How Temperature, Pressure, and Water Quality Affect How Much Water Your Home RO Unit Makes
If your home reverse osmosis unit is rated for 50 gallons per day production, that means that it will, theoretically, produce water at the rate of 50 gallons in 24 hours, or about 2 gallons per hour. What it will actually produce is only remotely related to its advertized production rate. It could be more, but it is often less. And it usually doesn’t matter. Unless you use the unit to fill an aquarium, you probably don’t need more than a couple of gallons per day anyway. As long that water comes out of the faucet when you request it, all is well.
When the membrane makers give the gallons-per-day figure for their membranes, they don’t take into account that the unit will most likely be used on an undersink reverse osmosis unit where it has to fill a pressurized storage tank. If the unit spends most of its time simply topping off a pressure tank when a pint or two of water has been taken out, its advertized production goes way down.
However, the three main variables that influence the final flow rate of the product water from an RO unit are inlet water pressure, the TDS (total dissolved solids) of the inlet water, and the temperature of the inlet water. With these, the membrane maker makes certain assumptions for residential membranes. The assumed numbers do not represent ideal conditions or even average conditions; they are simply numbers that have been agreed upon to provide a standard by which membrane production performance can be measured. Here are the assumptions:
Inlet Water Pressure: 60 psi.
Inlet Water Temperature: 77 degrees F.
Inlet Water Total Dissolved Solids: 500 ppm.
The following chart shows how each of these affects the actual amount of water that comes from your unit.
| Variable | How This Affects Performance | Discussion |
| Water Temperature: 77%. | As the temperature goes down, production goes down, sharply. As the temperature goes up, so does production. | 77 degrees F. is higher than water temperature in most areas of the country. It, of course, varies considerably by the season, so you might notice that your RO unit makes more water in summer than in winter. (If your water source is a deep well, the season won’t matter much.) Note also that as temperature and consequently production goes down, the overall TDS rejection rate of the membrane goes up. That is, when low temperature causes the unit to produce less water, it actually makes better water. It isn’t practical to try to control inlet water temperature for residential units other than by a simple fix like adding more tubing to the inlet water line to allow exposure to ambient temperature as the water enters the RO unit. This might speed production up a bit in the winter. |
| Inlet Pressure: 60 psi. | As pressure goes up, production goes up; as pressure goes down, production goes down. | 60 psi is a fairly common pressure for city water, but most wells run between 30 and 50 psi. A residential membrane makes little water at 30 psi. This variable is the easiest to control. Adding a booster pump in front of the unit will increase inlet pressure to about 80 psi and water production will go up significantly. |
| TDS of Inlet Water. | As feedwater TDS goes up, RO production goes down; as feedwater TDS goes down, production goes up. | 500 PPM TDS is higher than most city water. Typical city water that comes from a lake, for example, might be 200 ppm. Your water is the water your RO unit has to deal with, and there is no practical way to alter its TDS before it enters the RO unit. (The unit will, of course, make a 90% plus reduction in the TDS coming out.) |
How to Determine the GPD Production of Your Home RO Unit
Expected production for home reverse osmosis units is usually stated in gallons per day (GPD). Many factors affect the production. These include inlet water pressure, water temperature, total dissolved solids (TDS), the condition of prefilters, etc.
Here’s an easy way to determine the actual production of your RO. The only tools needed are a standard household measuring cup (or any measuring device that has a milliliter, or ml, scale) and a watch or clock with a second hand.
1. For undersink units, turn off the valve at the top of the tank to isolate the tank from the system, then lock the dispensing faucet open and let the unit produce into the sink for a minute or so. The drip or small stream you see is the actual production of the unit—how fast it is making water. (For countertop units, just start the unit and let it produce water for four or five minutes until a steady production rate is established.)
2. Using the watch and measuring cup, get an accurate measure of how much water the unit produces in milliliters into the cup in one minute.
3. Multiply the result by 0.38 to convert milliliters per minute to gallons per day. The result is how many gallons your reverse osmosis unit will produce if it runs for 24 hours.
Example: If your unit is making 50 milliliters per minutes, multiply 50 X 0.38. The result is 19. Your unit is producing water at the rate of 19 gallons per day.
