A lot of resources have been created for best practices when conducting a Manual J calculation—and for good reason, since a Manual J is a protocol that is dependent on the user’s inputs and assumptions. Anecdotal evidence from across the industry suggests that the majority of Manual J calculations are oversized, so let’s identify common Manual J errors that contribute to system oversizing.

Use of Safety Factors

Safety factors are integral throughout any type of system design application, as they serve to provide a buffer between theoretical estimates and practical uncertainties. In engineering, they are often applied to load bearing infrastructure such as bridges, which are intentionally built much stronger than needed for normal usage to allow for emergency situations, unexpected loads, misuse, or degradation. However, the use of safety factors is not necessary when performing a Manual J calculation. Research indicates that the Manual J procedure provides an inherent safety factor to the estimate due to the use of conservative assumptions in its methodology. When the load calculation is based on accurate information, no type of safety factor is required at all. However, when there is uncertainty pertaining to aspects of the home’s construction, it is tempting to use safety factors to correct for any ‘errors’ in the load calculation. In this scenario, it is best for the designer to conduct the relevant tests needed to fill the gaps. Alternatively, the designer may choose to make conservative estimates about the performance of systems under consideration. If taking the latter approach, it is integral to document all assumptions and make sure that all stakeholders comprehend and agree with these assumptions before proceeding with the design.

Air Infiltration Rates

When it comes to Manual J’s, the air infiltration rate is one of the most sensitive variables as it can constitute anywhere from 20-40% of the heating load calculation. Therefore, any errors in input variables drastically affect the final estimate. While Table 5A from the Manual J documentation lays out a simplified way to estimate infiltration rates, blower door tests are the preferred way to obtain those results. If the Table 5A method is being used, it is essential to account for factors such as the home’s age and existing weatherization measures when approximating how ‘leaky’ a home is. If local codes contain a fresh air requirement (typically given in ACH), it is incorrect to assume the infiltration rate meets this requirement and use it as an input value for Manual J’s. Lastly, rule-of-thumb estimates should not be applied to infiltration input values given the variability between home infiltration rates.