This section is from the book "Plumbing Problems", by The Sanitary Engineer. Also available from Amazon: Plumbing Problems, or Questions, Answers and Descriptions Relating to House Drainage and Plumbing.
Q. Is it safe to put stop-cocks on the hot-water pipes from a boiler, so that in case of bursting pipe, or other reason, water can be shut off? If not, what is the reason, and how can precautions be taken?
A. It is safer to have no stop-cocks on hot-water pipes. If the stop-cock on the cold-supply pipe to the boiler is closed, there would be no pressure on the hot-water pipes, which pipes could be emptied by opening the hot-water faucet over the sink.
 
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