INSTALLED GROUND JOINT UNION Left Side Piping (Standard) Right Side Piping (Alternate) AUTOMATIC GAS VALVE DRIP LEG DRIP LEG MANUAL MAIN SHUT−OFF VALVE (With 1/8 in. NPT Plugged Tap Shown) MANUAL MAIN SHUT−OFF VALVE (With 1/8 in. NPT ...

Page 25 Leak Check After gas piping is completed, carefully check all pipingconnections (factory− and field−installed) for gas leaks. Usea leak detecting solution or other preferred means. NOTE − If emergency shutoff is necessary, shut off the mainmanual gas valve and disconnect the main power to th...

Page 26 Before connecting the thermostat, check to make sure thewires will be long enough for servicing at a later date. Makesure that thermostat wire is long enough to facilitate futureremoval of blower for service. Complete the wiring connections to the equipment. Use theprovided unit wiring diagr...

Page 27 1 − When there is a call for heat, W1 of the thermostat energizes W of the furnace control with 24VAC. 2 − Integrated control runs a self−check. S10 primary limit, S47 rollout switch and S21 secondary limit contacts are found to be closed. Call forheat can continue. 3 − Integrated control (A...

Page 28 TYPICAL ML180UH FIELD WIRING DIAGRAM FIGURE 33 IGNITOR *R *NOTE − R" REQUIRED ON SOME OUTDOOR UNITS FIGURE 34 INTEGRATED CONTROL (Automatic Hot Surface Ignition System) 3 AMP, 32 VAC FUSE HEAT FAN OFF DELAY JUMPER TERMINAL DESIGNATIONS HUMLINEXFMREACCOOLHEATPARKFLAMENEUTRALS Humidifier (...

Page 29 Unit Start−Up FOR YOUR SAFETY READ BEFORE LIGHTING WARNING Do not use this furnace if any part has been under-water. A flood−damaged furnace is extremely dan-gerous. Attempts to use the furnace can result infire or explosion. Immediately call a qualified ser-vice technician to inspect the fu...

Page 30 11 − Are flame rollout switches tripped? If flame rollout switches are tripped, call the service technician for in-spection. Heating Sequence Of Operation(follow steps below or see Figure 32 for more detail) 1 − When thermostat calls for heat, combustion air blower starts. 2 − Combustion air...

Page 31 TABLE 12 Manifold Pressure Settings at all Altitudes Model Input Size Gas 0−2000 ft 2105−4500 ft. 4501−7500 ft 7501 − 10,000 ft Line Pressure in.wg. Min Max 045 Nat 3.5 3.2 3.0 3.5 4.5 13.0 LP/propane 10.0 10.0 10.0 10.0 11.0 13.0 070 Nat 3.5 3.2 2.8 3.5 4.5 13.0 LP/propane 10.0 10.0 10.0 10...

Page 33 BLOWER DATA ML180UH045P36A PERFORMANCE (Less Filter) External Static Pressure in. w.g. Air Volume / Watts at Various Blower Speeds High Medium- High Medium- Low Low cfm Watts cfm Watts cfm Watts cfm Watts 0.00 1670 711 1335 614 1140 485 925 388 0.10 1620 663 1335 577 1130 470 940 376 0.20 15...

Page 35 Service WARNING ELECTRICAL SHOCK, FIRE, OR EXPLOSION HAZARD. Failure to follow safety warnings exactly could resultin dangerous operation, serious injury, death orproperty damage.Improper servicing could result in dangerous opera-tion, serious injury, death, or property damage.Before servici...

Page 39 Integrated Control Diagnostic Codes DIAGNOSTIC CODES Make sure to Identify LED’S Correctly. LED #1 (Red) LED #2 (Green) DESCRIPTION SIMULTANEOUS SLOW FLASH SIMULTANEOUS SLOW FLASH Power on − Normal operation.Also signaled during cooling and continuous fan. SIMULTANEOUS FAST FLASH SIMULTANEOU...

Page 40 Troubleshooting: Heating Sequence of Operation HEATING SEQUENCE OF OPERATION NORMAL HEATING MODE ABNORMAL HEATING MODE CONTROL SELF−CHECK OKAY? NORMAL OPERATION: LED #1 −− SLOW FLASHLED #2 −− SLOW FLASH NO YES YES GAS VALVE OFF. COMBUSTION AIR INDUCER OFF. INDOOR BLOWER DELAY OFF. LED #1 ONL...

Page 41 Troubleshooting: Heating Sequence of Operation (Continued) HEATING SEQUENCE CONTINUED NORMAL HEATING MODE ABNORMAL HEATING MODE FLAME RECTIFICATION CURRENT CHECK. CAN FLAME BE PROVEN WITHIN 4 SECONDS AFTER GAS VALVE OPENS? (0.5 microamps) FLAME PRESENT? INDOOR BLOWER ON AFTER 30−SECOND DELAY...

Page 42 Troubleshooting: Cooling Sequence of Operation COOLING SEQUENCE OF OPERATION NORMAL COOLING MODE ABNORMAL COOLING MODE IGNITION CONTROL MAIN POWER ON. CONTROL SELF DIAGNOSTIC CHECK. IS CONTROL OPERATING NORMALLY? YES SIGNAL POLARITY REVERSED AT LED. CONTROL WILL CONTINUE TO CALL FOR COOLING ...

Page 43 Troubleshooting: Continuous Fan Sequence of Operation CONTINUOUS HEAT SPEED FAN SEQUENCE OF OPERATION LED: SLOW FLASH RATE REMAINS UNCHANGED THROUGHOUT SEQUENCE. MANUAL FAN SELECTION MADE AT THERMOSTAT. CONTROL (G) ENERGIZES SYSTEM FAN AT HEAT SPEED. EAC TERMINAL IS ENERGIZED. THERMOSTAT CAL...