Carrier 37HS specifications Ð Constant Volume Control Arrangement, Ð VAV Control Arrangement for

SYSTEM-POWERED APPLICATIONS

Constant Volume (CV) Cooling Ð (Function No. 1.) This is the most basic operating con®guration. The control arrange- ment consists of the volume controller and the ®lter. The unit maintains a steady ¯ow of primary air at the quantity set on the volume controller over a range of supply pressures. Fig- ure 43 shows constant volume controls mounted on the Modu- line unit.

CV Heating (Function No. 1.) Constant volume heating con- trols are the same as for CV cooling.

Variable Air Volume (VAV) Cooling Ð (Function No. 2 and 3.) The addition of a cooling thermostat to the constant vol- ume controls allows the unit to vary the ¯ow of primary air. The unit will provide just enough air¯ow to satisfy the ther- mostat setting at existing load conditions, up to the maxi- mum ¯ow set on the volume controller. The cooling thermo- stat is direct acting (DA); thus the branch pressure output from the thermostat increases as the space temperature in- creases. Both diffuser-mounted and wall-mounted variations are available. Figure 44 shows the system-powered VAV con- trols (with diffuser thermostat) mounted on the unit.

VAV Cooling With Warm-Up Ð (Function 4.) During an ex- tended off period (overnight or during a holiday), the space temperature will often be lowered. It is necessary to provide heated air, temporarily, to reestablish comfortable tempera- tures when occupancy resumes. Since the cooling thermo- stats are satis®ed at the reduced temperature, the units will be shut off and the system will not be able to deliver warm air. It is necessary, therefore, to provide a means of tempo- rarily overriding the cooling thermostat. System-powered warm-up is achieved by adding a warm-up switch to the VAV cooling control arrangement (Fig. 45). The warm-up switch, located inside the unit plenum, closes when it senses that warm air is being supplied to the unit. This causes the bel- lows to bleed, opening the unit. This condition is maintained until cool air is returned to the system and the warm-up switch, sensing cool supply air, returns control to the thermostat.

Where all Moduline units on a main duct-run are pro- vided with thermostats for variable air volume control, it is often difficult to get warm air to the end units on a run; with the units in shutoff there is no signi®cant ¯ow which will trigger the warm-up switch. Solutions to this situation are found on page 58 in the Control Operating Sequences, VAV Cooling with Warm-Up section.

Fig. 43 Ð Constant Volume Control

Arrangement

Fig. 45 Ð VAV Control Arrangement for

System-Powered Warm-Up

Fig. 44 Ð Variable Air Volume Control

Arrangement

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