Do Gas Fireplaces Have a Flue? 5 Things To Know

I’ve been to homes where people have gas fireplaces with a flue and also to homes with flueless gas fireplaces. Now, one might think that gas fireplaces with a flue are just old models, and the newer ones come without a flue – or maybe it’s the other way around. Regardless, let’s not guess and instead get the real take on whether gas fireplaces have a flue.

Some gas fireplaces have a flue, while others don’t. The ones that come without a flue are called ventless gas fireplaces, whereas those with a flue are called vented gas fireplaces. Both vented and ventless gas fireplaces are good options, and you should pick the one that best suits your needs.

In this article, I’ll give you a comprehensive overview of flues used (or not used) with gas fireplaces. Following that, I’ve also shared a few differences between vented and ventless gas fireplaces. This should help you understand all you need to know about flues and gas fireplaces.

Things To Know About Flues and Gas Fireplaces

Ventless gas fireplaces don’t have a flue, while vented fireplaces do. 

So how does a ventless gas fireplace exhaust the waste air? Moreover, if ventless gas fireplaces can function without a flue, why do we need them anyway? 

Does having (or not having) a flue affect the functionality and performance of a gas fireplace?

These are just some of the most common questions regarding gas fireplaces and flues. If you also have any of these questions in mind, don’t worry. I’ve tackled all of these queries and much more in the following section.

1. Ventless Gas Fireplaces Can Function Without a Flue

Ventless or vent-free gas fireplaces don’t have a flue for air intake or output. They work by taking in fresh air from the room where they’re placed and then exhaust the waste air back into the room.

But wait, isn’t that dangerous? Won’t ventless gas fireplaces degrade the air quality of the room?

Ventless gas fireplaces are extremely efficient and cleanly burn the fuel using the oxygen available in the room and then release carbon dioxide (CO2), carbon monoxide (CO), and water vapor back into their surroundings.

Of the mentioned by-products, CO is the most dangerous as it can lead to carbon monoxide poisoning. However, it’s generally released in trace amounts, and manufacturers claim it shouldn’t cause too much of an issue. 

On the flip side, many building science experts claim that it can degrade indoor air quality, especially if the room where it’s installed lacks proper ventilation. 

Additionally, apart from the potential danger related to CO, the room’s oxygen levels will gradually take a dip with extended use, especially if it’s a tightly sealed-off space. 

Considering all these factors, I’d recommend that you should only install a ventless gas fireplace in a room or space with proper ventilation. This will help the waste air leave the room and allow fresh air to come in, keeping the oxygen levels at a safe range.

2. Natural Vent Gas Fireplaces Use Your Chimney as a Flue

Natural vent gas fireplaces, also known as B-vent gas fireplaces, don’t have a dedicated flue to exhaust the waste air. Instead, you need to place them inside one of the masonry fireplaces in your home, and it’ll use the associated chimney as its flue.

So what if your home doesn’t have a masonry fireplace or attached chimneys? In that case, avoid getting a natural vent gas fireplace and instead pick a ventless or direct vent gas fireplace.

While functioning, the fireplace will use air in the room as a source of oxygen to burn the fire. The waste air – which is hot – will rise up and escape your home through the chimney.

In terms of design, a natural vent gas fireplace doesn’t directly connect with the chimney. Instead, it’ll have a big opening at the top, specifically designed to capture the waste air and guide it upwards through the chimney and eventually out of your house.

3. Direct Vent Gas Fireplaces Has a Dedicated Flue

Direct vent gas fireplaces have dedicated vent(s) for taking in air for combustion and exhausting waste air. It’ll use the oxygen from the outside air to burn the fire and also exhaust the waste air out of your home.

In this way, it’s a completely sealed-off system that doesn’t impact the air quality of the room or space where it’s placed – except for making it warmer, of course. Because of this, it’s also one of the most preferred forms of room heating in the market. 

Design-wise, direct vent gas fireplaces might have two separate vents in place – one for the air intake and the other for the flue. However, this is most common with older models. Modern direct vent gas fireplaces will usually use a single pipe for air intake that’ll also double as a flue for air exhaust.

Generally, at the time of installation, you’ll have the flexibility to choose how many ventilation pipes you want and where you want them placed. 

Make sure to thoroughly plan this out because moving an already-installed direct vent gas fireplace is difficult and extremely expensive.

4. Routinely Service the Flue on Your Gas Fireplace

Many people believe that gas fireplaces don’t get dirty and that they don’t need routine maintenance or servicing.

Now, it is true that gas fireplace flues tend to get less dirty than wood fireplaces as there’s barely any soot build-up or ash formation. Even so, it’s wise to get it serviced at least once a year to make sure everything is working okay.

For example, during the summer season, when you stop using the gas fireplace for an extended period, insects can start building nests inside the flue. It’s not uncommon to find moth nests, hornet nests, or beehives in a flue. 

Additionally, as the flue sucks in fresh air from the outside and/or exhausts waste air outdoors, there’s a chance of dust and debris building up inside the flue, especially if you don’t have a straight flue and it’s set up in an L-shaped configuration.

A flue that’s not working properly can prevent the waste air from leaving your house and may also have a leak from where the waste air re-enters your house.

You should preferably get the flue checked annually along with the gas fireplace as the same technician can do both jobs. I’d suggest booking the fireplace technician ahead of winter as they become super busy during the cold season, and it’ll be hard to find someone to look at your system.

5. Old Flues Need To Be Replaced

Flues will start to get old and damaged over the years and lose their efficiency in eliminating waste air. When this happens, your gas fireplace will also burn less efficiently because of ventilation trouble.

Here’s a list of common problems you can face from having an old flue:

• An old flue can lead to mold formation. It’s important to note that gas fireplaces release water vapor as a by-product. If the flue can’t properly exhaust the water vapor outside, it can cause mold formation either inside the flue or on the walls near the flue.
• A leaky flue can increase your risk of carbon monoxide poisoning. Old flues can have slightly loose joints, which can cause the waste air to leak into your room. As you can imagine, this may lead to severe health complications if ignored.
• Cold drafts can come in through an old flue. As the flue becomes less efficient at exhausting the waste air, it’ll allow the cold air from the outside to travel through it and into your room. This not only interferes with the flame but also reduces the efficiency of the fireplace in heating your room.

As such, I highly recommend that you replace your old flues with a new one to make sure you get the optimal heating experience from your gas fireplace.

When Should You Open the Flue on Your Gas Fireplace?

You should open the flue on your gas fireplace if it’s currently in use and housing a burning flame. You should also keep the flue open if the pilot light is turned on, even if the fireplace isn’t actively burning.

Both direct-vent and natural vent gas fireplaces can emit dangerously high levels of carbon monoxide as a by-product of the fire, leading to carbon monoxide poisoning.

On the other hand, if you keep the pilot light turned on, but the flue closed off, it can lead to a build-up of propane or natural gas (whichever gas is used as fuel) inside the fireplace, or worse, in your room. This can easily result in a devastating accident such as a structure fire.

As such, you must keep your flue open when your gas fireplace is running – whether the flame is burning or just the pilot light is on.

That said, it’s pretty straightforward to open/close a flue. Most modern direct vent gas fireplaces are designed to handle this automatically. 

With older models, as well as with natural vent gas fireplaces, you might need to manually open the flue damper to make sure the waste air gets properly exhausted. Just remember that the handle to open/close the damper can get hot. As such, you should wear hand protection like oven mitts when operating the damper.

You can close the flue after turning off the gas fireplace and the pilot light. It’s also recommended that you keep the flue closed if you plan on not using the fireplace for an extended period – either during summer or when you’re off on holidays.

Vented vs. Ventless Gas Fireplaces: Major Differences

The main difference between a vented and a ventless gas fireplace is that one has a flue while the other doesn’t. However, the lack (or presence) of a flue can have significant consequences making the heating appliances distinctly different from one another.

The presence of a flue can influence the cost of installing the fireplace, its overall size, heating efficiency, safety, ease of installation, maintainability, as well as the aesthetics of the fire.

I’ve put together a table that should give you a detailed look at how vented and ventless gas fireplaces compare to one another:

Key Takeaways

Depending on their type, gas fireplaces might or might not have a flue.

Ventless gas fireplaces don’t have a flue. They efficiently burn the fuel (gas), leading to minimal toxic emissions, and don’t degrade the air quality of the room where it’s placed.

Natural vent gas fireplaces don’t have a dedicated flue. They need to be installed inside a masonry fireplace where it can use the provided chimney to exhaust the waste air.

Finally, we have direct vent gas fireplaces, which come with a dedicated flue and ventilation system used for both fresh air intake and waste air exhaust. 

Steve Rajeckas

Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he’s not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.