What Size Whole House Reverse Osmosis System Do You Need?

What Size Whole House RO System Do You Need?

Crystal Quest® pairs its 500 to 2,500 gallon-per-day (GPD) whole house reverse osmosis systems with homes of one to four bathrooms. That's a wide range. Where your home lands inside it comes down to four numbers you can work out yourself: how much water your household uses on a busy day, how cold your water gets in winter, how much you draw during the morning rush, and how many fixtures run at once.

Bathroom count alone won't get you there. Two homes with the same floor plan can need very different systems if one runs on 50°F well water and the other on warmer city water. Cold water can cut a membrane's output by close to half.

The short version: add up your daily use, pad it for your busiest day, correct it for your coldest water, then size the tank for the morning rush and the pump for the fixtures running together.

In the worked example below, a family of four lands on a 750 GPD system with 68°F water and a 1,000 GPD system with 50°F well water, each with a 220-gallon storage tank.

If you're still deciding whether whole house RO is the right call at all, start with our look at whole house reverse osmosis pros and cons. This guide assumes you've decided and want to get the size right.

The Four Parts You're Sizing

A whole house RO system isn't one number. It's four parts that have to agree with each other.

Get the first one wrong and the tank runs low every evening. Get the pump wrong and the second shower goes weak. The steps below take them in order.

Step 1: Estimate Your Daily Demand

The EPA's WaterSense program puts average use at 82 gallons per person per day at home, based on USGS estimates (see its water statistics and facts). For a family of four, that's about 328 gallons a day.

Your own bill is a better number than a national average. Divide the gallons on a billing statement by the days in that billing period. If your utility bills in cubic feet, one cubic foot is about 7.48 gallons, so a unit of 100 cubic feet is roughly 748 gallons. On a private well without a meter, the national average is a reasonable place to start.

Count only what the RO will actually feed. That 82-gallon figure covers everything used at home, and the same EPA page notes that outdoor use accounts for more than 30 percent of total household water use on average. If your outdoor spigots and irrigation stay on untreated water, your RO demand could come in well under 82 gallons a person. The examples below keep the full figure, which errs toward a bigger system.

Step 2: Add a Safety Margin for Your Busiest Day

Your average day isn't the day that runs the tank dry. Laundry day, a houseful of guests over the holidays, or a teenager's 20-minute shower is. A system sized to the average falls behind on exactly those days.

Our commercial RO sizing guide uses the common engineering practice of rating a system at roughly 120 to 200 percent of peak daily demand. The same logic works at home, with one difference: most households don't know their peak day, so start from the average and let the margin cover the busy days. It also leaves room for the gradual drop in output as membranes age between replacements.

For the family of four, a 1.5 multiplier turns 328 gallons into about 492 gallons a day. That's the output you need your system to actually deliver. What it's rated for is the next step.

Step 3: Correct for Cold Water

Here's the step that changes the answer the most. A membrane's GPD rating comes from standard test conditions, and your water almost never matches them. All Crystal Quest whole house RO systems are rated with 77°F water and total dissolved solids under 2,000 ppm, per the whole house reverse osmosis system specifications.

Colder water is thicker, so it takes more pressure to push through the membrane and less comes out the other side. The Water Quality Association's guide to RO performance and sizing puts the drop at approximately 3 percent for every degree Celsius, and notes that the lower temperatures and line pressures typical of private wells slow production.

Applied as a straight-line rule of thumb, that looks like this:

Treat the table as an estimate. The precise figure for a given system comes from the membrane's temperature correction chart, so ask for it at your feed temperature. To find that temperature, run your cold tap for a few minutes in the middle of winter and check it with a kitchen thermometer.

Now the family of four. On 50°F well water, a 500 GPD system would make roughly 275 gallons a day. That's less than their average use, never mind the 492 they need on a busy day. To deliver 492 gallons at 55 percent of rating, they need about 895 GPD of rated capacity, which rounds up to a 1,000 GPD system. The same family on 68°F city water needs about 579 GPD of rating, so a 750 GPD system covers them.

Step 4: Size the Storage Tank for the Morning Rush

The membrane makes water at a steady trickle. The tank is what lets four people shower in the same hour. So the tank has two jobs: cover the gap between what you draw in your busiest stretch and what the RO makes during it, then refill before the next one.

Cover the Rush

Say the family's morning is four 10-minute showers inside two hours. Standard showerheads use 2.5 gallons per minute and WaterSense-labeled models use no more than 2.0, according to the EPA's showerhead guidance. That's 80 to 100 gallons for showers alone, before toilets, sinks or a load of laundry.

Meanwhile the 1,000 GPD system on 50°F water is making about 550 gallons a day, or roughly 23 gallons an hour. Over those two hours it adds about 46 gallons. The tank has to supply the rest: about 34 to 54 gallons for the showers alone, or at most about a quarter of a 220-gallon tank, before toilets, sinks and laundry add their share.

Keep in mind you never get the tank's full rated volume. The low float switch shuts the delivery pump off when the water level gets low, which protects the pump from running dry. Our walkthrough on installing float switches on a whole house RO system shows how the high and low floats work.

Refill Before the Next Rush

At 23 gallons an hour, refilling a 220-gallon tank takes up to about 10 hours. That works for a household whose heavy use falls in the morning and evening, with the RO catching up overnight and during the day. If your peaks come closer together, you need more membrane, more tank, or both.

That trade-off is the whole game. A smaller membrane can get by with a bigger tank, and a larger membrane lets you run a smaller one. Crystal Quest's storage tank kits pair up with system size like this, per the product specifications:

Measure the door opening to your mechanical room before you order. A 42-inch tank doesn't fit through every basement door.

Step 5: Match the Delivery Pump to Simultaneous Use

The pump is sized in gallons per minute, not gallons per day. What matters is how many fixtures run at the same moment.

Two showers at 2.0 to 2.5 gallons per minute plus a bathroom faucet at 1.5 (the WaterSense limit for bathroom faucets) come to 5.5 to 6.5 gallons per minute. That's before the washing machine or dishwasher starts filling. Add up your own worst realistic moment and ask for the pump's flow at the pressure you want at the tap. If the pump can't keep up, the fixture that opens last is the one that goes weak.

Crystal Quest storage tank kits include the delivery pump and the float switches that control it, so the tank and pump come sized as a set.

Step 6: Check the Feed Water and the Drain

An RO system doesn't turn every gallon it takes in into purified water. Part of it carries the rejected minerals to drain. Crystal Quest RO systems produce at minimum 1 gallon of purified water for every 1 gallon sent to drain, and usually 2 to 3 gallons purified per gallon of waste.

That changes two things. If the family uses 328 purified gallons on a typical day, roughly 110 to 165 more gallons go to drain, and up to 328 at the minimum. Their well and well pump have to supply all of it, and the drain has to take it, whether that's a floor drain, a sewer line or a septic system.

Check your supply against the system's minimums too. From the specification sheet, plus one WQA caution:

• Feed pressure: at least 45 psi, with a minimum feed flow of 1 to 10 gallons per minute depending on the model.
• Hardness: no more than 15 grains per gallon. Harder water needs softening ahead of the RO.
• Total dissolved solids: up to 1,000 to 2,000 ppm depending on the model.
• Iron and manganese: the WQA guide above notes these can foul a membrane even at low levels and should generally be removed before RO treatment.
• Power: systems come standard as 110V, with 220V available on request.

You can't check any of that without a recent water test. If you're on a well, our guide to testing well water covers what to test for. Once the system's running, the whole house RO maintenance guide keeps it producing what you sized it for.

Putting It Together: Two Worked Examples

Here's the same family of four run through every step, once on a cold well and once on city water.

Same family, same house, and the cold well needs a system one size up. That's why a sizing answer that starts and stops at bathroom count can leave a well owner short every winter. For what each size costs, see what impacts the cost of a whole house RO system.

If your water runs close to the hardness or dissolved solids limits, carries iron or manganese, or your home has five or more bathrooms, bring your test results and usage to a specialist before you pick a size. Those are the cases where pretreatment and system choice matter as much as the math.