When central heating was new, the Dead Men would have to prove to the customer that the system would work during the winter. If they couldn't do this, they couldn't get paid. So they came up with this test to prove a system's potential. Note that on a 90-degree summer's day, the Dead Man would have to raise the temperature inside the house to 123 degrees to prove its worth. I like to think that this is the origin of the expression "sweating for your money."