On the RO/DI the best pressure for the unit is 60psi at 65F to 72F, in most house pressure is somewhere 40psi to 80psi. When the temperature is less then 50F the water is heavier as it gets colder. If the unit rate 90gpd it might be doing 45gpd cause the water temp. The flow restrictor may need to change to compensate the lost on GPD. But then in near summer time you need to change the flow restrictor. Because the water temp will go up. If you have lest pressure then 40psi you will need a booster pump or greater then 80psi you will need a regulator. To test it out the GPD here what you got to do or go to this wed site
http://www.spectrapure.com/manuals/DELUXE_CSPDI_SYSTEM.pdf
Membrane Output Calculation Example
What is the expected GPD from a 75 GPD System at 40 psi pressure and 60?F
water temperature?
PCF = 40 ? 60 = 0.666
TCF = 0.754 (from Table 1)
Expected GPD = 75 ? 0.666 ? 0.754 = 37.7 GPD ? 15%
37.7 GPD would be the Actual Production Rate
Membrane Output Calculation
Membranes produce the rated gallons per day (GPD) at 60 psi (4.1 bars) operating
pressure, 77?F (25?C) operating temperature and 500 ppm total dissolved
solids.
Membrane output gallons per day (GPD) depends on operating pressure, water
temperature and the ppm TDS in the feed water.
Expected GPD = Rated GPD ? PCF ? TCF
PCF is the pressure correction factor
TCF is the temperature correction factor
Calculation of Pressure Correction Factor (PCF): The output (GPD) from
the membrane is directly proportional to the applied pressure.
Note: The membrane is rated to produce the rated GPD at 60 psi. For any pressure
other than 60 psi the output GPD is multipled by the PCF.
PCF = Line Pressure (in psi) ? 60
Calculation of Temperature Correction Factor (TCF): The output (GPD)
also decreases with decrease in temperature. This is because water viscosity
increases with decrease in water temperature.
Temperature Correction Factor Table (TCF)
?F /?C TCF ?F \?C TCF ?F \?C TCF
41.0 /5 0.521 59.0 /15 0.730 77.0 /25 1.000
42.8 /6 0.540 60.8 /16 0.754 78.8 /26 1.031
44.6 /7 0.560 62.6 /17 0.779 80.6 /27 1.063
46.4 /8 0.578 64.4 /18 0.804 82.4 /28 1.094
48.2 /9 0.598 66.2 /19 0.830 84.2 /29 1.127
50.0 /10 0.620 68.0 /20 0.857 86.0 /30 1.161
51.8 /11 0.640 69.8 /21 0.884 87.8 /31 1.196
53.6 /12 0.661 71.6 /22 0.912 89.6 /32 1.232
55.4 /13 0.684 73.4 /23 0.941 91.4 /33 1.267
57.2/14 0.707 75.2 /24 0.970 93.2 /34 1.304