Unvented Gas Heaters

Unvented Gas Heater Overview

Unvented gas heaters combust natural gas or propane while releasing all combustion products into conditioned space. These units achieve near-theoretical efficiency (99-99.5%) since no heat escapes through venting systems, but introduce water vapor, carbon dioxide, and trace contaminants into indoor air.

Safety systems including oxygen depletion sensors (ODS) and strict installation codes limit applications and capacities. Maximum allowed input typically restricted to 40,000 BTU/hr with specific room volume requirements. Several states and jurisdictions prohibit unvented heaters entirely due to indoor air quality concerns.

Combustion Products and Indoor Air Quality

Stoichiometric Combustion

Complete combustion of natural gas (primarily methane):

$$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + \text{heat}$$

Each 1,000 BTU input produces approximately:

• 0.06 lb CO₂ (11.7 ft³ at standard conditions)
• 0.075 lb H₂O (1.5 pints liquid equivalent)

For 30,000 BTU/hr heater operating 6 hours:

• Total CO₂: 10.8 lb (2,106 ft³)
• Total H₂O: 13.5 lb (1.6 gallons)

Oxygen Consumption

Combustion consumes oxygen from room air. Volumetric oxygen consumption:

$$V_{O_2} = \frac{Q_{input} \cdot 10.3}{LHV}$$

Where:

• $V_{O_2}$ = oxygen consumption rate (ft³/hr)
• $Q_{input}$ = heater input (BTU/hr)
• $LHV$ = lower heating value (1,013 BTU/ft³ for natural gas)
• 10.3 = stoichiometric constant for methane

30,000 BTU/hr heater consumes 305 ft³/hr oxygen, or 64 ft³/hr from standard air (21% oxygen concentration).

Carbon Monoxide Production

Properly adjusted unvented heaters produce minimal CO (<20 ppm in combustion chamber). However, any of these conditions increase CO:

• Insufficient primary air adjustment
• Flame impingement on cold surfaces
• Contaminated air supply (dust, lint, chemical vapors)
• Low-quality fuel (excess propane heavy ends)

CO production increases exponentially when oxygen drops below 19.5%:

$$[CO] \propto e^{-k \cdot [O_2]}$$

ODS systems shut down before dangerous CO levels develop.

Moisture Production

Water vapor from combustion raises indoor humidity. For tight construction (0.3 ACH), 30,000 BTU/hr heater increases relative humidity by:

$$\Delta RH = \frac{m_{H_2O}}{\rho_{air} \cdot V \cdot ACH} \cdot \frac{VP_{sat}(T)}{P_{atm}}$$

In 1,500 ft³ space:

• Moisture addition: 1.125 lb/hr
• Humidity increase: 4-8% RH depending on temperature

Condensation risk on cold surfaces (windows, exterior walls) during continuous operation in cold climates.

Catalytic Heaters

Catalytic Combustion Process

Platinum or palladium catalyst oxidizes fuel at temperatures below visible flame threshold (300-650°F versus 1,200-1,800°F for free flame). Surface catalysis provides flameless combustion with lower peak temperatures.

Reaction Mechanism Fuel molecules adsorb on catalyst surface, dissociate, react with oxygen, and desorb as CO₂ and H₂O:

1. CH₄ + 2* → CH₃* + H* (adsorption and dissociation)
2. CH₃* + O* → CH₂O* + H*
3. CH₂O* + O* → CO₂ + H₂O (oxidation and desorption)

Where * represents active catalyst site.

Performance Characteristics

Heat Output Distribution

• 60-75% radiant heat transfer
• 25-40% convective heat transfer
• Lower convection fraction than free-flame heaters

Surface temperature: 900-1,200°F Radiant efficiency: 50-65% of total output

Capacity Range

• Typical input: 4,000-20,000 BTU/hr
• Maximum input: 30,000 BTU/hr (code restricted)
• Turndown ratio: 2:1 to 3:1 on adjustable models

Catalyst Life and Degradation

Deactivation Mechanisms

Poisoning: Sulfur, silicones, halides permanently bind to active sites:

• Aerosol sprays containing silicone
• Sulfur compounds in propane
• Chlorine from cleaners

Catalyst life with clean fuel: 8,000-15,000 hours With contaminants: 2,000-5,000 hours

Sintering: High-temperature exposure reduces surface area:

$$A_{active} = A_0 \cdot e^{-kt}$$

Where:

• $A_{active}$ = active surface area at time t
• $A_0$ = initial surface area
• $k$ = sintering rate constant
• $t$ = exposure time at elevated temperature

Fouling: Physical blockage from dust and debris reduces efficiency by 10-20% before requiring cleaning.

Installation Requirements

Clearances

• Rear wall: 0-4 inches (rear wall mounting typical)
• Side walls: 6-12 inches
• Ceiling: 36-42 inches
• Floor: 18-24 inches
• Drapes, furniture: 36 inches

Ventilation Despite being unvented, room must have:

• Permanent opening to adjacent space OR
• Outdoor air opening (1 in² per 1,000 BTU/hr minimum)

Blue Flame Unvented Heaters

Burner Design

Multi-port ribbon burner or individual port injectors create diffusion flames with secondary air entrainment. Blue flame indicates complete combustion with minimal yellow tip luminosity.

Flame Characteristics Temperature distribution:

• Inner cone: 2,700-3,000°F
• Outer cone: 2,200-2,500°F
• Average effective: 1,400-1,800°F

Heat Distribution

• 20-35% radiant component
• 65-80% convective component
• Relies on natural circulation or optional fan

Thermostatic Control

Built-in thermostat maintains room temperature ±2-4°F. Gas valve modulates between pilot (500 BTU/hr) and full input.

Three-stage operation common:

• Pilot only: 500 BTU/hr
• Low fire: 40-50% of rated input
• High fire: 100% rated input

Capacity and Coverage

Coverage assumes well-insulated space with 8-foot ceilings.

Fan-Equipped Models

Optional convection blower increases heat distribution:

• Airflow: 100-200 CFM
• Power consumption: 25-75 watts
• Noise level: 35-45 dBA

Blower extends coverage area by 20-30% through improved air circulation and reduced ceiling stratification.

Infrared Radiant Unvented Heaters

Radiant Plate Technology

Burner heats ceramic or refractory plate to 1,400-1,800°F. Hot surface radiates infrared energy directly to objects and occupants without significant air heating.

Radiant Heat Transfer Stefan-Boltzmann radiation:

$$Q_{rad} = \epsilon \sigma A (T_s^4 - T_r^4)$$

Where:

• $Q_{rad}$ = radiant heat transfer rate (W)
• $\epsilon$ = surface emissivity (0.85-0.92 for ceramic)
• $\sigma$ = Stefan-Boltzmann constant (5.67×10⁻⁸ W/m²·K⁴)
• $A$ = radiating surface area (m²)
• $T_s$ = surface absolute temperature (K)
• $T_r$ = receiving surface temperature (K)

Radiant Fraction Well-designed infrared heaters deliver:

• 60-70% radiant component
• 30-40% convective component

Radiant effectiveness depends on view factor between heater and target surfaces.

Performance Advantages

Comfort at Lower Air Temperatures Radiant heating allows occupied zone comfort with air temperature 2-4°F lower than convective systems:

$$PMV_{radiant} < PMV_{convective}$$

At equal mean radiant temperature (MRT) despite lower dry-bulb.

Zone Heating Effectiveness Directs heat to occupied areas rather than heating entire space volume. Effective for:

• Large rooms with high ceilings
• Intermittently occupied spaces
• Areas with high infiltration

Safety Features

Hot Surface Guards Protective grilles prevent contact with radiating elements:

• Temperature at guard surface: 150-200°F
• Minimum wire spacing: 0.5 inches
• Must withstand 25 lb point load

Tip-Over Protection Gravity-actuated or mercury switch kills gas flow when unit tilts beyond 15-20 degrees. Manual reset required after tip-over event.

Oxygen Depletion Sensor (ODS) Systems

Operating Principle

ODS uses thermocouple in room air stream to monitor oxygen concentration indirectly. Flame temperature correlates with oxygen availability:

$$T_{flame} = f([O_2])$$

As oxygen depletes from 20.9% to 18%, flame cooling reduces thermocouple millivoltage output.

Sensor Design

Thermocouple Configuration

• Position: In pilot flame, exposed to room air
• Type: Thermopile (multiple thermocouples in series)
• Hot junction: In pilot flame
• Cold junction: Ambient air

Response Characteristics Voltage output versus oxygen concentration:

• 20.9% O₂ (normal): 500-750 mV
• 19.5% O₂: 450-600 mV
• 18.0% O₂: 350-450 mV
• Gas valve dropout: <350 mV

Shutoff Response Time

ODS must shut off heater before oxygen drops to hazardous levels:

Required response: Shutoff between 18.0-19.5% oxygen Typical response: Shutoff at 18.5% oxygen

Time constant for sensor response:

$$\tau = \frac{m c_p}{h A}$$

Where:

• $\tau$ = thermal time constant (seconds)
• $m$ = thermocouple mass (kg)
• $c_p$ = specific heat (J/kg·K)
• $h$ = convection coefficient (W/m²·K)
• $A$ = surface area (m²)

Fast response required for small room applications: $\tau$ < 30 seconds

Testing and Verification

Annual testing procedure:

1. Install calibrated oxygen meter in room
2. Operate heater in sealed room of known volume
3. Monitor oxygen level and heater operation
4. Verify shutoff occurs between 18-19.5% oxygen
5. Test manual reset and restart capability

Room Volume and Ventilation Requirements

Minimum Room Volume

ANSI Z21.11.2 specifies minimum space volume:

$$V_{min} = \frac{Q_{input}}{d}$$

Where:

• $V_{min}$ = minimum room volume (ft³)
• $Q_{input}$ = heater input rate (BTU/hr)
• $d$ = factor based on heater type and room height

Standard Factors (per 1,000 BTU/hr)

For 30,000 BTU/hr heater in confined space with 8 ft ceiling: $V_{min} = 30 \times 100 = 3,000$ ft³

Infiltration Air Supply

Natural infiltration provides combustion air and dilutes combustion products. Required infiltration:

$$ACH_{min} = \frac{Q_{input} \cdot 21}{V \cdot 0.075 \cdot 1,100}$$

For 30,000 BTU/hr in 3,000 ft³ space: $ACH_{min} = 0.25$ air changes per hour

Tight construction may require supplemental ventilation openings.

Ventilation Openings

Two-Opening Method Each opening minimum 1 in² per 1,000 BTU/hr:

• One opening within 12 inches of ceiling
• One opening within 12 inches of floor
• Opens to outdoor air or unconfined space

For 30,000 BTU/hr: 30 in² per opening

Single-Opening Method One opening at floor or ceiling level:

• 1 in² per 3,000 BTU/hr
• Opens directly to outdoors

For 30,000 BTU/hr: 10 in² opening

Maximum Input Limitations

Code Restrictions

International Mechanical Code (IMC)

• Bedrooms: Unvented heaters prohibited
• Bathrooms: Prohibited
• Living spaces: 40,000 BTU/hr maximum per room

NFPA 54 (National Fuel Gas Code)

• Maximum single heater: 40,000 BTU/hr input
• Aggregate maximum per dwelling: Varies by jurisdiction

State and Local Codes Jurisdictions prohibiting or restricting unvented heaters:

• California: Prohibited in new construction
• Massachusetts: Prohibited
• Several Canadian provinces: Prohibited
• Many localities: Require special permits

Practical Application Limits

Even where code-compliant, consider limiting input to:

• 10,000 BTU/hr in small bedrooms (if allowed)
• 20,000 BTU/hr in typical rooms
• 30,000 BTU/hr in large, open spaces
• 40,000 BTU/hr only in very large, open areas with good natural ventilation

Safety Certifications and Standards

ANSI Standards

ANSI Z21.11.2 Unvented room heaters must comply with:

• ODS performance requirements
• Combustion efficiency standards
• Structural and thermal requirements
• Control system requirements
• Installation instruction requirements

Testing Requirements

• 100-hour endurance test
• CO production monitoring
• ODS shutoff verification
• Tip-over switch testing (portable models)
• Temperature rise measurements

Listing and Labeling

All unvented heaters must be:

• Listed by recognized testing laboratory (UL, CSA, ETL)
• Labeled with input rating and fuel type
• Provided with installation instructions
• Certified to current ANSI standards

Label must include warnings:

• “FOR USE ONLY IN UNCONFINED SPACES”
• Minimum room volume requirements
• Prohibition on use in bedrooms or bathrooms
• Carbon monoxide detector recommendation

Installation Best Practices

Location Selection

Avoid Locations With

• Poor air circulation
• Confined spaces below code minimums
• Proximity to bedrooms
• High moisture areas
• Chemical storage areas
• Heavy dust or lint generation

Preferred Locations

• Central to heated area
• Good natural air circulation
• Permanent outdoor air supply
• Away from ignition sources for flammable vapors

Combustion Air Contamination

Substances to Avoid

• Halogenated hydrocarbons (refrigerants, degreasers)
• Silicones (furniture polish, hairspray)
• Sulfur compounds (well water off-gassing)
• Excessive dust or lint
• Spray paint or adhesive fumes

Catalytic heaters particularly sensitive to air quality.

Carbon Monoxide Detection

Despite ODS protection, install CO detectors:

• Within 10 feet of unvented heater
• In each bedroom
• On each floor level
• Test monthly, replace per manufacturer schedule

Detector placement height:

• Wall-mounted: 5 feet above floor
• Ceiling-mounted: Minimum 12 inches from ceiling in dead air space

Maintenance and Troubleshooting

Annual Maintenance

Combustion Analysis Measure in room air near heater:

• CO level: <10 ppm during operation
• O₂ level: Should not drop below 19.5% in confined space
• Flame appearance: Blue with minimal yellow tipping

Component Inspection

• Burner ports: Clean, no blockage or corrosion
• Pilot assembly: Proper flame impingement on ODS
• Gas pressure: 3.5 in. w.c. natural gas, 11 in. w.c. propane
• ODS sensor: Clean, no deformation
• Catalytic pad: No contamination or physical damage (catalytic models)

Common Issues

Heater Shuts Off During Operation

• ODS activation (check room ventilation)
• Contaminated combustion air
• Low gas pressure
• Failed thermocouple/thermopile

Odor During Operation

• New heater break-in period (normal for first 4-6 hours)
• Dust on burner (clean)
• Incomplete combustion (adjust air shutter)
• Contaminated gas or air supply

Uneven Heating

• Undersized for space
• Poor air circulation (add fan)
• Excessive stratification (reduce ceiling height or improve mixing)

Regular maintenance and proper application ensures safe, efficient operation within equipment and code limitations.