By Rick Andrew
Have you ever taken a shower when the water pressure was really low? The water barely trickled from the showerhead, leading to a maddening and time consuming ordeal, especially when it came to getting the shampoo out of your hair. In the middle of it all, you were doubtlessly wondering what the heck the problem was with the water pressure! Many things can contribute to low water pressure. Was the source pressure too low? Was water running from too many outlets at the same time? Were pipes in the house undersized? Was there a clogged point of entry (POE) filter? Or was there an undersized POE filter or water softener?
Pressure drop is an important consideration when installing any POE water treatment equipment. There are few factors that will lead to unhappy customers for water treatment dealers faster than undersized equipment causing unacceptable pressure drop. One bad shower is all it takes!
Many of us understand the dynamics of pressure drop and the causes of it. Friction with pipes and fittings and increases in elevation are to blame. Keeping these causes in mind, plumbers and water treatment dealers are typically careful to make sure pipes and equipment are not undersized. Modern code requirements help to keep undersizing from occurring, to the point (in some jurisdictions) where some believe the codes go too far.
Let’s explore how the measurements and calculations behind pressure drop work: how pressure drop is measured and how it can be calculated.
POE systems measurements
NSF/ANSI 42, 44 and 53 all require that pressure drop of POE systems be measured. This process is relatively straightforward. Essentially, highly calibrated pressure gauges are installed immediately upstream and downstream of the system. The inlet pressure for the measurement is set using a regulator designed to have low pressure drop. For all POE systems except water softeners, the inlet pressure is set to 30 psig; for water softeners, it is set to 35 ± 5 psig.
Prior to conducting the measurement, the media must be conditioned. For all POE systems except water softeners, the media is conditioned according to manufacturers’ instructions.
Cation exchange water softeners are exhausted by introducing water containing 20 grains per gallon hardness at a flow rate of four gallons per minute per cubic foot of cation exchange resin until the point where the treated water contains 10 grains per gallon hardness. Then they are regenerated at the manufacturer’s lowest recommended salt setting and conditioning is complete.
The flow rate for the measurement of pressure drop is set using a valve. For all POE systems except water softeners, the flow rate for measurement is the rated service flow. For water softeners, measurements are taken across a range of flow rates from 20 to 140 percent of the manufacturer’s recommended maximum service flow rate. Additionally, pressure drop data is gathered at these flow rates on softeners without the cation exchange media. The pressure drop of the media alone can be calculated as the difference between the pressure drop of the softener and the pressure drop of the softener without the media at a given flow rate. This additional data at various flow rates enables calculation of pressure drop for similar but untested softeners, as will be discussed below.
The pressure drop is determined by taking the difference in pressure of the two gauges at the system inlet and system outlet. 
See Figure 1 for an example of this measurement.
Significance of pressure drop
As many readers already know, pressure drop increases with flow rate. That is to say, the higher the flow rate, the more friction there is between the water and whatever it is flowing through. This results in a concern that the pressure drop of a POE system not be too great at a flow rate sufficient for the entire household.
In terms of NSF/ANSI 42, 44 and 53, the pressure drop of the POE system may not exceed 15 psig at the rated service flow. The concept here is that if the POE system is ‘robbing’ the house of no more than 15 psig of pressure, then the POE system is sized sufficiently large to deliver that rated service flow to the house. Codes will dictate the rated service flow required for a given size of house, depending on the number of bathrooms, etc. Code requirements vary by jurisdiction, so be sure to know the local code requirements.
Water softener calculations
NSF/ANSI 44 for residential cation exchange water softeners contains a section that allows calculation of pressure drop for water softeners that are similar to one that was tested for pressure drop. There are quite a few requirements to determine that a softener is similar enough to ensure validity of the calculation; these are presented in Figure 2.
When all of these requirements have been met, the pressure drop of a non-tested or ‘candidate’ softener can be calculated by taking the required pressure drop measurements of the tested softener with and without the cation exchange media and applying the formula presented in Figure 3.
Accurate measurements = successful results
Installers of POE systems are very familiar with pressure drop, one of the critical elements they must consider for any job. Everyone dislikes those dreaded showers with low water pressure and installers are doing everything they can to prevent that from happening due to equipment they have installed.
Some of the data they use in this effort is pressure drop information related to the POE systems they are installing. Without accurate measurements, this information would have little value. Keeping this in mind, the NSF Joint Committee on Drinking Water Treatment Units has specified detailed methods for measuring and calculating pressure drop of POE systems. These methods and the accurate measurements resulting from using them, help ensure that POE systems can be properly sized and installed to meet code requirements; that installers of POE systems can have happy customers and that the dreaded low pressure shower will not be caused by an improperly installed POE system!
About the author
Rick Andrew is the Operations Manager of the NSF Drinking Water Treatment Units Program. Prior to joining NSF, his previous experience was in the area of analytical and environmental chemistry consulting. Andrew has a bachelor’s degree in chemistry and an MBA from the University of Michigan. He can be reached at 1-800-NSF-MARK or email: [email protected].