Around 200 Ideal Boilers workers in Hull took the first of four planned one-day strikes on Wednesday of last week.
The Unite and GMB union members are fighting for better pay. But the bosses have refused to increase their pay offer despite earning large profits.
The workers are very active and organised by a works committee from across the plant.
Around 70 pickets gathered outside the factory’s three gates. Strikers include both office and shop floor staff.
The unions plan one day walkouts for the following three Wednesdays. Workers say more action could be called if bosses don’t come back with a better pay offer.