+---
+title: Furnaces
+category: Mechanical / Heating & Cooling Plant
+toc_depth: 3
+description: >
+ When to use: Gas-fired warm-air (central forced-air) furnaces serving as primary heating equipment in small commercial, light-industrial, and institutional buildings, generally up to 225,000 Btu/hr input. Covers upflow, downflow/counterflow, and horizontal cabinet furnaces; non-condensing (80% AFUE) and condensing (90%+ AFUE) types; natural gas and propane firing; standing-pilot and electronic (hot-surface or direct-spark) ignition; single-stage, two-stage, and modulating burner arrangements; all NFPA 54/IFGC vent categories (I, III, IV); and accessories furnished with the furnace (filters, integral controls, ignition and safety systems, matched cooling-coil cabinets).
+
+ Not intended for: Hydronic and steam boilers (see [[sync/boilers]]); ceiling- or wall-suspended unit heaters without ductwork (see [[sync/unit-heaters]]); split-system and packaged DX cooling or heat-pump equipment (see [[sync/split-system-air-conditioners]] and [[sync/condensing-units]]); duct accessories downstream of the furnace plenum such as mixing boxes, dampers, and VAV terminals (see [[sync/duct-accessories]]); fuel-gas piping and LP storage upstream of the furnace gas valve (see [[sync/fuel-gas-piping]]); thermostats, DDC controllers, and BAS tie-ins beyond the furnace's integral controls (see [[sync/hvac-controls-instrumentation]]); commercial warm-air furnaces rated at or above 225,000 Btu/hr input (DOE commercial CWAF category under 10 CFR 431.77); and oil-fired or electric-resistance warm-air furnaces.
+---
+
+# Scope {toc}
+
+## This standard covers the equipment, accessories, integral controls, ignition and flame-safety systems, vent and combustion-air interface, fuel-gas connection at the furnace, condensate handling, electrical connection, installation, startup, and testing of gas-fired warm-air central furnaces used as primary heating equipment. {note}
+
+## The scope of this standard begins at the furnace's gas-valve inlet, electrical disconnect, and return-air connection, and terminates at the supply-air plenum connection, the vent connector, and the condensate drain stub-out. {note}
+
+## This standard applies to furnaces rated up to 225,000 Btu/hr input. At or above 225,000 Btu/hr input, a furnace is a DOE "commercial warm air furnace" governed by 10 CFR 431.77 (thermal-efficiency basis) rather than the AFUE basis used here, and is outside this standard's scope. {note}
+
+## Furnace selection, capacity, quantity, location, airflow configuration, and zoning are [[drawing: as indicated on the mechanical equipment schedules and mechanical floor plans]].
+
+## This standard establishes the materials, performance, efficiency, controls, and installation requirements that govern those drawings. {note}
+
+## Fuel-gas piping, pressure regulation, and LP storage upstream of the furnace gas valve shall be coordinated with this standard at the furnace inlet.
+
+### Fuel-gas piping, pressure regulation, and LP storage upstream of the furnace gas valve are the subject of [[sync/fuel-gas-piping]]. {note}
+
+## Ductwork, plenums, registers, and duct-mounted accessories downstream of the furnace are the subject of [[sync/hvac-ductwork]] and [[sync/duct-accessories]]; this standard governs only the furnace's external static-pressure rating and listed plenum clearances. {note}
+
+## Where a furnace is matched with an evaporator coil for summer cooling, the cooling equipment is the subject of [[sync/condensing-units]] and [[sync/split-system-air-conditioners]]; this standard governs the furnace cabinet, the coil-cabinet interface, and the airflow required for the cooling mode. {note}
+
+# Referenced Standards {toc}
+
+## Equipment, accessories, and installation shall comply with the latest adopted edition of each of the following unless a specific edition is cited.
+
+## Where referenced standards conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
+
+## The applicable mechanical code and the applicable fuel-gas code (NFPA 54 / IFGC as adopted by the jurisdiction) shall take precedence over all other references on any matter directly addressed by those codes.
+
+| Standard | Title |
+|----------|-------|
+| ANSI Z21.47 / CSA 2.3 (2021) | Gas-Fired Central Furnaces |
+| UL 1995 / CSA C22.2 No. 236 | Heating and Cooling Equipment |
+| NFPA 54 / ANSI Z223.1 | National Fuel Gas Code |
+| IFGC | International Fuel Gas Code (Chapter 5, vent sizing) |
+| NFPA 70 | National Electrical Code (Article 424 for supplemental electric heat) |
+| ANSI/ASHRAE/IES 90.1 | Energy Standard for Sites and Buildings Except Low-Rise Residential Buildings (Table 6.8 heating minimums) |
+| ASHRAE Standard 103 | Methods of Testing for Annual Fuel Utilization Efficiency of Residential Central Furnaces and Boilers |
+| 10 CFR Part 430 Subpart B | DOE Energy Conservation Standards for Consumer Furnaces (AFUE) |
+| 10 CFR Part 431.77 | DOE Energy Conservation Standards for Commercial Warm Air Furnaces (thermal efficiency; equipment at or above 225,000 Btu/hr) |
+| ACCA Manual J / Manual N | Residential and Commercial Load Calculation |
+| ACCA Manual S | Residential Equipment Selection |
+| ACCA Manual D | Residential Duct Systems |
+| AHRI 210/240 | Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment (matched furnace/coil systems) |
+| SMACNA HVAC Duct Construction Standards | HVAC Duct Construction Standards — Metal and Flexible |
+| ENERGY STAR | Program Requirements for Central Gas Furnaces (95% AFUE minimum, gas) |
+
+# Submittals {toc}
+
+## Action Submittals {toc}
+
+### The Contractor shall submit the following for the Engineer's review prior to procurement and installation.
+
+### Work shall not proceed on any furnace until the corresponding submittals have been reviewed and returned.
+
+- Product data for each furnace, indicating manufacturer, model number, airflow configuration (upflow, downflow, horizontal-left, horizontal-right), fuel type (natural gas or LP), input and output rating (Btu/hr), AFUE or thermal efficiency, vent category (I, III, or IV), permitted vent material and maximum equivalent vent length, ignition type, burner staging, external static-pressure rating, listed temperature-rise range, blower motor type and horsepower, electrical characteristics, and applicable certification listings (ANSI Z21.47 / CSA 2.3, UL 1995)
+- Manufacturer's certified ratings establishing compliance with the project efficiency floor (ASHRAE 90.1 Table 6.8 or the specified ENERGY STAR level), with AFUE or thermal efficiency stated at the rated firing condition
+- Equipment selection calculations demonstrating that furnace input is sized to the ACCA Manual J / Manual N design heat-loss load within the limits of this standard, including any high-altitude de-rate applied at the installed elevation
+- Fan-performance data and blower-speed table showing available external static pressure across the operating airflow range, with the selected heating and cooling airflow and the resulting temperature rise within the listed min-max range
+- Venting and combustion-air plan for each furnace, including vent category, vent material and diameter, total equivalent vent length, intake and exhaust termination locations and clearances, common-venting calculations where furnaces share a vent, and condensate routing for Category IV equipment
+- Fuel-gas connection details, including required inlet pressure at the furnace, manifold pressure, gas-train components (manual shutoff, sediment trap, union), and any second-stage regulator required where the gas service is delivered at elevated pressure, coordinated with [[sync/fuel-gas-piping]]
+- Electrical connection details, including circuit ampacity, overcurrent protection, disconnect location and rating, and equipment grounding and bonding coordinated with [[sync/grounding-and-bonding]]
+- Controls interface description, identifying whether the furnace control board interfaces with the thermostat only or with the building automation system, and the interface type (24 VAC hardwired contacts, 0-10 VDC, or communicating protocol), coordinated with [[sync/hvac-controls-instrumentation]] and [[sync/building-automation-system]]
+- Coil-cabinet and matched-coil data where the furnace is paired with cooling, including AHRI 210/240 matched-system ratings, coil airflow, and condensate-drain provisions
+- Filter data, including filter type, dimensions, MERV rating, and the rack or cabinet arrangement
+- Manufacturer's installation, operation, and maintenance instructions and the startup / commissioning checklist
+- LP conversion kit data and qualified-installer certification where the furnace is field-converted to propane
+
+```datasheet
+label: Action Submittals Required
+type: checkbox
+options:
+ - "Furnace product data (configuration, fuel, input/output, efficiency, vent category)"
+ - "Certified efficiency ratings vs. project floor"
+ - "Equipment selection / load-match calculations (incl. altitude de-rate)"
+ - "Fan-performance and blower-speed table with temperature rise"
+ - "Venting and combustion-air plan"
+ - "Fuel-gas connection details"
+ - "Electrical connection details"
+ - "Controls interface description (thermostat vs. BAS)"
+ - "Coil-cabinet / matched-coil data (AHRI 210/240)"
+ - "Filter data (type, size, MERV)"
+ - "Installation/O&M instructions and startup checklist"
+ - "LP conversion kit and qualified-installer certification"
+default: "Furnace product data (configuration, fuel, input/output, efficiency, vent category)"
+```
+
+## Closeout Submittals {toc}
+
+### The Contractor shall submit the following upon completion, before final acceptance.
+
+### The Contractor shall submit the completed startup and combustion-analysis report for each furnace.
+
+- Completed manufacturer's startup checklist for each furnace, signed and dated
+- Combustion-analysis report for each furnace recording measured carbon monoxide (air-free), flue-gas temperature, manifold gas pressure, and inlet gas pressure at the firing condition
+- Measured external static pressure and supply-air temperature rise for each furnace, confirming the rise falls within the listed range
+- Record drawings showing the as-installed furnace locations, vent routing and terminations, and condensate routing
+- Operation and maintenance manuals, including filter type and replacement schedule, ignition and limit-control adjustment, and seasonal startup and shutdown procedures
+- Warranty documentation for the heat exchanger and the balance of the furnace
+
+```datasheet
+label: Closeout Submittals Required
+type: checkbox
+options:
+ - "Signed startup checklist per furnace"
+ - "Combustion-analysis report (CO air-free, flue temp, gas pressures)"
+ - "Measured static pressure and temperature rise per furnace"
+ - "Record drawings (furnace, vent, condensate)"
+ - "Operation and maintenance manuals"
+ - "Heat-exchanger and balance-of-furnace warranty documentation"
+default: "Signed startup checklist per furnace"
+```
+
+# Quality Assurance {toc}
+
+## Each furnace shall be listed and labeled by a Nationally Recognized Testing Laboratory to ANSI Z21.47 / CSA 2.3, or to UL 1995 / CSA C22.2 No. 236 where applicable to the equipment category.
+
+## Each furnace shall bear a permanent rating plate stating the input rating, output or heating capacity, AFUE or thermal efficiency, fuel type and manifold pressure, electrical characteristics, vent category, listed clearances to combustibles, and the listed temperature-rise range.
+
+## The installing contractor shall be experienced in the installation and startup of gas-fired warm-air furnaces.
+
+## The installing contractor shall hold any gas-appliance installation license required by the jurisdiction.
+
+## Field conversion between natural gas and propane shall be performed only by personnel qualified by the furnace manufacturer, using the manufacturer's listed conversion kit for the specific model.
+
+## Startup of each furnace shall be performed in accordance with the manufacturer's instructions, including verification of gas pressure, combustion, temperature rise, and operation of all flame-safety and limit controls.
+
+## Furnaces, vent materials, and condensate-handling components shall be the product of a single manufacturer's listed system to the extent the listing requires matched components, and substitution of unlisted vent or condensate components is prohibited.
+
+# Environmental and Service Conditions {toc}
+
+## The design heating load shall be established by an ACCA Manual J (residential) or Manual N (light commercial) calculation.
+
+## Furnace input shall be sized to the calculated design heat-loss load and shall not exceed 125% of that load.
+
+## Furnaces installed above 2,000 ft elevation shall be de-rated for altitude in accordance with the manufacturer's instructions, or furnished with the manufacturer's high-altitude kit.
+
+### Atmospheric and combustion conditions change with elevation; a gas furnace rated at sea level produces less usable output and risks incomplete combustion at altitude unless de-rated, typically on the order of 4% per 1,000 ft above sea level. {note}
+
+### The rated input used for equipment selection shall be the input at the installed elevation, not the sea-level nameplate input.
+
+## The furnace external static-pressure rating shall equal or exceed the design external static pressure of the connected supply-and-return duct system.
+
+### Residential-lineage furnaces are typically rated at 0.5 in. w.g. external static pressure, while light-commercial high-static models are rated at 0.8 in. w.g. to 1.0 in. w.g. {note}
+
+### A residential-rated furnace shall not be applied to a commercial multi-zone duct system whose design external static pressure exceeds the furnace's rating.
+
+## Combustion air shall be provided in accordance with NFPA 54 Chapter 9 for the installed condition (confined space, unconfined space, or direct-vent sealed combustion).
+
+### A confined-space installation requires combustion-air openings sized at 1 in.² of free area per 1,000 Btu/hr of total appliance input drawn from outdoors, and an unconfined space requires a room volume of at least 50 ft³ per 1,000 Btu/hr of total input. {note}
+
+### Combustion-air opening sizes and locations shall be shown on the mechanical drawings and shall not be conveyed by general note alone.
+
+## Direct-vent (sealed-combustion) furnaces shall be specified where the mechanical space is of tight construction, is unvented, or cannot provide combustion-air openings by another listed means.
+
+```datasheet
+label: Design External Static Pressure
+type: range
+unit: in. w.g.
+min: 0.3
+max: 1.0
+step: 0.1
+default: 0.5
+```
+
+```datasheet
+label: Installed Elevation (for altitude de-rate)
+type: range
+unit: ft
+min: 0
+max: 8000
+step: 500
+default: 0
+```
+
+```datasheet
+label: Combustion Air Source
+type: radio
+options:
+ - "Unconfined space (room volume meets NFPA 54)"
+ - "Confined space with outdoor combustion-air openings"
+ - "Direct-vent sealed combustion (concentric or two-pipe)"
+default: "Confined space with outdoor combustion-air openings"
+```
+
+# Furnace Type and Configuration {toc}
+
+## The furnace airflow configuration shall match the supply-and-return duct arrangement and the furnace location.
+
+### Upflow furnaces draw return air at the bottom and discharge supply air from the top and are the most common commercial arrangement; downflow/counterflow furnaces reverse this for plenums below the floor in slab-on-grade or crawl-space layouts; horizontal furnaces lie on their side for attic and crawl-space installations. {note}
+
+### Where the cooling coil must be located in the supply plenum below the furnace, a downflow configuration shall be specified to provide proper coil drainage and to avoid refrigerant migration.
+
+## The efficiency tier (non-condensing or condensing) shall be selected in coordination with the vent category, condensate handling, and the project efficiency floor.
+
+### Non-condensing furnaces operate in the 80% AFUE range, reject flue gas above its dew point, and vent through Category I or Category III systems; condensing furnaces operate at 90% AFUE and above, recover latent heat by condensing flue gas, and vent through Category IV systems with continuous condensate disposal. {note}
+
+## The burner staging shall be selected to suit the load profile of the building.
+
+### Single-stage burners fire at one rate; two-stage burners fire at a reduced and a full rate; modulating burners vary firing rate continuously and, paired with an ECM blower, deliver the steadiest supply temperature and the highest seasonal efficiency in variable-load commercial applications. {note}
+
+## The ignition system shall be a hot-surface igniter or direct-spark electronic ignition. Standing-pilot ignition shall be used only where specifically indicated and permitted by the jurisdiction.
+
+```datasheet
+label: Airflow Configuration
+type: radio
+options:
+ - "Upflow"
+ - "Downflow / counterflow"
+ - "Horizontal-left"
+ - "Horizontal-right"
+default: "Upflow"
+```
+
+```datasheet
+label: Efficiency Tier
+type: radio
+options:
+ - "Condensing (90%+ AFUE, Category IV)"
+ - "Non-condensing (80% AFUE, Category I/III)"
+default: "Condensing (90%+ AFUE, Category IV)"
+```
+
+```datasheet
+label: Burner Staging
+type: radio
+options:
+ - "Single-stage"
+ - "Two-stage"
+ - "Modulating"
+default: "Two-stage"
+```
+
+```datasheet
+label: Ignition Type
+type: radio
+options:
+ - "Hot-surface igniter (HSI)"
+ - "Direct-spark ignition"
+ - "Standing pilot (where permitted)"
+default: "Hot-surface igniter (HSI)"
+```
+
+# Fuel and Capacity {toc}
+
+## The furnace shall be furnished and configured for the project fuel type (natural gas or propane).
+
+### The gas valve, burner orifices, and manifold pressure differ between natural gas and propane; a furnace furnished for the wrong fuel, or field-converted with incorrect orifices, produces incomplete combustion, elevated carbon monoxide, or flame rollout. {note}
+
+## The furnace gas valve shall be sized and configured for the manifold pressure listed for the selected fuel.
+
+### Natural gas manifold pressure is typically 3.5 in. w.c.; propane manifold pressure is typically 10 in. w.c. {note}
+
+## The required inlet gas pressure at the furnace shall be coordinated with [[sync/fuel-gas-piping]] and shown on the contract documents.
+
+### Natural gas is typically delivered at the furnace inlet between 5 in. w.c. and 14 in. w.c.; propane is typically delivered between 11 in. w.c. and 14 in. w.c. {note}
+
+### Where the gas service is delivered at elevated pressure (for example 2 psig), a second-stage regulator shall be specified and coordinated so that pressure at the furnace inlet falls within the listed range.
+
+## The furnace input rating shall be selected from the manufacturer's standard offering to match the calculated design load at the installed elevation, within this standard's input range of 40,000 Btu/hr to 225,000 Btu/hr.
+
+```datasheet
+label: Fuel Type
+type: radio
+options:
+ - "Natural gas"
+ - "Propane (LP)"
+default: "Natural gas"
+```
+
+```datasheet
+label: Furnace Input Capacity
+type: range
+unit: Btu/hr
+min: 40000
+max: 225000
+step: 5000
+default: 80000
+```
+
+```datasheet
+label: Manifold Pressure
+type: range
+unit: in. w.c.
+min: 3.0
+max: 11.0
+step: 0.5
+default: 3.5
+```
+
+```datasheet
+label: Required Inlet Gas Pressure at Furnace
+type: range
+unit: in. w.c.
+min: 5.0
+max: 14.0
+step: 0.5
+default: 7.0
+```
+
+# Efficiency {toc}
+
+## Each furnace shall meet or exceed the minimum heating efficiency required by ASHRAE 90.1 Table 6.8 for its capacity category, and shall not be less efficient than the federal minimum AFUE of 80% for non-weatherized gas furnaces.
+
+## For new commercial construction in Climate Zones 4 through 8, condensing furnaces of at least 95% AFUE should be specified.
+
+### The DOE northern-climate condensing rule has been subject to legal challenge; ASHRAE 90.1 Table 6.8 and the ENERGY STAR 95% AFUE level are the stable specification levers for driving condensing equipment without relying on the contested federal regional standard. {note}
+
+## Where ENERGY STAR certification is required by the project, each gas furnace shall be ENERGY STAR certified at no less than 95% AFUE.
+
+## The AFUE or thermal efficiency stated on the submittal shall be the certified rating at the furnace's rated firing condition.
+
+### Annual fuel utilization efficiency (AFUE) is established by the ASHRAE 103 test method and applies to furnaces below 225,000 Btu/hr input; thermal efficiency applies to commercial warm-air furnaces at or above that threshold, which are outside this standard. {note}
+
+```datasheet
+label: Minimum AFUE
+type: range
+unit: "%"
+min: 80
+max: 98
+step: 1
+default: 80
+```
+
+```datasheet
+label: ENERGY STAR Certification Required
+type: radio
+options:
+ - "Yes (95% AFUE minimum)"
+ - "No (meet ASHRAE 90.1 minimum)"
+default: "No (meet ASHRAE 90.1 minimum)"
+```
+
+# Cabinet and Heat Exchanger {toc}
+
+## The furnace cabinet shall be of galvanized or pre-painted steel with factory-applied internal insulation, and shall provide service access to the burner, heat exchanger, blower, and control compartment.
+
+## The primary heat exchanger shall be the manufacturer's standard for the selected efficiency tier (aluminized or stainless steel for non-condensing; corrosion-resistant stainless steel for the secondary condensing heat exchanger).
+
+### The secondary heat exchanger in a condensing furnace operates continuously in the presence of acidic condensate. {note}
+
+### The secondary heat exchanger in a condensing furnace shall be of a material listed for condensate service.
+
+## The heat exchanger shall be factory-tested for leakage in accordance with ANSI Z21.47.
+
+## The furnace shall not be installed if the heat exchanger shows evidence of cracking, leakage, or corrosion on receipt.
+
+## The blower shall be the type indicated (PSC or ECM); ECM blowers shall be furnished where modulating or variable-capacity operation is specified.
+
+### An ECM (electronically commutated) blower maintains airflow against varying static pressure and consumes less energy at part load, which is why it is paired with two-stage and modulating furnaces. {note}
+
+## The blower speed shall be set at startup so that the supply-air temperature rise falls within the range listed on the furnace rating plate.
+
+### Each furnace lists a minimum and maximum temperature rise (for example 35°F to 65°F, or 45°F to 75°F); operating below the minimum risks condensation in a non-condensing heat exchanger, and operating above the maximum trips the high-limit and shortens heat-exchanger life. {note}
+
+```datasheet
+label: Blower Motor Type
+type: radio
+options:
+ - "ECM variable-speed"
+ - "PSC multi-speed"
+default: "ECM variable-speed"
+```
+
+```datasheet
+label: Listed Temperature Rise Range
+type: select
+options:
+ - "35°F to 65°F"
+ - "40°F to 70°F"
+ - "45°F to 75°F"
+ - "50°F to 80°F"
+default: "45°F to 75°F"
+```
+
+# Venting and Condensate {toc}
+
+## The vent system shall match the furnace's listed vent category, and the vent material shall be the type listed for that category.
+
+### Category I furnaces vent by gravity or induced draft through Type B vent; Category III positive-pressure non-condensing furnaces vent through listed AL29-4C stainless; Category IV positive-pressure condensing furnaces vent through PVC, CPVC, or polypropylene listed for the furnace. {note}
+
+### A Category IV condensing furnace shall not be vented through a Category I (Type B) vent.
+
+### Installing a condensing furnace on a Type B vent destroys the vent within a single heating season because condensing flue gas attacks the gravity-vent liner; this is a common and severe field error. {note}
+
+## The vent material, diameter, total equivalent length, and termination shall be specified to match the specific furnace listing and the connected vent run.
+
+## Sidewall and direct-vent terminations shall comply with NFPA 54 clearance requirements, including a minimum of 12 in. above grade and a minimum of 4 ft from building openings such as windows, doors, and air intakes.
+
+### Termination clearances shall be shown on the mechanical drawings to prevent field conflict at the building exterior.
+
+## Condensate from a Category IV furnace shall be collected and routed continuously, by gravity or by a listed condensate pump, to a floor drain or to a condensate neutralizer where required by the jurisdiction.
+
+### Condensing-furnace condensate is acidic, with a pH of approximately 3.2 to 5.5, and a typical production of 1.0 to 2.5 gallons per hour per 100,000 Btu/hr of input; a continuously pitched drain line and, where required, an acid neutralizer protect the drainage system. {note}
+
+### A condensate drain line exposed to freezing temperatures shall be protected against freezing, and an outdoor or unconditioned-space drain line shall not be routed where it can freeze and block.
+
+```datasheet
+label: Vent Category and Material
+type: radio
+options:
+ - "Category IV — PVC / CPVC / polypropylene (condensing)"
+ - "Category III — AL29-4C stainless (positive-pressure non-condensing)"
+ - "Category I — Type B vent (gravity / induced draft)"
+default: "Category IV — PVC / CPVC / polypropylene (condensing)"
+```
+
+```datasheet
+label: Vent Termination Type
+type: radio
+options:
+ - "Sidewall (two-pipe or concentric)"
+ - "Vertical through roof"
+default: "Sidewall (two-pipe or concentric)"
+```
+
+```datasheet
+label: Condensate Disposal (Category IV)
+type: radio
+options:
+ - "Gravity to floor drain"
+ - "Condensate pump to drain"
+ - "Neutralizer then floor drain"
+default: "Gravity to floor drain"
+```
+
+```datasheet
+label: Condensate Neutralizer Required
+type: radio
+options:
+ - "No"
+ - "Yes (per jurisdiction)"
+default: "No"
+```
+
+# Controls and Ignition {toc}
+
+## The furnace shall be furnished with an integrated control board providing ignition sequencing, blower control, and flame-safety supervision.
+
+## The furnace shall include, at minimum, a flame-sensing or flame-rectification proving circuit, a primary high-temperature limit, a rollout safety switch, and a pressure or airflow proving switch for the induced-draft or power-vent blower.
+
+### These interlocks prove combustion airflow, prove flame, and shut off gas on overtemperature or flame rollout; each is a code-required safety function, not an option. {note}
+
+## The thermostat or control interface shall match the burner staging and the cooling configuration.
+
+### A single-stage heating-only furnace uses a basic 24 VAC thermostat; two-stage heating with cooling and humidification requires a multi-wire thermostat; modulating and communicating furnaces use a 0-10 VDC or a manufacturer's communicating interface. {note}
+
+## Where the furnace integrates with the building automation system, the interface method (24 VAC hardwired contacts, 0-10 VDC analog, or communicating protocol) shall be specified before the furnace is ordered and coordinated with [[sync/hvac-controls-instrumentation]] and [[sync/building-automation-system]].
+
+### Factory-installed communicating control boards are not standard on all models; specifying the interface late forces field workarounds or re-ordering. {note}
+
+```datasheet
+label: Control Interface
+type: radio
+options:
+ - "24 VAC thermostat (heating only)"
+ - "24 VAC multi-stage thermostat (heat/cool/HUM)"
+ - "0-10 VDC analog to BAS"
+ - "Communicating protocol to BAS"
+default: "24 VAC multi-stage thermostat (heat/cool/HUM)"
+```
+
+# Filters and Cooling-Coil Cabinet {toc}
+
+## The furnace shall be furnished with a return-air filter rack or cabinet accepting a filter of the specified type and rating.
+
+## The filter shall be a minimum of MERV 8 unless a higher rating is indicated, and the filter rack shall accept the filter without bypass.
+
+### Filter pressure drop counts against the furnace's available external static pressure; a high-MERV filter on a residential-rated furnace can consume the entire static budget and cause nuisance high-limit trips. {note}
+
+## Where summer cooling is provided, a matched cooling-coil cabinet shall be furnished and arranged for the airflow configuration, with the coil downstream of the furnace heat exchanger in the supply airstream.
+
+## The matched furnace-and-coil system shall be rated in accordance with AHRI 210/240, and the furnace blower airflow shall meet the coil's rated cooling airflow.
+
+```datasheet
+label: Filter MERV Rating
+type: select
+options:
+ - "MERV 8"
+ - "MERV 11"
+ - "MERV 13"
+default: "MERV 8"
+```
+
+```datasheet
+label: Cooling-Coil Cabinet
+type: radio
+options:
+ - "None (heating only)"
+ - "Cased coil furnished with furnace"
+ - "Coil by cooling-equipment supplier (coordinate)"
+default: "None (heating only)"
+```
+
+# Testing {toc}
+
+## Each furnace shall be started, tested, and adjusted in accordance with the manufacturer's instructions before the system is placed in service.
+
+## A combustion analysis shall be performed on each furnace at startup.
+
+## The measured air-free carbon monoxide shall not exceed 400 ppm.
+
+## The manifold gas pressure and the inlet gas pressure shall be measured and adjusted to the values listed for the selected fuel.
+
+## The supply-air temperature rise shall be measured and the blower speed adjusted so the rise falls within the listed range.
+
+## The external static pressure shall be measured across the furnace and compared against the furnace rating and the design value.
+
+## The operation of each flame-safety and limit control shall be verified, including flame proving, high-limit trip, rollout switch, and pressure or airflow proving switch.
+
+## Where the furnace is part of a system requiring air balancing, the airflow shall be set and documented under [[sync/testing-adjusting-and-balancing]].
+
+```datasheet
+label: Field Tests Required
+type: checkbox
+options:
+ - "Combustion analysis (CO air-free ≤ 400 ppm)"
+ - "Manifold and inlet gas pressure measured and set"
+ - "Supply-air temperature rise within listed range"
+ - "External static pressure measured"
+ - "Flame-safety and limit controls verified"
+ - "Airflow balanced and documented"
+default: "Combustion analysis (CO air-free ≤ 400 ppm)"
+```
+
+# Installation {toc}
+
+## The furnace shall be installed in accordance with its listing, the manufacturer's instructions, NFPA 54 / IFGC, and NFPA 70.
+
+## The furnace shall be installed with the clearances to combustibles shown on its listing label, and where the furnace is not listed for a clearance, the clearances of NFPA 54 Table 10.2 shall apply.
+
+### Service clearance is required even for a zero-clearance-listed furnace. {note}
+
+### A minimum front service clearance of 24 in. shall be maintained and shall be shown on the mechanical drawings.
+
+### The mechanical room or closet shall be dimensioned to include the furnace service clearance, not merely the equipment envelope.
+
+## The furnace shall be set level on a non-combustible base or platform appropriate to the configuration, and a horizontal furnace shall be supported along its length per the manufacturer's instructions.
+
+## The return-air connection shall be made so that no return air is drawn from the combustion zone or the space containing the furnace where prohibited by NFPA 54.
+
+## The supply plenum and ductwork connected to the furnace shall maintain the listed clearance to combustibles, coordinated with [[sync/hvac-ductwork]] and the SMACNA HVAC Duct Construction Standards.
+
+## A disconnecting means shall be provided within sight of the furnace in accordance with NFPA 70.
+
+## A gas shutoff valve, sediment trap, and union shall be provided at the furnace in accordance with NFPA 54, coordinated with [[sync/fuel-gas-piping]].
+
+## The condensate drain shall be installed with continuous pitch to the point of disposal and shall include the trap or neutralizer required by the furnace listing and the jurisdiction.
+
+```datasheet
+label: Mounting Configuration
+type: radio
+options:
+ - "Floor-mounted on non-combustible base"
+ - "Suspended / platform-mounted (horizontal)"
+ - "Closet-mounted with clearances"
+default: "Floor-mounted on non-combustible base"
+```
+
+```datasheet
+label: Minimum Front Service Clearance
+type: range
+unit: in.
+min: 24
+max: 36
+step: 3
+default: 24
+```
+
+# Delivery, Storage, and Handling {toc}
+
+## Furnaces shall be delivered in the manufacturer's original packaging with the rating plate and installation instructions intact.
+
+## Furnaces shall be stored indoors, upright per the packaging markings, and protected from moisture, dust, and physical damage until installation.
+
+## Furnaces shall be inspected on receipt for shipping damage, and a furnace with a damaged cabinet, heat exchanger, or controls shall not be installed.
+
+# Warranty {toc}
+
+## The manufacturer shall warrant the heat exchanger against defects in materials and workmanship for a minimum of 10 years from the date of substantial completion.
+
+## The manufacturer shall warrant the balance of the furnace against defects in materials and workmanship for a minimum of 5 years from the date of substantial completion.
+
+## The Contractor shall warrant the installation, including venting, gas connection, condensate handling, and electrical connection, for a minimum of 1 year from the date of substantial completion.
+
+```datasheet
+label: Heat-Exchanger Warranty Period
+type: range
+unit: years
+min: 10
+max: 20
+step: 5
+default: 10
+```
+
+```datasheet
+label: Balance-of-Furnace Warranty Period
+type: range
+unit: years
+min: 5
+max: 10
+step: 1
+default: 5
+```
+
+# Spare Parts {toc}
+
+## The Contractor shall furnish one complete set of replacement filters for each furnace, of the specified type and rating.
+
+## The Contractor shall furnish one spare hot-surface igniter or spark electrode for each furnace model installed where electronic ignition is used.
+
+## The Contractor shall furnish one spare flame sensor for each furnace model installed.
+
+```datasheet
+label: Spare Parts to Furnish
+type: checkbox
+options:
+ - "One set of replacement filters per furnace"
+ - "One spare igniter / spark electrode per model"
+ - "One spare flame sensor per model"
+default: "One set of replacement filters per furnace"
+```