System Design

Discharge Damper

The static pressure in the duct to the Therma-Fuser is maintained by dampering air flow between fan and low pressure duct. Typical applications include (1) uncontrolled fan or (2) a fan that has controls that do not lower cfm to the minimum leakage of the Therma-Fuser.

Discharge dampers are not recommended for DX equipment without unloading.

Zone Damper

Static pressure control is achieved by dampering air flow to low pressure ductwork. A typical example is a large system in which supply main ducts operate at pressures higher than desired for quiet Therma-Fuser operation, about .25" wg.

In variable air volume systems, fan capacity control should be considered for energy reduction. Also, some fans may have a pressure capability that could damage ductwork if totally closed off. For more on fan control, see option F of Options of Supply Air Static Pressure Control To Provide Pressure Independence At Therma-Fusers.

Bypass Damper

Static pressure control is achieved by bypassing air from the supply duct to the return. Typical applications include (1) uncontrolled fan capacity or (2) a fan that has controls that do not reduce capacity to the minimum leakage of the Therma-Fusers.

Bypass dampers should be sized to handle an air volume equal to the total turndown of the Therma-Fusers.

Example 1: 12 Therma-Fuser model TB-10". Design air flow 450 cfm each. Turndown to tight shut off. Size the bypass damper for 5400 cfm.

Example 2: 5 Therma-Fuser model TF-HC-8". Design air flow 275 cfm each. Minimum flow stops set at 20% of design air flow. Size the bypass damper for 1100 cfm.

Duct Design

If the Therma-Fusers are to deliver nominal capacity at .15" wg in the Therma-Fuser neck and if a maximum of .25" wg is to be held for quiet operation, the duct between points X and Y should be sized for a maximum pressure drop of .1" wg. In an existing system, if the pressure drop between X and Y exceeds .1" wg consider additional dampering, relief or a parallel duct. If Therma-Fuser design is below nominal, say .05" wg in the Therma-Fuser neck the duct between X and Y can be sized for a maximum pressure drop of .2" wg.