How Vent-Free Heating Works
Vent-free gas products are gas appliances that are designed to be installed and operated without the addition of a vent or chimney. They are intended to provide supplemental heat to a home. Because there is no vent, flue, or chimney required, all of the heat generated by the product goes directly into the home. That’s why vent-free products are 99 percent energy efficient.
Historically it has been believed that only a hard, blue flame is allowable on gas appliances. However, recent technology has emerged to enable yellow flames that have clean combustion. Several hearth products incorporate this aesthetic feature.
The general rule has been that there is not to be any flame impingement on the logs, yet several manufacturers have impingement on the front log on their sets. How can they do this? Recent technological developments have made flame impingement on the front logs possible. Vent-free appliances with front log impingement typically utilize both blue flame and yellow flame burner design technologies. The blue flame burners impinge and provide a significant source of radiant heat and a glowing effect or charcoal appearance. The yellow flames in the rear of the appliance do not normally impinge on the logs, but rather burn between the logs and twigs, completing the “wood-burning” aesthetic effect of the appliance.
Oxygen Detection Safety Pilot (ODS)
A “Circuit Breaker” For Gas Supply
Safety is built into every vent-free gas product. Every vent-free unit sold in the U.S. comes with a precision-engineered Oxygen Detection Safety Pilot (ODS).
The ODS automatically reacts to a reduction in the room’s oxygen supply. If the oxygen level approaches 18%, the ODS shuts off the gas supply, eliminating any possibility of danger well before an unsafe level is approached. What’s more, the vent-free appliance can only be re-ignited manually, and only when the room air is back to normal.
ODS technology originated in Europe, and has been used in European gas heating appliances for more than half a century with an outstanding record of safety. According to ODS manufacturers, no documented deaths have been attributed to emissions from an ODS-equipped product. With more than ten million vent-free units operating in American homes and more than 50 million worldwide, that is an exemplary safety record.
Technology Behind The ODS Operating System
Thermocouple: A device consisting of two pieces of dissimilar metals joined together at one end (hot junction). When the hot junction is heated, the thermocouple produces DC voltage to the other end. It powers thermoelectric gas valves and when used is a safety feature on gas hearth products.
Millivolt (mV): Unit of electromotive force equal to one one-thousandth of a volt.
Orifice: The opening in a device whereby the flow of gas is controlled and through which the gas is discharged to either a pilot burner or main burner.
Piezo igniter: A device that delivers an igniting spark by means of pressure on a crystal.
Normal Operation: 20.9 Percent Oxygen – The flame touches the tip of the Thermocouple, generating the necessary millivoltage needed to hold the gas valve open.
Oxygen Level Drops: 19 Percent Oxygen – The flame begins to lift off the precision Pilot Burner. The Thermocouple begins to cool.
Safety Shutdown: 18 Percent Oxygen – The unstable pilot flame goes out, causing the Thermocouple to cool. The Thermocouple stops generating the electricity needed to hold the spring-loaded solenoid valve open. The valve shuts the vent-free gas heater down and cannot be started, until the oxygen level in the room returns to normal.
How The ODS Works
The ODS system consists of three main components:
- A precisely designed, oxygen sensitive pilot burner that regulates flame characteristics.
- A thermocouple positioned in the mantle of the pilot flame.
- A safety shut-off valve.
The pilot is designed to be stable within a very narrow operating range. The thermocouple responds to changes in the pilot flame characteristics and, when heated, generates a milli- voltage, which keeps the gas supply valve in the open position.
If low levels of oxygen are detected by the ODS system, the flame extinguishes. The loss of flame causes the thermocouple to cool. This cooling reduces the mill voltage, which causes the gas valve to return to its normally closed position, thus turning off the fuel supply to the appliance. The unit will not operate until the living space is properly ventilated and adequate oxygen is introduced, and the appliance is manually restarted.
Every ODS system contains a precision orifice. This orifice will disintegrate with any attempt at drilling it out to enlarge the pilot flame. Additionally, it is not interchangeable with a normal standing pilot. Furthermore, single fuel, propane and natural gas vent-free products cannot be converted from one type of gas to another. Dual Fuel heaters must be set on the specific fuel by the installer.
Safety Features and Child Protection
- Besides the ODS valve, every vent-free heating appliance is also equipped with an automatic shut-off valve, which shuts off the gas flow if the pilot extinguishes or the gas flow is interrupted in any way.
- Burners that are precision-engineered to produce the cleanest, most complete gas burn, with no lift-off or flashback when subjected to low input rates, low pressure or drafts.
- A non-adjustable pressure regulator that prevents over firing in case of increased gas pressure, must be used with any vent-free heater.
- An automatic ignition device, usually a Piezo igniter, which eliminates the need for matches in starting the pilot flame.
- Defined cabinet temperature limits for wall mounted heaters.
- A defined distance from combustible surfaces for all gas appliance categories.
- An enclosed burner chamber and stainless steel front panel protects the user from flames.