Key Takeaways
- Heat Pump Oversizing: Prioritizing heating capacity without considering cooling and dehumidification needs leads to comfort problems and inefficient operation
- Manual S Guidelines: Single-stage systems allow 115% oversizing, two-stage 120%, and variable-speed 130% – exceeding these limits causes issues
- Latent Load Challenge: Modern high-efficiency equipment has reduced dehumidification capacity (26% less) compared to older systems, making proper sizing critical
- Variable Speed Misconception: Multistage and variable-speed systems cannot compensate for oversizing because capacity reduction affects both sensible and latent removal equally
Electrification, Heat Pumps and the Oversizing Issue
The last few years have seen air source heat pumps (referred to simply as heat pumps throughout this post) sales overtake fossil fuel furnace sales in the US. It’s no secret that the push for electrification and installing heat pumps over fossil fuel furnaces has been growing and becoming the norm, even in climates that once would never have considered installing a heat pump.
While we enjoy this excitement for heat pump technology and the growing number of systems installed, we also find that with it comes the need for better education and reformed sizing and installation practices.
Understanding the Oversizing Challenge
One practice that seems to be happening more frequently is the oversizing of heat pumps in climates where there is both a heat load and a latent cooling load (also referred to as “Condition A” in Manual S (N1-5 Heat Pump Sizing Condition)).
I get asked about the “how” and “why” this happens and what the ramifications are. The simple answer to the “why” is that the greater portion of the heat load that is covered by the heat pump, the lesser the need for reliance on resistance heat. Put more simply: decrease the thermal balance point, increase the savings on heating your home.
When we fall below the balance point, we need supplemental heat, and for conventional heat pump systems, that is electric resistance heat. Understanding how heat pump reversing valves work is fundamental to grasping why proper sizing matters for both heating and cooling modes.
Common Field Mistakes and Their Impact
Secondly, we see instances where a load is performed incorrectly by not doing due diligence during the site survey process. There have been cases where folks will “manipulate” the information being entered in an effort to increase the overall BTU load to ensure they don’t end up with an undersized system.
It’s as if there is a mistrust in the calculations being performed, so they “help it out” because they don’t want an angry customer. I’ve heard many of our industry’s top voices involved in the science of load calculations say that the calculations are already very conservative and don’t need our help in this area. Some have even called these “hidden BTUs.”
For accurate sizing without manipulation, consider using performance-based heat load calculation methods that rely on actual energy consumption data rather than theoretical estimates.
The Variable Speed Misconception
Understanding Modern Equipment Characteristics
Following Manual S Guidelines
Conclusion
In closing, I don’t think folks set out to size systems incorrectly or oversize from a lack of desire to do right by the customer – it’s likely the opposite. It’s probable that we don’t want to take a chance on having a customer be cold in their home, and we want them to use the minimum amount of energy possible to be comfortable.
Sizing toward the heat load is an easy trap to fall into. If you couple that with a misunderstanding of latent load and how sensible and latent splits shift during turndown, it’s easy to see how this oversizing practice happens. I know I have been guilty of this in the past myself and judge no one for it, but I do implore everyone to take the time to get educated and let’s do our best to apply this knowledge in the field starting today.
For a simplified approach to accurate heat load calculations that takes just seconds, check out the Heat Load Factor method that uses real-world data to prevent oversizing issues.
