Nikki Krueger on the Flow Lab Podcast

When a Right Sized AC Still Leaves You Sticky: Diagnostics and Tools That Actually Fix Humidity

 

In a recent Flow Lab conversation with Nikki Krueger from Santa Fe, we walked through a case that probably looks a lot like calls you get.

Load calculations were solid. The air conditioner was right sized. Runtime on design days was around 85 percent. On paper, it was the textbook example you would use in a training class.

Inside the house, relative humidity was still living north of 60 percent.

From the homeowner’s point of view, that reads as “the new system does not work.” From a technician’s point of view, it is a reminder that sensible and latent are not the same job, and that a “correct” cooling design can still leave you with an uncontrolled moisture problem.

This post is for the people who get called in after the complaint. HVAC techs, home performance pros, energy auditors. The folks who have to sort out whether they are dealing with a load problem, a system problem, an envelope problem, or all of the above.


Humidity problems are system problems, not just equipment problems

You already know the physics, so we will keep it high level. Humidity issues are almost never a single point failure. They are the interaction of:

  • outdoor moisture levels and dew point

  • enclosure leakage and connections to wet zones

  • ventilation strategy

  • equipment type, staging, and control logic

  • internal moisture loads and occupant behavior

In a “green grass” climate, the outdoor air is a moisture source all summer. If the building is even moderately leaky, or if the ventilation design brings in outdoor air without any latent strategy, you are constantly importing water vapor.

Modern equipment can cut both ways. High efficiency systems and variable capacity can give you long, low sensible cycles with nice cold coils. That is helpful. At the same time, very efficient envelopes and glazing reduce sensible load, which means the system simply does not need to run long on mild or shoulder season days. When runtime drops out, latent control disappears unless you have another strategy in play.

From an auditing or diagnostic standpoint, that means you cannot just stare at tonnage and SHR and call it a day. You need to see how the building and the system are behaving together. That starts with data.


Do not go hunting without numbers

If the only information you have is “it feels sticky,” you will end up chasing your tail.

At minimum, you want:

  • continuous indoor RH and temperature data for at least a few days

  • some sense of runtime, staging, or compressor demand

  • blower door results and basic zonal pressures

  • duct leakage numbers and key room to room pressures

Most smart thermostats will give you temperature and runtime. Some track RH. If they do not, a couple of cheap loggers in representative locations work fine. What you are looking for is pattern. Does RH climb at night when the system shuts down. Does it spike with rain events. Does it float high all the time regardless of load.

Once you have that, bring in the test gear.

A blower door is your first filter. You are not just chasing a code number. You are trying to answer questions like:

  • How leaky is this enclosure compared to typical stock in this climate.

  • How connected is the living space to the crawlspace, basement, or attic.

  • Does a zonal pressure test tell you that the wet lower level is essentially part of the conditioned volume.

If you are an energy auditor or home performance contractor using Retrotec gear already, this is the same equipment you use every day. The difference is that you are now reading the results with latent load in mind instead of only thinking about kWh or ACH50 targets.

Next, put a duct tester and a manometer to work. Return leaks from humid zones are a latent freight train. Supply leaks can leave certain rooms under delivered and under dehumidified. Room to room pressure imbalances and exhaust appliance operation can drive infiltration right when the house least needs it.

By the time you are done with envelope and duct diagnostics, you should know whether most of the moisture is entering through the shell, through the distribution system, or mostly being generated internally. That makes equipment decisions much easier.


When the AC really is doing its job

There are plenty of cases where the cooling system is not the villain. Airflow is correct, coil temperatures are reasonable, and runtime on design days is what you would expect. You fix obvious duct and envelope issues and the house is still spending too much time at 60 percent RH or higher.

At that point, you can keep trying to wring more latent control out of the AC, or you can admit that you need another tool.

In tighter, well insulated homes in mixed humid and humid climates, a lot of us are landing on the same conclusion. Cooling and dehumidification need to be treated as related but separate design problems. Cooling covers the sensible load. A dedicated dehumidifier handles the latent load that does not get picked up in the normal run of the system.

Units like Santa Fe dehumidifiers are built exactly for this gap. They pull moisture directly out of the airstream on their own control schedule, then put warm, dry air back into the space or the ductwork. They do not care what the thermostat is doing. They care what the relative humidity is doing.

From a design standpoint, the sequence tends to look like this:

  1. Use blower door and duct testing to reduce uncontrolled latent sources as much as is practical for the scope and budget.

  2. Verify that the existing system is actually delivering design airflow and is not fighting itself with silly pressure problems.

  3. Bring in a dedicated dehumidifier when the building and climate tell you that even a well behaved cooling system will not keep RH in the target range.

You can make that decision with confidence when you have the numbers in front of you.


Designing and installing dehumidification that works in the real world

Once you are in dehumidifier territory, details matter. This is where a lot of field failures come from.

You are not just dropping a box in a basement. You are trying to control moisture across a whole volume. That means you need air mixing and you need controls where they see representative conditions. Nikki talked about the “microclimate” problem that happens when you dump all of the dry air right next to the onboard sensor. The unit satisfies locally and things still feel wet ten feet away.

Simple ducting solves a lot of that. In a retrofit basement or crawlspace, that might just mean flexing the discharge ten or fifteen feet away from the return so the air has to move across the volume. In a fully ducted system, you can tie the discharge into a supply trunk and use existing distribution to mix the air. On the return side, give the dehumidifier its own return path rather than tying into the main return in a way that reheats the coil and re evaporates condensate.

For tight, efficient homes that are already running ductless for cooling, ventilating dehumidifiers can often stand in as the missing ductwork. You keep the sensible side on the wall, then use the dehu and a simple duct layout to pull from and supply to the right parts of the house.

The common thread is that you treat the dehumidifier as part of the system, not an appliance someone picked up at a box store. You design it, you commission it, and you verify its impact with the same tools you used on the front end.


The upside of doing this the “long way”

Approaching humidity complaints this way takes more effort than telling someone they need a bigger unit or a “humidity package.” It also does a few things that are good for your business.

You separate yourself from contractors who are still winging it. You can explain your findings with blower door numbers, duct leakage reports, RH graphs, and actual measurements. You are no longer arguing about feelings. You are walking a client through data.

You make better product decisions. It becomes very clear when a Retrotec blower door test, duct diagnostics, envelope work, and a Santa Fe dehumidifier form a coherent plan instead of a pile of guesses.

And over time, you build a library of case studies in your own head. This is what a wet house over a vented crawl looks like on the gauge. This is what a leaky return in a humid attic does to indoor RH. This is what happens when you close the gaps, balance the system, and add dehumidification only when the building actually needs it.


Want to go further with this

If you are getting more “my house is cold but clammy” calls, a few concrete next steps:

  • Listen to the Flow Lab episode with Nikki Krueger for more detailed examples of latent problems that did not match the homeowner’s first story, and how she reads the data behind them.

  • Make sure your blower door and duct testing are part of your comfort and humidity workflow, not just something you pull out for code or program work. Link your reports and visuals to the moisture story, not only to energy.

  • Get familiar with the Santa Fe line or similar dedicated dehumidifiers so you know what is available when the building and the climate tell you that cooling alone is not enough.

Humidity complaints are not going away, especially as homes get tighter and equipment gets more efficient. The upside is that you already own most of the tools you need. The shift is to think of them as comfort and moisture diagnostics, not just energy diagnostics, and to give humidity its own plan instead of expecting the AC to carry all of the load on its own.

Want to hear the full story?

Listen to our conversation with Nikki Krueger on The Flow Lab Podcast – now streaming wherever you get your podcasts.

Flow Lab Podcast

  

 

The Flow Lab Podcast by Retrotec

Retrotec has launched The Flow Lab podcast to connect you with the leading minds within the building science industry.  We interview everyone from long-time experts to the newest innovators, exploring the latest technology advancements and industry news. Subscribe on the Retrotec YouTube channel and wherever you listen to podcasts. Like and subscribe to become part of the Flow Lab community!