Water softeners and tankless water heaters

Install Water Softener is an independent referral service, not an installer or plumber. When you send the form, your details go to an independent local water treatment pro who can contact you and prepare a quote.

What hard water does to water heaters

Water utilities say hard water can cause some types of scaling in pipes and water heaters, and on plumbing fixtures, and describe scale building on pipes. Your utility’s yearly report gives the hardness for your area. That is reason enough to know your hardness when you have a tankless heater.

Start with the heater’s manual

For what your heater needs, check the heater’s manual. Look in it for what it says about water hardness, descaling or flushing, and water treatment. The manual, not a general rule, is what to follow for your heater.

Then get your hardness figure. On city water, it is in the utility’s yearly report; on a well, it comes from a lab test. One grain per gallon is about 17 mg/L, and the U.S. Geological Survey calls more than 180 mg/L very hard. Compare your figure with what the manual says.

Where the softener goes

For a softener to protect a water heater, the water reaching the heater has to pass through the softener first. That means the softener sits on the main line before the line branches to the heater. If the heater is fed from a branch upstream of the softener, it gets hard water whatever the softener does. Ask the pro to confirm the order in your house, and to show you where the branch to the heater leaves the main line. If the heater sits far from where the main line enters, the plumbing between them is part of the plan.

Settings matter

A softener has to be set for your hardness. Water utilities commonly note that some softening systems can increase the corrosivity of the water, which can damage the anode in a tank water heater. For a tankless heater, check the heater’s manual about softened water, and ask the softener pro how they will set the system for your water and your heater.

Signs scale is building up

Scale in a heater is out of sight, so look at the rest of the house for clues. Crust on faucets and showerheads, spots on glassware and soap that will not lather all point to hard water, as the U.S. Geological Survey and the utilities describe. If you see those, the water reaching your heater is hard too, unless a softener already treats it. If your heater shows a service message, check the heater’s manual for what it means and what to do.

Softener or another treatment

A softener removes hardness, which is what scale is made of. It is not the only product sold for heaters: you may see scale-reduction cartridges and salt-free conditioners offered too. Those are different technologies from a salt-based softener, and the standard for home softeners, NSF/ANSI 44, covers home softeners that use a resin regenerated with sodium or potassium chloride. Ask what any device is certified for, and check the heater’s manual for what it asks for. The salt-free page has the questions to ask.

Questions to ask

• What does my heater’s manual say about hardness and treated water?
• What is my hardness, and from what source?
• Will the softener sit before the branch to the heater?
• How will you set the softener for my water and heater?
• Is the softener certified to NSF/ANSI 44?
• Does the heater still need flushing on the maker’s schedule with a softener in place?
• Will the softener also treat the cold water to the kitchen and bathrooms, or only the heater?
• If I add a filter as well, where does it go in relation to the softener and the heater?

Planning both at once

If you are replacing a tank heater with a tankless one, it can make sense to plan the softener at the same time, because both jobs happen near the main line and the plumber is already there. Ask for one quote that covers both, with the order on the line shown. The whole house installation page covers sizing and placement, and the plumber’s page covers the connections and permits.