Direct heating products are vented products and can use a millivolt powered system, or they may require a 24-volt system. Millivolt systems use the pilot light to provide the small amount of electricity to control the unit while a 24-volt system requires an external (110-volt) electric power source to operate.

Direct heating systems are designed to provide heat to an area and can be installed in a floor, in or on the wall, or as a room heater, requiring a traditional venting system to exhaust combustion byproducts outdoors. Many of these traditional heating products have been in service for years and remain in use today. A large number of the units sold today are replacements or retrofits with a similar type of heater. New technologies incorporated into direct heating equipment are making these products safer and more efficient.

Highly Efficiency Direct Vent Products
Highly efficient direct heating products are identified by their higher Annual Fuel Utilization Efficiency (AFUE) ratings. These products will have an AFUE rating of 80 percent or higher. To achieve these efficiencies, the products have an induced draft system and are direct vented. The induced draft system brings fresh air into the heater for combustion and then forces the flue gases through the vent to the outdoors. The direct vent system will require most installations to be on an outside wall so as to properly vent the heater. These highly efficient units require an external power source to operate.

Gravity-Type Direct Heating Equipment
Gravity-type wall furnaces provide heat to the space through convection and come in single wall or dual wall configurations to heat one or two rooms on either side of the wall. Gravity wall furnaces operate on a millivolt system and do not require an external electricity source for operation. A single wall application will usually be installed between 16” centered studs or can be surface mounted. Other options for single wall applications include fan kits to help circulate the warm air throughout the room and rear register kits to divert heated air to another area through the back side of the heater. A dual wall gravity wall furnace must be mounted between the wall studs to simultaneously heat two adjacent rooms. This type of gravity wall furnace can also include an optional fan kit, on either side, to help promote circulation of heated air.

Gravity-type direct vent heaters must be installed on an outside wall of the area or space being heated. The direct vent heater has a sealed combustion system and operates on a millivolt system. The vent configuration (provided with the unit) for direct vent heaters is a concentric (pipe within a pipe) vent system with a termination cap. The concentric vent system consists of an outer fresh-air intake pipe and an inner pipe to exhaust combustion gases.

Counterflow Wall Furnaces
Counterflow and direct vent counterflow wall furnaces are similar in that they can be surface mounted or installed inside the wall. These wall furnaces require special venting materials (B vent or L vent) that are attached to the top of the heater and vented through the roof, allowing these furnaces to be installed on an interior or exterior wall. The direct vent counterflow wall furnace must be installed on an outside wall, with the vent materials typically supplied with the furnace. Both of these wall furnace styles have a built-in blower that forces the heated air out of the bottom portion of the heater and into the area to be heated. Due to the blower configurations, these furnaces operate on 24-volt systems and require connection to 110 volt power source to operate. There are optional side and rear register kits available to disperse heated air to the side or the rear of the heater.

Room Heaters and Floor Furnaces
Other direct heating furnace options include floor furnaces and vented room heaters. A floor furnace is a unit installed in a floor opening. The unit uses a traditional venting configuration with the exhaust vent running under the floor or in a crawl space or basement area to the outside wall and up above  the roofline. Vented room heaters are installed within the room or area to be heated and require a vent to exhaust from the back of the unit and then up and outside the roof. Floor furnaces and vented room heaters can also be vented through an existing chimney using a chimney liner system. These units also operate on millivolt systems requiring no electricity unless a blower option or fan kit is used.

Some benefits of choosing direct heating products include economics and ease of installation. They are normally installed in the area to be heated, thus eliminating costs associated with the installation of ductwork. An additional benefit is no heat loss through ductwork.

Use the questions below to help guide you to the appropriate direct heating solution. What is the arrangement of the area to be heated?

• What is the necessary heating load for the area to be heated?
• How many BTUs are necessary to heat the space?  The answer to this can be found by using the sizing chart available to calculate the approximate amount of heating capacity necessary for your application. Refer to ACCA Manual J., or to Selecting the Right Furnace.
• Is the installation needed on an exterior wall or on an interior wall?  Review description and application for product types (see above).
• Or is it necessary for the heater to be installed below grade? Refer to local codes.
• What type of gas will be connected to the unit — natural gas or propane gas?  Most direct heating equipment is designed for use with natural or propane gas, and most are adaptable with an optional conversion kit. Make sure you know before making any gas connections.

Finding the right direct heating solution for your application is the key! Direct heating equipment manufacturers offer accessories that can enhance the performance of your equipment. Some heating products have blower fans built in to help circulate the heated air away from the unit and into the area being heated, while others have an optional kit. A few direct heating equipment products allow you to add a register to the side or rear of the unit to provide heated air to an additional living space. Direct vented units can be installed on thicker exterior walls; some can accommodate various lengths while others require an optional vent extension kit. Other accessories can help make the installation of your direct heating product a better fit for your customer’s home.

The installation of any gas-fired appliance, including a direct heating product, is governed first by the manufacturer’s installation instructions. These are often more stringent than local building codes; however, you must check with the local building codes to ensure all requirements are met. If there are no local codes that govern the installation of a direct heating product, then the installation must follow the requirements of the National Fuel Gas Code, NFPA 54 (National Fire Protection Association). NFPA 54 is the code by which all gas-fired products should be installed. It also is a resource for sizing gas lines, clearances, and other venting requirements necessary for the proper installation of direct heating equipment.

Safety
In North America, the industry product safety standards covering vented direct heating equipment are the American National Standards/CSA Standards for Vented Gas-Fired Space Heating Appliances, ANSI Z21.86/CSA 2.32, and Vented Gas Fireplace Heaters, ANSI Z21.88/CSA 2.33. These are minimum safety standards that are intended to ensure that a product and the manufacturer’s production practices and techniques meet certain safety design and performance criteria. These standards apply to newly produced space heaters for permanent connection to the building fuel supply system. All direct heating equipment manufacturers test their products through a third-party, Nationally Recognized Testing Laboratory (NRTL). Installation codes, such as the National Fuel Gas Code, also require that vented gas-fired space heaters and fireplaces be listed to the appropriate standard before it can be installed in the home.

Energy Efficiency and Performance
Many manufacturers participate in AHRI’s voluntary certification program for direct heating equipment. AHRI’s certification program serves as a “seal of approval” that direct heating equipment meets the manufacturer’s claimed efficiency level. For many years, the Department of Energy (DOE) has required minimum efficiency levels for many direct heating products, and AHRI’s program ensures that participating manufacturers’ products meet or exceed DOE’s minimum efficiency levels. Consumers and contractors are encouraged to visit the AHRI Directory of Certified Product Performance to find listed products and to learn more about AHRI’s certification programs.

Please click on each term below to access its description:

24 volt
A type of power necessary for some direct heating equipment to operate. These units usually have a transformer that “steps down” the standard household current to operate the heater.

AFUE (Annual Fuel Utilization Efficiency)
The ratio of annual output energy to annual input energy, which includes any non heating-season pilot input loss and does not include electric energy.

AHRI
The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) is the trade association representing manufacturers of air conditioning, heating, commercial refrigeration, and water heating equipment. An internationally recognized advocate for the industry, AHRI develops standards for and certifies the performance of many of these products.

ANSI
The American National Standards Institute (ANSI) is a private, non-profit organization whose mission is to enhance U.S. global competitiveness and the American quality of life by promoting, facilitating, and safeguarding the integrity of the voluntary standardization and conformity assessment system.

ASHRAE
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) is a non-profit technical organization whose members influence the direction of heating, ventilation, air-conditioning and refrigeration (HVAC&R) technology by creating industry standards and recommended procedures and guidelines, developing research, and writing technical information.

Blower
A device that assists in moving air through or away from heater and into area being heated.

BTU (British thermal unit)
Amount of heat needed to raise one pound of water through one degree Fahrenheit.

Chimney liner system
A system of venting material that properly sizes an existing chimney, allowing gas heating equipment to be vented into the chimney, which enhances proper operation and venting.

Convection
A process of heat transfer through a gas or liquid by bulk motion of hotter material into a cooler region.

Conversion kit
This kit allows the conversion of a gas valve from one gas (Nat to LP or LP to Nat) to another.

CSA International
Tests products for compliance to national and international standards and issues certification marks for those products.

Direct vent wall furnace
A system consisting of an appliance, combustion air, and flue gas connections between the appliance and the outdoor atmosphere, and a vent cap supplied by the manufacturer and constructed so all air for combustion is obtained from the outdoor atmosphere and all flue gases are discharged to the outdoor atmosphere.

DOE (Department of Energy)
A United States government agency which addresses U.S. energy, environmental, and nuclear challenges through transformative science and technology.

Flue
The general term for the passages and conduits through which flue gases pass from the combustion chamber to the outer air.

Induced-draft burner
A burner that depends on the draft induced by a fan for its proper operation.

Manual J
An HVAC load calculation manual used in the industry to help determine the amount of BTU’s necessary to heat an area.

Millivolt
This system uses dissimilar metals to create a spark for ignition which will operate as long as there is a flame that impinges the power pile generator creating adequate energy to open and close gas valve to satisfy the stat.

NFPA (National Fire Protection Association)
A worldwide leader in fire, electrical, building, and life safety. The mission of the international nonprofit organization founded in 1896 is to reduce the worldwide burden of fire and other hazards on the quality of life by providing and advocating consensus codes and standards, research, training, and education.

NFPA 54
National Fuel Gas Code, the safety benchmark standard for fuel gas installations and venting of gas appliances.

Rear register kit
An accessory for a conventional vented unit that allows a unit to provide heat to another area by accessing the back portion of the heater with the kit.

Side outlet register
An accessory for a counterflow vented unit that provides heat to another area by attaching the side of the heater with the register to the adjacent area to be heated.

Vent extension kit
Vent materials for direct vent units that allow unit to be installed on walls that are thicker than standard wall thickness.

Vent terminal
The fitting vent cap at the end of the vent pipe that directs the flue products to the outdoor atmosphere and protects against downdraft.

Vented wall furnace
A self-contained vented appliance complete with grilles or equivalent. It is designed for incorporation in or permanent attachment to the structure of a building and for furnishing heated air circulated by gravity convection or by a fan directly into the space to be heated through openings in the casing. Such appliances shall not be provided with duct extensions beyond the vertical and horizontal limits of the casing proper, although boots no more than 10-inches beyond the horizontal limits of the casing may be permitted to extend through walls of nominal thickness.