When it comes to designing and sizing a heat pump system, more is not always better. While undersized systems fail to deliver the required amount of heating, oversized systems can consume excess energy, increase stress on the system, and lead to uncomfortable indoor temperature swings. Inappropriately sized systems have a much higher risk of leading to customer complaints and callbacks when the systems are not performing as expected. One of the most important steps in ensuring correct system sizing is completing a heat load calculation, the procedure for which is outlined by the ACCA’s Manual J. While it was once considered optional for HVAC projects, it is increasingly becoming the industry standard to perform a heat load calculation for every project. The Manual J instructions are rather straightforward but there are some key factors to keep in mind.
Before beginning a Manual J load calculation, it is important to first forget the traditional rule of thumb measurement of 400-600 square feet per ton of heating. Data from 40 heat pump installations across the US shows that most projects require significantly less heating than estimated, as shown on the graph below. The average of these forty projects is closer to 1,300 square feet per heating ton, but even then, there is a significant variation between different projects. This is important to keep in mind because it is easy to allow bias to impact the quality of the load estimate. Additionally, since many HVAC experts consider the load estimates returned from the Manual J to be oversized to begin with, it is better to be as accurate as possible when performing the calculations.
Another important factor is to ensure that the building orientation and glass surface area match the plans and compliance documentation, as this has an impact on system zoning and ensuring sufficient airflow is delivered to each room. It might also be beneficial to use best- and worst-case load estimates for cookie-cutter designs that have varying site orientations. When it comes to glass area, all documented window U-values and solar heat gain coefficients (SHGCs) should be considered, as well as any internal shades where applicable. It is also important to avoid the use of ‘safety factors,’ as the Manual J already has them built in. Any attempt to add onto that will likely result in higher costs and inappropriate system sizing. Ultimately, the Manual J is a comprehensive engineering tool that is valuable in improving the quality of heat pump installations when used correctly.