The Trane Vapor System is a combined atmospheric and natural vacuum system installed for residential heating. In this system, the air and water return runs in the same direction and is practically the same length as the supply main. When properly worked out, this feature gives the same effect as though each convector radiator were placed at an equal distance from the boiler. This tends to synchronize the heating effect of all the heat emitting units in the system.

When the fire is started in the boiler of the Trane system, the water becomes heated and steam is formed which flows through the supply main and enters the radiators, displacing the air which is heavier than the steam. The air and condensation drain from the radiators through radiator traps and return piping to a point near the boiler where the air is exhausted through the quick vents and float vents at the end of the steam and return mains. The water is returned to the boiler by the direct return trap.

As the rooms become warm, less steam is condensed and the pressure in the boiler begins to rise. The rising pressure causes the damper regulator to operate, closing and opening the drafts and maintaining just the amount of pressure necessary for proper heating.

When the fire in the boiler becomes lower, condensation forms and air is prevented from entering the system and a vacuum is created. As a result, the operation changes from atmospheric to natural vacuum; hence the name “combined atmospheric and natural vacuum system.” The reduced pressure of the vacuum allows the water to boil and furnish steam to the radiators at a lower temperature; a decided economy when the fire is low.

It is very important that the steam connection to the return trap be taken from the steam space of the boiler and not from the supply piping of the header. The top of the direct return trap must be placed at least 22 inches above the water line of the boiler. In no case should the top of the trap be less than 4 inches below the air and water return main. The dimensions from the water line to the end of the mains and the top of the return trap are the minimum allowable. Greater clearance above the water line should be employed where possible. Where the ends of the steam and return mains occur in remote parts of the building away such main must always be vented before dropping to the wet return. The vent pipe must be installed from the piping below the trap up to the return main when a wet return is used. If a dry return is used, the vent pipe may be omitted.

If desired, a Hartford connection may be used between the inlet to the boiler and the return connections from the steam and return mains. The recommended size for vertical piping immediately below the float vents and the quick vents is 1/4 inch pipe at least 6 inches long. This affords a separate chamber for the air and water. The trap and strainer may be omitted provided the lower main is at least 18 inches above the water line and the return is connected directly into the return header of the boiler.

When the motor operated steam valves are used on the mains, the boiler connections should be arranged properly. An equalizer line is required to equalize between the steam main and the return main valve when a vacuum forms in the former after the motor operated valve closes. A swing check valve prevents the flow of steam into the return main. Sometimes reversed circulation resulting from rapid condensation of steam will tend to create greater vacuum in the steam main than in the return main.

A typical convector radiator used in the Trane system is equipped with and angle valve and angle trap having horizontal laterals below the floor. In the Trane system, the upper unit has a vertical trap and a straightway valve concealed within the convector radiator. The lower unit is equipped with an angle valve and trap with a downfeed riser dripped through the angle trap.