Sunday, December 13, 2009

Gas Furnace Start Up Tips

by Phil Rains

As technicians, during colder months you will be involved with the start up of gas furnaces for proper heating of homes. This will involve both visual inspections and effective start up procedures. We will discuss a few of these in this article.

Prior to the start-up of a gas furnace there are several things that you should verify. These are usually items that can be accomplished by visual inspection.

First, verify the type of fuel being utilized. As simple as this may sound, it is an important item to check. The furnace nameplate will typically detail the type of fuel that should be used. Almost all manufacturers today will produce furnaces that are set-up to use natural gas. These furnaces can easily be converted to LP gas operation by installing the correct LP conversion kit. Part of this kit may be a sticker or a new rating plate that identifies the converted furnace as an LP gas furnace.

Always verify the power supply to the furnace. All electrical connections must conform to the manufacturer’s installation and operation instructions as well as the National Electric Code and any and all local codes.

A HOT wire, a NEUTRAL and a GROUND wire of not less than #14 AWG should typically be run to the furnace from a separate 15 Amp power supply through a disconnect switch. The switch should break the black or hot leg of the 115-volt power supply.

The furnace must be properly grounded as per the National Electrical Code for safety purposes; it also should be grounded to ensure proper operation of any electronic components. The furnace can be grounded to the main panel through the power supply wiring or to a separate ground rod properly installed near the furnace.

The power supply to the furnace connections must be between 104 VAC and 127 VAC during furnace operation for acceptable performance. Field wiring connections must be made inside the furnace connection box. A suitable strain relief should be used at the point the wires exit the furnace casing. Copper conductors must be used. Line voltage wires should conform to temperature limitations of 63°F rise and be sized for the unit maximum amps.

Check and verify proper gas supply piping size. The gas supply must be properly sized to handle the combined appliance load, or run the gas pipe directly from the gas meter or LP gas regulator to the furnace. Inadequately sized gas supply piping could interfere with proper operation of the gas furnace. Additionally, proper gas supply piping installation should always follow local codes and ordinances. In their absence, you should follow the criteria of the National Fuel Gas Code (NFGC). The NFGC is the American National Standard that applies to the installation of fuel gas piping systems and fuel gas utilization equipment that are supplied with natural gas; manufactured gas; liquefied petroleum gas (LPG), in the vapor phase only.

Prior to correctly sizing and properly installing supply gas piping, you should review several considerations. Some of these considerations include, but are not limited to the following: furnace input, pressure drop, length of pipe, pipe fittings, and specific gravity of the fuel.

It is recommended that a manual equipment shutoff valve be installed in the gas supply line outside the furnace. Locate the valve as close to the furnace as possible where it is readily accessible. Also, it is recommended that you always install a drip-leg and a union.

Install a shut-off valve

Install a drip leg

Install a union

In order to ensure proper, safe and efficient operation of a gas furnace, you must pay careful attention to the venting system. You should always review the manufacturer’s installation and operation instructions for the furnace. These instructions will provide the necessary information on the proper venting requirements for the particular furnace.

All gas furnaces are classified by categories. The category of each furnace is shown on the rating plate and this category may determine the type of venting material required for the furnace. The chart below shows the flue gas conditions for each category for use when selecting the vent material.

Most furnaces today are either Category I or Category IV.

Category I furnace vent installations should be in accordance with the National Fuel Gas Code. Category I furnaces are defined as central furnaces that operate a non-positive vent static pressure and with a flue loss not less than 17 percent. These furnaces are approved for common-venting and multi-story venting with other fan-assisted or draft hood equipped appliances in accordance with the NFGC. The venting material should be only Type B or Type L double wall vent pipe.

Category IV appliances include all condensing furnaces. They may be single pipe, direct vent, or dual certified furnaces. Single pipe condensing furnaces require ventilation openings to provide air for proper combustion and ventilation of flue gases. Direct vent condensing furnaces use outside air for combustion ONLY, and must take this air from the same atmospheric pressure zone as the vent pipe.

Dual certified condensing furnaces can be installed as direct vent furnaces using outside air for combustion, or the furnace can use air from inside the structure for combustion. The INLET pipe is optional. If combustion air comes from inside the structure, adequate make up air MUST be provided to compensate for oxygen burned. If combustion air is drawn from outside the structure, it MUST be taken from the same atmospheric pressure zone as the vent pipe.

Category IV furnace vent installations should be usually Schedule 40 PVC unless otherwise specified by the manufacturer. Sizes may range from 2” to 3” and the allowable length will be determined by the manufacturer and is dependent on Btu input and size of the pipe. Always refer to the manufacturer’s data when determining actual size vent required.

Always verify adequate combustion air supply for any furnace. Combustion air is supplied to the furnace to ensure complete combustion of the fuel gas. The combustion process requires a large amount of air.

This air for combustion can be supplied from either inside the structure or from outside. If all air for combustion is supplied from inside the space, it must be determined if that space is a confined space or an unconfined space. A confined space is an area that has less than 50 Cubic Feet of volume for every 1,000 Btu/h Input rating of ALL gas appliances installed in that space.

An unconfined space is an area that has greater than 50 Cubic Feet of volume for every 1,000 Btu/h input rating. Use the previous information in the definitions to determine if the space where the furnace you are starting has an adequate air supply.

On any new installation be sure to check for properly converted furnaces when the furnace is installed in other than the “as shipped” configuration.

Condensate drains are required on all condensing furnaces. This type furnace removes sensible and latent heat from the products of combustion. Removal of the latent heat results in condensation of the water vapor. The condensate is removed from the furnace through the drains in the plastic transition and the vent fitting. The drains connect to the externally mounted (internally on up flow models) condensate drain trap on the side of the furnace. Be sure that the drain is installed correctly and is piped according to the Manufacturer’s requirements and any local codes (drains external to the furnace and drain termination).

Phil Rains
Copyright © Phil Rains

About the Author: Phil Rains is Master Trainer/Technical Developer for He has over 35 years of HVAC and Refrigeration experience in installation, service, and training. He is NATE-certified in 5 areas, a member of ASHRAE and RSES, and ACCA EPIC-Certified in Residential and Commercial Design. He also holds a Universal Classification in EPA 608.
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