4 reasons there is mold in vents across your home – from excess moisture to incorrect thermostat settings, here's what to sort out first

While common, mold anywhere around your home is a serious issue which calls for urgent attention. Not only is it damaging to your space, it's also dangerous for your health, causing allergic reactions, respiratory issues and allergies.

If you've found mold growing in vents across your home, our HVAC pros are on hand to help you resolve the causes, and how to clear it.

Being up to speed with any bad habits that are increasing the risk of mold in your home will also help you stop it from returning and affecting the air quality of your home.

4 reasons there is mold growing in vents across your home

1. Your thermostat settings are wrong

According to Danny Pen, president of New Era Plumbing & HVAC, common thermostat mistakes could be leading to mold in your vents.

He explains, 'An unexpected thing that could encourage mold on vents is your thermostat setting. If your fan is set to "On" instead of "Auto", it runs continuously, even when the AC isn't cooling. This means moisture lingers in the ducts instead of being drained away.'

Therefore, Danny recommends always setting the fan to Auto, so it cycles moisture out, rather than trapping it inside. This should avoid any common thermostat problems, while effectively helping to get rid of mold and mildew.

2. Moisture and condensation

It's no secret that with moisture comes mold, which could be the reason you need to clean mold in your air vents.

David Lewis, owner of Mission AC & Plumbing, advises, 'Mold is usually caused by condensation happening inside the HVAC unit. If your system isn't insulated, or if humidity levels are high, it's a perfect breeding ground for mold.'

As David highlights, investing in one of the best dehumidifiers, such as the Absoke Dehumidifier available at Walmart, is a brilliant way to avoid mold growing on your air vents.

'Leaks in duct work can actually pull in humid air, which will make mold worse,' he adds. 'Duct sealing services can help resolve this.'

If you'd rather not call in a pro, you can also fix these leaks yourself, using duct seal such as the Rectorseal Duct Seal Compound available at Amazon, which is a maintenance task that will lower your energy bills, too.

Nick Barber, manager at Utilities Now, explains, 'Too much moisture in your HVAC system is not only a health problem – it's an energy efficiency problem. Mold growth restricts airflow, which makes your heating and cooling system work harder,' driving up rather than cutting your energy bills.

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3. Your air filter is too advanced

This one may sound counterintuitive, but HVAC expert Danny says that homeowners, particularly those who are very concerned about improving indoor air quality and preventing mold, are often surprised to learn that their air filter might be too good.

He says, 'HEPA and high-MERV filters are great, but if your system isn’t designed for them, they can restrict airflow, creating moisture buildup,' and leading to mold spreading through your vents.

Therefore, if you use a MERV 13+ filter, which is the highest MERV value safe for residential homes, Danny recommends checking with your HVAC tech to ensure your system can handle it.

'Otherwise, a MERV 8-11 might be a better balance between filtration and airflow,' he adds.

An HVAC tech will be able to advise you on whether you should make the switch, as well as remove and re-install your filters where necessary.

4. Your ducts are too large

Another thing Danny recommends checking if you're spotting signs of mold and mildew in your home is checking your duct size, as one that is too large may be the reason for mold on your vents.

He says, 'An over-sized duct makes air move too slowly, leaving moisture behind. This is especially common in older homes, or ones with recent HVAC replacements.'

If you're not sure whether this is the issue you're encountering, Danny suggests asking an HVAC pro to check the airflow in your home with a static pressure test. This will determine whether resizing the ducts will improve air circulation, and therefore prevent mold from growing in your vents.

FAQs

Does heating prevent mold?

While heating can help to prevent mold growth, by reducing dampness and moisture, it's usually not enough on its own, but rather should be used in conjunction with other measures.

When preventing mold growth even when leaving a property unheated, taking steps to winterize the plumbing system, improve ventilation and insulate doors and windows can effectively keep common types of household mold at bay.

Does opening windows help with mold?

Opening windows is an effective method to stop condensation on windows, and therefore is a great way to help with mold that doesn't cost a cent. Even opening windows just once a day will allow mold spores to escape, and let any patches of moisture dry out thoroughly. Using the German shock ventilation method is quick, easy and effective, plus it won't suck all the heating out of your home.

'Installing good air filters, sealing duct leaks, and removing excess moisture are all small steps that make a big difference – not just in preventing mold, but in getting your HVAC system to run better and last longer,' says HVAC expert Barber. 'It's all connected. The more efficiently your system runs, the less you pay, and the healthier your indoor air will be.'

There are still bad habits that increase the risk of your mold in your home, though, that it's important to be aware of, such as allowing wet items to stay damp, and neglecting proper ventilation.