Swamp Coolers and Indoor Humidity

Start with the fact that makes this page necessary. Spokane summers are not humid. From June through early September outdoor relative humidity here typically runs between 20 and 35 percent, and August averages about half an inch of rain. Muggy days are close to nonexistent.

That is exactly why evaporative coolers work so well in the Inland Northwest, and it is also why the usual national advice about summer mold does not apply here. Every article written for the Southeast tells you that summer humidity comes in from outside and that you should run a dehumidifier. In Spokane, if your indoor relative humidity is high in July, the water is not coming from outdoors. It is coming from something inside the building.

A swamp cooler is the most common of those somethings, and almost nobody in the local HVAC conversation mentions it, because the people who sell and service them have no reason to.

How an evaporative cooler actually works

It pulls outside air through wet pads. Water evaporating off those pads absorbs heat from the air, and the air comes out cooler. The energy to do that comes from the phase change, which means the water has to go somewhere, and where it goes is into the air stream and then into your house.

This is not a defect. It is the operating principle. A swamp cooler cools by humidifying. There is no version of it that cools without adding moisture, which is the whole reason it only works in a dry climate.

Two consequences follow, and both are mold-relevant.

Consequence one: it needs the house open, and people close it

An evaporative cooler is a once-through system. It continuously pushes outdoor air in, so an equal volume of now-humid indoor air has to continuously leave. That is why the instructions say to leave windows cracked, typically on the far side of the house from the cooler.

What happens in practice is that people close the house up, either for security, or because of wildfire smoke, or simply because closing windows is what you do with air conditioning and the habit carries over. Now the moisture has nowhere to go. Indoor relative humidity climbs through the day and keeps climbing, and rooms furthest from the exhaust path get the worst of it.

The rooms that suffer are the closed ones. A spare bedroom with the door shut, a closet on an exterior wall, the back of a walk-in pantry. Those spaces are cooler than the rest of the house and they get no airflow, which means the relative humidity in them is higher than whatever your thermostat says. Surfaces in a cool, still, humid corner are where growth starts.

Consequence two: the unit itself is a wet warm box

A swamp cooler holds standing water in a reservoir, circulates it over cellulose or aspen pads, and sits in the sun. That is a growth environment by any definition, and it is connected directly to your supply air.

The recurring findings, in rough order of how often we see them:

• Pads left in for multiple seasons. They hold mineral scale and organic debris and they stay damp. They are consumables and they are cheap.
• A reservoir that never gets drained or cleaned, with sediment in the bottom and a bleed-off line that was capped to save water.
• The plenum and duct immediately downstream, which sees the wettest air in the system and in a rooftop unit is often uninsulated sheet metal.
• Ceiling material around the register, particularly on a down-discharge rooftop unit where condensation runs back down.
• A unit left uncovered over winter, which is an opening through your roof or wall for six months, collecting snowmelt and delivering it into the duct.

The tell that separates this from other causes

The pattern is seasonal and it points at the cooler. Odor or visible growth that starts a few weeks into cooling season and improves in the fall. Growth concentrated in rooms served by the cooler, or worst in the closed-off rooms. A musty smell that is strongest right when the unit kicks on and fades after it has been running. Condensation on ductwork or around the register that nobody remembers seeing before.

Compare that to a crawl space problem, which is usually worst in the shoulder seasons and does not correlate with the cooler running, or to a plumbing leak, which does not care what month it is.

What to do before you call anyone

1. Buy a hygrometer. They cost about fifteen dollars. Put one in the room that smells and one in a room that does not. You want to know the actual number, and you want it in the problem room rather than the hallway. Indoor relative humidity sustained above roughly 60 percent is the range where growth becomes likely on ordinary surfaces.
2. Open the exhaust path. Crack windows on the opposite side of the house from the cooler, and open interior doors to the rooms that are getting damp. A closed door is the problem in a lot of these houses.
3. Change the pads and clean the reservoir. If you cannot remember the last time, that is the answer.
4. Look at the ceiling and duct around the register for staining or soft spots.
5. Try turning it off for a few days during a stretch you can tolerate, and see whether the smell changes. That is a free diagnostic and it is often conclusive.

When testing is worth it

Testing earns its cost here when the simple steps have not resolved it, when there is visible growth you want identified before you decide how much to tear out, when someone in the house is reacting and you want to know what is actually airborne in the rooms they use, or when you are buying a house with a rooftop evaporative unit and want the duct and ceiling assembly looked at before you own it.

An assessment in this situation includes psychrometric readings by zone, meaning temperature, relative humidity and dew point in each affected room rather than one number for the whole house, because the differences between rooms are the diagnosis. Moisture readings on the ceiling and wall assemblies around registers. Thermal imaging along duct runs to find condensation. A look at the cooler itself and at where it discharges. Air samples with a mandatory outdoor control, because an indoor spore count is meaningless without knowing what the air outside was doing at that hour.

Pricing

Why an independent tester

Neither Washington nor Idaho licenses mold assessment or mold remediation. The Washington Department of Health states plainly that there are no certification requirements and that anyone can call themselves a mold specialist. New York made it illegal for one company to both assess and remediate the same job under Labor Law Article 32. Nothing like that exists here, which means the company telling you how bad it is and the company selling the fix are frequently the same company.

We test only. No remediation, no reconstruction, no referral fees from the companies that do that work. The report reads the same whether the answer is that your house is fine or that something needs to be opened up.

Should you replace it with air conditioning?

That is an HVAC question and not ours to sell. What we can tell you is the moisture consequence: refrigerated air conditioning removes water from indoor air, an evaporative cooler adds it, and in a house that has had recurring summer humidity problems that difference is real. Some people run a swamp cooler for most of the season and a mini split for the worst weeks. Whether that trade makes sense for your house is between you and a mechanical contractor.

What we will say is that if you keep the swamp cooler, the maintenance is not optional and the windows have to be open. Those two things prevent most of what we get called about.

Frequently asked questions

Does a swamp cooler cause mold? It can, though not by itself. It raises indoor humidity by design, and when the house is closed up so the moisture cannot exhaust, relative humidity climbs into the range where growth starts. The unit's own pads and reservoir can also grow biological material that the fan then distributes.

What indoor humidity is too high? Sustained readings above roughly 60 percent relative humidity are where growth becomes likely on ordinary surfaces. Below about 50 percent you are generally fine. Measure in the room that smells, not in the hallway, because still cool rooms read much higher than the rest of the house.

My swamp cooler smells musty when it starts up. Is that dangerous? It is a signal worth acting on rather than an emergency. Most of the time it is the pads and the reservoir, which is inexpensive to correct. If the smell persists after the pads are changed and the reservoir is cleaned, the duct and the ceiling assembly around the register are the next places to look.

Should I cover the cooler in winter? Yes. An uncovered unit is an opening into your duct system for the entire wet half of the year, and snowmelt entering there ends up in the ceiling assembly. Drain the reservoir and the supply line as well, or it freezes.

Is summer humidity in Spokane ever from outdoor air? Very rarely. Summer outdoor humidity here typically runs 20 to 35 percent and August averages about half an inch of rain. If indoor humidity is high in July, look for an indoor source: the cooler, a crawl space, a plumbing leak, or a drain.

Can you test the cooler itself? We can sample surfaces on the pads, the reservoir and the plenum, and we can measure what is in the air of the rooms it serves. What we will not do is service or replace the unit, because we do not sell that work.