size an RO, you must first determine your water requirements. Most people use between 50 and 75 gallons of water per day. This includes drinking, food preparation, cleaning, clothes washing, toilet use, showers etc. We recommend you choose the higher number (75 gallons per day per person) for sizing the system.
Once you have decided to purchase an RO system that will produce all the water in the house, you will need to answer the following questions.
How much water do I need?
To size an RO, you must first determine your water requirements. Most people use between 50 and 75 gallons of water per day. This includes drinking, food preparation, cleaning, clothes washing, toilet use, showers, etc. We recommend you choose the higher number (75 gallons per day per person) for sizing the system. We also recommend you consider bypassing the clothes washer and toilets if possible to conserve water.
Example – A family of four will need to have 4 X 75 = 300 gallons per day.
How much storage should I have?
Because RO systems typically produce water at a rate lower than it is used, the RO will need to run during times of no water usage and store water for later use. The storage should hold at least half of your daily household consumption. This will allow for peak use during the morning and evening. We recommend that you size the storage so it can hold a one-day supply.
How large an RO do I need?
Reverse Osmosis units are expensive so it is important not to oversize the unit. If the unit is too small, it will have to run excessively. We recommend sizing the RO so it does not need to run for more than 4-5 hours per day. This sizing will reduce wear and tear on the pump and motor, provide for unexpected peaks, and allow for increased future demand.
There are a number of factors that affect the output of an RO. You should be aware that the rated output of the RO is seldom the actual output in the field. Cold water
and high total dissolved solids (TDS) will reduce this output.
Have the system designer tell you what the actual output will be under the conditions your RO will be operating.
What features should the RO have?
Large RO systems tend to have more standard features and options. Standard features on whole house RO systems usually include.
• Liquid filled system operating pressure gauge
•5 micron cartridge pre-filter
• Solenoid feed water shut-off
• Adjustable system pressure regulator
Typical recommended options available (in order of
• Pre-filter pressure gauge
• Low pressure cut off switch
• Recycle valve
• Flow meters for permeate(treated water) and
• Pretreatment lockout control
• Permeate quality monitor
• Auto flush
Pre-filter pressure gauges- will allow you to monitor the pressure change as the water passes through the prefilter. A large difference in pressure indicates a dirty filter. If the filter is not changed before it becomes too dirty, the RO will shut down or be damaged.
A low-pressure cut-off switch- is essential to prevent damage to the RO pressure pump. This switch will turn the RO off if there is not enough water provided to the
RO pump. There are two general types of switches used to do this-
• The first type automatically resets itself when water pressure returns. This type of switch can cause damage to the pump if the shutdown was caused by a dirty pre-filter.
With this type of switch, a dirty pre-filter will cause the RO pump to cycle on and off rapidly and over heat.
• The second type of switch will not reset itself or will only reset after an extended period of time(typically minutes or hours). This type of switch will prevent damage to the RO pump even if the pre-filter is the cause of the low-pressure condition.
A recycle valve- will reduce water use. An RO that does not have a recycle valve will waste 4-5 gallons of water for every gallon of water produced. With a recycle valve,
this waste can be reduced to 1 or fewer gallons of water wasted for every gallon of water produced.
Flow meters- will allow you to monitor RO performance and adjust the efficiency.