astat reverses the control action of the room thermostat on the water valve and

Astat reverses the control action of the room

This preview shows page 191 - 194 out of 401 pages.

astat reverses the control action of the room thermostat on the water valve and de-energizes the electric heater circuit. Only heat- ing by hot water is then available. The electric heater is de-ener- gized whenever the fan is not operating. The system mode changeover decision is less critical but still involves the numerous variables previously mentioned. More often than not, the pivotal rationale for the decision is economic. Owners are reluctant to incur monthly electrical demand charges for short-period operation of their refrigeration plants. Therefore, the changeover decision is governed by the calendar—so many months on cooling and so many on heating. Not infrequently, the months when cooling will be available become a lease provision, but many tenants will not accept such a lease restriction. 8.4.3 Two-Pipe Systems with Total Electric Heat If electric heating is economically justifiable throughout the year, the need to change water supply from cooling to heating and vice versa is eliminated. Unit control is simplified; reversa1 of the room thermostat action is unnecessary. Chilled-water circulation and refrigeration can be discontinued when there is little or no need for cooling, and zoning of distribution circuits is unnecessary. A significant advantage is that the electric power source for heating and air circulation can be via the tenant connection and can thus be separately metered. For some occupancies, in some jurisdictions, this is a legal requirement as an energy conservation measure. 8.5 FOUR-PIPE SYSTEMS Figure 8-4 shows a schematic layout of a typical four-pipe sys- tem, including basic system controls. Providing two independent water distribution systems—one dedicated to chilled water and the other to hot water—permits either cooling or heating by individual room terminals throughout the year. Each terminal unit is equipped with a heating and a cooling coil (or a dual-use coil) and two
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192 A LL -W ATER S YSTEMS Figure 8-4. Piping schematic for a four-pipe fan-coil system (Carrier 1965).
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A IR -C ONDITIONING S YSTEM D ESIGN M ANUAL 193 water-flow control valves (or equivalent coil bypass control) and is connected to the distribution system by four pipes—supply and return for both chilled and hot water. Both valves are controlled in sequence by the room thermostat. A dead band of several degrees is normally incorporated into the control. In this band, both valves are closed. This feature prevents valve cycling with frequent shifts between heating and cooling and reduces energy use. In practice, the lowest and quietest unit fan speed is adequate most of the time. The four-pipe arrangement and the two-pipe arrangement with total electric heating offer significant advantages over the simple two-pipe concept, such as year-round availability of heating and cooling for individual space temperature control, elimination of zoning cost and complexity, and simpler changeover decisions.
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