Tankless Water Heater on Well Water: Pressure, Hard Water, and What to Install First

Short answer: A tankless water heater works on well water if the well holds at least 20 psi (Rinnai) or 15 psi (Navien) at the heater, hardness stays under 200 mg/L, and iron stays under 0.3 mg/L. Most Texas aquifers run harder than that, so test the water and soften it before the heater goes in.

On a working well, pressure and flow are rarely the problem. Water chemistry is.

Can you use a tankless water heater with well water?

Yes. Neither Rinnai nor Navien prohibits well water. Both set minimum pressure, minimum flow, and water-quality limits that the supply must meet, and a private well is your job to keep inside them.

The three things to check, in order of how often they cause trouble on Texas wells:

• Hardness and dissolved solids, which cause scale on the heat exchanger.
• Sediment and iron, which clog the inlet screen and stain or foul the unit.
• Pressure and flow, which decide whether the burner fires and stays lit.

What water pressure does a tankless water heater need on a well?

Rinnai’s RE series lists a 20 psi minimum and recommends 30 to 80 psi. Navien’s NPE-A2 series lists 15 to 150 psi. A typical well pressure switch set at 30/50 meets both at the heater, before filters are added.

Well systems swing between a cut-in and cut-out pressure. Amtrol’s Well-X-Trol sizing card lists the three common pressure switch settings as 30/50, 40/60, and 50/70 psig. The number that matters for a tankless unit is the low one, because that is what the heater sees just before the pump kicks on.

Will water filters drop the pressure too low? (Worked example)

They can. HomeGuide says each treatment system typically drops pressure 2 to 10 psi. Stack two or three on a 30/50 switch and the low end can fall under the heater’s minimum.

Here is our worked example, using HomeGuide’s 2 to 10 psi per system range and the specs above:

The worst case assumes 10 psi lost per system. Well-sized systems may lose far less. Our decision rule: if you add treatment, put a pressure gauge after the last filter and confirm it stays at or above 30 psi at cut-in, which is the bottom of Rinnai’s recommended range. If it does not, the fix is usually a higher switch setting, provided your pump and tank are rated for it. Note that Amtrol’s card shows less drawdown per tank at higher settings (a WX-202 holds 6.2 gallons at 30/50 but 4.7 at 50/70), so the pump will cycle more.

What flow rate does a tankless water heater need?

To fire, a Rinnai RE needs 0.4 gpm and a Navien NPE-A2 needs 0.5 gpm. To keep up with your house, the heater’s output at your temperature rise has to beat the fixtures you run at the same time.

ENERGY STAR’s tankless buying guide lists average hot water demands of 2.5 gpm for a shower or tub, 3.3 gpm for a clothes washer, 2.2 gpm for a kitchen or bathroom sink, and 1.3 gpm for a dishwasher.

Output drops as the temperature rise goes up. Navien’s NPE-240A2 page lists 11.2 gpm at a 35°F rise, 6.5 gpm at 60°F, and 4.9 gpm at 80°F.

Worked example: if your well water comes in at 60°F and you want 120°F at the tap, you need a 60°F rise. At that rise the NPE-240A2 delivers 6.5 gpm, which covers two showers (5.0 gpm) plus a sink (2.2 gpm) only if one of them is throttled. Measure your actual inlet temperature with a thermometer at a cold tap after running it, because well water temperature varies by depth and season.

A low-yield well is a different problem. If your well cannot keep up with a long shower, see our guide to a storage tank for a low-yield well.

How hard is Texas well water?

Very hard in most of the state. A USGS summary of about 30,000 Texas water analyses found median hardness above 250 mg/L in five of the seven principal aquifers. Rinnai requires treatment above 200 mg/L.

Hardness and dissolved-solids medians come from the USGS report Texas Ground-Water Quality (Open-File Report 87-754), based on samples from 1900 to 1986. The treatment column applies the hardness table in Rinnai’s SENSEI RX manual:

• 0 to 60 mg/L: no treatment.
• 61 to 160 mg/L: a softener or Rinnai Scale Cutter is recommended.
• 161 to 200 mg/L: strongly recommended.
• 200+ mg/L: required.

Two Texas nuances the median hides. First, the USGS says Gulf Coast and Carrizo-Wilcox water is hard at shallow depths and softens with depth as sodium replaces calcium, so a shallow well there can test much harder than 80 mg/L. Second, the same Rinnai table caps total dissolved solids at 500 mg/L, and the median Trinity, Edwards-Trinity, and alluvium wells exceed that too. A softener swaps calcium and magnesium for sodium, as HomeGuide explains, so it will not lower a high dissolved-solids reading. Ask the water treatment dealer about that number specifically.

Does hard water void a tankless water heater warranty?

It can leave you without coverage. Rinnai’s manual says replacement of components due to water quality damage is not covered by the warranty.

Rinnai’s water-quality table also lists iron up to 0.3 mg/L, pH 6.5 to 8.5, dissolved solids up to 500 mg/L, and total hardness up to 200 mg/L, taken from EPA secondary drinking water standards. Read the water-quality section of your model’s manual before you buy.

What about sediment and iron in well water?

Filter sediment before the heater and remove iron above 0.3 mg/L. Rinnai says the water must be free of sand, dirt, and other contaminants, and its units have an inlet filter that needs regular cleaning.

HomeGuide lists a whole-house sediment filter at $150 to $500 installed, with cartridges at $5 to $30 changed every one to three months. For iron, it lists $1,000 to $3,500 installed and says iron above 1 to 2 ppm should get a dedicated iron filter ahead of the softener, because iron fouls softener resin. Iron and manganese are common in some Texas aquifers. The USGS report notes iron limits Carrizo-Wilcox water in parts of East Texas.

What should you install first on a well before a tankless heater?

Test the water first, then install treatment in this order: sediment filter, acid neutralizer if pH is low, iron filter if needed, softener, then UV if bacteria are present. The tankless heater goes after all of it.

That sequence is the treatment order HomeGuide gives for well systems, where each stage protects the one after it. A standard lab panel covering bacteria, nitrates, pH, hardness, iron, manganese, and dissolved solids runs $100 to $200. Use this decision table with your results:

Replacing the well pump too? Do that first, so the pressure switch is set before the heater is sized. Our well pump replacement cost guide covers Texas pricing, and the Texas well drilling hub has more on Texas wells.

How much does a tankless water heater on well water cost?

Plan on about $1,400 to $3,900 for the heater installed, plus $1,200 to $3,800 for a softener and a few hundred dollars for a sediment filter. Annual flushing adds $150 to $350 if you hire it out.

Rural Texas homes often run tankless units on propane. Angi estimates about $350 a year in propane to run a tankless heater versus about $200 for natural gas, so the fuel line matters as much as the water line.

How often should you flush a tankless heater on well water?

At least once a year. Rinnai’s table calls for a yearly flush on residential units with hardness from 61 to 200 mg/L, and HomeGuide suggests every 6 to 9 months on hard water.

Rinnai’s procedure circulates four gallons of undiluted food-grade white vinegar through the unit for at least one hour at four gallons per minute, then rinses it and cleans the cold-water inlet filter. That needs isolation valves on the hot and cold lines, so make sure your installer includes them.

What happens to a tankless heater on a well when the power goes out?

You lose hot water and, usually, all water. The well pump stops, and the heater’s controls need 120V power. The Rinnai RE manual lists a 120V AC supply.

In a freeze, that matters. Rinnai’s SENSEI RX manual says to drain the unit if power is lost in freezing weather, notes that freeze damage is not covered by warranty, and offers an optional Drain Down Kit that drains the heater automatically on power loss. For water through an outage, size backup power for the pump first. Our guides to a generator for a well pump and a power station for a well pump walk through the math.

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Frequently asked questions

Do I need a water softener for a tankless water heater on well water?

Usually, in Texas. Rinnai requires a softener or scale inhibitor above 200 mg/L hardness and recommends one from 61 mg/L up. USGS data put the median Trinity, Edwards, Edwards-Trinity, and Ogallala well above 250 mg/L. Test your well first, because shallow and deep wells can differ a lot.

What is the minimum water pressure for a tankless water heater?

Rinnai’s RE series lists 20 psi minimum and recommends 30 to 80 psi. Navien’s NPE-A2 series lists 15 to 150 psi. On a well, check pressure at the heater when the pump is about to kick on, after all filters, since each filter can drop pressure by 2 to 10 psi.

Can sediment from a well damage a tankless water heater?

Yes. Rinnai says the supply must be free of sand, dirt, and other contaminants, and sediment clogs the heater’s inlet filter. A whole-house sediment filter costs about $150 to $500 installed, according to HomeGuide, with cartridges changed every one to three months depending on how much sediment your well produces.

Will a 30/50 pressure switch work with a tankless water heater?

Yes, on its own. A 30/50 switch keeps pressure at or above 30 psi, the bottom of Rinnai’s recommended range. Add a sediment filter and softener, though, and the low end can drop below 20 psi in a worst case. Many owners raise the switch to 40/60 if their pump and tank allow it.

How often should I descale a tankless water heater with hard well water?

At least yearly. Rinnai calls for one flush a year on residential units with hardness from 61 to 200 mg/L, and HomeGuide suggests every 6 to 9 months on hard water. A professional flush costs $150 to $350, or you can buy a reusable kit for $100 to $300.