More than 1.25 million vent-free space heaters, fireplaces, logs, and stoves that operate on natural gas or propane are sold every year in America. But gas appliances produce water vapor, and that can bring high humidity into the air if  there’s not enough ventilation. So could these vent-free gas appliances increase the humidity enough to promote mold growth?

A lot of people aren’t too familiar with the controversy around mold and vent-free gas products, so it helps to look at the problem by going backwards.

  • Mold grows because of water.
  • Water exists in the air as a gas in the form of water vapor. Relative humidity is how much water vapor there is in the air.
  • Water vapor can get into the air by evaporating from a number of sources in your home.
  • One of those sources is the burning of natural gasses or fossil fuels. Burning fossil fuels produces heat but this also breaks them down into carbon dioxide and water vapor.
  • If this water vapor is being ventilated outside the home, then it’s going to increase the relative humidity through the house.
  • So this is why there’s a concern that vent-free gas appliances may contribute to mold growth.

When a study was created by the Vent-Free Gas Products Alliance of the Gas Appliance Manufacturers Association (GAMA) and performed by Risk Sciences, LLC they found that vent-free gas products do not generate enough humidity levels to foster mold growth. In the study, gas products were left on for twice as long as they were normally used according to a previous customer survey. After 100,000 steps and stages, it showed that the relative humidity did not increase past 70% in 99% of cases. The study took into account factors like room volume, air exchange rate, temperature, initial relative humidity, and normal human activities.

But saying that vent-free gas appliances couldn’t produce enough humidity to create mold is too broad of a statement. Burning fossil fuels produces water vapor – there is no question about that. The study concludes that vent-free appliances typically do not raise humidity levels past 70% in “normal circumstances.” But what Risk Sciences defines as normal may not be the same as all consumers.

What if the relative humidity is already at 69% naturally? What if there is no air exchange and the room is tightly sealed? What if the gas appliances are turned on high and left on for too long? What if the temperature is too low (high temperature reduces humidity)? There are so many factors that could allow gas products to raise humidity past 70%. Consumers need to be aware that the only solid conclusions we can make about gas products is that water vapor is produced when we use them and, unless it is ventilated, this will increase the humidity.