This article was originally published in the September/October 1996 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.


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Home Energy Magazine Online September/October 1996



Unvented Gas Space Heaters: Drainless Sinks?

The other day I went to the building supply store looking for materials for my kitchen remodel. I was attracted to the new low-water-use drainless sinks. The salesperson pointed out that the cost of installation would be low, since I would not have to install a drain. This story may sound far-fetched, but it's similar to what some gas appliance manufacturers are promoting with the sales of unvented or vent-free gas space heater and gas log sets. Unvented gas appliances emit water vapor and combustion gases into the living space. Even in a loose house, a small, heavily used, unvented gas heater can cause toxic gases to build up to a level that may be dangerous for occupants.

The industry stands behind the products as safe, and increasing numbers of U.S. states allow them. Nationwide, gas space heater sales are dominated by an ever-expanding unvented heater market (about 1/2 million units will be sold in 1996). The new vent-free appliances on the market have several attractive features. They are efficient-99% of the combusted fuel energy is released indoors (although 9% of the energy remains trapped in water vapor until it condenses on cold surfaces). They are inexpensive to manufacture and install. Most of the units are attractively designed-their flames, visible behind a screen or glass door, flicker over glowing ceramic logs.

Since 1980, unvented gas space heaters have been improved by the addition of oxygen depletion sensors (ODS), which cause the heaters to shut off when oxygen (O2) levels drop to 18.5% (ambient air contains 20.9% O2), and by improved burner designs, which are supposed to reduce the chance of catastrophic increases in carbon monoxide (CO) emissions. Industry spokespeople dismiss concerns about other combustion by-products, claiming that a room would become unbearably hot before dangerous levels of combustion products could build up.

  ANSI Standard Radiant Fiber Matrix Blue Flame Infrared
CO (g/kJ) 51.5 2-28 19±8 48±27
NO2 (g/kJ) 8.46 1-9 11±1 5±1
Table 1. Pollutant emission rates in micrograms per kilojoule (µg/kJ) for three types of unvented gas heaters, based on laboratory tests. The lower emission (radiant fiber matrix) heaters are also the most expensive.

In the 1980s, prompted by fatalities due to CO poisoning from unvented gas heaters, the U.S. Department of Energy and the Consumer Product Safety Commission collected emissions information. This research found that gas combustion produces the obvious carbon dioxide (CO2) and water, along with an array of gaseous chemical compounds including CO, nitrogen dioxide (NO2), respirable suspended particles (RSP), and formaldehyde. The CO and NO2 emission rates of several of the tested heater types are shown in Table 1. Other research found emissions of acid aerosols and a huge range of volatile and semivolatile organic compounds, including irritating organic acids and carcinogenic polycyclic aromatic hydrocarbons.

Manufacturers of unvented gas heaters defend their devices by pointing out that they comply with the current standard from the American National Standards Institute (ANSI). However, this standard is quite permissive, allowing pollutant emissions ten times higher than the low levels possible with current technology.

In vented heaters, the venting system isolates residents from combustion products and removes these wastes. These flues serve as a crucial safety feature in the case of a burner malfunction. Even with this safety feature, several hundred deaths and thousands of non-fatal CO poisonings occur annually in the U.S. when burners and vent systems both malfunction.

By designing out the flue, manufacturers of unvented gas heaters rely only upon their ability to produce a tamperproof, accident-proof, low-emission burner. Changes in burner configuration, fuel composition, burner adjustment, and altitude can all lead to catastrophic changes in the production of CO and other unwanted products. If there is any problem-a design flaw; an error in installation, maintenance, or repair; aging; or misuse-the only safety factor protecting the user is the ODS. In the case where a burner becomes poorly tuned, CO concentrations can reach dangerous levels long before the ODS turns the heater off. With many millions of these heaters installed, even a very low failure rate translates into large numbers of people at risk.

Very little is known about the way today's unvented gas heaters are actually operated in homes: there are no recent studies of use patterns and indoor air quality in these homes. However, it can be expected that in cold climates, these heaters will be run until steady-state concentrations of combustion gases are reached, without the room becoming unbearably hot. Further, those who prefer high indoor temperatures (surveys have frequently observed temperatures as high as 90oF in homes of the elderly) will operate their heaters for long periods of time. In addition, the appliances are likely to be used in some situations as a primary heater.

Pollutant Burner rate (Btu/h) Steady-state pollutant concentrations in tight and loose houses Standards
0.35 ACH 1.0 ACH
CO2 (ppm) 10,000 3,200 1,400 1,000 ppm (ASHRAE)*; 5,000 ppm
40,000 11,700 4,400 (U.S. OSHA)
CO (ppm) 10,000 4.7 1.7 200 ppm over 15 min (U.S. OSHA); 
40,000 18.9 6.6 35 ppm over 8 hrs (U.S. OSHA); 35 ppm over 1 hr (U.S. EPA outdoor); 9 ppm over 8 hrs (U.S. EPA outdoor)
NO2 (ppm) 10,000 0.15 0.09 0.25 ppm over 1 hr (CA EPA); 
40,000 0.59 0.37 0.05 ppm over 1 year (U.S. EPA)
Respirable Particles (RSP, µg/m3) 10,000 42 15 150 µg/m3 over 24 hrs (U.S. EPA); 
40,000 170 59 50 µg/m3 over 1 year (U.S. EPA)
* ASHRAE's CO2 concentration standard is not a health or safety standard. It is a rule of thumb for ventilation system designers that, if CO2 levels exceed this level, more ventilation is needed. 
Table 2. Calculated steady-state indoor pollutant concentrations due to continuous operation of a vent-free gas heater in a 300 m3 (11,000 ft3) house. These numbers assume
  • Properly working, correctly installed heaters with emission rates equal to the ANSI standard.
  • No other indoor pollutant sources and unpolluted outdoor air.
  • NO2 reactivity rate of 0.8/hr.
  • There is no ANSI standard for respirable particle emissions. A typical emission rate of 0.4 µg/kJ is assumed.

If these heaters are operated until combustion gas levels reach a steady-state in the room air, indoor air quality will be compromised. Table 2 shows calculated steady-state indoor pollutant concentrations in a small home running an unvented gas heater that just meets ANSI standards. All of the scenarios show significant degradation of indoor air quality, without room oxygen levels getting low enough to trigger the ODS.

There are no pollution standards for residential indoor air, but the U.S. Environmental Protection Agency (U.S. EPA), the U.S. Occupational Safety and Health Administration (U.S. OSHA), and other agencies have made guidelines for ambient and workplace air. Continuous use of a 40,000 Btu per hour heater, as would be necessary in a cold climate, causes indoor pollutant levels to exceed these government-specified air quality and health standards. Even the best case shows CO and NO2 levels that may be unhealthy for children, pregnant women, the elderly, or individuals with existing health problems, such as heart disease or asthma.

Even benign gases can be a problem if not adequately vented. A shower emits about 300 grams per hour of water into the air and is typically used for less than one hour daily. Building codes require operable windows or installed ventilation fans to remove this vapor. In contrast, about 400 grams of water vapor is produced per 10,000 Btu of fuel consumed. This water, if not removed by ventilation, will condense on cold surfaces such as windows and wall cavities. In moderate and moist climates, the accumulation of moisture leads to mildew and fungal growth. Fungal colonies in building materials cause rot and decay, and produce spores that can cause allergic reactions. Putting all this moisture in the living space, unvented gas heaters would truly seem to be like drainless sinks.

-Michael G. Apte

Michael G. Apte is a staff research associate at Lawrence Berkeley National Laboratory in Berkeley, California and a doctoral candidate at the University of California-Berkeley School of Public Health.


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