Kitchen Exhaust After Heavy Rain: Water at the Hood, a Tripped Fan

A commercial kitchen is sealed against the weather everywhere except one place: the exhaust. The duct runs from the hood straight out through a roof, a facade or a riser, and at the far end it is open to the sky. The fan that pulls the air is often out there too, on a roof or a bracket on an external wall, next to its isolator switch. So when a heavy storm comes through, the kitchen exhaust is the part of the kitchen that meets it first.

Rain usually shows up in one of two ways, often the morning after a storm: water where there should not be any, dripping from the hood or staining a ceiling tile, or a fan that has tripped and will not stay on. Both are worth understanding before anyone starts replacing parts, because the colour of the water and the way the fan fails tell you most of what you need to know.

First: water and electrics, so be careful

• Reset a tripped breaker once, at most. If it trips again, leave it off. A fan that trips after rain may have water where the power connects to it, and repeated resets are how a motor that only needed drying out becomes one that needs replacing.
• Do not open a wet isolator or control panel, and do not open the fan housing with the system live.
• Do not send kitchen staff onto a wet roof to look at the fan. A photograph from a safe spot is enough for now.
• If water is running near a light fitting or anything electrical in the ceiling, switch that circuit off at the board and keep people clear.
• Cut the menu if the fan is off. A line with no extraction fills with smoke and heat quickly.

The general order for any ventilation fault is in commercial kitchen ventilation troubleshooting. What follows is what is different when the weather is the thing that changed.

How rain gets into a kitchen exhaust

While the fan runs, air is leaving the discharge fast. Once the kitchen closes and the fan is switched off, the duct is simply a pipe open to the sky, and a Singapore downpour at night has hours to find its way in. The usual routes are:

• The discharge itself. A terminal with no rain protection, or one whose cap or cowl has corroded, come loose or been knocked off. Getting this right is a design question: a cap that throws the exhaust back down onto the roof keeps rain out but defeats the height the discharge was given, which is its own source of smell complaints. How the discharge should be arranged is covered in where your kitchen exhaust actually discharges.
• A side-facing louvre in the path of wind-driven rain. A facade discharge that stays dry in light rain can take water in a storm blowing straight at it.
• Joints and connections outdoors. A duct seam on the roof, or the flexible connection between the fan and the duct, that has split or lost its seal lets water in every time it rains.
• The penetration. Where the duct passes through the roof or an external wall, the seal around it can fail. Water then runs down the outside of the duct into the ceiling, which looks quite different from water that came down the inside.

Read the water before you call anyone

Where the water appears, and what it looks like, points to the route.

Brown, greasy water dripping from the hood, the filters or the grease channel after rain. Rain came down the inside of the duct, picking up grease on the way. The entry point is somewhere upstream: the discharge, a roof joint or the fan connection. It is also a sign there is grease in the duct for the water to carry, which is worth knowing on its own.

Clean water, or a tide-mark stain on a ceiling tile beside the duct rather than under the hood. Water came down the outside of the duct, most likely through the seal where it passes through the roof or wall. That is a waterproofing problem at the penetration. Depending on your lease, fixing it may involve your landlord or building management as well as whoever maintains the exhaust, so find out who owns that seal before the next storm.

Water or a brown stain at one point along the duct run, well away from the hood. Water is collecting at a low point in the duct and escaping through a joint there. A horizontal run should have a slight fall back to a drain point; a run that dips around a beam holds whatever gets in. Why that happens is explained in what your kitchen exhaust duct is made of, and why it leaks grease.

Dripping that follows cooking, not the weather. If the drip is worst at the end of a busy service and the weather has been dry, it is more likely condensate and liquefied grease than rain. The test is simple: note the date and time every time it happens, and compare against when it rained. Two weeks of notes usually settle the argument.

When the fan trips after a storm

An outdoor fan has electrical connections exposed to the weather: the motor's terminal box, the cable entry and the isolator beside the fan. Each is sealed against water, and each seal ages in the sun. When one fails, water reaches a live connection and the protection trips, which is exactly what it is there to do.

The pattern is what gives it away. A fan that trips during or just after heavy rain, and then runs normally once things have dried out, has not fixed itself. The water is still getting in each time it rains, and every wet trip leaves moisture and corrosion behind. It tends to get worse with each storm, until the day it does not come back on at all, usually in the middle of service.

So even when the fan is running again the next morning, report it. A technician should look at the isolator, the cable entry and the motor terminal box for water and corrosion, and check the motor and its connections are dry and sound before it is relied on. If the isolator enclosure or the seal is the problem, that is often a small repair. Left alone, it can become a motor.

What water does inside a greasy duct

The photograph at the top is from our project portfolio: the inside of an exhaust duct section on one of our jobs, with rust and dark staining along the seams and the floor of the duct. Kitchen ductwork carries hot, moist air every day, so it is built to cope with some moisture. What it copes with badly is water that sits: at a seam, at a low point, against a layer of grease that holds it there.

That is where corrosion starts, and a corroded seam eventually leaks, first water and then grease. A duct that has been taking in rain for a few years can end up needing sections replaced, not just a clean. It is far cheaper to stop the water at the discharge or the roof joint than to replace the duct it has been sitting in.

A check worth doing before the wet months

Singapore gets heavy thunderstorms all year, but the inter-monsoon months, which include October and November, are known for heavy afternoon and evening storms, and the Northeast Monsoon that follows brings spells of prolonged heavy rain from December. If your exhaust has never been looked at with the weather in mind, the next few weeks are a good time. You do not need to climb anything to start:

1. Know where your discharge and fan actually are, and who holds the key to reach them. If nobody knows, that is the first thing to find out.
2. Ask for photographs of the discharge, the fan and its isolator at your next service visit, and keep them. A cap that is missing, a louvre facing the weather, an isolator cover that is cracked or taped shut: all show clearly in a photo.
3. Look at the ceiling tiles along the duct route for old tide marks. A stain that is already there tells you where the next leak will be.
4. Check the grease cup the morning after a storm. If it is fuller or more watery than after a normal night, water is getting in upstream.
5. Keep a simple log: date, weather, where water appeared, whether the fan tripped, and how many times it was reset.

A scheduled maintenance program is the natural place for these checks. Mechanical checks on motors, bearings, belts and control panels are already part of it, and a documented report with photographs means a weathered seal is caught while it is still a small job.

What to send when you call

• When it happened, and whether it followed heavy rain.
• Where the water appeared, and whether it was brown and greasy or clean.
• Whether the fan tripped, and how many times it was reset.
• Where the fan sits (roof, facade, riser) and who holds access.
• Photographs of the water, the ceiling and, if it can be done safely, the discharge and the fan.
• Whether the kitchen is still cooking.

That lets the first visit arrive with the right parts. We repair and replace motors, MV fans and fan parts, control panels and starters, and ducting and hoods, whoever installed the system, and because we keep motors, fans and parts in stock, a common failure is often fixed on the first visit. If the fan has simply stopped and there has been no rain, start with kitchen exhaust fan not working instead.

The short version

• The exhaust is the one part of the kitchen open to the weather, at the discharge and often at the fan.
• Brown greasy water at the hood means rain came down the inside of the duct. Clean water beside the duct means it came down the outside, through the roof or wall seal.
• A fan that trips in the rain and recovers when dry is a seal problem that gets worse. Reset once at most, and report it.
• Water that sits in a greasy duct starts corrosion at the seams. Stop it at the source.
• Photograph the discharge, fan and isolator before the wet months, and keep a log of every wet-weather drip or trip.

For a kitchen exhaust repair after a storm, or to have the discharge, fan and roof connections checked before the wet months, WhatsApp us on 8989 2833 with what happened, where your fan sits and whether the kitchen is still cooking. We are on 24/7 standby for breakdowns. Scope and pricing follow an inspection.