Gas fireplaces with vents can dry out the air in your home. However, vent-free gas fireplaces may do the exact opposite, actually increasing the humidity in your home. For best results, adjust the time you have your vented gas fireplace on and keep vent-free gas fireplaces on for a maximum of three hours at a time.
In fact, the whole question winds up being fairly complicated. Gas fireplaces often add moisture to your home while simultaneously drying out the air. In the end, several factors–including fireplace type and how it’s used–factor into whether, in your particular case, your fireplace is humidifying the air more or whether it’s drying it out more. Let’s explore this interesting and multi-faceted question.
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Do Gas Fireplaces Make the Air Dry or Create Moisture?
The answer is: change the “or”. Repeat the question, but replace the “or” with an “and”. Gas fireplaces make the air dry and create moisture–that is the answer to the question.
Gas fireplaces create moisture in the air, while simultaneously drying the air out. Of course, to fully understand what that means, we’ll have to dive in deeper.
Let’s actually start with traditional wood-burning fireplaces. Wood-burning fireplaces are a big part of why you might have heard the rumor about fireplaces drying out the air. And that is because, in the case of traditional fireplaces, the rumor is true.
A traditional wood-burning fireplace requires ventilation with the outside. This might be in the form of a chimney, flue, or other vent. Regardless, it is an open hole, which allows air from outside to come into your house.
The cold air from outside is dry. So as your chimney or vent allows in the fresh air, it is also allowing in dry air.
Additionally, the warmth of the fire itself dries out the air. If the fire is too hot, it can actually remove water vapor from the air.
(But usually, the main culprit in dry air is leaking from the outside–letting in very dry cold winter air.)
If you have a vented gas fireplace, both those same problems apply. The fireplace has a vent/hole to the outside, letting in dry air, and if the fire gets too warm, that heat can remove water vapor from the air.
But there is an important way in which gas fireplaces differ from traditional wood-burning ones. And that is the chemical makeup of the combustion.
You see, unlike when wood is burned, natural gas combustion combines oxygen molecules in the air with methane’s carbon and hydrogen and produces two byproducts: CO2 (also known as carbon dioxide) and H2O (also called water).
Did you hear that? The process of burning natural gas literally makes water. On a chemical level, after using a gas fireplace, more water will now exist in the world than did before.
So when using a vented gas fireplace, the combustion/fire in the fireplace will chemically create water, adding humidity to the air. The vent will then allow some of that air to escape outside and allow some dry air from outside to come into your home, thereby drying the air.
Overall, a vented gas fireplace tends to dry out the air more than it moistens it.
And dry air can be a real problem, according to Mayo Clinic.
If you want both a gas fireplace and humid air, however, there is another option: vent-free gas fireplaces.
As the name says, vent-free gas fireplaces have no vent, no chimney, no exposed hole allowing air in from the outside. So, in this case, the fire adds moisture to the room–and that moisture stays. There is dry air outside your house, but, assuming your home is well-insulated, that air stays outside.
So in short: a vented gas fireplace will most likely dry the air in your home, while a vent-free gas fireplace will likely humidify your home.
Can Built-Up Moisture Damage Fireplaces?
Built-up moisture can damage fireplaces, rot wood, damage metal, weaken structures, and foster mold.
Additionally, even if the moisture hasn’t caused damage to your fireplace, the presence of extra moisture can be a sign that your fireplace is already damaged. If you’re using a vented fireplace and your room is increasingly humid, that could indicate a clogged chimney or vent, which is more than just a mechanical property problem. Your chimney or vent filters dangerous chemicals out of your house.
So if your chimney/vent is clogged, there is a risk that carbon monoxide and other hazardous chemicals are coming into your home. These chemicals can, in better situations, cause headaches and nausea, but in worse scenarios, kill you.
(Which might be a good time to mention: it is never a bad idea to have a carbon monoxide alarm! Especially with a gas fireplace! Carbon monoxide is odorless and tasteless).
How Do You Prevent Moisture from Building Up?
To keep an eye on moisture in your home, you can buy a hygrometer. This device will measure your home’s humidity levels, letting you know for sure whether you are starting to have too much moisture in your air.
As we talked about earlier, if cold air from outside is allowed into your home, it will dry out your home’s atmosphere. This means that if you are indeed getting too much moisture in your home, one effective way of fighting this is to open a window. Dry winter air will blow in and lower the overall humidity of your home.
How Long Can You Leave a Gas Fireplace On?
Similarly, how long you can leave your gas fireplace on depends on whether it is vented or vent-free.
A vented gas fireplace can be left on indefinitely. However, a vent-free gas fireplace should only be used for up to three hours at a time.
A vented fireplace can run indefinitely if it has been properly maintained. If the vents are functioning properly, any poisonous fumes should be filtered outside.
But a vent-free fireplace is a little more problematic. While with “perfect” combustion, such a fireplace will only produce heat, carbon dioxide, and water. If at any point there is a lack of oxygen in the air, the fireplace will create carbon monoxide–which is odorless, tasteless, and deadly.
The longer the fireplace runs, and the more oxygen it burns, the more likely it is to run into such a scenario and produce this deadly poison. For this reason, you should only leave a vent-free fireplace running for a maximum of three hours. And you should never leave the fireplace running while you sleep, as you risk carbon monoxide poisoning.
No matter what type of fireplace you own, however, for safety, it is best to monitor the fire at all times, and please make sure your fireplace is properly, professionally maintained, and that you have, in the same room as your fireplace, a working carbon monoxide detector, which you test regularly.