Big Blue Water Filter: Complete Guide to 10" and 20" Housings
Sediment Filtration for Well Water
Big Blue Water Filter: Complete Guide to 10" and 20" Housings
A "Big Blue" is the most common whole-house sediment filter in residential water treatment, and for good reason: the oversized 4.5-inch diameter housing handles higher flow rates than standard cartridge filters without crushing your water pressure. But most homeowners pick the wrong size, install it in the wrong spot, or use a cartridge type that clogs in weeks. After 30+ years recommending and troubleshooting these systems, this guide covers everything you need to know to get it right the first time.
Want the full picture on sediment filtration? Start with our Complete Guide to Sediment Filters for Well Water. For the broader treatment overview, see our Complete Guide to Well Water Filtration Systems.
The Short Version
A Big Blue water filter is a large-format cartridge housing that removes sediment, sand, silt, and rust from your home's water supply. Here's what you need to know:
- Two standard sizes: 4.5" x 10" and 4.5" x 20". Both use the same diameter cartridges, but the 20" version lasts roughly twice as long between changes and handles higher flow rates.
- 10" Big Blue kit: $165 from Mid Atlantic Water. Best for smaller homes (1 to 3 bathrooms) or as a secondary polishing filter.
- 20" Big Blue kit: $195 from Mid Atlantic Water. Best for most homes. Longer cartridge life, better flow, and fewer changeouts per year.
- Replacement cartridges: The Pentek WP5BB97P 5-micron polyspun filter ($45) is what we include in every kit and recommend exclusively. Replace every 3 to 6 months depending on sediment levels.
- Where it goes: After the pressure tank, before every other treatment system (acid neutralizer, iron filter, softener, UV). It protects the valves and media in those downstream systems.
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Which Big Blue Filter Size Do You Need?
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What This Article Covers
- What Is a Big Blue Water Filter?
- 10" vs 20" Big Blue: Which Size Do You Need?
- How Big Blue Filter Housings Work
- Cartridge Types: Pleated vs Wound vs Melt-Blown
- When to Replace Your Big Blue Cartridge
- Installation: Where the Big Blue Goes in Your System
- Our Big Blue Kits and Replacement Filters
- 5 Common Big Blue Mistakes
- Frequently Asked Questions
What Is a Big Blue Water Filter?
"Big Blue" is a term coined by Pentair (under their Pentek brand) for a line of oversized filter housings. Over the decades, the name became generic across the water treatment industry, much like "Kleenex" for tissues. Today, any 4.5-inch diameter cartridge housing is commonly called a "Big Blue," regardless of the manufacturer.
What makes a Big Blue different from a standard cartridge filter? Size. A standard "slim line" filter housing uses 2.5-inch diameter cartridges. A Big Blue uses 4.5-inch diameter cartridges, which means:
- Higher flow rates: The larger surface area creates less resistance, so water flows through without the pressure drops you get with slim line housings. This is the main reason we use them.
- Longer cartridge life: More media surface area means more capacity before the cartridge clogs and needs replacing.
- Better for whole-house applications: Slim line filters are fine under a kitchen sink. For whole-house filtration where every faucet, shower, and appliance depends on it, you need the flow capacity of a Big Blue.
Big Blue housings come in two lengths: 10 inches and 20 inches. Both accept the same 4.5-inch diameter cartridges, just in different lengths. The choice between them comes down to your household size, sediment load, and how often you want to change the cartridge.
10" vs 20" Big Blue: Which Size Do You Need?
This is the most common question we hear. Here's the honest answer: the 20" is the better choice for most homes. But the 10" has its place. Let's break down exactly when each one makes sense.
| Spec | 10" Big Blue | 20" Big Blue |
|---|---|---|
| Cartridge dimensions | 4.5" x 9.75" | 4.5" x 20" |
| Peak flow rate | ~10 GPM | ~15 GPM |
| Sediment capacity | Standard (1x) | Roughly 2x the 10" cartridge |
| Cartridge life | 2 to 4 months typical | 4 to 6 months typical |
| Kit price (MAW) | $165 | $195 |
| Included in kit | Housing with pressure relief, mounting bracket, wrench, 5-micron polyspun cartridge | |
| Port size | 1" NPT | 1" NPT |
| Best for | 1-2 people, light sediment, tight spaces, secondary/polishing filter | 3+ people, moderate to heavy sediment, primary whole-house filter |
Our Recommendation
For most well water homes, go with the 20" Big Blue kit ($195). The $30 difference pays for itself immediately in longer cartridge life. The 20" housing flows better, lasts longer between changes, and gives your downstream treatment equipment better protection. Every time a customer calls asking which size, the answer is the same: if you have the vertical clearance to mount it, go with the 20".
When the 10" Makes Sense
- Tight installation spaces: If you only have about 15" of vertical clearance below your plumbing run, the 10" housing fits where the 20" won't.
- Very small household or vacation home: With 1 to 2 people and light water usage, the 10" provides enough capacity.
- Secondary filter: Some customers install a 10" Big Blue after their treatment system as a final polishing filter. It catches any remaining fine particles before the water reaches your faucets.
How Big Blue Filter Housings Work
The concept is straightforward: water enters the housing through the top inlet port, passes through the cartridge from the outside in, and exits through the center core and out the top outlet port. Any particles larger than the cartridge's micron rating get trapped in the filter media.
The Mechanics
- Water enters through the inlet port on the housing cap (top). This is the "raw" unfiltered water from your well (after the pressure tank).
- Water flows down into the sump (the blue canister) where the cartridge sits.
- Water passes through the cartridge from the outside surface inward. This is where filtration happens. Sediment, sand, silt, rust, and other particles get trapped in the filter media.
- Filtered water rises through the center core of the cartridge and exits through the outlet port on the cap.
- Clean water continues to the rest of your plumbing and treatment systems.
The housing cap has a pressure relief button on top. Before you unscrew the sump to change a cartridge, you press this button to release the internal pressure. The included wrench fits the sump for removal. The whole cartridge change takes about 5 minutes once you've done it the first time.
Why 5 Microns?
A micron (micrometer) is one millionth of a meter. A 5-micron filter traps particles down to 5 microns in size, which includes virtually all visible sediment: sand, silt, rust flakes, and pipe scale. For reference, a human hair is about 70 microns. Five microns is fine enough to protect every downstream treatment system (iron filters, acid neutralizers, water softeners) without creating unnecessary flow restriction.
We sell every Big Blue kit with a 5-micron polyspun cartridge, and that's all we recommend. Going finer (1 micron) catches more particles but clogs much faster and restricts flow. Going coarser (20 or 50 microns) lets too much through to adequately protect your equipment. Five microns hits the sweet spot for well water applications.
A Big Blue Is Not an Iron Filter
A common misconception: a sediment filter does not remove dissolved iron, manganese, hardness, acidity, bacteria, or hydrogen sulfide. It catches physical particles only. If your water test shows iron above 0.3 ppm, low pH, or bacteria, you need additional treatment systems. The Big Blue protects those systems, but it does not replace them. See our Complete Guide to Well Water Filtration Systems for what each system does.
Cartridge Types: Pleated vs Wound vs Melt-Blown
All Big Blue housings use the same standard 4.5-inch diameter cartridges, but the cartridges themselves come in three main construction types. The differences matter more than most people realize.
Melt-Blown (Polyspun)
What we usePolypropylene fibers thermally bonded into a gradient density structure. The outer layers catch larger particles; the inner layers catch progressively finer ones. True depth filtration.
Pros:
- Highest dirt-holding capacity
- Consistent micron rating throughout
- No binders or adhesives that can leach
- Won't harbor bacteria (unlike cotton)
- Low pressure drop even when loaded
Cons:
- Cannot be washed or reused
- Must be replaced when spent
String-Wound
Budget optionCotton or polypropylene string tightly wound around a central core. The varying tension creates a depth filter effect. An older technology that still works, but with tradeoffs.
Pros:
- Lower cost per cartridge
- Widely available
- Decent general-purpose filtration
Cons:
- Cotton versions can harbor bacteria
- Less consistent micron rating
- Lower dirt-holding capacity vs melt-blown
- Higher pressure drop when loaded
- Cannot be washed or reused
Pleated
Specialty usePolyester or cellulose media folded into pleats for maximum surface area. A surface filter (not depth), meaning particles collect on the outside only. Can sometimes be rinsed and reused.
Pros:
- Higher flow rate per square inch
- Can be rinsed (not indefinitely)
- Good for light, uniform sediment
Cons:
- Surface-only filtration clogs faster with mixed sediment
- Lower total dirt-holding capacity
- Fine particles embed in pleats permanently
- More expensive per cartridge
Why We Use Melt-Blown (Polyspun) Exclusively
After testing all three types across thousands of installations over 30+ years, melt-blown polyspun cartridges consistently outperform the alternatives for well water sediment. The gradient density design holds more dirt before clogging, the polypropylene won't harbor bacteria, and the pressure drop stays low even as the cartridge loads up. The Pentek WP5BB97P ($45) is the specific cartridge we stock and include in every kit. It fits any standard 4.5" x 9.75" Big Blue housing.
When to Replace Your Big Blue Cartridge
There is no universal replacement schedule because it depends entirely on your water. A home with light sediment might go 6 months between changes. A home with heavy sand after a storm might need a new cartridge in 6 weeks. Here's how to know when it's time.
The Pressure Drop Method (Most Reliable)
Install a pressure gauge before and after the Big Blue housing (many homeowners already have a gauge on the pressure tank). Monitor the difference between the two readings. When the pressure drop across the filter increases by 5 to 10 PSI from its clean baseline, it's time to change the cartridge.
Cartridge Condition by Pressure Drop
Difference between inlet and outlet pressure readings
Other Signs It's Time to Replace
- Visible sediment returning: If you see particles at your faucets or in the toilet tank, the cartridge may be channeling (water finding a path around the loaded media).
- Low water pressure throughout the house: A clogged cartridge restricts flow to every fixture. If pressure drops suddenly, check the Big Blue first.
- Cartridge is visibly dark brown or caked: When you remove the cartridge and it's uniformly discolored and heavy, it did its job. Replace it.
- It's been 6 months: Even if pressure seems fine, replace the cartridge at least every 6 months to prevent bacterial growth on the loaded media.
Typical Replacement Intervals
| Sediment Level | 10" Big Blue | 20" Big Blue |
|---|---|---|
| Light (clear water, minimal particles) | 4 to 6 months | 5 to 6 months |
| Moderate (occasional cloudiness, screen buildup) | 2 to 4 months | 3 to 5 months |
| Heavy (sandy/silty water, frequent clogging) | 1 to 2 months | 2 to 3 months |
If you're changing cartridges more than once a month, a Big Blue alone may not be enough. Consider adding a spin-down sediment filter ($145) before the Big Blue to knock out the largest particles first. Or, if sediment is truly extreme and persistent, a backwashing sediment filter ($1,895) handles heavy loads without any cartridge changes at all.
Installation: Where the Big Blue Goes in Your System
Placement matters. The Big Blue sediment filter goes in a specific position in your water treatment chain, and installing it in the wrong spot is one of the most common mistakes we see.
The Correct Order
For a well water system, the treatment sequence runs in this order:
The Big Blue goes right after the pressure tank and before everything else. Its job is to catch sediment coming out of the well before it reaches any treatment equipment. Sand, silt, and rust particles can damage control valves, clog media beds, and reduce the effectiveness of every system downstream.
Not every home needs every system in that chain. You might only have a Big Blue and an acid neutralizer. Or a Big Blue, iron filter, and softener. The sequence stays the same regardless; you just skip what you don't need.
DIY Installation Tips
Installing a Big Blue is one of the simplest plumbing jobs in water treatment. The kit includes everything you need except the pipe fittings to connect to your plumbing.
- Mount the bracket securely: The housing gets heavy when full of water. Use the included bracket and mount it to a stud or solid backing, not just drywall.
- Leave clearance below: You need enough room to unscrew the sump downward for cartridge changes. For a 10" housing, plan for about 15" of clearance below the cap. For a 20" housing, plan for about 25".
- Use the correct fittings: Both the 10" and 20" kits have 1" NPT (National Pipe Thread) ports. If your main water line is 3/4", use a 3/4" to 1" adapter. Do not reduce to 3/4" fittings on both sides of the housing; this creates a bottleneck that defeats the purpose of the Big Blue's high-flow design.
- Install a bypass: Adding a simple three-valve bypass lets you isolate the Big Blue for cartridge changes without shutting off water to the whole house. This isn't required, but it makes life easier.
- Orient the ports correctly: Water flows in through the inlet (usually marked "IN" on the cap) and out through the outlet. Check the arrows on the housing. Installing it backward won't damage anything, but it bypasses the cartridge entirely.
Don't Skip the Sediment Filter
We recommend a Big Blue before any treatment system, regardless of how clean you think your well water is. Sediment levels can change seasonally (heavy rain, spring runoff, construction nearby) and a single slug of sand can damage a control valve that costs hundreds to replace. At $165 to $195, a Big Blue is cheap insurance for equipment that costs $1,000 or more. As Aidan tells every customer: "I would put a Big Blue in front of everything."
Our Big Blue Kits and Replacement Filters
We sell Big Blue kits as complete packages because buying the components separately leads to mismatched parts, missing wrenches, and brackets that don't fit. Every kit ships free, arrives ready to install, and includes a pre-installed 5-micron polyspun cartridge.
| Product | What's Included | Price | Best For |
|---|---|---|---|
| 10" Big Blue Sediment Filter Kit | 4.5" x 10" housing with pressure relief, bracket, wrench, 5-micron polyspun cartridge | $165 | Small households, tight spaces, polishing filter |
| 20" Big Blue Sediment Filter Kit | 4.5" x 20" housing with pressure relief, bracket, wrench, 5-micron polyspun cartridge | $195 | Most homes (recommended for 3+ people) |
| Pentek WP5BB97P Replacement Filter | 5-micron melt-blown polyspun cartridge (fits 4.5" x 9.75" housings) | $45 | Replacement for the 10" Big Blue kit |
"Worked out great, easy install"
Not sure which kit you need? Call Aidan at 800-460-5810 or email your water test to support@midatlanticwater.net. He'll recommend the right size and cartridge based on your specific water conditions.
5 Common Big Blue Mistakes (and How to Avoid Them)
1. Installing It After the Treatment System Instead of Before
The Big Blue goes between the pressure tank and your first treatment system. If you put it after the iron filter or softener, sediment has already entered those systems and done its damage. The whole point is to protect downstream equipment.
2. Using the Wrong Micron Rating
Homeowners sometimes buy 1-micron cartridges thinking "finer is better." In practice, a 1-micron cartridge clogs dramatically faster, creates higher pressure drops, and provides minimal additional benefit for whole-house sediment filtration. Five microns is the right balance for well water. Save the 1-micron filtration for under-sink point-of-use systems.
3. Not Changing the Cartridge Often Enough
A clogged cartridge does more than reduce pressure. It can channel, meaning water forces a path through the loaded media and starts carrying sediment through to your plumbing. An overloaded cartridge can also become a breeding ground for bacteria. Change it on schedule or when pressure drops indicate it's time.
4. Using Slim Line Housings for Whole-House Filtration
Standard 2.5-inch slim line housings are designed for under-sink and low-flow applications. When used for whole-house filtration, they create pressure drops that affect every fixture in the home, especially when multiple faucets run simultaneously. The 4.5-inch Big Blue housing exists specifically to handle whole-house flow rates without restricting pressure.
5. Forgetting to Press the Pressure Relief Button Before Changing the Cartridge
The housing is under full system pressure (typically 40 to 60 PSI). If you try to unscrew the sump without releasing pressure first, it won't budge. Or worse, it releases suddenly and sprays water. Always press the red pressure relief button on top of the cap, wait for the hissing to stop, and then use the wrench to unscrew the sump. If you have a bypass installed, close the inlet valve first.
Frequently Asked Questions
What does a Big Blue water filter remove?
A Big Blue sediment filter removes physical particles from your water: sand, silt, rust, pipe scale, dirt, and other suspended solids. With a 5-micron cartridge, it traps particles down to 5 microns (about 14 times smaller than a human hair). It does not remove dissolved contaminants like iron, manganese, hardness minerals, bacteria, or chemicals. For those, you need dedicated treatment systems. The Big Blue protects those systems by keeping sediment out of their valves and media.
How often should I change my Big Blue filter cartridge?
Every 3 to 6 months for most homes, but it varies by sediment load. The most reliable method is to monitor your pressure drop: when the difference between the pressure before and after the housing increases by 5 to 10 PSI over the baseline, it's time to change. With heavy sediment, you may need to change every 1 to 2 months. With very light sediment, you might go the full 6 months. Regardless of pressure readings, replace the cartridge at least every 6 months to prevent bacterial growth.
Can I wash and reuse a Big Blue filter cartridge?
Melt-blown (polyspun) and string-wound cartridges cannot be effectively cleaned. The sediment embeds throughout the depth of the media, and rinsing only clears the surface layer. Pleated cartridges can sometimes be rinsed and reused a few times, but their performance degrades with each cleaning. For consistent filtration quality, replace with a new cartridge. At $45 for the Pentek replacement filter, the cost of a fresh cartridge is worth the reliability.
What size Big Blue filter do I need for my house?
For most homes with 3 or more people, go with the 20" Big Blue ($195). It handles higher flow rates and the cartridge lasts roughly twice as long as the 10" version. The 10" Big Blue ($165) works well for small households (1 to 2 people), vacation homes, or tight installation spaces. If you have heavy sediment, the 20" is strongly recommended regardless of household size.
Where does the Big Blue go in my water treatment system?
Right after the pressure tank and before any other treatment equipment. The standard order is: well, pressure tank, Big Blue sediment filter, acid neutralizer (if needed), iron filter (if needed), water softener (if needed), carbon filter (if needed), UV system (if needed), then the home. The Big Blue's job is to catch sediment before it reaches your treatment systems and protects their valves and media beds.
Is a Big Blue filter the same as a whole-house water filter?
A Big Blue is one type of whole-house water filter, specifically designed for sediment removal. The term "whole-house water filter" is broader and can include carbon filters (for taste, odor, and chemicals), iron filters, UV systems, and more. The Big Blue is the sediment-specific component. Most complete well water treatment setups include a Big Blue as the first stage in a multi-system chain.
Do I need a Big Blue filter if I have a spin-down filter?
In most cases, yes. Spin-down filters (like the Rusco) are excellent for catching large particles (sand, grit, and debris) before the pressure tank, but they typically filter to 100 or 60 microns. A Big Blue with a 5-micron cartridge catches the finer sediment that passes through the spin-down. If your well produces significant sand or grit, using both (spin-down first, then Big Blue) is the ideal setup. The spin-down handles the large stuff and the Big Blue polishes the rest.
Will a Big Blue filter reduce my water pressure?
A clean Big Blue cartridge creates minimal pressure drop (1 to 3 PSI). You should not notice any difference at your fixtures. As the cartridge loads with sediment, the pressure drop gradually increases. That is normal and expected. When it reaches 5 to 10 PSI above the clean baseline, replace the cartridge and pressure returns to normal. If you experience a sudden, significant pressure drop, the cartridge likely clogged from a sediment surge (common after heavy storms or well pump work).
Keep Reading
- Sediment Filter Micron Ratings Explained — what 1, 5, 10, 20, and 50 micron mean for your Big Blue cartridge
- Best Sediment Filter for Well Water — cartridge, spin-down, and backwashing compared
- Spin-Down Sediment Filter Guide — the Rusco pre-filter that pairs with a Big Blue
- Complete Guide to Well Water Filtration Systems — how every treatment system works together
About the Author: Aidan has been in the water treatment industry for over 30 years, specializing in well water filtration for homeowners across the United States. Mid Atlantic Water is a wholesale distributor that ships commercial-grade water treatment systems directly to homeowners, cutting out the dealer markup and commissioned salespeople. Every recommendation in this article is based on field results, not theory.
Need help choosing the right filter? Call 800-460-5810 · Email support@midatlanticwater.net