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Mike Lamb

Direct and non-direct venting

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I don't think you described it correctly.

You're talking about a mechanical draft vs. a direct vent, yes? They're two different animals.

The one pipe/two pipe part isn't the distinction.

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I'm assuming (and I may be wrong) that the mechanical draft means it's a Cat 3. They run under positive pressure, so maybe that's the reason for the different vent termination clearances.

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I don't think you described it correctly.

You're talking about a mechanical draft vs. a direct vent, yes? They're two different animals.

The one pipe/two pipe part isn't the distinction.

I think Mike is describing it correctly. The rule being quoted from NFPA 54 is repreated in all of the other gas codes as well.

A forced-vent by itself can pressurize an area around the vent outlet, whereas when it is accompanied by its combustion air intake, the pressures of the two offset each other. One foot is considered sufficient clearance for the gasses to dissipate in the atmosphere just as with a direct vent appliance.

Many Category IV appliances can be installed either way. The instructions might allow you to draw combustion air from indoors or from a location remote from the vent termination, in which case the rule for 4 ft. below or to the side will apply. That same appliance might be allowed to have an installation where the combustion air pipe terminates a foot from the vent, and the vent can terminate under a deck or a foot from an openable window.

With these appliances, you always need the installation instructions to know if it was installed correctly. Part of the code and the installation instructions is that the instructions must be left with the appliance. A good installer will make some sort of pocket to hold them on the outside of the appliance. At a minimum, they should tape the envelope containing the instructions to the side of the appliance. I've also seen some installers who create a sheet metal pocket to hold the things.

I'm told the most common venting defect these days is failing to use the correct primer on the joints of the plastic vent pipes.

Douglas Hansen

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Interesting.

What is the distinction of mechanical draft versus direct vent?

Our need for logical categories has us wanting to pick one of these sets of rules and apply a particular installation to those rules. However, it isn't a question that has to be answered when we follow the appliance manufacturer's instructions. As an example, some appliances allow the two pipes to be joined into a coaxial plastic pipe so that you only have one sidewall exit. Those termination kits are easy to visualize as "direct vent" but they don't have to be called that. That same appliance may also have an instruction allowing two pipes with a certain minimum or maximum distance between their terminations, or a single vent pipe with combustion air from indoors.

The workmanship in the attached photos is terrible.

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DH

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kurt Posted - May 29 2011 : 10:12:14 PM

Interesting.

What is the distinction of mechanical draft versus direct vent?

I understand the distinction to be: mechanical (fan assisted) draft vs. natural (gravity) draft. That is to say........a direct vent appliance could utilize a mechanical draft or a natural draft. The term direct vent describes an appliance with a sealed combustion chamber, drawing all combustion air from outside the heated space-regardless of the type of exhaust vent.......yes? no?

..........Greg

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kurt Posted - May 29 2011 : 10:12:14 PM

Interesting.

What is the distinction of mechanical draft versus direct vent?

I understand the distinction to be: mechanical (fan assisted) draft vs. natural (gravity) draft. That is to say........a direct vent appliance could utilize a mechanical draft or a natural draft. The term direct vent describes an appliance with a sealed combustion chamber, drawing all combustion air from outside the heated space-regardless of the type of exhaust vent.......yes? no?

..........Greg

That sounds right. These definitions are from the IRC.

DIRECT-VENT APPLIANCE. A fuel-burning appliance

with a sealed combustion system that draws all air for combustion

from the outside atmosphere and discharges all flue gases

to the outside atmosphere.

MECHANICAL DRAFT SYSTEM. A venting system

designed to remove flue or vent gases by mechanical means,

that consists of an induced draft portion under nonpositive

static pressure or a forced draft portion under positive static

pressure.

Forced-draft venting system. A portion of a venting system

using a fan or other mechanical means to cause the

removal offlue or vent gases under positive static pressure.

Induced draft venting system. A portion of a venting system

using a fan or other mechanical means to cause the

removal of flue or vent gases under nonpositive static vent

pressure.

Power venting system. A portion of a venting system using

a fan or other mechanical means to cause the removal offlue

or vent gases under positive static vent pressure

Maybe since a non-direct vent uses inside air for combustion it creates a more of a likelihood that air is being drawn in through house openings.

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