This section is from the book "The Gardener's Monthly And Horticulturist V18", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
I have had thirty years' practical experience in heating plant houses with flues, steam and hot water, and do not hesitate to say that hot water is by far the best for plant growth. The best boilers are those of Hitchings & Co. and Weathered of all I have had in use. My advice is to use plenty of pipe for heating to produce healthy plants. I don't like the water to exceed 160 degrees. My advice is to keep pipes 4 to 6 feet above the boiler, and to connect the feed pipe with the flow, to take away the steam generated in the boiler. Many suppose the hot water drives the cold, but it is just the reverse - it is the balance of power. Cold water being the heavier, the more the pipes are elevated above the boiler the quicker the circulation.