By: Bo DeAngelo

Accurate heat‑loss calculations are foundational to proper system design—yet, in practice, most systems installed today are significantly oversized. As gas absorption heat pumps (GAHPs) continue gaining traction for their high heating efficiencies and low operating costs, right-sizing becomes even more critical to ensure optimal performance.

Where Traditional Heat‑Loss Calculations Fall Short

When designing a residential system, contractors typically base heating load on an outdoor design temperature paired with a standard indoor design temperature—often 70°F. In many U.S. climates, this results in a temperature differential of roughly 68°F, which drives the calculated rate of heat loss.

However, most heat‑loss models remain highly conservative. They rarely account for real-world internal gains such as:

Appliances (oven, refrigerator, microwave)
Electronics (laptops, TVs)
Lighting and solar gains
People and pets

As a result, heat‑loss values often overshoot the true load by 15–30%, before equipment selection even begins.

Oversizing: A Common Industry Habit

Even with a conservative heat loss number in hand, oversizing often increases further when contractors select from standard equipment increments—40K, 60K, 80K, 100K BTU/hr, etc.

If a home shows a 53,000 BTU/hr load, many installations end up with an 80,000 BTU/hr furnace because:

It’s the next available size in the chosen brand lineup, or
The next size down seems “too small,” despite real-world buffer factors

In reality, even a 60,000 BTU furnace at 90% efficiency—delivering 54,000 BTU/hr—would closely match the load and still benefit from the calculation’s inherent conservatism. Yet contractors often default to larger equipment.

If the same 60,000 BTU model were selected in an 80% AFUE version, the output would drop to 48,000 BTU/hr. Many fear this would be undersized—but in most cases, it still meets the home’s true heating requirements.

Across North America, most heating systems are oversized by 20–50%, even when load calculations are performed.

Why This Matters for Gas Absorption Heat Pumps

GAHP systems thrive when correctly matched to the actual building load. Oversizing can:

Reduce efficiency by shortening run times
Increase cycling frequency
Undermine GAHP’s modulation and heat recovery advantages
Add unnecessary installation cost
Reduce ROI and customer satisfaction

Because GAHPs deliver high heating output with dramatically reduced fuel input, right-sizing ensures that the system operates within its optimal efficiency window—particularly during part-load conditions where absorption technology excels.

Takeaway for GAHP-Focused Contractors

As gas absorption heat pumps become a more prominent part of the high-performance HVAC toolkit, contractors should revisit their assumptions around sizing. A well-executed heat‑loss calculation—combined with an understanding of internal gains and typical oversizing patterns—helps ensure GAHP systems deliver their full potential in efficiency, comfort, and longevity.

The opportunity? Contractors who embrace accurate sizing practices will not only improve system performance—they’ll also differentiate themselves in a growing market where precision and system optimization matter more than ever.