Upflow and Downflow furnace schematics (C) Carson Dunlop AssociatesHeating Furnace Clearance Distances
Fire clearances, working space, other

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Warm air furnace installation clearances & fire clearances:

What are the required clearance distances around a heating furnace: here we provide a table of recommended working space and fire clearances for gas fired heating furnaces.

This article series answers just about any question about forced air or warm air furnace central heating system troubleshooting, inspection, diagnosis, and repairs in residential buildings and homes.

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Warm Air Furnace Clearance Distances

Horizontal warm air furnace in a crawl space (C) Daniel FriedmanGas Furnace Clearance Distances

There are several gas or oil fired heating furnace clearance distance types:

  1. Clearance from combustibles - ranging from 0" to 18" to 36" (typically) depending on the fuel type (electric, gas, oil).
  2. Working space or clearance distances - that may allow 0 on 1-3 sides but always requires working space at least at the front, or at front + rear if access to a blower or filter are at the unit rear side.

    Typically for gas furnaces there must be a minimum of 24" front accessibility to the furnace, and any access door or panel must have enough clearance space to permit removal of the largest component in the equipment.
  3. Direct vent terminal clearance distances: distance from the exhaust vent at the exterior wall to building features such as gas meter outlet, windows, doors, balconies, building corners, clearance over a driveway or paved sidwewalk.

    These range from 0" to 7 ft. - this set of distances is surely not what your maintenance guys were discussing regarding your clothes dryer. Details are at GAS APPLIANCE DIRECT VENT CLEARANCES
  4. Flue vent connector clearance distances: distance from the flue vent connector of "flue pipe" to combustibles
  5. Chimney clearances: for heaters vented into a metal or masonry chimney, clearance distances around the chimney and chimney top including chimney top to roof.
  6. Location related clearances: for heaters installed in a garage or in another location subject to hazards of gas accumulation at floor level or subject to impact by a vehicle, protection and elevation above the floor are required.

Typical Minimum Clearance Distances for Gas Furnaces

Clearance Topic Clearance Distance Comment / Reference
General furnace clearances & working space
Must meet all manufacturer's clearance specifications Various See I&O Manual for the specific brand & model heater
Must meet all local & national building, fire, electrical, fuel-gas codes   Check with local code officials
Service / working space - front 24" Notes 1, 2
Service / working space - other sides, panels   Note 2
Combustion air openings & nearest closed panel or door 3" Note 3, do not block air flow into combustion air intake
Combustion air & ventilation openings   not blocked by anything, including snow
Space for addition of cooling coil   Note 4
Attic installations   Notes 6, 7, service access to front, limited framing contact, may be suspended
Burner & ignition above garage floor 18" Note 5
Crawl space installations   Note 8
Clearance to combustibles, model dependent
Sides to combustible surface 0 - 1" Note 1
Front to combustible surface 3 - 18" Note 1
Top to combustible surface 1" Note 1
Flue, condensing furnace, plastic vent 0" Notes 1, 10.
Flue through masonry chimney w/ other appliances, stainless steel type 29-4C   Note 11
Condensate drain elevation 4-6" Note 9
Attic installation of horizontal unit   Note 7

Notes & References for the table above:

1. AMERICAN STANDARD GAS FURNACE INSTALLATION MANUAL [PDF] (2008), American Standard Heating & Air Conditioning, A341624P07, Upflow / Horizontal gas furnaces, Downflow / Horizontal gas furnaces, Models CUX1B040A9241A CUX1B060A9361A CUX1B080A9421A CUX1C100A9481A CUX1D100A9601A CUX1D120A9601A CDX1B040A9241A CDX1B060A9361A CDX1B080A9421A CDX1C100A9481A CDX1D120A9601A Pub. No. 41-5016-07

2. Any access door or panel must permit removal of the largest component in the heater at that location

3. Combustion air requirements: COMBUSTION AIR DEFECTS

4. Allow space above supply plenum or above the furnace to add a cooling coil including installation work space if not installed at time of furnace installation

5. Garage installations: Furnaces installed in a garage must place burners & ignition source no less than 18" above the floor & furnace must be protected from physical damage from vehicles

6. Attic installations: A horizontal gas furnace installation in an attic 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 (see manufacturer's I&O manual).

Line contact is only permissible between lines formed by intersections of the top and two sides of the furnace casing and building joists, studs, or framing.

7. Attic instllations, suspended: The horizontal furnace may also be suspended from the joists using 3/8" all-thread rods with pieces of angle iron underneath the furnace to form a hanging rack at both ends and the midpoint. The rods need to be of sufficient length to allow for proper clearances from combustible materials. The angle iron needs to be at least 32" in length to allow for access to service panels.

8. Crawl space installations: The furnace may be placed horizontally in a crawl space on a pad or other noncombustible material which will raise the unit for sufficient protection from moisture.

9. Condensate drain elevation space: A horizontal furnace in a crawl space must also be elevated approximately 4-6 inches to allow clearance for the condensate drain to exit the cabinet in the horizontal position.

10. Condensing furnace venting: American Gas Association has certified the design of condensing furnaces for a minimum of 0" clearance from combustible materials with a single wall plastic vent pipe.

See DIRECT VENTS / SIDE WALL VENTS for a complete table of recommended clearance distances from a direct vent gas appliance terminal to other building components or features.


11. Masonry flue or shared flue venting: Where the system is routed to the outdoors through an existing masonry chimney containing flue products from another gas appliance, or where required by local codes, then 3" venting of Type 29-4C stainless steel must be used in place of PVC material.

These furnaces have been classified as CATEGORY IV furnaces in accordance with the latest edition of ANSI Z21.47 • CAN/CGA- 2.3 Standards. Category IV furnaces operate with positive vent pressure and with a vent gas temperature less than 140° F above the dewpoint. These conditions require special venting systems, which must be gas tight and water tight.

99. WALL-MOUNTED FURNACES - separate clearance distance requirements are given for wall-mount heating furnaces


On 2017-07-19 by (mod) re: clothes dryer location vs gas furnace clearance distances


The two worries, I suspect, nagging at your maintenance guys were

1. lint and dust from the clothes dryer may clog the combustion air inlet of your gas fired heating furnace, causing unsafe conditions and risking fatal carbon monoxide poisoning.

2. They might miss a chance to sell you a lint filter that you not only don't need, but that depending on the type and installation might itself be a serious fire hazard.

If your clothes dryer is properly and safely vented to the building exterior, the release of lint around the dryer itself ought to be minimal and not a safety hazard.

Start with a practical examination: is there dust visible in, on, or around your gas fired heating equipment? Is that dust dryer lint? If so, then a combination of housecleaning and an inspection of the dryer for dust and lint leaks is in order regardless of the dryer's exact placement.

Two feet more or less distance change from a clothes dryer that's leaking lint would not make one iota of difference in safety.

So where is the issue with clothes dryer distance to the gas furnace?

The answer is working space or fire clearances from combustibles that will be specified in the gas furnace installation manual for your specific heater. If the dryer is installed too close to the heater it violates the manufacturer's instructions, may prevent proper heater maintenance, and might be unsafe.

Using an American Standard Gas Furnace installation manual as an example, since you don't name your furnace brand and model, the word "clearance" appears 50 times in the document, showing that the manufacturer indeed has a worry about safe clearance distances for the heater.


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