Oversizing is one of the most common errors in residential HVAC. A bigger system does not equal better comfort. In fact, oversizing can cause poor humidity control, short-cycling, and higher energy bills. Here are the top three mistakes contractors make and how to prevent them.
1. Ignoring the Sensible/Latent Split
Many contractors focus only on total BTU load. Manual J calculates both sensible and latent loads. Installing a unit that meets total BTUs but fails on sensible cooling can leave a house hot, humid, or uncomfortable.
Solution: Use Manual S verification to check that your chosen equipment matches the required sensible and latent ratios.
2. Using Default Inputs Without Verification
Relying on generic values for insulation, windows, or duct location may underestimate or overestimate the load.
Solution: Input accurate dimensions and properties. If possible, use blower door test data to replace default infiltration values. Load Calc Guru flags deviations from defaults.
3. Overlooking Duct Losses
Ductwork in unconditioned spaces or leaky ducts can dramatically increase load requirements. Without factoring this in, you risk oversizing.
Solution: Include all duct gains/losses in Manual J calculations. Load Calc Guru automates this, calculating sensible gains and losses based on duct location and leakage class.
Why This Matters
Oversized systems:
- Cycle too frequently, increasing wear and maintenance.
- Fail to properly dehumidify, creating discomfort and mold risk.
- Run inefficiently, raising utility costs.
Load Calc Guru ensures you calculate the correct load and select equipment that fits the design, protecting your reputation and your clients’ comfort.
Summary
- Verify sensible vs. latent requirements.
- Avoid default input shortcuts.
- Factor in duct losses.
Properly sized equipment saves money, improves comfort, and meets inspection standards every time.