Fitting filtration ahead of a tankless water heater is straightforward once two things are settled: what the filter is protecting against, and what flow it has to pass without becoming a restriction. Tankless units are flow sensitive by design, so a filter that would be unremarkable elsewhere in the house can stop the heater firing properly if it is undersized.
Do not throttle the appliance
Tankless heaters need a minimum flow to fire and modulate, and they are usually installed on a branch that also serves showers and taps at the same time. A slim cartridge housing with a modest flow rating, fitted on that branch, becomes the bottleneck as soon as two fixtures run, and the household experiences it as a heater that cycles or delivers lukewarm water. Size the housing generously: a large diameter body flows far better, holds more dirt and lasts longer between changes, and the extra cost is trivial next to the heater it is protecting.
The sediment specification
The heat exchanger has narrow passages and an inlet screen, so what you are keeping out is grit, rust flakes and pipe scale lifted from the supply. A moderate rating does that without blocking every month; a very fine cartridge is the wrong instinct here because it restricts flow and exhausts quickly for no additional protection. On well water, stage it: a coarse cartridge or a spin down screen first, a finer one behind it. Check the heater's own inlet screen at the same interval, because it silts up quietly and produces the same symptoms as a blocked filter.
Scale is the real threat and needs a different stage
Filtration does nothing about dissolved hardness, and hardness is what shortens a tankless heater's life. The USGS classifies water at 121 to 180 mg/L as calcium carbonate as hard and above 180 as very hard, and in those bands a rapidly heated narrow passage deposits scale quickly. An ion exchange softener upstream is the reliable protection. Where a softener will not fit or is not wanted, a template assisted crystallization unit or scale inhibitor dosing reduces adhesion instead, which is less complete but better than nothing.
Order, isolation and the descaling routine
Sediment filtration goes first, softening or scale control next, then the heater. Fit isolation valves either side of the filter so cartridge changes do not mean shutting the house down, and fit the heater's service valves at installation so the periodic descaling flush is a twenty minute job rather than a plumbing exercise. Keep the flush on the manufacturer's schedule even with softened water. Our tankless softener guide covers sizing the softener for the heater's warranty conditions and the household's water use.
Questions people ask about water filter for tankless water heater
What micron rating should the filter be?
A moderate rating that catches grit and flakes without restricting flow. Very fine cartridges throttle the heater and exhaust quickly for no real gain.
Does a filter protect against scale?
No. Scale comes from dissolved hardness, which passes through any filter. That needs a softener or a scale control device instead.
Where does the filter go?
Upstream of the heater, with any softener or scale control between the filter and the appliance, and isolation valves either side of the filter.
Why is my tankless heater running lukewarm?
Often a flow restriction rather than a heater fault. Check the filter cartridge and the heater's inlet screen before assuming the unit is failing.