HVAC School - For Techs, By Techs
WHY SQUARE FOOTAGE ≠ TONNAGE - Short #276
· Summarized
Key takeaways
- Square footage is no longer a reliable measure for determining HVAC tonnage in modern homes.
- Modern homes are better insulated, leading to increased humidity complications and the need for accurate load calculations.
- ACCA Manual J serves as a guide but can be cumbersome and conservative; practical installation often requires a balance.
- The future of HVAC will focus more on managing latent loads and encouraging systems that operate continuously for energy efficiency.
- Ultimately, HVAC solutions should focus on ensuring homeowner comfort and health, not just adhering to outdated rules.
Summary
Understanding HVAC Sizing: Square Footage vs. Tonnage
In this episode, Bryan explores the common misconception that square footage equals tonnage in HVAC calculations. Traditionally, the rule of thumb suggests sizing a house at 500 square feet per ton based on the characteristics of older homes, which were leaky and allowed moisture to escape. However, this approach is outdated as modern homes are built with far better insulation, which affects energy loads and moisture retention.
The Impact of Modern Building Practices
Bryan discusses how the shift to well-insulated homes has resulted in smaller sensible loads but increased moisture issues. The inability to escape excess moisture leads to problems like mold and discomfort in modern living spaces. This shift necessitates a different approach than the old sizing rules, as oversizing HVAC systems today can lead to short cycling and other issues.
The Manual J Dilemma
The conversation turns to ACCA Manual J, a standard method for calculating load that aims to eliminate guesswork in HVAC sizing. While it provides a more accurate calculation than traditional methods, it comes with a steep learning curve and is often overly conservative. This poses challenges for installers who prioritize practical solutions.
Future Directions in HVAC System Management
Looking ahead, Bryan indicates a shift towards a focus on latent loads and peak load management as energy efficiency standards evolve. With advancements in technology and smarter HVAC systems, there's potential for constant operation that maintains comfort without significant spikes in energy consumption. Ideally, systems won't bombard the grid at peak hours but operate steadily.
Shifting the Paradigm: Comfort Over Calculations
Bryan emphasizes the importance of prioritizing the comfort of the end user, rather than adhering strictly to engineering standards. The key takeaway is ensuring that the end result meets the comfort needs of homeowners, which includes managing humidity and ensuring efficiency without sacrificing comfort.
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Show notes
In this short podcast episode, Bryan dives a bit into equipment sizing rules of thumb and why square footage does NOT equal tonnage in today's world. Many rules of thumb exist in the industry, and one is a load calculation rule stating that you can size the HVAC for a house at 500 square feet per ton.
Old houses are leaky and poorly insulated compared to new homes, which results in large energy loads but allows the homes to dry themselves out, as moisture could leak out before it could cause trouble indoors. Large loads and leaky envelopes made 500-600 square feet per ton a sensible rule.
Homes built within the last few decades have a lot more insulation and are tighter, and they have smaller sensible heat loads. However, they're a lot more moisture-prone, especially when moisture can't escape via proper ventilation paths. The 500 square-foot rule of thumb overshoots the latent capacity and leads to short cycling due to oversized equipment. Enter ACCA Manual J, which presents a load calculation method that is very good, but it has barriers to entry; it is very rigorous, has a learning curve, and can be a hassle. One thing is clear, though: load management is key, especially latent load management.
At this time, we measure energy efficiency in terms of metrics like SEER, but the future is pointing to peak load management as the answer: getting the right power draw at the right moments instead of high general efficiency. Modernizing hot deck-cold deck systems with steady-state, constantly running systems might be the way to go, especially if we utilize energy storage and modern variable-speed technologies.
The new rule of thumb is to think like a building scientist and apply new tools to concepts that have stood the test of time. Low peak loads and steady-state operation are often the way to go with high latent loads in homes built to the most recent building codes and standards.
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