The connections shown in Figs. 96 and 97 are used with the overhead system shown in Fig. 90.
Where the connection is of the form shown at the left in Fig. 90, the cooler water from the radiators is discharged into the supply pipe again, so that the water furnished to the radiators on the lower floors is at a lower temperature, and the amount of heating surface must be correspondingly increased to make up for this loos. as already de scribed for the circuit system.
For example, if in the case of Fig. 90 we assume the water to leave at 180 degrees and return at 160, we shall have a drop in tem perature of 10 degrees on each floor; that is, the water will enter the radiator on the second floor at 180 degrees and leave it at 170, and will enter the radiator on the first floor at 170 and leave it at 160.
The average temperatures will be 175 and 165, respectively. The efficiency in the first case will be 175 — 70 = 105; and 105 X 1.5 = 157. In the second case, 165 — 70 = 95; and 95 X 1.5 = 142; so that the radiator on the first floor will have to be larger than that on the second floor in the ratio of 157 to 142, in order to do the same work.
This is approximately an increase of 10 per cent for each story downward to offset the cooling effect; but in practice the supply drops are made of such size that only a part of the water is by-passed through the radiators. For this reason an increase of 5 per cent for each story downward is probably sufficient in ordinary cases.
Where the radiators discharge into a separate return as in the case of Fig. 88, or those at the right in Fig. 90, we may assume the tempera ture of the water to be the same on all floors, and give the radiators an equal efficiency.
In a dwelling-house of two stories, no difference would be made in the sizes of radiators on the two floors; but in the case of a tall office build ing, corrections would necessarily be made as above described.
Where circulation coils are used, they should be of a form which will tend to produce a flow of water through them. Figs. 98, 99, and 100 show different ways of making up and connecting these coils.
In Figs. 98 and 100, suppl; pipes may be ether drops or risers; and in the former case the return in Fig.100 may be carried back, if desired, into the supply drop, as shown by the dotted lines.
The object of this arrangement is to secure the advantages of a hot-water system for moderate temperatures, and of steam heating for extremely cold weather.
As less radiating surface is required for steam heating, there is an advantage due to the reduction in first cost. This is of consider able importance, as a heating system must be designed of such dimen sions as to be capable of warming a building in the coldest weather; and this involves the expenditure of a considerable amount for ing surfaces, which are needed only at rare intervals. A combination system of hot-water and steam heating requires, first, a heater or boiler which will answer for either purpose; second, a system of piping which will permit the circulation of either steam or hot water; and third, the use of radiators which are adapted to both kinds of heating. These requirements will be met by using a steam boiler provided with all the fittings required for steam heating, but so arranged that the damper regulator may be closed by means of valves when the system is to be used for hot-water heating. The addition of an expansion tank is required, which must be so arranged that it can be shut off when the system is used for steam heating. The system of piping shown in Fig. 88 is best adapted for a combination system, although an overhead distribution as shown in Fig. 90 may be used by shutting off the vent and overflow pipes, and placing air-valves on the radiators.
While this system has many advantages in the way of cost over the complete hot-water system, the labor of changing from steam to hot water will in some cases be trouble some; and should the connections to the expansion tank not be opened, serious re sults would follow.