A gas ventilation interlock is a simple idea with serious consequences. If the air a gas appliance depends on stops, the gas stops too. When interlocks are missing, bypassed or faulty, the result can be a build-up of carbon monoxide or unburnt gas in a plant room, an oven or a kitchen.This guide explains what an interlock does, where you'll find one, which standards apply, how it should be tested, and why "CP16 regulations" is a common mix-up.
An interlock links the gas supply to the ventilation system. A sensor proves that the fan is running and air is moving, usually an air pressure or airflow switch, and sometimes a fan current monitor. Only then does it allow an automatic safety shut-off valve on the gas supply to open. If the airflow fails, the valve closes and the appliances shut down. The gas should not come back on by itself when the fan restarts. Someone has to reset it deliberately, once the cause has been checked.Many interlock panels also connect to the fire alarm, so the gas shuts off when the alarm sounds. Some include a valve proving or pressure proving function, which checks the pipework for leaks before the gas valve is allowed to open.
Larger boiler installations often rely on fans to bring in combustion air and remove heat. Where mechanical ventilation is used, the boiler installation standards (BS 6644 for hot water boilers up to 1.8 MW net input, and IGEM/UP/10 for flued appliances in industrial and commercial premises) require the burners to be interlocked with the fans, so they shut down if the airflow fails.
Process plant frequently depends on exhaust and supply fans, both to support combustion and to remove fumes or solvent vapour. On ovens and spray booths, the burner control system should prove the airflow and purge the chamber before ignition, and shut down if airflow is lost. These safety functions are part of the equipment's design, and they need to be tested when the burner is serviced.
Gas catering equipment under an extract canopy must be interlocked with the ventilation. The standard is IGEM/UP/19, read alongside BS 6173. On new installations this often includes monitoring the carbon dioxide level in the kitchen. If your site has a gas kitchen or canteen, the interlock is checked as part of the CP42 catering inspection, which we can issue.
You'll sometimes see interlocks described as "CP16 regulations". That's a mix-up. A CP16 is the certificate for gas testing and purging, which records that pipework has been tightness tested and purged. It does become relevant when an interlock is installed or altered, because fitting a safety shut-off valve means breaking into the gas pipework, and the installation then has to be tested and purged and a CP16 issued. The interlock requirements themselves come from the standards above and the manufacturer's instructions.
An interlock that has never been tested may not work when it's needed. A proper test includes:
Interlocks should be tested whenever the appliances they protect are serviced, and after any change to the ventilation or the gas installation.
The faults we see most often are blocked filters and dirty fans reducing airflow, air pressure switches that have drifted out of adjustment, damaged sensing tubes, solenoid valves that stick, and panels that have been bypassed after nuisance tripping. Most can be fixed quickly once they are found. The danger lies in not knowing about them.
No. Interlocks are needed where an appliance depends on mechanical ventilation or extraction for its safe operation. Where ventilation is natural and meets the standard, there may be nothing to interlock. An engineer can assess your site.
Yes. Older systems can usually be upgraded to current standards. Any work on the gas pipework is followed by a tightness test and purge.
At least whenever the protected appliances are serviced, which for most sites is once a year, and after any change to the ventilation or the gas supply.
Yes. We test airflow proving and shutdown as part of process burner servicing on ovens, dryers and spray booths.
If you aren't sure whether your interlocks work, or whether you need one, get in touch.SMR Gas Maintenance Ltd
Unit 10 Higher Walton Mill, Cann Bridge Street, Higher Walton, Preston PR5 4DJ
📞 07882 898492 (24/7 commercial emergency call-out)
📧 andy@smrgasservices.co.uk
🌐 www.smrgas.co.uk
Gas Safe Register No. 562186. Industrial and commercial work only.