I attended a great seminar on this topic today presented by Austin Energy’s Green Building Program and I thought I’d share the highlights. First of all, don’t read this post if you live outside of Austin’s Hot/Humid climate zone. I have a few folks who are regular followers from all over the US and this topic is not for you!
So, when we talk about a HVAC system in Austin, TX we know that our main issue here is cooling. In this cooling dominated climate our AC systems deal with two types of cooling loads.
#1 Sensible Load which means the dry bulb temperature of the air. Pull out your standard thermometer and take a reading, this is the dry bulb temperature.
#2 Latent Load which means the wet bulb temperature. In layman’s terms this is the cooling energy needed to pull moisture out of the air by condensing it on the AC coil and sending it down the condensate drain.
The main premise of the discussion today was that most houses do a good job of controlling the temperature of the house (sensible load), but only do a fair job of dealing with the humidity issues (latent load). Remember that we are most comfortable at 72-78 degrees F, AND a Relative Humidity of 40-60% RH. I’ve experienced this at my own house where I’m usually comfortable at 77 degrees at night but sometimes I’m hot at 77. I’ll go turn down the thermostat to 74-75 and I’m closer to comfortable again. This lack of comfort stems from higher humidity in the house. My AC will only run when the thermostat tells it that the temperature is too hot. The AC kicks on for a short time, cools the house, but never controls the humidity thus I’m still hot at 77 degrees even though 2 nights ago that was perfectly comfortable.
3 Ways to Deal with Latent Load in Austin
1- Build a very tight envelope & use good exhaust for humidity generating activities (showers/ cooking)
2- Run your AC (not always effective, more on this later)
3- Add a stand-alone Dehumidifier to your HVAC system
Why is the AC system not always good at controlling humidity?
-Oversized tonnage of the AC system leads to short run times. If you think about this like cars it’s like using a V12 engine to power your compact car. The engine barely has to work to get you up to speed then shuts off quickly. These short run times will remove the sensible heat (temperature) but don’t run long enough to remove the latent load (humidity). Remember that the compressor outside moves the sensible heat to the outside and the condensate drain removes the latent heat as water exits your house through condensation on the AC coil.
-Many times of the year the temperature only needs to go down a few degrees so even a correctly sized AC isn’t able to dehumidify.
-Runtime of AC equipment is vitally important because it takes 4-12 minutes before moisture will condense on the AC coil inside your house so no moisture is actually removed from your house till that moisture starts to accumulate and flow down the condensate drain. To really remove maximum latent heat (moisture) an AC system should run for 35-40 minutes before shutting off. This is a major reason that it’s better to slightly undersize an AC than oversize.
PHEW! So we’ve spelled out why a traditional HVAC system might leave us uncomfortable. Next post we’ll talk about some solutions to this Latent load. -Matt Risinger