This section is from the book "The Construction Of The Small House", by H. Vandervoort Walsh. Also available from Amazon: The construction of the small house.

The basis of calculating the required size of any one of the systems previously mentioned is to assume that a certain temperature of heat is to be maintained when the weather is zero, and then by means of the laws of heat transmission estimate the quantity of heat lost per hour from the house. The amount of heat lost per hour is, of course, the quantity which the heating system must supply. Knowing this, a system is installed which is capable of supplying this heat loss.

In such devices as the warm-air furnace the required size can be computed directly to meet the heat loss, but where radiators are used the required sizes of these must first be determined to offset the losses from the rooms in which they are installed, and then the size of the heater must be estimated to supply sufficient heat to the radiators and to make up for the losses of heat through the distributing-pipes.

The usual temperature to which the small house is heated when it is zero outside is 70 degrees Fahrenheit. It is then assumed that a certain quantity of heat is lost through the walls of the house by radiation and convection and conduction, and another quantity lost by the leakage of warm air out through the window-cracks. (The quantity of heat is measured in British thermal units, called B. T. U.'s.)

To understand the manner by which heat is lost through the exterior walls, it is necessary to know the meaning of radiation, convection, and conduction.

By standing before an open fire the heat given off by radiation can be observed by shutting it off with a piece of paper held between the face and the fire. This is the transmission of the heat through the ether, and is similar to the transmission of light, since this heat will pass through glass, like light.

Convection of heat is illustrated by heating air in one place and transferring that air to another place, where it will give up its heat to surrounding bodies.

Conduction of heat is illustrated by heating the end of an iron rod and noticing that the heat will eventually be transmitted along the length of it to the other end.

The heat within a house escapes from the interior to the colder atmosphere of the exterior through the walls, by radiation through the glass windows and the substance of the walls, by the convection action of the warm air of the interior giving up its heat to the interior face of the wall and the cold air of the exterior extracting this heat from the exterior face and carrying it off, and also by the action of conduction of the materials of which the wall is composed.

The quantity of heat lost is measured by the number of B. T. U.'s lost through one square foot of the wall each hour. As the window-glass loses heat through it more quickly than the wall, it is necessary to calculate this separately. The process, then, for estimating the heat loss from a room is as follows:

1. Estimate the number of square feet of exposed wall surface in the room, including windows.

2. Subtract from the above the area of the windows to find the net wall area.

3. Multiply this net wall area by the number of B. T. U.'s which the wall loses per square foot of surface for each hour.

TYPE OF WALL | Zero outside and 70 degrees inside - Number of B. T. U.'s lost for each square foot of wall surface each hour |

Brick wall, furred and plastered: | |

8" thick................ | 2I.O |

12" thick................ | 17.5 |

Frame wall, sheathed, clapboarded, and plastered.. | 21.7 (with building-paper use 20.3) |

Hollow-tile wall and concrete and stone have factors about the same as for the furred brick wall.

4. Add to this the number of B. T. U.'s lost per hour through the windows. This is determined by multiplying the area of the windows by the heat loss in B. T. U.'s per hour for each square foot of window, which is 78.8 for single windows, and where storm-windows are added it is 31.5 B. T. U.'s.

5. This total sum is the number of B. T. U.'s lost through walls and windows for each hour.

6. To this must be added the heat lost by leakage through the window-cracks. This is secured by measuring the length of window-cracks on the side which has the greatest length of crack and multiplying this by 168, or the number of B. T. U.'s lost each hour for each linear foot of window-crack. For very tight windows reduce above to 84.

7. The total of all the above gives the number of B. T. U.'s lost each hour from the room when the outside temperature is zero and the inside is 70 degrees Fahrenheit.

Knowing the quantity of heat lost per hour, a radiator must be installed which will supply this amount per hour. As the average steam-radiator supplies about 250 B. T. U.'s per hour from each square foot of its surface, the number of square feet required for a radiator to be installed in the room can be found by dividing 250 into the number of B. T. U.'s which were found to be lost from the room each hour.

A hot-water radiator gives off about 150 B. T. U.'s per hour for each square foot of surface, so that the radiator is generally about one-third larger than the steam-radiator.

Knowing the required number of feet of radiation for the radiator, the proper size can be selected from the manufacturer's catalogue.

By lumping the total number of square feet of radiation for all the radiators throughout the house together and adding 35 per cent to this to make up for loss through pipes and underrating of boilers, the size of the boiler can be selected from the catalogue to fit this need.

To estimate the size of a warm-air furnace, the total quantity of heat lost from all the rooms of the house should be calculated in the same way, and then 25 per cent added to allow for cold attics and exposure. This quantity should then be multiplied by 2.4 and divided by 8,000 to find the number of pounds of coal which will be required to be burned per hour. By dividing this amount by 5, the grate area of the required furnace can be found, and the correct size selected from the manufacturer's catalogue.

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