Why is magnetic grit still getting past my sediment filters?

Since that grit is magnetic, you are almost certainly looking at magnetite or iron scale. This usally happens when you have an older steel well casing that is starting to flake or if there is high iron content in the water reacting with the pump components. Even if the water tastes fine, those tiny black particles are heavy and abrasive, which is why they are chewing trough your appliance screens and threatening your tankless heater.

If this stuff is bypassing your main filter, you have likely got a few things going on. First, check the filter housing to ensure the bypass valve isn't partially open. But more likely, you are using a filter with too high of a micron rating. Most people run a 20 or 50 micron filter, but magnetite particles can be much smaller than that. When your pump kicks on and creates a surge of pressure, it can actualy force those particles right through a standard string-wound or cheap pleated filter. To stop this, you need to step up your filtration setup. I woud install a **spin-down sediment trapper** right where the line comes in from the well. Get one with a stainless steel screen and a manual flush valve. It will catch the heavy grit before it even reaches your main filter. If the particles are still getting through, look into a dedicated magnetic iron filter or a sludge remover. These use high-strength magnets to pull those metallic flakes out of the water stream before they hit your plumbing.

For the tankless unit specifically, you realy should be running a 5-micron pleated filter. Anything larger is just asking for a clogged heat exchanger. Those solenoids in your washer are also incredibly sensitive, so if you keep seeing grit there, it means your primary stage is failing. Switch to a **dual-gradient filter cartridge**; it has a tight 5-micron inner core and a larger outer layer. It will handle the load better without killing your water pressure or letting that magnetic junk through.

That bit about the 5-micron filter makes me wonder about the water pressure. If I switch from a 20 to a 5, am I going to lose a ton of flow when I'm trying to shower? I'm allready worried about my tankless heater acting up if the pressure gets too low. Also, about those magnetic filters—coud I just buy some strong magnets and strap them to the outside of the copper pipe before the main filter as a temporary fix? Or does the water move too fast for that to acutally grab anything?

Strapping magnets to the outside of your copper won’t do a thing. The water velocity is too high and the magnetic field won't penetrate enought to catch moving grit trough the pipe wall. If you're scared of pressure loss with a 5-micron, switch to a 4.5" x 20" Big Blue housing. The massive surface area keeps the flow rate high even with a tight filter. Another thing—check your pump depth. If it was replaced in 2020, they might have set it too low. If it's sitting in the 'sludge' at the bottom of the casing, you’ll never win the sediment battle. Raising it just 5 or 10 feet can make a world of difference.

Check your pressure tank too. If the bladder is toast and the pump is short-cycling, it's gonna stir up the well every single time it kicks on. That constant starting and stopping creates a water hammer effect that knocks more scale loose from the casing. If your pump is cycling every 30 seconds when you're just running a sink, that’s your problem. Fixing the cycle rate will stop alot of that grit from getting pulled up in the first place.

Have you checked if your pump has a soft-start controller or a VFD? If that pump is just a standard single-speed slamming on, the sheer torque and sudden suction can create a scouring effect inside the well casing. It basicly vibrates the pipe and knocks that magnetite loose right as the water starts moving.

Iu2019d also suggest a multi-stage step-down array. Instead of one filter doing all the heavy lifting, run a 50-micron spin-down, then a 20-micron, and finally your 5-micron. It sounds like overkill, but it saves your pressure and makes the 5-micron last way longer. If that grit is as fine as I think, a single stage just gets overwhelmed and unloads the sediment when the pressure spikes.