Do You Really Need a Whole-House Filter AND an Under-Sink RO?
Published by Jared Beviano · Water Wizards Filtration
This is the question I get more than almost any other. Someone has just learned their water has PFOS above the EPA limit, or they've seen the EWG database for the first time, and they're ready to do something about it. They call me, I tell them what I'd recommend, and then there's a pause.
"Wait — do I need both? Can't I just do one?"
It's a fair question. The short answer is: it depends on what's in your water and how you're being exposed to it. The longer answer is that whole-house filters and under-sink RO systems solve different problems — and in South Florida, most households genuinely need both.
Here's why.
They're Not Doing the Same Thing
This is the core of the confusion. People assume a whole-house filter and an under-sink RO are the same product at different scales. They're not. They work differently, remove different things, and protect you from different exposure routes.
An under-sink RO is a drinking water system. Water passes through a semi-permeable membrane that blocks virtually everything — PFAS, arsenic, nitrates, chromium-6, lead, fluoride, dissolved salts. What comes out the other side is extremely pure water. You fill water bottles from it, run your coffee maker from it, cook with it. It serves one or two faucets. It doesn't do anything for the water in your shower, your washing machine, or your ice maker on the other side of the kitchen.
A whole-house carbon filter treats every tap in the house. It removes chlorine, chloramines, and the disinfection byproducts they form — trihalomethanes (TTHMs) and haloacetic acids (HAAs). It significantly improves taste and odor throughout the home. What it doesn't do effectively: remove PFAS, nitrates, arsenic, or chromium-6. For those contaminants, you need RO.
So why does this matter? Because you're exposed to your water in more ways than drinking it.
The Shower Problem Nobody Talks About
Here's something most people don't consider: you absorb more chlorine and disinfection byproducts through your skin and lungs in a ten-minute shower than you do drinking two glasses of unfiltered water.
That's not speculation — it's documented in peer-reviewed research. Hot shower water volatilizes chloramines and TTHMs. You're inhaling them. Your skin is absorbing them through open pores in warm water. A shower is essentially a chloramine inhalation chamber.
Miami-Dade WASD uses chloramine disinfection. So does Palm Beach County Water Utilities. So does Broward County Water and Wastewater Services. Every municipal water customer in South Florida is showering in chloraminated water.
An under-sink RO does nothing for this. The RO is under your kitchen sink. Your shower is across the house. The water reaching your showerhead has never been through your RO system.
A whole-house catalytic carbon filter addresses this. It treats water at the point of entry — the main line coming into your house — before it reaches any tap, any shower, any bathtub.
What Each System Actually Protects Against
Let me make this concrete for South Florida specifically.
Contaminants in South Florida water and what removes them:
| Contaminant | Whole-House Carbon Filter |
Under-Sink RO |
|---|---|---|
| PFOS / PFOA (forever chemicals) | No | 90–99% |
| Arsenic | No | 90–96% |
| Nitrates | No | 85–95% |
| Chromium-6 | No | 95–99% |
| Lead | Minimal | 95–98% |
| Chlorine taste & odor | Yes | Yes |
| Chloramines | Yes | Pre-filters |
| TTHMs — drinking | Yes | Yes |
| TTHMs — shower / inhaled | Yes | Doesn't reach shower |
| HAAs (haloacetic acids) | Yes | Yes |
| Hard water / scale | No | No |
Look at that table carefully. The contaminants that the RO removes brilliantly — PFAS, arsenic, nitrates, chromium-6 — the carbon filter misses entirely. The exposure route that the carbon filter protects against — shower absorption, inhalation — the RO never reaches.
They're not redundant. They're covering different ground.
The Case for RO Only (When It Makes Sense)
I want to be honest here: there are households where an under-sink RO alone is the right call.
If your primary concern is PFAS, arsenic, or nitrates — and you're on municipal water where chloramine exposure is lower because the distribution system is shorter — an RO addresses the most urgent chemical concerns at a lower cost.
If budget is genuinely constrained, an under-sink RO at $400–900 installed is where I'd start. It addresses the contaminants with the most serious health implications at the point where ingestion happens. You can add a whole-house filter later.
If you're a renter who can get permission for under-sink installation but not a whole-house system, an RO gets you most of the way there for drinking water.
The RO is not optional if your water has PFAS above guidelines, arsenic, nitrates, or chromium-6. That's non-negotiable. The question is whether to add a whole-house carbon filter on top of it.
The Case for Whole-House Only (Almost Never Right in South Florida)
I'm going to be direct: for most South Florida households, a whole-house carbon filter without an RO is not a sufficient solution.
A whole-house carbon filter improves taste, removes chlorine and chloramines, and reduces disinfection byproducts. That's real value. But if your water has PFAS — and Miami-Dade WASD does, at 4.7× the EPA's legal limit — a carbon filter is not removing it. You're drinking better-tasting PFAS water.
The one scenario where whole-house only makes sense: your water quality concerns are genuinely limited to taste, odor, and chlorine. No PFAS. No arsenic. No elevated nitrates. If your utility has a clean EWG profile and your only complaint is the pool taste, a whole-house carbon filter solves that.
Check EWG's database for your specific utility before deciding. The answer isn't the same for every water system.
What About Hard Water?
Neither system handles it.
I say this because it comes up in almost every conversation. A customer will ask about RO and whole-house filtration, we'll talk through the options, and then I'll test their water and find 18 GPG hardness. Neither the RO nor the carbon filter touches that.
Scale from hard water destroys water heaters in 5–7 years in South Florida. It clogs showerheads, ruins dishwashers, leaves white deposits on everything. The damage is slow and cumulative and expensive.
A water softener is a third system — separate from both the RO and the whole-house filter. If your water is hard (anything above 7 GPG is considered hard; South Florida municipal water ranges from 15 to 22+ GPG), a softener is part of the complete picture.
The full setup I install most often in South Florida homes:
Water softener — at the point of entry, treats the whole house
Whole-house catalytic carbon filter — after the softener, removes chlorine, chloramines, TTHMs/HAAs
Under-sink RO — at the kitchen sink, handles PFAS, arsenic, nitrates, chromium-6, final polishing
Each one does something the others don't. Together, they cover everything.
The Real Cost Comparison
People sometimes balk at the cost of all three systems. Let me put it in context.
Water softener: $1,400–2,500 installed. Annual salt cost: $200–400.
Whole-house catalytic carbon filter: $1,200–2,200 installed. Annual maintenance: $150–250.
Under-sink RO: $400–900 installed. Annual maintenance: $100–200.
Total installed: $3,000–5,600. Annual operating: $450–850.
Now compare to what unfiltered water costs the average South Florida household:
Bottled water (family of 4, drinking only): $600–1,200/year
Hard water appliance damage and replacement: $1,500–2,500/year
Plumber calls for scale-related issues: $300–800/year
Skin/hair products to compensate for hard water: $200–500/year
Total annual cost of doing nothing: $2,600–5,000/year.
The complete treatment system pays for itself in 1–2 years and then runs at a fraction of the ongoing cost. Most of my customers who do the full system stop buying bottled water, extend the life of their appliances significantly, and notice the difference in their skin and hair within weeks.
The Question That Determines Your Answer
Here's how I frame it for every customer:
What's your exposure concern — what you drink, or what you shower in, or both?
If you're worried about PFAS, arsenic, nitrates, and chromium-6 — the stuff that causes cancer and other serious health problems with long-term ingestion — you need an RO. That's where ingestion happens.
If you're worried about chloramines, TTHMs, and shower absorption — the stuff that affects skin, hair, respiratory health, and daily quality of life — you need a whole-house carbon filter. That's where showering happens.
If you're worried about both, which is the case for virtually every South Florida home I walk into, you need both.
The only way to know what you're actually dealing with is to test your water. Not guess based on which city you're in. Not assume because your neighbor had good results. Test your specific water, at your specific address, and design the system around what's actually there.
We do that test for free. Thirty minutes at your kitchen sink and you'll know exactly what you're working with.
