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Duct cleaning - troubleshooting
A gas interlock that keeps shutting off the hobs is doing its safety job: it will only let the gas flow while it can prove the kitchen's extraction is running, and if it cannot, it cuts the gas. So repeated shut-offs mean the interlock cannot prove enough airflow - often because a grease-restricted duct or a failing fan has dropped the airflow below the threshold. Here is why your gas interlock shuts off the hobs, and what to check.
The short answer
A gas interlock is a safety system that links the kitchen's gas supply to its extraction, so that gas can only flow to the cooking appliances while the extraction is proven to be running. It proves the extraction using an air pressure switch fitted in the ductwork, which senses the airflow the extract fan produces; while the airflow is adequate, the switch tells the interlock the extraction is running and the gas is allowed to flow, but if the airflow drops below the switch's threshold, the switch signals that the extraction is not proven and the interlock closes the gas solenoid valve, cutting the gas - so the hobs go off. This is the interlock doing exactly its job: it will not let gas burn without proven extraction, because a kitchen burning gas without extraction could accumulate dangerous combustion products. So when the interlock keeps shutting off the hobs, it means it cannot prove enough airflow - the airflow it senses has dropped below the threshold, or the sensing itself has failed. The common causes are: the extract fan not running, running too slowly, or failing; the ductwork severely restricted - very often by grease build-up - so the airflow has fallen below the threshold even with the fan running; the make-up air inadequate, so the extraction is starved and the airflow is low; the pressure switch mis-set (threshold too high) or faulty; or a fault in the interlock wiring or controls. In a working kitchen, a grease-restricted duct dropping the airflow, or a failing or slow fan, are among the most common causes - and the grease-restriction one is cleanable. So the interlock is not malfunctioning when it shuts off the hobs; it is reporting that it cannot prove the extraction, and the thing to find is why the airflow it senses has dropped - starting with the fan and the ductwork.
Key points
What the interlock is doing
To understand why the interlock shuts off the hobs, start with what it is for. A gas interlock is a safety system required for commercial gas kitchens (under BS 6173), and its purpose is to ensure that gas cooking appliances cannot operate unless the kitchen's extraction is running. It does this by linking the gas supply to the extraction: gas is only allowed to flow to the appliances while the extraction is proven to be working, and if the extraction stops or falls short, the interlock cuts the gas. The reason is safety - a kitchen burning gas without adequate extraction could accumulate the products of combustion, including carbon monoxide, and could let heat, fumes and moisture build up dangerously - so the interlock makes sure the gas cannot burn without the extraction clearing what it produces. So the interlock shutting off the hobs is not a fault; it is the safety system working as designed, refusing to let gas flow when it cannot confirm the extraction is running.
This reframes the problem. When the interlock keeps shutting off the hobs, the interlock is not malfunctioning (usually) - it is correctly reporting that it cannot prove the extraction is running, and cutting the gas as it is meant to. So the question is not how do I stop the interlock cutting the gas but why can the interlock not prove the extraction is running, because that is the real problem the shut-offs are flagging. Trying to bypass or override the interlock to keep the hobs on would defeat the safety system and is dangerous and unlawful; the right response is to find and fix the reason the extraction is not being proven. So the interlock shutting off the hobs is a symptom pointing to a problem with the extraction or its sensing, and the way to stop the shut-offs is to fix that underlying problem - not to fight the interlock, which is doing its job. Understanding this is the key to responding correctly: the interlock is the messenger, and the message is the extraction is not proven.
How it proves the extraction
The interlock proves the extraction is running using an air pressure switch fitted in the ductwork, and understanding this is central to understanding the shut-offs. The pressure switch is a sensor in the duct that detects the airflow the extract fan produces - effectively sensing the pressure the moving air creates. While the extract fan is running and producing adequate airflow, the switch senses that airflow and tells the interlock the extraction is proven, so the gas is allowed to flow. If the airflow drops below the switch's set threshold - because the fan has stopped, slowed or failed, or the duct has become restricted - the switch no longer senses adequate airflow, so it signals that the extraction is not proven, and the interlock cuts the gas. So the whole system turns on the airflow the switch senses: enough airflow, gas allowed; too little, gas cut.
This means the interlock shutting off the hobs is, specifically, the pressure switch not sensing enough airflow. And that can happen for two broad reasons: either the airflow really has dropped (the fan or the duct), or the sensing has gone wrong (the switch or its setting). If the airflow really has dropped, the interlock is correctly cutting the gas because the extraction genuinely is not moving enough air - a real problem to fix. If the sensing has gone wrong (a faulty switch, or one set to too high a threshold so it trips even with adequate airflow), the interlock is cutting the gas because it wrongly believes the airflow is inadequate - a fault in the sensing to fix. So the pressure switch is the crux: the shut-offs happen because it is not sensing enough airflow, and the diagnosis is whether that is because the airflow really is too low, or because the switch or its setting is at fault. Both lead back to the same practical question - why is the switch not sensing adequate airflow - which the checks below address, starting with the most common causes.
The common causes
The common causes of an interlock shutting off the hobs, in roughly the order worth checking in a working kitchen, are as follows. First, the extract fan: if the fan is not running, running too slowly, or failing, it is not producing the airflow the switch needs to sense, so the interlock cuts the gas. A fan that trips, or has slowed (perhaps itself because of grease - see below), is a direct cause. Second, and very commonly, the ductwork restricted by grease: if the duct is severely clogged with grease, the airflow through it drops - even with the fan running - and can fall below the switch's threshold, so the switch cannot prove the extraction and the interlock cuts the gas. This is a common cause in a working kitchen precisely because grease build-up is what kitchens produce, and it is the cleanable one: clearing the grease-restricted duct restores the airflow, so the switch senses it again and the shut-offs stop. So a grease-clogged duct is a prime suspect, and one that cleaning fixes.
Third, inadequate make-up air: because the extraction can only move air as fast as make-up air comes in, a kitchen starved of make-up air has a low extraction airflow, which can fall below the switch's threshold - so a make-up air problem can cause interlock shut-offs by starving the airflow the switch senses. Fourth, the pressure switch itself: it may be mis-set (its threshold too high, so it trips even with adequate airflow), or faulty (failing to sense correctly), or its sensing tube blocked - any of which makes it fail to prove the extraction even when the airflow is fine. Fifth, a fault in the interlock wiring or control panel. So the causes range from the airflow genuinely being too low (fan, grease-restricted duct, make-up air) to the sensing being at fault (switch, wiring). In a working kitchen, the airflow causes - especially a grease-restricted duct and a failing or slow fan - are among the most common, which is why checking the fan and the ductwork comes first. The grease-restriction cause is both common and cleanable, so it is often where the answer lies, and where checking should start.
What to check, and who
The sensible order for tackling an interlock that keeps shutting off the hobs is to check the airflow causes first, then the interlock and switch. Start with the extract fan (is it running, at full speed, not tripping or failing?) and the ductwork (is it grease-restricted, reducing the airflow?), because these are the common causes in a working kitchen, and a grease-clogged duct in particular is both a frequent cause and one that cleaning fixes. If the duct is grease-restricted, clearing it restores the airflow, so the pressure switch senses it again and the interlock stops cutting the gas - a clean solving the problem. Checking and clearing the ductwork, and confirming the fan is running properly, is where to start because it addresses the most likely and most fixable causes, and cleaning the extract is needed for fire safety anyway.
If the fan is running well and the ductwork is clear but the interlock still shuts off the hobs, the cause is more likely the make-up air (starving the airflow), the pressure switch (mis-set, faulty, or its sensing blocked), or the interlock wiring and controls - and these need a competent person to diagnose: a gas engineer or the interlock installer for the switch and controls, and a ventilation engineer for the make-up air balance. Because the interlock is a gas-safety system, faults in it should be handled by qualified people, not bypassed or overridden - overriding the interlock to keep the hobs on defeats the safety system and is dangerous and unlawful. So the approach is: check and clear the fan and ductwork first (the common, cleanable airflow causes), and if that does not resolve it, get the make-up air, the switch and the interlock checked professionally. Never respond to repeated shut-offs by bypassing the interlock; respond by finding why the extraction is not being proven and fixing it - which, often enough, is a grease-restricted duct that cleaning puts right.
The takeaway
A gas interlock shuts off the hobs when it cannot prove the kitchen's extraction is running - which is its safety job, since gas must not burn without extraction. It proves the extraction with an air pressure switch in the ductwork that senses the fan's airflow; while the airflow is adequate the gas is allowed to flow, but if the airflow drops below the switch's threshold, the switch cannot prove the extraction and the interlock cuts the gas. So repeated shut-offs mean the interlock cannot sense enough airflow - either because the airflow really has dropped, or because the sensing has failed. The interlock is not malfunctioning; it is reporting that the extraction is not proven.
The common causes, worth checking in this order, are: the extract fan not running, slow or failing; the ductwork severely restricted by grease, dropping the airflow below the threshold even with the fan running (a common cause in a working kitchen, and the cleanable one); inadequate make-up air starving the extraction airflow; a mis-set or faulty pressure switch; and a fault in the interlock wiring or controls. So check the fan and the ductwork first - a grease-restricted duct is a frequent cause, and clearing it restores the airflow so the switch senses it again and the shut-offs stop. If the fan and duct are fine, get the make-up air, switch and interlock checked by a qualified person. Never bypass the interlock to keep the hobs on - that defeats a gas-safety system and is dangerous and unlawful. Find why the extraction is not being proven, and fix it - often, that is a grease-clogged duct that cleaning puts right.
Questions
Usually not - more often it is correctly doing its job. The interlock only lets gas flow while it can prove the extraction is running, and it cuts the gas when it cannot. So repeated shut-offs usually mean it genuinely cannot prove enough airflow - because the airflow has dropped (a stopped or slow fan, a grease-restricted duct, inadequate make-up air) rather than because the interlock itself is broken. The interlock is the messenger reporting that the extraction is not proven. Occasionally the fault is in the interlock's own sensing (a mis-set or faulty pressure switch, or a wiring fault), which does need fixing - but even then the interlock is not malfunctioning in cutting the gas; it is acting on a sensing fault. So treat the shut-offs as a signal about the extraction, not an assumption that the interlock is broken.
Yes - it is one of the common causes in a working kitchen. The interlock proves the extraction with a pressure switch that senses the airflow in the ductwork; if the duct is severely restricted by grease build-up, the airflow through it drops, even with the fan running, and can fall below the switch's threshold. When that happens, the switch can no longer prove the extraction, so the interlock cuts the gas and the hobs go off. This is common because grease build-up is exactly what a working kitchen produces, and it is the cleanable cause: clearing the grease-restricted duct restores the airflow, so the switch senses it again and the shut-offs stop. So a dirty duct is a prime suspect, and one that duct cleaning fixes - which is why checking and clearing the ductwork is a sensible first step.
No - never. The interlock is a gas-safety system that ensures the gas cannot burn without proven extraction, precisely because a kitchen burning gas without extraction could accumulate dangerous combustion products, including carbon monoxide. Bypassing or overriding it to keep the hobs on defeats that protection and is dangerous and unlawful. If the interlock keeps cutting the gas, it is telling you the extraction is not being proven - which is a real problem to fix, not an inconvenience to override. The right response is to find why the extraction is not proven (check the fan and ductwork, then the make-up air, switch and interlock) and fix it. Bypassing the interlock removes the safety it provides at exactly the moment it is telling you something is wrong, which is the worst time to do so.
Check the airflow causes first, because they are the most common in a working kitchen and include the cleanable one. Start with the extract fan - is it running, at full speed, not tripping or failing? A stopped or slow fan does not produce the airflow the interlock needs to sense. Then the ductwork - is it restricted by grease, dropping the airflow below the switch's threshold? A grease-clogged duct is a frequent cause, and clearing it restores the airflow so the interlock can prove the extraction again. So checking and clearing the fan and ductwork addresses the most likely and most fixable causes, and cleaning the extract is needed for fire safety anyway. If the fan and duct are fine, the make-up air, pressure switch and interlock wiring are the next things to check, by a qualified person.
A duct cleaner (and a check of the fan) addresses the common airflow causes: a grease-restricted duct dropping the airflow, which cleaning fixes, and confirming the fan is running properly. So if the shut-offs are caused by a grease-clogged duct or a fan issue, cleaning and fan attention resolve them. If the fan is running well and the ductwork is clear but the interlock still cuts the gas, the cause is more likely the make-up air balance, the pressure switch (mis-set, faulty, or its sensing blocked), or the interlock wiring and controls - and these need a qualified gas engineer or the interlock installer, because the interlock is a gas-safety system. So start with the duct cleaner and fan for the common cleanable causes; bring in the gas engineer for the interlock, switch and controls if the airflow is confirmed adequate but the shut-offs continue.
Yes. The extraction can only move air as fast as make-up air comes in, so a kitchen starved of make-up air has a reduced extraction airflow - and if that airflow falls below the pressure switch's threshold, the switch cannot prove the extraction and the interlock cuts the gas. So a make-up air problem can cause shut-offs by starving the airflow the switch senses, even if the fan is running and the duct is clear. This is why, if the fan and ductwork check out, the make-up air balance is worth investigating: the extraction and make-up air are a balanced pair, and a shortfall on the supply side weakens the extraction airflow just as a restricted duct does. A ventilation engineer can check whether the make-up air is adequate and balanced to the extract, which may be the cause if the more obvious airflow problems are ruled out.
A grease-restricted duct dropping the airflow is a common, cleanable reason a gas interlock cuts the hobs - we clear the ductwork so the interlock can prove the extraction again. Ask us to check yours.
