Williams Monterey Wall Furnace © IAPWall-Mounted Furnaces & Heaters
Inspect, trouble shoot, repair gas-fired wall heaters

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Wall mounted direct-venting gas-fired heating furnaces.

This article describes wall mounted furnaces used for home heating. Usually these are gas-fired direct-vent units such as the gas furnaces described in the text below.

We describe the installation fire clearances to combustibles, the use of an auxiliary blower unit, and the ability of the gas furnace to deliver heat without requiring external electrical power, permitting an area to be heated during a power outage or where electricity may simply not be available.

We discuss wall mounted furnace choices, installation, troubleshooting, safe flue gas venting, repairs. How do direct-vent gas fueled wall furnaces work? Use of auxiliary blowers with wall furnaces. Operation of the wall furnace during a power outage. Clearances to combustibles for wall-mount gas furnaces.

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Direct-Vent Wall-Mounted Gas Furnaces

Williams wall furnaceShown at page top and at left are examples of gas-fired wall furnaces used for heating residential homes or in some cases mobile homes. These example furnaces are Williams wall furnaces. Shown at left: Williams Furnace Corporation 22K/30K BTUh direct vent furnace.

[Click to enlarge any image]

The gas-fueled direct-vent wall-mounted furnaces described here are typically installed where point-source heat will be adequate to warm a living area and where no chimney is required (the units vent directly through a side wall).

Typical wall mount furnaces like the ones described here offer an added advantage of being able to provide heat during a power outage, as they can work without electricity. The units all incorporate an automatic thermostat that can be used to control heat output.

Typical direct-vent gas fueled furnaces operate with output ranges between 15 KBTUh and 50KBTUh. The heaters vary in width from as little as 14" to about 36" and in height from about 27" to nearly 6. 5 feet depending on the manufacturer and model.

If your application needs greater heating capacity, higher BTUh output gas-fueled heaters may be sold by the same manufacturer in alternative models that require top-venting rather than venting horizontally directly through the building wall, offering outputs up to 65,000 BTUh.

Gas furnace models designed to operate at high altitude are also available. When evaluating the BTUh output of these gas-fired heaters, Trane notes that the input BTUH ratings are for elevations up to 2000 ft and that BTUH ratings (for U.S. installations) should be de-rated by 4% per 1000 feet above sea level.

For Canadian installations of such gas heaters Trane's BTUH input ratings are for up to 4,500 ft. and are de-rated 4% for elevations above 4,500 feet above sea level. Adjustments to the BTUH data for other manufacturers may vary. - Trane (2003).

Auxiliary Blower Unit for Direct Vent Wall-Mount Gas Furnaces

A high-capacity blower fan, provided as an accessory, does require 115VAC and is sold as an additional feature that can increase the heat output and heat delivery speed for these units. But they can also work by convection without operating the blower fan.

Sunroom conversion to heated space using wall mount gas furnace (C) Daniel Friedman KL RS 1983 The gas heater can operate with or without the auxiliary blower if one is installed, so its presence does not prevent the delivery of heat should there be a power loss.

The auxiliary blower mount location varies but is typically attached to the top of the furnace and includes separate on-off or automatic controls. Some wall mount furnaces also include a rear-outlet register suitable for some installations.

I [DJF] first installed one of these direct vented, wall-mounted gas furnaces in an un-heated sunroom addition of a Fishkill, New York Home in 1983. What had been a porch that was later enclosed had been an un-heated area of the home shown at left (red arrow).

Shortly after the project was complete, the owners experienced a power outage that lasted several days.

They reported being quite happy to be able to retreat to the newly-completed and warm sunroom of the home where the heater operated without a requirement for external electrical power.

Wall Mount Furnace Installation Specifications

Direct vent gas furnace operation, Williams Furnace CorporationUsing the Williams Furnace Company models as an example, typical clearance distances are

Larger, taller wall-mount furnaces of this type can be installed with zero clearance to the finished floor and as closely as 2-inches to the ceiling or overhang, depending on the furnace model.

[Click to enlarge any image]

The wall furnace vents the LP or natural gas fuel combustion gases through a horizontal wall vent to the building exterior. Because the furnace is designed to accept combustion air from outside and to vent combustion gases directly back to the outdoors, the combustion chamber is sealed from room air, minimizing the risk of escape of flue gases or dangerous carbon monoxide to the room interior.

The wall furnace vent installation should include a vent shield and a vent-cap guard to protect against entry into the vent by birds or other animals - an event that could occur when the unit is idle and the vent is cool, and an event that could block the flue, preventing safe operation of the unit.

For thicker building walls the horizontal vent can be extended up to 24-inches. For very thin building walls (such as in a mobile home where permitted by local codes), a thin-wall collar kit adapts the heater's vent system to walls from 1" to 5" in thickness.

Watch out: both for safety and to avoid a building code violation, check with your local building department about building permit requirements, inspections, and code compliance before purchasing much less installing any heating systems in your home.

Direct-Vent Wall Mount Gas Furnace Operation

The illustration at left, adapted from Williams Furnace Company, illustrates both the heating of room air by the gas furnace (the red area) and the direct-venting through the side wall (yellow areas).

On a call for heat a piezoelectric igniter lights the LP gas or natural gas gas burner inside of the combustion chamber. Combustion air for the gas burner enters from outside (blue arrows around the exhaust vent in the right of the illustration), and combustion gases exit through the wall, passing through the exhaust vent to outdoors (large yellow arrows).

Cooler room air enters at the bottom of the furnace (dark blue arrows at the bottom of the heater), and is warmed by heat transferred through a steel heat exchanger to the room air as it passes upwards by natural convection to exit into the room through the front and top of the heater (red area and arrows).

If an auxiliary 115VAC blower unit is installed the blower increases the rate of air movement through the furnace.

Exhaust vent sizes vary by manufacturer and by heater type.

The power-vented the Trane units we reviewed used round vent diameters of just 2-3" while naturally-vented units used larger exhaust vents. Note that direct-vent gas furnaces that use power-venting will require 120VAC at all times.

Inspecting Gas Fired Wall Mount Heaters

Below left: unsafe gas venting of a heater at a mobile home. Below right, oil fired heater in a mobile home closet. [Click to enlarge any image]

Unsafe gas vent on a mobile home heater (C) Daniel Friedman

Watch out: Casey et als (1999) reported that the heating equipment most-often found in unsafe conditions in homes were gas-fired wall heaters.

Oil furnace in a mobile home (C) Daniel Friedman

Watch out: This heating system inspection including any tests that are performed cannot be a complete, comprehensive nor technically exhaustive inspection nor safety test of the equipment but it can reduce risks to building occupants. Remember to check for the presence of CO2 and Smoke detectors or alarms.

Our photos above show that goofy installation of heaters in a mobile home closet are not confined to gas fired units.

At above right, the heater in this mobile home closet was oil-fired.


See HEATING FURNACE INSPECTION GUIDE for a complete index to inspecting & troubleshooting warm air heating systems.

Direct Vent Gas Furnace Product Sources & Inspection / Installation Information


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