Mould in HVAC Ducts: Signs, Causes and What to Do

Mould in HVAC ducts almost always means moisture is getting into the system from somewhere. Cleaning the ducts without finding that source usually fails, because the growth comes back. If you see dark growth inside a duct or at a register, smell a musty odour when the furnace or air conditioner runs, or have had a leak or flood near your ductwork, the first job is to find the water. The second is to decide how much of the system is affected. This guide covers both, in the order you will face them in an Ottawa home.

TL;DR

• Mould needs moisture. Fix the leak, condensation or humidity source first, or the mould will return after any cleaning.
• A musty smell, visible spots at registers and unexplained respiratory symptoms are warning signs, but none of them confirms mould inside the ducts on its own.
• Health Canada does not recommend routine mould testing. A visual inspection and a plan to fix the moisture are worth more than a lab report.
• Do not let anyone fog or spray biocides into your ducts. Mechanical removal is the accepted approach, and badly affected duct sections are often cheaper to replace than clean.
• Large areas of growth (more than 3 square metres) call for trained professionals with containment.

Table of Contents

• Why mould grows in ductwork
• Signs of mould in air ducts
• Do you need mould testing for ducts?
• Cleaning, replacing or leaving it alone
• What professional duct mould remediation involves
• What it costs in Ottawa
• Keeping mould out of your ducts
• Our perspective: treat the building, not just the duct
• Need help in Ottawa?
• FAQ
• Sources

Why mould grows in ductwork

Mould spores are part of normal indoor and outdoor air. They only become a problem when they land on something damp and stay there. Health Canada’s guide to moisture and mould says mould will typically grow indoors where moisture is present, from everyday sources such as showering, laundry and cooking, as well as leaks and floods.

In a duct system, the moisture usually arrives in one of these ways:

• Condensation on the air conditioner coil or drain pan. Cooling air produces water. A clogged drain line, a pan that does not drain or poorly insulated supply ducts in a hot attic or basement can leave surfaces wet.
• Humidifier problems. A furnace-mounted humidifier set too high, or one that leaks or drips into the plenum, wets the inside of the ductwork in winter.
• Water damage near the ducts. A basement flood, a burst pipe, a roof leak into an attic run or a wet crawl space can soak duct insulation or open seams.
• Leaky ducts. Gaps pull humid air from unconditioned spaces into the system, where it condenses on cold metal.
• Overall high humidity. Poor bathroom and kitchen ventilation, an oversized air conditioner that cools quickly without removing moisture, or a damp basement raise the humidity the whole house sees.

Duct metal itself does not feed mould. Dust, lint, internal fibreglass lining and duct board do. That is why the growth tends to start in lined sections, at the coil and at joints where dust and moisture meet.

For a look at how persistent leaks and high humidity cause mould in residential heating and cooling systems in a very humid climate, a home inspection company in coastal Alabama explains how humidity drives mould risk. Ottawa’s climate is different, with cold, dry winters and humid summers, but the mechanism is the same.

Signs of mould in air ducts

People search for symptoms first, so here is what is worth paying attention to, and how much each sign tells you.

The health side needs some care. According to Ontario’s guidance on mould in workplace buildings, the most common moulds are generally not hazardous to healthy people, but they can cause symptoms such as coughing, congestion and asthma aggravation in people with respiratory conditions. If someone in your home has asthma, allergies or a weakened immune system and symptoms ease when they leave the house, talk to your family doctor. Do not diagnose the ducts from symptoms alone.

How to look without making it worse

1. Turn the system off at the thermostat before you open anything.
2. Remove a register cover and shine a flashlight into the duct. Look for fuzzy or spotty growth, wet insulation, water stains and rust.
3. Look at the furnace cabinet, the coil and the drain pan if you can reach them safely. Check any humidifier for leaks or mineral buildup.
4. Photograph what you find, with a ruler or tape measure in frame for scale.
5. Stop if you see growth that looks larger than about a sheet of plywood, or if the material is wet and crumbling.

Running the fan to see whether the smell gets worse is fine. Poking at or vacuuming visible growth with a household vacuum is not. It spreads spores through the house.

Do you need mould testing for ducts?

Usually not. Health Canada does not recommend routine testing for mould, and says it is rarely of value in building a remediation plan. If you can see mould or you know about a moisture problem, you already have the information that matters. Finding and fixing the water is the only way to prevent it from coming back.

Air sampling is even weaker as a verdict. Health Canada’s guidance for indoor air quality professionals does not recommend air sampling alone to judge whether remediation worked. A visual check that surfaces are clean of dust and debris is the more reliable test, often called a white glove test. Ask your contractor to do it before they close up any containment.

There are cases where testing helps: an insurance claim, a dispute over a real estate sale, a workplace with occupants who are reporting illness, or a need for a written record that a job was completed. In those cases it supports a decision. It does not replace one. If you want to understand what a clearance check involves, our mould clearance test guide explains what to expect.

Cleaning, replacing or leaving it alone

There is a lot of marketing around duct cleaning, and it helps to know what independent Canadian sources say.

CMHC’s About Your House publication on duct cleaning notes that duct cleaning does not significantly improve air quality or cut heating costs. It can make sense after major renovation work when construction dust has gotten into the system, or when there is a blockage that restricts furnace airflow. The same publication points out that no products are registered under the Pest Control Products Act for residential duct cleaning that effectively remove mould, and that biocide fogging or spraying is not appropriate. If a contractor offers it, walk away.

The Canadian Centre for Occupational Health and Safety (CCOHS) gives the working rules for mould in HVAC systems:

• Turn the system off and let it dry completely before cleaning begins.
• Clean mechanically with HEPA vacuuming and detergent, not chemical disinfectants.
• Avoid bleach as a remedy, since it can create its own health concerns for occupants.

So which option fits which situation?

CMHC also notes that, in many cases, replacing duct sections affected by significant mould costs less than trying to clean them. That matches how we see it in practice. Flexible duct and lined sections are cheap compared with a second round of remediation.

The 1 and 3 square metre guideline

CCOHS describes small, isolated contamination of less than 1 square metre as something a homeowner can often clean with soap, water and detergent. Large-scale contamination, more than 3 square metres, should be handled by trained professionals who use dust suppression and containment so spores do not spread. For large areas that includes full-face HEPA respirators, protective clothing and negative pressure created with HEPA-filtered exhaust fans.

Ducts complicate that guideline, because mould inside a duct can be moving through the whole house while the visible patch is small. If your system has been running with mould in it, or if you find it in more than one place, treat it as bigger than it looks. Our article on why large mould jobs call for remediation rather than removal explains the difference in approach.

What professional duct mould remediation involves

A well-run job follows a sequence. If a contractor skips steps, that is a red flag.

1. Find and document the moisture source. Roof, plumbing, drain line, humidifier, ductwork leaks, basement humidity. Without this the rest is temporary.
2. Shut down and isolate the system. The furnace or air conditioner stays off, registers are sealed, and the work area is contained.
3. Control dust and use negative pressure. HEPA-filtered air machines keep spores from drifting into living areas.
4. Remove what cannot be cleaned. Wet or mouldy insulation, lined duct sections, filters and porous material are bagged and taken out.
5. Clean what can be cleaned mechanically. HEPA vacuuming and detergent wiping of metal surfaces. No fogging.
6. Dry and repair. Seal leaky joints, fix the drain, correct the humidifier, insulate cold surfaces.
7. Verify and document. A visual check for remaining dust or debris, with photographs and a written record.

Basements are a common place for ducts to pick up moisture. If yours had a leak or flood, our basement mould remediation guide covers the building side. If the ducts run through an attic, see attic mould remediation. Building managers weighing wider system risks can read our guide to HVAC contamination risk.

What it costs in Ottawa

Duct mould is rarely a standalone price, because the cost depends on how much of the system and building is affected and on how the moisture source is repaired. Here are the published ranges for our related mould work, which give you a sense of scale. Your own quote depends on the site.

Sections of duct replaced by an HVAC contractor, a new drain line or humidifier repair are separate trades and are priced separately. If a flood or leak is behind the mould, check your policy early. Our article on when home insurance covers mould damage explains what to ask.

Keeping mould out of your ducts

Prevention is moisture control, and most of it is cheap.

• Keep indoor humidity in range. Health Canada guidance points to relative humidity between 30 and 55% to discourage mould. CCOHS notes that in winter indoor humidity should be lower, around 20 to 40%, which matters in Ottawa when it is very cold outside and windows start to fog.
• Use bathroom and kitchen exhaust fans and make sure they vent outdoors.
• Change furnace filters regularly. Government guidance for occupants recommends changing furnace filters every 3 months, along with HEPA vacuuming and humidity control, as the simpler route to better air than duct cleaning.
• Check the air conditioner drain at the start of every cooling season, and look at the coil and pan.
• Seal and insulate ducts that run through attics, crawl spaces and unfinished basements.
• Fix leaks right away. Mould will typically grow where moisture is present, so a fast response matters. See Health Canada’s advice on reducing humidity and mould.
• Choose moisture-resistant materials when you renovate. Our piece on mould-resistant renovation materials goes through the options.

Our perspective: treat the building, not just the duct

We have spent more than 25 years working in Ottawa homes, and the pattern is consistent. When ducts have mould, the duct is the symptom. The cause is a drain that backed up, a humidifier set too high, a leaking roof or a damp basement that nobody connected to the furnace.

That is why we start with the moisture source and write it into the quote. We prefer itemized, line-by-line quotes so you can see what is containment, what is removal, what is cleaning and what is repair. We do not fog ducts with biocides, and we would not recommend anyone who does. Where a section of duct is badly affected, we will tell you plainly that replacement is the better value, and then coordinate the repair so the job is closed up properly with a clean record of what was done.

Since we also handle abatement, demolition and renovation, we can deal with the whole problem in one visit when it goes beyond the ducts. That means wet drywall behind a register boot, insulation in a flooded basement or an older home where the mould job meets a bigger renovation. Often those jobs also need testing for asbestos in old materials before anything is disturbed, and we are licensed to do that work too.

If your ducts look fine and you simply want reassurance, we will say so. Not every musty smell needs a contractor.

— Jason

Need help in Ottawa?

If you have found growth in your ducts, or a flood or leak has affected your system, our mould remediation service covers assessment, containment, removal and documentation across Ottawa and the surrounding area. We offer free estimates, and estimates come back within 24 hours. You can also get in touch through our main site if you are not sure whether your situation needs remediation or an HVAC technician first.

Property owners with a workplace or commercial building have additional duties. Under Ontario’s Occupational Health and Safety Act, employers must take every reasonable precaution to protect workers, and that includes mould hazards.

FAQ

Is it normal to have mould in AC ducts?

It is common enough that it should not alarm you, but it is not something to ignore. Mould spores exist in all indoor air, so small amounts of dust-borne spores in a duct are expected. Active growth means moisture is present, and that needs to be found and fixed.

What kills mould in air ducts?

Removing the moisture and physically removing the mould. CCOHS recommends HEPA vacuuming and detergent over biocides and bleach, and CMHC says no duct-cleaning biocides are registered in Canada for effectively removing mould. Porous or lined duct sections that are mouldy are usually better replaced.

Can you clean mould out of an HVAC system?

Sometimes. Smooth metal surfaces can be cleaned mechanically after the system has been turned off and fully dried. Lined ducts, duct board and wet insulation generally cannot be cleaned reliably, and large contamination should go to trained professionals with containment.

Do I need to test my ducts for mould?

Usually not. Health Canada does not recommend routine mould testing, and says that testing is rarely useful for planning remediation. Testing can still help with an insurance claim, a sale dispute or a record of completed work.

What are the symptoms of mould in air ducts?

The common signs are a musty smell when the system runs, visible spots at registers and water stains near the furnace or coil. Some people with asthma or allergies notice coughing or congestion at home. These symptoms are not specific to mould, so speak with a doctor about health concerns and a qualified contractor about the ducts.

Sources

• Guide to addressing moisture and mould indoors, Health Canada
• Reducing humidity, moisture and mould, Health Canada
• Guidance for indoor air quality professionals, Health Canada
• Indoor air quality: moulds and fungi, CCOHS
• Mould guide, CCOHS
• About your house: duct cleaning, CMHC
• Occupant handbook: indoor air quality, Government of Canada
• Alert: mould in workplace buildings, Ontario

Recommended

• HVAC contamination risk: a building manager’s guide
• Attic mould remediation: what to do and when to call a pro
• Cut Mold Recurrence: Mold Resistant Materials Help Ottawa Homes Dry
• Mould Remediation in Ottawa: A Homeowner’s Guide