Ragweed Season Is Here: How Your HVAC Can Help With Late-Summer Allergies

Somebody in our office sneezes around the middle of August, somebody else says "ragweed," and that's how I know the season has turned. A week or two later the phones catch up, and a good share of the calls aren't about broken equipment at all. They're people asking what their furnace and air conditioner can do about the fact that they can't breathe in their own house.

Fair question. Your HVAC system can help a lot with pollen indoors. It can also make things worse if it's set up wrong, and there's a whole category of things people buy for allergies that do nothing at all.

What You're Actually Dealing With

Ragweed is the second big pollen season of the year in West Michigan. Tree pollen runs in the spring, grass through early summer, and then ragweed takes over from roughly the middle of August until the first hard frost kills the plants off, which around here is usually sometime in October. So figure on two months, give or take.

What makes it rough is that ragweed pollen is tiny and it travels. It's not the yellow dust that coats your car in May. It's fine enough to stay airborne for hours, fine enough to ride in on the breeze from fields miles away, and fine enough to slip through screens and a cheap filter. Then it settles on every surface in the house and gets stirred back up every time somebody walks through the room.

And it doesn't come alone. Late August around here is humid, and humidity means mold. Mold spores are a second allergen riding along with the ragweed, and for a lot of people it's the combination that really knocks them down. That matters for what follows, because your air conditioner is one of the few things in the house that fights both.

The Filter Is Where This Starts

If you do one thing this month, look at your furnace filter. Not the vent covers or a plug-in gadget, the filter.

Every bit of air your system moves passes through it, which makes it the single most effective allergen catcher in the house. But only if it's the right grade and only if it's not already full. Two things to get right:

Grade. Filters are rated by MERV, and higher catches finer particles. For pollen, MERV 8 is the floor and MERV 11 is a solid step up. The cheap fiberglass filters at the hardware store, the ones you can see daylight through, are MERV 1 to 4 and they exist to protect the blower, not your lungs. Ragweed goes right through them.

The trap is going too high. MERV 13 sounds better on the box, and it does catch finer stuff, but it also restricts airflow noticeably. On a newer, properly ducted system that's fine. On an older furnace, or one with an undersized return, a MERV 13 filter can starve the blower enough to freeze the AC coil in summer and trip the high-limit in winter. I've pulled MERV 13 filters out of systems where the homeowner's allergy fix had turned into a no-cool call. If you want to go that high, ask us first. There's usually a way to do it, sometimes with a thicker media filter cabinet that gets the filtration without the restriction, but it's not a grab-and-go decision.

Timing. In a normal month, most homes are fine on a one-to-three-month change interval. Ragweed season is not a normal month. The filter is catching more, and a filter that's loading up with pollen is both less effective and harder on the blower. Pull it every couple of weeks and hold it up to a light. If you can't see through it, change it. It's the cheapest allergy relief there is, and the whole discussion of intervals is in how often to change your furnace filter.

Fan Setting: On or Auto?

There's a setting on the thermostat that says Auto or On, and August is when people ask me about it.

On Auto, the blower only runs when the system is heating or cooling. On On, it runs constantly, which means air is passing through the filter around the clock whether the AC is running or not. More passes through the filter, more pollen caught. That's the argument for it, and it's a real one.

The catch is summer humidity. When the AC cycles off, the indoor coil is wet with the moisture it just pulled out of the air. If the fan keeps running, it blows across that wet coil and evaporates some of the moisture right back into the house. In a dry climate nobody would care. In West Michigan in August, it can nudge indoor humidity up a couple of points, and a house that's already fighting mugginess doesn't need help. Higher humidity also means happier mold, which was allergen number two.

So my honest advice is to try it and pay attention. Run the fan on On for a few days and see how the house feels. If it's noticeably clammier, go back to Auto and lean on a good filter changed more often. If it feels fine, leave it on through the season. Some newer thermostats have a circulate setting that runs the fan part of every hour, which is a decent middle ground.

Humidity Is the Other Half

I said the AC fights both allergens, and this is why. A properly working air conditioner is a dehumidifier. Every cooling cycle wrings water out of the air, and holding indoor humidity down under about 50 percent takes away what mold and dust mites need to thrive. That's a real allergy benefit that has nothing to do with the filter.

Where this goes wrong is a system that can't hold humidity down. An oversized AC that cools the air fast and shuts off never runs long enough to dry anything. A system with a dirty coil or a low charge doesn't pull much moisture. And a house with a damp basement is fighting a losing battle no matter what the AC does. If your house reads 74 on the thermostat and still feels sticky, and the allergies are worse in the basement, that's a humidity problem before it's a filter problem, and it's covered in the whole-house dehumidifier guide.

What Doesn't Help

I'd rather save you the money.

Duct cleaning. People assume the ducts are full of pollen. They mostly aren't. Pollen gets caught in the filter or settles in the rooms; very little of it accumulates in the ductwork. Duct cleaning has its place — after a renovation, after a rodent problem, when there's visible debris or growth — but as an allergy fix it's a poor return. There's a whole post on when duct cleaning is worth it if you want the full argument.

Vent filters. Those thin filter pads that go behind the register covers. They restrict airflow at the wrong end of the system, they load up fast, and they do a fraction of what the main filter does. Skip them.

Ozone "purifiers." Anything that advertises ozone as the cleaning mechanism. Ozone is a lung irritant, which is the last thing an allergy sufferer needs. Avoid these entirely.

Scented anything. Plug-ins, sprays, candles marketed for "fresh air." They add stuff to the air. They don't remove it.

When a Whole-House Purifier Actually Makes Sense

For someone who genuinely suffers, not sniffles but real, season-long misery, a whole-house air cleaner installed on the return side of the furnace is worth a conversation. These are either a deep media cabinet holding a thick pleated filter that catches far more than a one-inch filter can without the airflow penalty, or an electronic or polarized-media unit that grabs finer particles still. Either way, they treat every room, not one, and they don't take up floor space or need a filter change every month.

I was in a home in Kentwood last week where the homeowner had three portable air purifiers going in three rooms and was still miserable. The furnace had a one-inch fiberglass filter in it. We swapped that for a MERV 11, and I told them to run that for two weeks and call me back before we talked about anything bigger. That's the order to do it in. Filter first, fan and humidity second, and a whole-house purifier only if those didn't get you where you need to be. Plenty of people never need the third step.

If you're not sure which camp you're in, an indoor air quality visit is a low-key way to find out. We look at the filter, the airflow, the humidity, and the house, and we tell you what would actually help and what wouldn't. Sometimes that's a twenty-dollar filter. I'm fine with that answer. The bigger picture on what's in your air and how to deal with it is in the indoor air quality guide.