This paper is dedicated to research of the impact of the limited heat source capacity on indoor temperature and energy consumption in serial nZEB residential buildings. This is an innovative aspect as it explores the potential design simplification for different locations, allowing for cost optimization and quicker construction timelines. The objective of this paper is to examine the impact of limited heat source capacity by utilizing thermal mass and optimizing the ventilation operation. Numerical results demonstrate that incorporating thermal mass increases heating energy consumption by up to 1%. The study addresses the impact of limited heating capacity on indoor temperatures and the need to manage ventilation’s impact during peak temperatures using simulation software IDA ICE. The study reveals that a limited heating capacity reduces energy consumption up to 2.6%, but may result in lower indoor temperatures. By optimizing ventilation strategies, energy consumption can be reduced from 2.4% to 4.4% compared to the suboptimal solution, and from 2.8% to 6.5% compared to the initial case. Parametric analysis reveals optimal ventilation operation change-over point at an outdoor temperature of −17 °C during winter. The research provides practical recommendations for adjusting heating schedules, selecting appropriate heating capacities and implementing optimal ventilation strategies.