GIS 22: Gas heating in hothouses and carbon dioxide dosing

Purpose

This information sheet provides guidelines about the controls required for safely working in hothouses where gas combustion products are present.

Background

A gas installation that is part of a hothouse is deemed to be a complex gas installation under the Gas Safety Act 1997.

An application for acceptance of complex gas installations and Type B appliances must be submitted to Energy Safe Victoria through GasTrac prior to gas supply being approved.

CO₂ dosing and exposure to combustion products

Raising the percentage of carbon dioxide (CO₂) in a hothouse can accelerate plant growth. This is called CO₂ dosing.

Hothouses are generally heated by in-ground hot water coils from a gas-fired water heater. Occasionally, direct-fired air heaters (where fanned air is directly heated by the burner flame) are used to raise both the hothouse’s temperature and CO₂ levels, with the CO₂ coming from the appliance’s combustion products.

Hothouses that use combustion products for CO₂ dosing must be adequately ventilated. Without sufficient fresh air, the gas appliance will produce more carbon monoxide (CO), which can be extremely hazardous.

Further information on safe working levels for carbon monoxide is also available in Gas Information Sheet 44.

Guidelines

Acceptance requirements for CO₂ dosing

There are different acceptance requirements which we oversee for CO₂ dosing.

CO₂ dosing using gas-fired water heaters

General acceptance requirements for the controlled release of combustion products into a hothouse using a gas-fired water heater (in-ground heating) are as follows:

• A hazard/risk assessment must be undertaken and mitigations planned.
  • Assessments should include an adequate Job Safety Analysis (JSA), Safe Work Method Statements (SWMS) and/or equivalent proposed emergency procedures which include adequate means of personal access and egress from the hothouse, and emergency communications.
  • Consideration should be given to the type and quantity of CO and CO₂ detectors to be used, their location, serviceable life and calibration requirements. Appropriate control systems are necessary to maintain a safe operating system.
• The appliance is required to pre-purge to atmosphere (outside the hothouse) prior to start-up of the appliance burner. Dampers in the appliance or flue controlling this function must be:
  • interlocked
  • in the correct position before pre-purging begins.
• The appliance’s main flame must be ‘proven’ (established and stable) before any combustion gases are allowed to enter the hothouse.
• The CO level in the appliance flue must not exceed 400 ppm.
• The environment within the hothouse must be continually monitored as part of the CO2 dosing system and the monitoring system must be interlocked to the appliance.
• The appliance must shutdown or its combustion products be directed to atmosphere if the hothouse levels reach:
  • 10 ppm CO, or
  • 3000 ppm CO₂.
• Automated ventilation openings in the hothouse must:
  • be interlocked with the gas control system
  • open to introduce fresh air when high CO levels are detected in the hothouse.

CO₂ dosing using direct-fired or mobile industrial heaters

Direct-fired heaters

General acceptance requirements for the controlled release of combustion products into a hothouse using a direct-fired appliance are as follows:

• the air supply must be taken from outside the building as per AS 3814, clause 5.7.2.
• the combustion product limits must comply with AS 3814, clause 5.7.7.1.
• the maximum gas consumption per cubic metre of room volume in a hothouse must not exceed the limits listed in AS/NZS 5601.1, clause 6.10.6.3, Table 6.10.6.3.

Mobile industrial heaters

Mobile industrial heaters are designed for industrial heating applications, not for CO₂ dosing. They are not recommended for use.

Mobile industrial heaters located indoors recirculate combustion products. They may create a hazardous environment without adequate ventilation.