HVAC School - For Techs, By Techs
The Hidden Cost of Killing Radiant Load - Short #260
· Summarized
Key takeaways
- Condensation occurs when surface temperature falls below dew point; radiant barriers can inadvertently cause this.
- Removing radiant heat can increase the risk of condensation due to reduced sensible load and longer runtimes.
- Proper system design must account for both radiant and latent loads to tackle moisture problems effectively.
- In high humidity areas, equipment must run sufficiently to manage both temperature and moisture.
Summary
Introduction to Radiant Load Issues
Bryan introduces the topic of condensating ducts, shedding light on how changes in the attic, specifically the installation of radiant barriers, can cause unexpected sweating of ductwork. He emphasizes that this podcast will address the misconceptions surrounding radiant load and its effects.
Understanding Radiant Heat and Dew Point
The podcast dives deeper into how radiant barriers affect attic conditions. Bryan explains that while radiant barriers cool attic temperatures and impact surface temperature, they do not alter dew point levels. This imbalance can lead to condensation on ducts if their temperatures drop below the surrounding dew point.
The Paradox of Adding Heat for Moisture Control
Bryan discusses how introducing heat can paradoxically make spaces drier by increasing sensible load, thereby enhancing the cooling system's ability to dehumidify. He explains that not only temperature but also humidity levels must be considered when analyzing moisture issues.
Importance of Proper System Design
The podcast concludes by stressing the importance of thoughtful system design when addressing moisture problems. Bryan warns against eliminating radiant heat blindly, as it may worsen condensation issues. Instead, he encourages understanding the interaction between sensible and latent loads to find practical solutions.
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Show notes
In this short podcast, Bryan talks about the hidden cost of killing radiant load, especially when it comes to installing radiant barriers in attics.
We sometimes get service calls about ducts sweating when they never used to sweat. If there are no problems with the system (i.e., the refrigerant charge, static pressure, airflow, supply air temperature, and blower speeds are all good), this type of situation can be a head-scratcher. Ask a customer if they've made any changes to the attic or equipment since the last service call, and there's a great chance they'll say something about adding a radiant barrier to keep the attic cooler.
Ducts pick up heat from the surfaces in the attic. Radiant heat can come from the sun, but visible light isn't required for radiation to happen, and it can occur between objects. A radiant barrier prevents solar heat absorbed by the roof decking from radiating onto attic surfaces, but it doesn't change the dew point. When attic surfaces are cooler and the cool air passes through, the surface temperature drops below the dew point. Condensation happens on the surface; when moisture accumulates, it can lead to other problems, like the dirty M word. Moisture issues happen with low sensible load and high latent load, especially in times like hurricane season in Florida.
Adding heat to a space also makes it drier because it acts as a reheat source and drops the relative humidity. It also adds to the sensible load, leading to longer runtimes (better for comfort but more expensive in terms of energy). When we remove that radiant heat, we increase the risk of condensation and short runtimes. If moisture problems are caused by a radiant barrier, then the barrier will need to come out or the attic will need to be encapsulated and dehumidified.
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