When introduced in 1917, the N class proved adept at hauling both passenger and freight services on the SECR. They were well liked by crews who appreciated the general robustness of the design, although the lightly built frames caused excessive vibration and rough riding on the footplate when worked hard. Despite there being little wrong with the original design, the N class’ full steaming potential was not realised because of the failure to capitalise upon a larger boiler, which was a direct consequence of Maunsell's standardisation policy. Instead, the SECR batch was trial-fitted with "stovepipe" chimneys in an attempt to improve draughting. This was initially applied to No. 812 in 1921, although two more were fitted to Nos. 817 and 819 during building because of a shortage of chimneys at Ashford Works. The chimney fitted to No. 819 was transferred to No. 818 sometime between 1921 and 1924, though all "stovepipes" had been replaced with the standard N class type by April 1927. Another trial saw the addition of a second slidebar to No. 825, which gave better support to the valve gear and helped protect it from spillage from the driving wheel sander fillers.