Fitting a tankless water heater and water softener at the same time is cheaper and cleaner than doing them a year apart, because both interrupt the same run of pipe near where the main enters the house. The work is mostly about order and layout: what sits upstream of what, where the softener drains, and which taps deliberately stay on raw water.
The order along the pipe
Working from the point of entry: the meter or pressure tank first, then any pressure regulator, then sediment filtration, then the softener, then the branch to the water heater. Everything that needs protecting from hardness sits downstream of the softener, and the tankless unit is the most valuable item on that list. Getting the order wrong is the most common fault on a combined job. A softener installed after the heater branch protects the fixtures and leaves the exchanger, the one component you were most worried about, running on raw hard water for its whole life.
The unsoftened cold tap, and why to plan it now
Ion exchange trades hardness minerals for sodium, and many households want at least one tap left on untreated water for drinking, cooking and filling pet bowls, and often an outside spigot for the garden. That branch has to be taken off upstream of the softener, and it is trivial while the pipe is open and awkward afterward. Decide before the work starts which kitchen tap, refrigerator line and hose bib stay raw, and mark them. If a drinking-water filter is going under that sink later, tee for it at the same time.
Drains, power and clearances
A salt-based softener needs a drain for regeneration discharge, with an air gap so nothing can siphon back, a brine tank overflow route, and a nearby outlet. A tankless heater needs gas or a substantial electrical supply, dedicated venting for gas models, and a condensate drain if it is a condensing unit. Together they need floor space and headroom in the same utility area, plus service clearance in front of both. Map the drain, the outlet, the vent path and the floor plan before anyone buys equipment, because a system that will not fit the room is a rework, not an inconvenience.
Valves and unions that pay for themselves
Fit a full bypass on the softener so it can be taken out of service without shutting the house down, and isolation valves with service ports on both the hot and cold connections of the tankless heater so the required descaling flush is an afternoon rather than a plumbing job. Use unions or press fittings at every appliance connection. None of this is expensive during a fresh install and all of it is expensive later, when the alternative is cutting into finished pipework to service something the manufacturer expects you to service yearly.
Commissioning: prove it before the trades leave
Once both are in, run the softener through a manual regeneration and confirm the brine draw and the discharge. Test the hardness at a treated tap and at the raw tap to prove the bypass is not passing and the branch is on the right side. Run the heater at full demand while a second fixture is open to confirm the softener is not throttling the flow it needs. Log the settings, the hardness number and the install date. Our water softener installation guide covers the sizing and setup checks in more detail.
Questions people ask about tankless water heater and water softener
Does the softener go before or after the tankless heater?
Before it, always. The softener sits on the main line upstream of the branch feeding the heater, so the exchanger never sees raw hardness and the treated water also serves the rest of the house.
Can one contractor do both installs?
Often yes, since both are plumbing work, but gas tankless units may need a licensed gas fitter and a vent inspection. Confirm who is pulling permits and who is responsible for the commissioning checks.
Should the kitchen cold tap be left unsoftened?
Many households do, for drinking and cooking and to feed a refrigerator or drinking-water filter. That branch has to be teed upstream of the softener, which is easy during the install and disruptive afterward.
What if there is no drain near the entry point?
You either run a discharge line to a suitable drain with an air gap, add a condensate pump arrangement, or choose a salt-free conditioner that needs no regeneration discharge at all. Solve it before ordering.