Category I Appliances

Category I Gas Appliances

Category I appliances are gas-fired equipment operating with negative vent pressure and non-condensing flue gas temperatures, representing the traditional and most common venting configuration for residential and commercial gas equipment. These appliances utilize draft hoods or barometric dampers to maintain atmospheric pressure at the appliance outlet, allowing safe venting through Type B gas vents or masonry chimneys.

Definition and Characteristics

ANSI Z21.47 Classification

Vent pressure: Negative (below atmospheric)

Flue gas temperature: Non-condensing (>275°F minimum at vent inlet)

Typical AFUE range: 78-83% for furnaces, 80-84% for boilers

Operating Principles

Draft hood operation:

Combustion products exit heat exchanger at 400-550°F
Products pass through draft hood relief opening
Room air (dilution air) mixes with flue gases
Mixture enters vent at 300-450°F, reduced concentration
Natural draft or fan assistance creates negative pressure

Pressure profile:

Combustion chamber: Slightly negative (-0.01 to -0.05 in. w.c.)
Draft hood inlet: Negative (-0.02 to -0.10 in. w.c.)
Draft hood outlet: Near atmospheric (±0.01 in. w.c.)
Vent system: Negative throughout (-0.02 to -0.10 in. w.c.)

Common Appliance Types

Residential Equipment

Gas furnaces:

Input: 40,000-200,000 Btu/h (residential)
AFUE: 78-80% (standard efficiency), 80-83% (mid-efficiency)
Draft hood integral to appliance
Vent outlet: 3-7 in. diameter

Gas-fired boilers:

Input: 50,000-400,000 Btu/h (residential)
AFUE: 80-84%
Draft hood or barometric damper
Vent outlet: 4-8 in. diameter

Water heaters:

Input: 30,000-75,000 Btu/h (residential)
Energy factor: 0.58-0.62 (atmospheric)
Integral draft hood
Vent outlet: 3-4 in. diameter typical

Commercial Equipment

Commercial furnaces:

Input: 200,000-500,000 Btu/h
Thermal efficiency: 80-83%
Power venter or barometric damper
Vent outlet: 8-12 in. diameter

Commercial boilers:

Input: 400,000-6,000,000 Btu/h
Combustion efficiency: 81-83%
Barometric draft control
Vent outlet: 10-24 in. diameter

Unit heaters:

Input: 30,000-400,000 Btu/h
Separated combustion chamber heaters require venting
Draft hood or power vent
Vent outlet: 3-10 in. diameter

Venting System Materials

Type B Gas Vent

Compatibility: Excellent for all Category I appliances

Sizing: Per NFPA 54 Appendix G tables

Advantages:

1 in. clearance to combustibles
Lightweight, economical
Proven performance
UL 441 listed

Limitations:

Negative pressure only
480°F maximum continuous
Aluminum liner susceptible to prolonged condensation

Masonry Chimney

Compatibility: Suitable for Category I appliances with proper liner

Requirements:

Clay tile liner (ASTM C315) or stainless steel liner (UL 1777)
Proper sizing to prevent excessive condensation
Common venting possible (with caution)

Advantages:

High capacity for large appliances
Handles multiple appliances
Long service life

Disadvantages:

Exterior chimneys may condense
Often oversized for modern efficient equipment
Higher cost than Type B vent

Class A Chimney (All-Fuel)

Compatibility: Over-designed for Category I appliances but acceptable

When used:

Multi-fuel capability desired (future wood stove)
Replacing masonry chimney
Very high capacity (commercial applications)

Considerations:

Higher cost than necessary
Better draft than Type B (may compensate for undersizing or altitude)

Single-Wall Connector

Use: Vent connector only (appliance to vent/chimney)

Not permitted for: Entire venting system, penetrations through building assemblies

Clearance: 18 in. to combustibles (6 in. with protection)

Maximum length: 75% of vertical vent height, typically 10 ft maximum

Draft Hood Function and Sizing

Purpose of Draft Hood

Constant draft provision:

Isolates appliance from vent system variations
Wind, barometric pressure changes don’t affect combustion
Prevents excessive draft from pulling too much air through burner

Dilution air:

Reduces flue gas temperature (~50% reduction)
Dilutes combustion products (CO₂ from ~10% to ~5%)
Increases total vent volume flow

Spillage prevention:

Relief opening allows room air if vent draft fails
Prevents backdrafting into combustion chamber
Visible spillage alerts to venting problem

Backdraft prevention:

Momentary vent downdrafts don’t enter appliance
Wind gusts, startup transients accommodated

Draft Hood Configuration

Integral (built-in):

Factory-installed on furnaces, water heaters
Sized for specific appliance
Cannot be removed or modified

Field-installed:

Required for some boilers, conversion burners
Sized to match appliance outlet diameter
6 in. minimum clearance above hood required

Double-acting barometric damper:

Alternative to draft hood on boilers
Weighted damper opens to admit dilution air
Also prevents excessive draft when open further
Adjusted for specific installation

Fan-Assisted Category I Appliances

Fan-Assisted Combustion (FAC)

Configuration:

Small blower assists combustion air delivery
Heat exchanger under slight positive pressure
Draft hood downstream of heat exchanger
Vent system remains negative pressure

Advantages:

More complete combustion (lower excess air)
Improved efficiency vs. atmospheric (80-83% AFUE)
Smaller vent connector possible
Better performance in tight buildings

Venting:

Draft hood makes appliance Category I
Type B vent or masonry chimney acceptable
NFPA 54 tables apply
Cannot use positive-pressure venting

Induced Draft

Configuration:

Fan at appliance outlet, before draft hood or barometric damper
Pulls combustion products through heat exchanger
Draft hood follows fan
Still Category I (negative pressure in vent)

Applications:

Some commercial boilers
Mid-efficiency (83% AFUE) equipment
Allows longer vent connectors

Sizing and Design

NFPA 54 Sizing Tables

Table selection based on:

Vent type (B, single-wall, masonry)
Vent height (6-50 ft typical range)
Connector configuration (length, rise)
Single appliance or common vent

Capacity factors:

Taller vent = higher capacity (more draft)
Shorter/longer connector = lower capacity (more resistance)
Steeper connector rise = higher capacity (better draft)
Type B vent > masonry > single-wall (for same size, height)

Example: 80,000 Btu/h furnace, 20 ft Type B vent height, 6 ft connector

Table G.2.4 lookup: 4 in. = 77,000 Btu/h (too small), 5 in. = 108,000 Btu/h ✓
Select 5 in. Type B vent

Common Venting Multiple Appliances

Requirements:

All appliances Category I
Located in same room/space
Smallest appliance connector enters above largest
Manifold connections within 18 in. of draft hoods
Special NFPA 54 common vent sizing tables (Appendix G.2)

Concerns:

When only one appliance fires, vent is oversized for that load
Oversized vent cools flue gases, potential condensation
Backdrafting risk to inactive appliance
Modern trend: Individual venting preferred

Example: Water heater (40,000 Btu/h) + furnace (100,000 Btu/h)

Individual vents: WH = 4 in., furnace = 5 in.
Common vent: Table G.2.5, combined 140,000 Btu/h = 6 in.

Installation Best Practices

Vent Connector

Material: Single-wall or Type B connector

Slope: Minimum ¼ in./ft rise toward vent

Length: Minimize for best performance, maximum 75% of height or 10 ft

Support: Every 3-4 ft to prevent sagging

Clearance: 18 in. single-wall, 1 in. Type B (residential)

Vertical Vent

Straight vertical preferred:

Minimizes resistance
Maximizes draft
Simplifies installation

Offsets when necessary:

Use 45° elbows when possible (less resistance than 90°)
Limit horizontal offset lengths
Support adequately to prevent joint separation

Termination:

3 ft above roof penetration minimum
2 ft above any roof within 10 ft
4 ft clearance from windows, doors, air intakes (varied directions)

Combustion Air Provisions

Indoor air (traditional):

Requires adequate infiltration or ventilation
Two openings to outdoors or large indoor space
NFPA 54 Chapter 9 requirements
Tight buildings may require mechanical makeup air

Outdoor air (direct):

Duct from outdoors to appliance area
Reduces building air usage
Improves draft reliability
Required in very tight construction

Troubleshooting

Draft Hood Spillage

Symptoms: Flue products escape draft hood during/after appliance startup

Common causes:

Blockage: Bird nest, ice, debris in vent or cap
Insufficient draft: Vent too short, too large (oversized)
Negative building pressure: Exhaust fans, inadequate combustion air
Downdraft: Wind, improper termination location/height

Diagnostic steps:

Check vent cap and vent interior for blockage
Verify vent sizing per NFPA 54 tables
Measure building pressure with appliance and exhaust fans operating
Observe on windy day, different wind directions
Smoke test to verify draft direction

Solutions:

Remove blockages, install cap with larger free area
Increase vent height if too short, reduce size if grossly oversized
Provide combustion air makeup, interlock with exhaust systems
Increase termination height, relocate away from problem areas

Condensation in Vent System

Symptoms: Water staining at joints, white efflorescence, deterioration of vent

Causes:

Oversized vent (low velocity, excessive cooling)
Exterior vent in cold climate without insulation
Short vent cycles, insufficient warmup time

Solutions:

Reduce vent diameter if excessively oversized
Insulate exterior vents or install within insulated chase
Verify proper appliance operation (excessive short cycling indicates other problems)

Category I appliances represent proven, reliable heating technology with straightforward venting requirements when properly sized and installed per NFPA 54 and manufacturer specifications.