This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
This is often used for heating greenhouses, churches, schoolrooms, &c
The following simple plan is adapted for a greenhouse. If the kitchen boiler is one that is fed from a cistern at the side of the fireplace, it may be utilized by connecting it by means of 3/4-in. or 1-in. iron or lead pipe with the cast-iron (2-in. or 3-in.) pipes in the greenhouse. If iron connecting-pipes are used, they could be screwed into the boiler with a nut on each outside to keep them watertight, by means of a grummet and red-lead paint. One should go into the boiler at the top and be connected with the top line of pipes, called the flow, and the other should go in at the bottom and be connected with the bottom line of pipes, called the return. If lead pipes are used they could be connected with the boiler and greenhouse-pipes by means of brass unions, to be purchased at any plumber"s. The pipes should rise from the boiler to the farthest end about 1 in. to the yard, and in the bend at that point should be screwed a 3/8-in. gas-tap, and from it a small lead pipe should be carried up to the roof inside. This tap should always be open, to allow any steam to escape. If the kitchen boiler is supplied from the top of the house, it is more satisfactory to put up a small gas-boiler, as the pressure of the water would try the joints and prevent the vent-tap being kept open.
The kitchen fire would, of course, be required to be kept in all night in frosty weather, and there should be taps on the connections between boiler and pipes, to shut off the heat when not required.