A whole-house or under-sink filter housing that drips from the seam almost always needs one part: the housing O-ring. It's the highest-failure, lowest-cost component in the whole system — and the one part the big-box stores never stock in the right size.
In Stock Now
151122 / OR-100 Buna-N O-ring — fits Pentek Big Blue housings and compatible 4.5" whole-house systems. $2.07, ships within 24 hours. Full replacement walkthrough: Big Blue housing leaking? The $2 fix.
Don't Know Your O-Ring Size? Measure the Old One
Housing rings are sized by inside diameter and cross-section thickness. Measure the old ring with calipers (not stretched), or measure the groove in the sump: groove ID = ring ID, cross-section ≈ groove depth ÷ 0.75. Then use our O-ring size calculator to confirm OD and groove math. We also stock standard Buna-N rings from 1/4" to 5-1/2" ID in the O-rings collection.
Why Buna-N for Water Filters
Buna-N (nitrile) handles pressurized cold water, chlorine exposure, and constant compression better than generic rubber — it's what the housing manufacturers spec. Compound details on the rubber compatibility chart.
Can't Find Your Size?
Email customerservice@rubberfeetwarehouse.com with your housing brand/model or the old ring's measurements — we match filter housing O-rings daily and will tell you honestly if we stock it. Maintenance companies and plumbers: bulk pricing available.
Replace It Before It Leaks
Housing O-rings take a permanent compression set after 2–3 years. Swap the ring with a filter change and keep a spare taped to the housing — a $2 ring is the cheapest flood insurance ever sold.