Fuel Oil Furnace - "Negative Chimney Draft"

After finding a fuel oil furnace a few weeks ago with a 16’ long horizontal vent connector and five elbows, I have been trying to nail down exactly what the specific requirements are.

My state, like most others probably, defers to NFPA 31 for venting of fuel oil furnaces. NFPA 31 states the following (among other things) for horizontal vent connectors …


I came across an article, linked to below, that seems to indicate any fuel oil furnace with a mechanical draft burner falls under the 75% rule and not the 10 foot rule. My question is; why does NFPA 31 refer to those with a mechanical draft burner as "requiring a negative chimney draft?" I know this can get technical, but my mind wants me to think that any non-direct vent fuel burning appliance would need negative chimney draft?

Same reason a fan-assisted (induced draft) gas appliance needs a chimney/vent with a negative draft. The fan in the oil burner is only to provide primary air and to help overcome the resistance of the heat exchanger. From there the [negative] draft of the chimney is needed to remove the combustion exhaust gases.

BTW, are you sure the long@$$ connector you found terminated into a chimney and not a power vent?


I agree, but the why does NFPA 31 separate out “natural draft” appliances and give them a shorter allowed horizontal vent connector? It would seem like natural draft appliances rely just as much on negative chimney draft?

It’s not shorter. If you have 20foot chimney, 75% of that is more than 10 feet.

In other words, in the case of natural draft burner that nobody uses anymore, you cannot exceed 10ft unless a fan is used… but with mechanical draft you can exceed.

Ok, so two scenarios, both with a 20’ chimney…
A natural draft fuel oil furnace (no burner fan) is only allowed a 10’ horizontal vent connector.
A fuel oil furnace with a mechanical draft burner is allowed a 15’ horizontal vent connector.

1 Like

Yeap, correct. BTW, that is the max and should not be done, the appliance needs to be located to the chimney as close as possible.

1 Like

Ok, I think I have this figured out now.
The NFPA isn’t saying that natural draft appliances don’t rely on negative chimney draft. They are limiting natural draft burners to 10 feet, but then saying even if you have a mechanical draft burner, it still relies on negative chimney chimney draft and we’ll give you a little more leeway but you are still limited.

Yeap. A natural draft oil burner will not burn without negative chimney draft, forget removing combustion gasses. It needs the draft of the chimney to give it primary air. Oxygen is needed to burn fuel :slight_smile: A mechanical draft burner will burn the oil without chimney but all the exhaust will go into the house :slight_smile:

1 Like

Well, they couldn’t have worded it any worse, in my opinion, lol. Was confusing the shit out of me for awhile because they have one requirement for “natural draft” and another for “those needing negative chimney draft.” Well, natural draft appliances “need negative chimney draft!” :rofl: :joy: :sweat_smile:

After thinking about the NFPA’s wording in these two requirements a bit more, it is starting to make sense why they wrote it the way they did.

The 2nd requirement is actually a refinement of the first. No matter how tall the chimney, natural draft fuel oil furnaces are limited to a 10’ horizontal vent connector.

Furthermore, any fuel oil furnace that relies on negative chimney pressure, including natural draft fuel oil furnaces, is limited to a horizontal vent connector length equal to 75% the height of the chimney.

It makes more sense when you think of the 2nd requirement as a refinement of the first.

You are over thinking it :slight_smile: Try this:

For [all other] appliances requiring a negative chimney draft…

The natural draft has no fan and if the connector is longer than 10feet, it is unlikely to work safely. So it cannot have a longer connector like one with a fan (all other), if the chimney is tall enough. If you read from the beginning of chapter 6, you will see that the first requirement is a chimney with a proper draft.

1 Like

Who, me? Lol. :rofl:

Thanks for your help Simon.