Abstract: The large-scale utilization of biomass combustion is impeded by the issues pertaining to fouling and slagging. A 30 kW biomass combustion experiment setup was built up to study fly ash deposition in a 4-row circular heat exchanger bundle under low-temperature wall conditions, focusing on the impact of heat exchanger bundle pitch changes on ash deposition. The experimental results demonstrated that an increase in transverse pitch (1.5D–3.0D) initially led to a decrease in fly ash deposition, followed by an increase. Conversely, increasing the longitudinal pitch (1.5D–3.0D) significantly raised ash deposition in the final two tube bundle rows. Throughout varying tube bundle pitches, the first two tube rows consistently had the higher deposition than the last two. Alkali metal content in deposition ash was directly correlated with ash deposition mass. The heat transfer coefficient of the tube bundle exhibited a proportional decrease in response to increasing fly ash deposition. Under these experimental conditions, the optimal performance in heat transfer and anti-ash accumulation was achieved when S_t/D = 2 and S_l/D = 2. The study provides a foundation for further research on the impact of heat exchanger tube shape and arrangement on ash deposition.