From this article:
Charles Haddon Spurgeon (1834-1892) is known as the ‘prince of preachers.’ He was a Baptist minister in England who saw enormous blessings from God upon his ministry. It is not an exaggeration to say that thousands came to Christ through his preaching. Some of services drew as many as 10,000 people at a time!
But Spurgeon never took credit for the success of his ministry. Instead, he always pointed to the hundreds of people who came before services and prayed for God’s blessing. He said any success he had come from God in answer to their prayers. Spurgeon was often fond of calling these prayer gatherings the church’s “boiler room.”
In Spurgeon’s time, steam was the power source of the day. Boiler rooms were the powerhouses, the driving forces of everything from vast machines in factories to household heating systems. Boiler rooms, however, were not pleasant places to visit. They were functional, dirty, and hot; often tucked away in the basement. Likewise, Spurgeon saw the prayers of his people as the spiritual power behind his preaching and ministry.
This is why he told his fellow pastors, “Brethren, we shall never see much change for the better in our churches in general till the prayer meeting occupies a higher place in the esteem of Christians.”