Checking External Static Pressure

External Static Pressure

External Static Pressure can best be deﬁned as the pressure difference (drop) between the Positive Pressure (discharge) and the Negative Pressure (intake) sides of the blower. External Static Pressure is developed by the blower as a result of resistance to airﬂow (Friction) in the air distribution system EXTERNAL to the VERT-I-PAK cabinet.

Resistance applied externally to the VERT-I-PAK (i.e. duct work, ﬁlters, etc.) on either the supply or return side of the system causes an INCREASE in External Static Pres- sure accompanied by a REDUCTION in airﬂow.

External Static Pressure is affected by two (2) factors.

1.Resistance to Airﬂow as already explained.

2.Blower Speed. Changing to a higher or lower blower speed will raise or lower the External Static Pressure accordingly.

Theseaffectsmustbeunderstoodandtakenintoconsideration when checking External Static Pressure/Airﬂow to insure that the system is operating within design conditions.

Operating a system with insufﬁcient or excessive airﬂow can cause a variety of different operating problems. Among these are reduced capacity, freezing evaporator coils, premature compressor and/or heating component failures. etc.

System airﬂow should always be veriﬁed upon completion of a new installation, or before a change-out, compressor replacement, or in the case of heat strip failure to insure that the failure was not caused by improper airﬂow.

1.Set up to measure external static pressure at the supply and return air.

2.Ensure the coil and ﬁlter are clean, and that all the registers are open.

3.Determine the external static pressure with the blower operating.

4.Refer to the Air Flow Data for your VERT-I-PAK system to ﬁnd the actual airﬂow for factory-selected fan speeds.

5.If the actual airﬂow is either too high or too low, the blower speed will need to be changed to appropriate setting or the ductwork will need to be reassessed and corrections made as required.

6.Select a speed, which most closely provides the required airﬂow for the system.

7.Recheck the external static pressure with the new speed. External static pressure (and actual airﬂow) will have changed to a higher or lower value depending upon speed selected. Recheck the actual airﬂow (at this "new" static pressure) to conﬁrm speed selection.

8.Repeat steps 8 and 9 (if necessary) until proper airﬂow has been obtained.

EXAMPLE: Airﬂow requirements are calculated as follows: (Having a wet coil creates additional resistance to airﬂow. This addit ional resistance must be taken into consideration to obtain accurate airﬂow information.