This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
The following table, taken from Hood's work on hot-water heating, may be given as showing the length of 4-in. piping required to heat 1000 cub. ft. of air per minute from 45° to 90° F., the temperature of the pipes being 200° F.
Temperature of External Air. | Temperature at which the House is to be kept. | |||||||||
45° | 50° | 55° | 60° | 65° | 70° | 75° | 80° | 85° | 90° | |
Number of Feet of 4-in. Pipe. | ||||||||||
10° | 126 | 150 | 174 | 200 | 229 | 259 | 292 | 328 | 367 | 409 |
20° | 91 | 112 | 135 | 160 | 187 | 216 | 247 | 281 | 318 | 358 |
30° | 54 | 75- | 97 | 120 | 145 | 173 | 202 | 234 | 269 | 307 |
32° | 47 | 67 | 89 | 112 | 137 | 164 | 193 | 225 | 259 | 296 |
40° | 18 | 37 | 58 | 80 | 104 | 129 | 157 | 187 | 220 | 255 |
50° | - - | - | 19 | 40 | 62 | 86 | 112 | 140 | 171 | 204 |
If a house containing 10,000 cub. ft. of air is to be kept at a temperature of 70° F., the external air being at 32° (freezing-point), the amount of piping required is found thus: Go down the column under 70° and find the figures opposite the given temperature of the external air, that is 32°. The figures 164 stand opposite this and beneath the 70°. Multiply 164 by 10, and the result 1640 represents the number of feet of 4-in piping-according to Hood's method. This, however, will scarcely do for horticultural purposes, as no one would dream of heating his hot-water pipes up to 200° F. - only twelve degrees below boiling-point. And, moreover, the length of piping cannot be varied at will, in accordance with the fluctuations of the external air. The quantity of piping is really regulated according to whether a structure is to be treated as a greenhouse or a hothouse, the latter requiring about twice as much piping as the former. Taking a house 100 ft. long, 12 ft. wide, and 8 ft. high to the ridge board with walls to the eaves 3 ft. high, we get a house with about 9000 ft. cubic capacity. If used as a greenhouse with a minimum winter temperature of 45° F., about 500 ft. of 4-in. piping will be sufficient in the usual way, but an extra 200 ft., making 700 ft. altogether, would maintain a temperature at a minimum of 50° to 55°. In a similar house, 1000 to 1200 ft. of 4-in. piping would maintain a stove temperature during the winter months without heating the pipes to more than 100° F.
Heating horticultural structures by steam is practised in America, where climatic conditions are different, but it is not likely to be adopted in Britain.
 
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