When Can A Fuel Vent Connect To A Masonry Chinmay?

Student here. I’m having a little trouble understanding this, please correct me!

As I understand things, connecting a fuel burning appliance to a masonry chimney was common when the appliances were around 60% efficient. Now that appliances are >80%, they need their own vent to the exterior of the building. This is because the combustion air can condense in the flue, potentially causing moisture problems (also containing toxic gases that may seep into the building).

Appliances that are 80% efficient, burning natural gas or propane must use a metal B-Vent.

Appliances that are 80% efficient, burning oil or gas, must use a metal L-Vent.

Appliances that are 90% efficient, must use a plastic PVC vent.

Appliances that use electricity as a “fuel” do not need venting to the exterior of the building.

Is this correct?

It’s not your job to “analyze” or determine code.

Simply, if the flue does not extend to the top of the chimney .call it out. If the utility shows up they will likely red tag it and shut off and lock the meter.

You did not mention that combustion gasses dumped in the chimney could diffuse back into the house (depending where the chimney is attached to the building.


Thanks for the reply.

I think you may have misunderstood… I’m trying to ensure I understand the venting requirements for a fuel-burning appliance connecting to a masonry chimney, not the flue component of a masonry chimney.

Are you telling us inspector should not call out mid-efficiency gas-fired furnace with a draft inducer dumping into clay-lined masonry chimney because that is not the home inspector’s job?

My SOP states:

Heating System:
Observe and report on visible components of vent systems, flues and chimneys.

What does that mean to you? what exactly is inspector to do to comply with this requirement?

Yes, David, you are confusing others with such comments, so hopefully you can explain it so we could all learn.

No, I did not say that. I said we are not required to give the okey dokie on the operation of the install of a flue into a chimney. I said if a metal flue doesn’t come out the top that it should be called out and checked by the utility who has jurisdiction on these things.

The utility has greater power over these things based on their requirement regardless of any code or HI opinion. We have no idea what their assessment would be. Client buys the house and comes home to a red tag on the front door and a locked meter, think they may have choice words for the HI?

When I was working in HVAC my company got a call from a little old lady who was using a gas heater vented into the chimney for heat. She called the gas company to light her pilot light that went out. They locked her meter because of chimney construction. We threw in a flue pipe but she was without heat for 48hrs.

Would you like this to happen on a house you just inspected?

That is quite different from what you said above. Yes, I completely agree, inspector should never claim something is in compliance, I see it too often. Inspector would say something like “all electrical circuits were sized correctly”. Inspector should only call things out that don’t appear to have been done correctly and or performing correctly. However, the OP was merely asking how things should be done when done correctly. Where did “not your job to analyze or determine code” come from? a lot of these safety and installation requirements are part of and or based on modern code. It’s impossible to call something out if we don’t know how it should be done. The better an inspector knows this the better s/he can advise their client of a potential issue. Without knowing how it should be done, often by code, inspector has only one option and that is to defer. At that point the entire report would just be: hire plumber, hire HVAC, hire roofer, hire electrician! now give me money money :slight_smile:


Simon touched on I believe what you’re looking for and please correct me if i’m wrong but an 80% furnace can vent to a masonry chimney as long as it’s a metal lined flue but it’s best to consult the manufacturers instructions pertaining to vent requirements.

I am also trying to get this venting stuff based on efficiency rating engrained into my brain Saman, for some reason there is a bunch of conflicting information out there on this topic.

The issue is that… too often, the source you are reading is either hard to understand, misleading/incorrect, and or incomplete. Have you tried to find information on how venting should be done? Google is your friend, you can find almost anything. Always try go to the source! if we’re talking about gas venting, start with NFPA 54, it will spell out exactly how to do it. Then if you see something different in the field and you don’t know if it’s a “local” thing, you can confirm with local building department.

Having said this:

  1. A draft-hood gas appliance can vent into a properly sized clay lined chimney, but this is best avoided and is rarely sized correctly. It can also vent into approved metal lined chimney.
  2. A fan-assisted category 1 appliance listed for use with a B-vent cannot typical vent into a clay-lined chimney, but can vent into a masonry chimney lined with an approved liner designed to vent b-vent gas appliance.

You need to first learn the 4 categories of appliances and how they differ. Then check NFPA 54 chapter 12 venting. Learn to use google to find credible sources. Let me know if something is still confusing. I could give you a good read, but I doubt you would read it because most people that ask questions like this too often just want “quick” answer. So, prove me wrong, start researching, start reading, start asking the “right” questions. Not just I’m confused. If you’re “just confused”, you will have to go to step 1 and start all over or pray the gods empower you with great wisdom that took others many years to acquire :smiley:


Thanks for the followup Simon, I am pretty good on most things HVAC but it’s the venting that has always got me!! If I can understand the reasoning for something rather than just random code speak then I find I remember these things much better. So the logic behind clay liner restrictions is that higher temperature flue gases take a toll on clay liners which is why they are allowed with a draft hood, because in that case the flue gases have cooled to some degree?

To understand the reasoning it’s important to understand the idea behind natural draft venting using a vent or a chimney. The idea is to create enough draft to expel the combustion gasses to the exterior without condensing the gasses inside of a flue. The issue with clay liners besides being compromised is that they are, too often, oversized (used for oil then switched to gas, used with hood draft appliance then switched to fan-assistated) and when properly sized to produce just enough draft, because they are not insulated like a double-wall b-vent, the combustion exhaust may be too cool and condense regardless. A metal liner does one and or several things: it 1) provides corrosion resistant protection that clay liners don’t have and corrects compromised clay liner and 2) if it’s a double-wall liner provides enough insulation to expel, otherwise too cool, gasses to the exterior without condensing them. When gasses are even too cool for a metal double-wall liner, we turn to plastics and or double-wall stainless steel to expel them as prescribed by the manufacturer when it’s 90%+ condensing appliance.


Thank you so much for taking the time to explain this Simon. So the problem is not in the clay liner itself but that the industry standard clay liners are generally too large to keep the flue gases insulated from condensing and due to the size also slows the draft.

Oversized, too often compromised, and not as corrosion resistant. Should ANY condensation occur, which would be almost impossible to avoid with a fan-assisted gas appliance over long-term operation, the corrosiveness of the condensate will eat at the liner and compromise it. Remember, if some condensate occurs inside of a stainless metal liner it can often dry out without issues but in a clay liner the liner begins to deteriorate much faster.


That makes a lot of sense. Thank you.

Simon, thanks for the info! I would be interested in the “good read”, when you get a chance! Also, I’m looking into the NFPA 54 regulations!

NFPA.org, access is free but you may need to create a free account and login.

This doc is pretty informative:
Venting-Gas-Appliances.pdf (887.9 KB)


Thanks Simon I was watching this thread, You are one smart dude I always read what you have to say, Thanks again

That was a fantastic article for understanding the venting nuances for gas appliances, very detailed and informative. Thanks again for the info!!

Call out a clay lined vent because the local utility company “might red tag it”? I would not take that approach in my area. Here in the greater Atlanta area, we have several natural gas service providers, some owned by the public and some not. Each will likely have different standards.

However, after reading the links provided on here I have a better understanding of why a modern appliance may not perform well if attached to a clay lined chimney. I think that is information my client could use. Furthermore, if I see indicators of condensation such as rust, corrosion, moisture staining etc. on a flue, I will consider those as visual clues.

They all have safety standards. Take it for what it’s worth…

Adding to the excellent information above regarding cooling of flue gasses:
if the flue gases cool too much due to the long cold clay pipe, cold air can sink back down the flue and set up eddy currents.

In a tight home, a nearby dryer can suck enough air out of the home to compromise a flue with a weak draft (as stated in the JLC article linked above).

In California the major gas provider PG&E has weatherization programs: they won’t by policy touch a home that vents a gas appliance into a masonry chimney.