Hanzlick & Associates HISTORY OF VENT SAFETY SHUT-OFF SYSTEMS

Introduction

The efficient and safe operation of a gas-fired appliance is dependent on a number of factors some of which are controlled by the overall design and construction of the appliance. Some of the most basic of these factors are:

· Products of combustion (flue gases) must be removed from the appliance and new air for combustion supplied. New air for combustion is mixed with the fuel (propane vapor) and combusted at the main burner. This process establishes a “natural” draft through the appliance which establishes the rate at which heat is extracted from the appliance through its heat exchanger. This establishes the overall operating efficiency of the appliance. In order for an appliance to operate at an efficient level on a consistent basis, the natural draft through an appliance must be maintained at as near a constant level as possible.

· Products of combustion for a properly adjusted gas appliance contain amounts of carbon dioxide, nitrogen, excess oxygen and water vapor. If the appliance is in poor condition, poorly maintained or installed incorrectly, products of combustion can also contain dangerous levels of carbon monoxide as well. For these reasons products of combustion must be removed from the occupied area and vented through a venting system to the outside atmosphere.

· The venting system an appliance is connected to can have a dramatic effect on the draft through an appliance. The draft rate through a vent system can increase or decrease depending on a number of variables some of which include:

· Height of the vent terminal above the appliance

· Size of the vent system

· The difference between inside and outside temperatures

· Adequate supply of new makeup air for combustion to the appliance

· Outside wind conditions

· Obstructions near the vent terminal

· The venting system must not be allowed to restrict or block the natural draft of combustion products through an appliance. If the natural draft through an appliance is blocked incomplete combustion and possible fire hazards are created.

· A continuous source of new combustion air containing oxygen must be supplied to the appliance to maintain proper combustion and operation of the appliance. If the supply of new combustion air is restricted incomplete combustion and generation of carbon monoxide can result.

The Draft Diverter

The draft diverter, or draft hood, is one method utilized to “couple” the gas appliance to the venting system and naturalize the effects that a vent system may have on the operation of the appliance.

The draft diverter is a device that is built into an appliance and is designed to:

· Provide for the ready escape of the flue gases from the appliance in the event of no draft, back draft, or stoppage beyond the draft hood in the vent system.

· Prevent a back draft from entering an appliance.

· Neutralize the effect of stack action of the gas vent upon the operation of the appliance.

In its most basic form a draft diverter is in the shape of a “box” with an inlet in the bottom, an outlet at the top and a “relief” opening on one side. A draft diverter may contain one or more baffles to control how the products of combustion flow through the draft diverter. In operation products of combustion from the appliance enter the draft diverter through the inlet, collect in the diverter box and exit the draft diverter through the outlet to the venting system. During normal operation “dilution” air, air from the area around the appliance, enters the draft diverter through the relief opening and mix with the products of combustion. This process of adding dilution air through the relief opening acts to allow the flow rate through the vent system to increase or decrease, depending on vent system design and outside atmospheric conditions, while maintaining a constant natural draft through the appliance.

The design of the draft diverter will also allow products of combustion to exit the appliance through the relief opening in the event that the vent system should become blocked or if a down draft or reverse draft should develop in the venting system.

Safety Problems Associated With The Draft Diverter

While a draft diverter provides an important safety function of protecting the appliance from the effects of the venting system, the basic design of the draft diverter presents a serious safety concern. Specifically a blocked vent or a down draft condition in the vent will cause products of combustion to “spill” out of the draft diverter relief opening

into the area around the appliance. This is normally not a problem if the blockage or down draft condition is caused by a temporary condition and the spillage only occurs for short periods of time.

However, if the spillage condition is allowed to continue for a prolonged period of time a serious safety condition will develop. Products of combustion will be drawn back into the appliance, replacing the normal supply of combustion air, causing the appliance to have “incomplete” combustion resulting in the production of carbon monoxide. This cycle, if allowed to continue, can generate sufficient amounts of carbon monoxide in the area around the appliance to cause a serious threat to human life.

Spillage of products of combustion from the draft diverter can be caused by:

· Blockage in the vent system

· Back draft in the vent system

An inadequate supply of combustion air can cause spillage. As oxygen is depleted in the combustion process, in the area around the appliance, new air containing oxygen for combustion will be drawn from the path of least resistance. This path of least resistance is normally through the venting system creating a back draft. This back draft causes the products of combustion to exit the appliance through the relief opening of the draft diverter.

The gas industry, and specifically the American Gas Association (AGA), has been aware of these problems for many years and has conducted numerous research activities in an attempt to eliminate these safety hazards. This research is documented in the following AGA publications:

· Report No. B-1124 - Effects Of Confined Space Installation On Central Gas Space Heating Equipment Performance, September, 1948

· Progress In Overcoming Venting And Air Supply Problems, Keith T. Davis, Chief Engineer, Bryant Heater Division, Affiliated Gas Equipment, Inc., presented at the AGA Domestic Gas Research and Utilization Conference, April, 1950

· Engineering Experiment Station Bulletin No. 427 - Outdoor-Air Supply and Ventilation of Furnace Closet Used With A Warm-Air Heating System, December, 1954

· Research Bulletin 67 - Combustion And Ventilation Air Supply To Gas Equipment In Small Rooms, August, 1954

· Research Bulletin 68 - Literature Review And Design Studies of Gas Appliance Venting Systems, April, 1955

· Research Report No. 1243 - A field Survey Of Gas Appliance Venting Conditions, February, 1956

· Research Report No. 1267 - A Field Survey Of Gas Appliance Venting Conditions Part II, March, 1957

· Research Report No. 1362 - Vent Cowl Performance And Location, November, 1963

· Investigation Report Project DA-10-HA - Investigation Of Draft Hood Spillage In A New Housing Development, December, 1967

One of the earliest indications of the gas industry’s concern over the spillage of combustion products from the draft diverter relief opening is documented in a patent (No. 2,184,983) dated October 9, 1935 by E. L. Tornquist for a gas burner control system. The control system described in this patent “provides a simple means for shutting down the burner or appliance whenever the products of combustion are spilled into the space surrounding the appliance, indicating a blocked chimney or a down-draft condition in which the continued operation of the burner becomes objectionable.”

The author of this patent goes into great detail discussing the hazards of allowing the appliance to operate for prolonged periods of time while spilling products of combustion back into the area around the appliance. He states “where a continued down-draft occurs, or when the chimney becomes blocked also creating a continuous condition, it is objectionable to permit the discharge of the products of combustion into the space surrounding the appliance for, in the first place, the presence of flue gases is in itself objectionable and, even though the draft hood and the appliance are arranged to operate satisfactorily with complete combustion where all of the products of combustion are vented through the draft hood, the continued spillage of the flue gases into, for example, a basement, soon results in at least a partial depletion of the oxygen content of the air in the basement. Thus, even though the appliance and the draft hood are correctly coordinated theoretically, the continued operation of the burner under these conditions, and especially where the source of oxygen for the burner is the air in the basement, may produce carbon monoxide in dangerous quantities”.

This patent describes a gas burner control system that utilizes a temperature actuated control switch located in the draft diverter relief opening to sense a spillage condition. This switch opens when it senses an increased temperature indicating spillage interrupting the electrical circuit to the gas burner. The methodology described in this patent matches very closely the methodology utilized in today’s gas appliances equipped with a vent safety shut-off system.

A different patent (No. 2,112,554) dated July 17, 1936 also discusses the same safety concerns and describes an “automatic control for fuel burning apparatus” that utilized a “fusible” link located in the draft hood relief opening. This fusible link was designed to open when it sensed an increase temperature indicating spillage.

Specifically, the purpose of a vent safety shut-off system is to sense a spillage condition at the draft diverter relief opening and shut down the operation of the appliance if prolonged spillage occurs.

One version of a vent safety shut-off system consists of a normally closed bi-metal snap acting temperature actuated control switch mounted in the area of the relief opening of the draft diverter. The temperature-actuated switch is mounted so that it senses the temperature around the dilution air inlet of the draft diverter. If hot products of combustion are relieved out through the draft diverter relief opening the temperature switch will open.

This temperature actuated switch is integrated into the gas valve electrical circuit so that when the temperature actuated switch opens it interrupts the electrical circuit to the gas valve causing the gas valve to close and shut down the operation of the gas appliance. The switch will close after it cools down to below it’s temperature set point and allow the gas appliance to operate again. The resetting of the switch is automatic and does not require any action by the user.

Vent safety shut-off systems in use today are independently engineered and designed by the appliance manufacturer for specific applications on specific appliances. Vent safety shut-off systems have been required on gas fired vented room heaters as certified under American National Standards Institute (ANSI) standard Z21.11.1 and on vented wall furnaces as certified under ANSI standard Z21.49 since the mid 1980’s.

The technology and components utilized in the vent safety shut-off system have been in existence since at least the 1930’s and possibly longer. As discussed above, the dangers associated with the continued operation of gas fired appliances under spillage conditions have been known since at least the 1930’s, as evidenced by a patent dated October 9, 1935 by E. L. Tornquist (No. 2,184,983), for a gas burner control system. In fact, this patent is very similar to the technology utilized in gas-fired wall furnaces which are currently being manufactured.

Research in the gas industry tended to focus on the design of the venting system and combustion air supply to minimize safety hazards associated with spillage. There are documented recommendations dating back to the 1935 patent (referenced above) to eliminate this safety hazard utilizing a very simple solution - utilizing a temperature actuated switch to sense spillage shutting down the operation of the appliance.

This solution is also discussed in the AGA Research Bulletin 67 dated August, 1954 - Combustion And Ventilation Air Supply To Gas Equipment In Small Rooms, pages 106-107. This document discusses utilizing a control operated by temperature and states

“These same controls might be called upon to function if draft hood spillage or air supply failure of one appliance is caused by another appliance. Thus down drafts ….. could actuate a temperature switch to interrupt the gas supply to the main burner of a furnace, and possibly turn on a signal light to call attention to the cause of failure. Use of such a signal is poor advertising, and it might be preferable simply to have control shut the appliance down, and force the owner to call a service man to investigate the cause of the trouble”.

The temperature-actuated switch utilized in the vent safety shut-off system in the current Williams Forsaire and Montery SRO Wall Furnaces is manufactured by Therm-O-Disc (Williams P/N P321127). The switch as manufactured by Therm-O-Disc is a single pole single throw (SPST) normally closed (NC) board mount type temperature actuated switch with a set point of 330º F. This type switch has been in existence sense at least the 1940’s and has been utilized on a routine basis as a limit switch in gas-fired appliances.

A review of the 1975 edition of the Gas Heating Controls Service Manual published by the American Gas Association indicates that there were components available in the 1960’s that could have been utilized in a vent safety shut-off system. This manual contains service information on heating systems and controls dating to the 1950’s. Examples include General Controls ITT, Honeywell Inc., and Robertshaw Controls Company.

A review of the catalog section of the 1955 Heating, Ventilating, Air Conditioning Guide, published by the American Society of Heating and Air Conditioning Engineers, lists the following manufacturers of temperature switches and other associated components that could have been utilized in a vent safety shut-off system.

Other companies known to exist in the 1950’s and manufacture temperature switches of this type include Therm-O-Disc and General Electric.

Based on product availability and industry knowledge existing in the 1960’s, at least two different technologies could have been utilized in the design of a vent safety shut-off system for gravity vented wall furnaces:

· Millivolt control system integrated with a temperature actuated limit switch or fusible link mounted in the draft diverter relief opening. Basic 250MV and 750MV systems existed which were capable of operating a gas valve through a temperature actuated limit switch and room thermostat.

This type of configuration is almost identical to the system utilized today.

· Mounting a thermostatic temperature bulb in the draft diverter relief opening. The thermostatic bulb would act to shut down the main gas control valve with an increase in temperature at the diverter opening.

Mounting a thermostatic temperature bulb in the draft diverter relief opening as the vent safety shut-off system is documented by the Consumer Products Safety Commission in a memorandum dated April 4, 1983. The CPSC reports in this memorandum that the president of the Gas Appliance Manufacturers Association (GAMA) in a letter dated March 15, 1983 states “such thermostatic bulbs might be relocated in the draft hood relief opening to act as a spill switch”.

Automatic gas control valves utilizing a hydraulic sensing bulb to sense room temperature, rather than an electrical thermostat, have been in existence since at least the 1950’s. One such valve, Robertshaw Unitrol model 110, was utilized in gas-fired room heaters and wall furnaces that were manufactured in the late 1950’s and early 1960’s.

A very simple but effective vent safety shut-off system could be implemented by simply moving the location of the hydraulic sensing bulb from its traditional location, normally at floor level sensing the temperature of the combustion air supply to the burner, to a location near the draft diverter relief opening where it would sense the temperature of the dilution air entering the draft diverter. At this location the appliance would operate normally until spillage occurred. Spillage of hot products of combustion would be sensed by the hydraulic sensing bulb causing the main burner to shut down eliminating the safety hazard.

Recent History Of The Vent Safety Shut-off System

Renewed concern over the safe use of gas-fired space heating equipment began to resurface during the oil embargoes in the mid to late 1970’s. It was during this time period that various energy conservation devices designed to increase operating efficiency of gas appliances were introduced. This concern was investigated by the Consumer Products Safety Commission and results reported in at least five (5) reports prepared for the CPSC:

· Hazard Analysis - Space Heaters published in February, 1975 (NIIC-0313-75-H003)

· Investigation of Safety Standards For Flame-Fired Furnaces, Hot Water Heaters, Clothes Dryers And Ranges published in July, 1975 (YG-5569-D-3)

· Unvented Gas Space Heaters - HIA Hazard Analysis Report published in July, 1978 (no CPSC number)

· Safety Devices For Gas-Fired Appliances published in May, 1980 (Calspan Report No. 6608-D-1)

· Hazard And Human Factors Analysis of Injuries Associated With Flame-Fired Appliances published in August, 1980 (CPSC-C-79-1204)

This investigation and analysis resulted in the requirement by the CPSC of an Oxygen Depletion Safety Shut-off System (ODS) for unvented gas-fired space heaters. This mandatory standard became effective in December, 1981. The standard did not address vent safety shut-off systems for vented space heaters.

Over the next two years various local building departments petitioned for a waiver from this requirement on the basis that:

· Their local requirements outlawing the installation of unvented space heaters was safer.

· This mandatory standard would require the local building departments to allow the installation of unvented space heaters resulting in unsafe situations.

· The use of vented space heaters was a more serious safety hazard and standards should be drafted addressing the hazards associated with vented space heaters.

The CPSC reported in a memorandum dated May 18, 1983 from Beatrice Harwood, EPHA to Douglas L. Noble, EX-P that “The risk of CO poisoning is 6 times higher in a vented gas heater than in an unvented heater, even one without an ODS device. 1979 estimates are of 130 deaths from vented heaters out of an estimated 3,253,000 in use, vs. 40 deaths from unvented heaters out of an estimated 5,394,000 in use”.

The CPSC voted on May 26, 1983 to revoke its mandatory requirement for an ODS on unvented space heaters.

Because of the attention drawn to the hazards associated with gas-fired space heaters by the Consumer Products Safety Commission, the Gas Appliance Manufacturers Association (GAMA) and the American Gas Association (AGA) began drafting proposed revisions to the Z21 ANSI Standard for vented gas-fired room heaters (Z21.11.1). This revision would require the use of a vent safety shut-off system designed to detect improper venting. This revised standard was released and became effective in 1984. Revisions to standards for similar appliances (vented wall furnaces) followed almost immediately.