FIELD NOTES · HOME WATER
Whole-House Filtration vs. Under-Sink RO: What You Actually Need
Not everyone needs the full whole-house system I built. Sometimes the right answer is a $200 under-sink unit — and sometimes it's neither until you've tested. Here's the honest comparison I wish someone had given me before I started spending.
Two completely different approaches
There are really only two ways to treat water at home, and they solve different problems. Point-of-entry means treating the water where it enters your home — a whole-house system, like mine, that filters every tap, shower, and appliance. Point-of-use means treating water at a single spot, most commonly a reverse-osmosis (RO) unit under the kitchen sink that handles just your drinking and cooking water.
Most people buy the wrong one for their situation, usually because they copied someone whose water and living situation were nothing like theirs.
What reverse osmosis actually does
An RO unit pushes water through an extremely fine membrane that strips out the large majority of dissolved solids, metals, and many contaminants ordinary filters miss. The water it produces is exceptionally clean. The trade-offs: it treats only the tap it's connected to, it works slowly into a small storage tank, and it sends some water to drain in the process. For drinking and cooking at one sink, though, it's hard to beat for the money.
What whole-house does that RO can't
A whole-house system protects your home, not just your glass. It treats the water your pipes, water heater, washing machine, and shower all see. For my water — acidic and chloramine-heavy — that mattered, because the corrosiveness was a threat to the plumbing itself, and the chloramine affected every shower, not just drinking water. RO at the kitchen sink would have done nothing for any of that.
Who should buy which
If you rent, or you mainly care about clean drinking and cooking water, get an under-sink RO system. You can't re-plumb a place you don't own, the install is a doable afternoon project, and it solves what most people care about most. You do not need the most expensive unit on the shelf.
If you own your home — especially with problem water like mine — a whole-house system is the better long-term call, because it protects expensive infrastructure that costs far more than the system to repair. Plenty of people run both: whole-house to protect the home, plus an RO at the kitchen sink for drinking water polished to the last step.
Before you buy either one: test
Here's the part nobody selling a system wants to say — test your water first. A cheap test kit and your city's water report will tell you whether you even have a chloramine, hardness, or metals problem. Don't buy a chloramine filter if you don't have chloramine. Match the tool to the actual problem, and you'll spend less and solve more. I've linked both tiers below so you can start wherever your results point you.
Side-by-side: which one fits you?
| What matters | Whole-house (point-of-entry) | Under-sink RO (point-of-use) |
|---|---|---|
| What it treats | Every tap, shower & appliance | One faucet — drinking & cooking |
| Best for | Homeowners with whole-home water problems | Renters & anyone focused on drinking water |
| Protects plumbing & water heater | Yes — pH correction + sediment control | No |
| Handles chloramine | Yes — catalytic carbon tank | Yes — RO membrane |
| Install effort | Plumbed in at the main water line | Doable afternoon project under the sink |
| Test your water first? | Always | Always |
Not sure which you need? Don't guess — test.
A cheap test kit plus your city's water report tells you exactly which problem you actually have.
Common questions
Do I need both a whole-house filter and an under-sink RO system?
Not necessarily, but plenty of people run both. A whole-house system protects your plumbing, water heater, and every shower, while an under-sink RO unit polishes your drinking and cooking water at one faucet. If you own a home with problem water and also want the cleanest possible drinking water, running both makes sense.
Why does whole-house filtration matter if RO already cleans my water?
Reverse osmosis only treats the single tap it is connected to, so it does nothing for the water your pipes, water heater, washing machine, and shower all see. For acidic or chloramine-heavy water, that whole-home exposure can corrode plumbing and affect every shower. A whole-house system treats the water where it enters the home, which RO at the kitchen sink cannot do.
Can renters use a whole-house filtration system?
Generally no, because you cannot re-plumb a place you do not own. Renters are usually better served by an under-sink RO system, which installs in a doable afternoon and handles drinking and cooking water. You also do not need the most expensive unit on the shelf to get clean water this way.
Should I test my water before buying any system?
Yes, always test first. A cheap test kit plus your city's water report will tell you whether you actually have a chloramine, hardness, or metals problem before you spend anything. Matching the tool to the real problem means you spend less and solve more, instead of buying a filter for a contaminant you do not have.