How to Flush (Descale) a Tankless Water Heater — and What the Service Valves Are For

What does flushing a tankless water heater actually do?

It dissolves the mineral scale that hard water leaves inside the heat exchanger, so heat gets into the water again instead of building up in the metal. Rinnai’s manual is blunt about the alternative: failure to flush the appliance will cause damage to the heat exchanger [1].

Hardness is the calcium and magnesium dissolved in water, and when hard water is heated, solid deposits of calcium carbonate form [7]. A tankless heater warms every drop it delivers in one compact heat exchanger as the water passes through, so that is where the deposits collect. Rinnai adds that scale build-up is caused by hard water and can be accelerated if the heater is set to a high temperature [1].

Noritz explains why it matters: excessive scale in the heat exchanger acts as an insulator and reduces the heat transfer into the water, and a dark grey colouring on the copper surface is a good sign of it [5]. Heat that cannot get into the water stays in the heat exchanger and the exhaust — which is what several of the error codes further down this page are watching for [1][5].

What are the valves under my tankless water heater for?

They let the heater be shut off from the house and flushed without cutting a pipe. There is one isolation valve on the cold line coming in and one on the hot line going out, and each has a small purge port of its own — a drain valve with a hose thread on it [1][4].

Rinnai’s manual describes the isolation valves supplied with its heater as the way to isolate the water heater from the building’s plumbing and give quick access for flushing the heat exchanger; the red valve goes on the hot outlet, the blue one on the cold inlet, and each drain valve is turned to where it can be reached [1]. Noritz notes that the flush ports on its isolator valves have 3/4-inch hose-thread connections [4]. The same part goes by several names — drain valve [1], service port [6], flush port [4] or purge port — and it is where the pump hose and the return hose connect.

The same valves are used for smaller jobs too. Rinnai’s maintenance list has the inlet water filter cleaned by closing the cold and hot isolation valves first, so the rest of the house keeps its water [1].

Two more things usually sit in the same spot. On a gas heater there is the gas shutoff: Rinnai says to turn off the gas at the manual gas control valve, usually located immediately below the heater, before any maintenance [1]. And there is a pressure relief valve, which Rinnai requires, says must not have any other valve between it and the heater, and says should only be tested by a licensed professional [1].

We install tankless heaters with an isolation valve kit. The photograph below is one of ours in Englewood: the isolation valves, the PEX lines and the gas shutoff under a Rinnai heater.

• Cold isolation valve (blue on Rinnai’s kit) — shuts off the water coming in [1]
• Hot isolation valve (red on Rinnai’s kit) — shuts off the water going out to the house [1]
• Purge (drain) port on each valve — where the flushing hoses connect [1][4]
• Gas shutoff, on gas models — turned off before maintenance [1]
• Pressure relief valve — required, with no other valve between it and the heater [1]

How do you flush a tankless water heater?

With the power and gas off and the house shut out by the isolation valves, a pump in a bucket pushes descaling solution in through the cold-side port, through the heat exchanger and out of the hot-side port back into the bucket, for the time the manual gives; then the heater is rinsed with clean water. This is what Rinnai’s own procedure sets out, in outline [1]:

• The controller switched off and the heater disconnected from electrical power; before any maintenance the manual also has the gas turned off at the manual gas valve [1]
• The shutoff valves on the hot and cold lines closed [1]
• The pump’s outlet hose on the cold-side service valve, a drain hose on the hot-side service valve, both ends in the pail [1]
• Four gallons of undiluted virgin, food-grade white vinegar in the pail [1]
• Both service valves opened and the vinegar circulated for at least one hour at four gallons a minute [1]
• The vinegar rinsed out with fresh water for five minutes, the inlet filter cleaned of residue, the valves returned to service and power restored [1]

Is the flushing procedure the same for every tankless heater?

The loop is the same; the details are not. Each manufacturer’s procedure differs in what goes in the bucket, for how long, and what has to happen inside the heater first.

Noritz’s procedure starts with the heater running so a technician can confirm, from the heater’s maintenance readings, that an internal bypass valve has closed; only then are the power, gas and water shut off and the solution circulated, for one hour [3]. Rheem’s flush kit instructions have the gas and both water supplies turned off, the gas kept off for the whole flush, and the controller switched off and on before unplugging so that an internal bypass valve closes [6]. Noritz also publishes separate descaling procedures for different groups of its own models [4].

That is why this page gives the outline and not a recipe. The manual for your model is the instruction; what is here is enough to know what a proper flush involves and to recognise one that skipped a step.

Should I use vinegar or a descaler in a tankless water heater?

Use exactly what your heater’s manufacturer names, because the manuals do not agree with each other. Rinnai’s procedure calls for undiluted virgin, food-grade white vinegar [1]. Rheem’s flush kit says to use virgin food-grade white vinegar and no other flushing solution [6]. Noritz’s procedure uses a calcium, lime and rust removal product mixed half and half with water [3], and its help page allows either pure vinegar or such a product [4].

Amounts and times differ as well: four gallons for at least an hour at four gallons a minute for Rinnai [1]; two gallons for 45 to 60 minutes for Rheem’s kit, with the note that some regions may need longer depending on build-up and hardness [6]; one gallon of product with one gallon of water for one hour for Noritz [3]. None of these documents offers a stronger homemade mix, and neither do we.

How often should a tankless water heater be flushed?

As often as your water’s hardness calls for. The manufacturers tie the interval to how hard the water is and whether it is treated, not to one calendar date for everyone.

Noritz prints a table for this. For homes, it lists no flushing for soft water (0–1 grains per gallon) or slightly hard water (1–3 gpg); once a year for moderately hard (3–7 gpg) and hard water (7–10 gpg) when no water treatment device is installed; and treatment required for very hard and extremely hard water [3]. The same page says damage from water harder than 12 grains per gallon (200 mg/L) is not covered by its limited warranty [3].

Rinnai lists total hardness up to 200 mg/L in its water quality guidelines [1]. Where the water is known to be hard, its manual says the water must be treated and may need a more frequent flushing schedule, and the heater has a Service Soon reminder that the installer sets to half a year, one year or two years — or switches off — based on the water conditions [1]. Separately, the manual calls for the heater to be inspected annually by a licensed professional [1].

For scale, the U.S. Geological Survey calls 0–60 mg/L soft, 61–120 moderately hard, 121–180 hard and anything above 180 very hard [7]. So the 200 mg/L limit both manufacturers draw already sits inside what the USGS calls very hard. To place your own house on that scale, start from your utility’s published figure; our article on Tampa Bay water hardness, linked below, lists each utility’s own number.

What does a scale or service code on a tankless heater mean?

Usually that the heat exchanger is due for a flush, or is overheating because scale is insulating it — but the codes differ from brand to brand and model to model, so read the one in your own manual.

The codes below are the ones in the documents we read. Several of the checks behind them — measuring sensor and switch resistance, inspecting the heat exchanger surface, checking gas pressure — are written for technicians [1][2][5], so an overheat code is a reason to have the heater looked at, not only reset.

• Rinnai SS (Service Soon) — a time-based reminder set at installation; it means it is time to flush and service the heater, and the manual says to contact your service provider [1]. The manual shows how to clear it — the On/Off button five times in five seconds — but clearing the reminder does not remove any scale [1].
• Rinnai LC on the RL75i and RL94i — scale build-up in the heat exchanger; the listed remedy is to flush the heat exchanger, or replace it [2].
• Rinnai 14 — heat exchanger overheat; the checks include looking for hot spots on the heat exchanger that indicate blockage by scale, and flushing [1].
• Noritz 94 — exhaust temperature too high on condensing models; scale build-up is the first common cause Noritz lists, and it can also bring other overheat codes such as 14 or 20 [5].

Can I flush my tankless water heater myself?

That depends on what your manufacturer says, and they do not say the same thing. Rinnai’s manual says the heater must be inspected annually by a licensed professional and that repairs and maintenance shall be performed by one [1]. Noritz says its descaling procedure is intended only for a qualified service professional or an authorized Noritz service representative, and that unauthorized use may void the warranty [3]. Rheem’s flush kit comes with instructions for its user, but they assume isolation valves are already installed and say to consult a plumbing professional if there are none [6].

Whoever does it, the manuals agree on the order: power off and gas off before the solution goes in [1][3][6]. The whole flush is done through the valves under the heater — none of these procedures has you open the gas valve or the burner compartment to do it.

If your heater has no isolation valves, it cannot be flushed this way until a set is installed on the water lines at the heater. Rinnai strongly recommends installing them so the heat exchanger can be flushed [1]; Rheem’s instructions send you to a plumbing professional [6].

Does scale damage void a tankless water heater warranty?

The manuals we read exclude it from cover. Rinnai’s says damage caused by lime build-up is not covered by the heater’s warranty [1], and its warranty lists improper maintenance — scale build-up is the example given — and damage caused by poor water quality among the things it does not cover [1]. Noritz’s descaling procedure says damage from water harder than 12 grains per gallon (200 mg/L), and from poor water quality, is not covered by its limited warranty [3].

Those are the manufacturers’ own words, not our reading of them. For what your heater’s warranty covers, read the warranty for your model or ask the manufacturer.

What this means in Clearwater, Tampa and across Tampa Bay

Water across Tampa Bay runs from soft to very hard depending on which utility supplies you, not on the city in your address. Our article on Tampa Bay water hardness lists each utility’s own figure; put yours next to the hardness limits in your heater’s manual and you will know whether the manufacturer expects the water to be treated, and how often it wants a flush.

We install tankless water heaters with an isolation valve kit, so the heater can be flushed later without cutting a pipe — the photographs on this page are from one of our installations in Englewood. We install and service water heaters across Clearwater, Tampa and the rest of Tampa Bay, and where hard water is the reason a heater keeps scaling, we also look at treating the water before it reaches the heater.

FAQ

Sources

We review and cite. We do not reproduce a paper or a manual — follow the links for the originals.

1. [1]Tankless Water Heater Installation and Operation Manual — RU199/RU180/RU160/RU130 (SE+ Series), U334-0771(02) — Rinnai America Corporation · 2019Manufacturer's manual · §2.1 isolation valve kit included; §3.2 water quality, total hardness up to 200 mg/L, hard water must be treated and may need more frequent flushing; §3.7 isolation valves (red hot, blue cold, drain valves); §3.8 pressure relief valve; §3.12/3.14 Service Soon interval; §4.5 codes 14 and SS; §5.1 annual inspection, maintenance by a licensed professional, gas and water off first; §5.3 flushing with 4 gal vinegar ≥1 h at 4 gpm; §6.6 warranty exclusions; read 2026-10-11
2. [2]Technical Data Sheet — RL75i, RL94i (U287-1817x01(00)) — Rinnai · 2011Manufacturer's manual · error code LC: scale build-up in heat exchanger, remedy flush or replace the heat exchanger; code 14 hot-spot check; for trained and qualified service technicians; read 2026-10-11
3. [3]Descale Procedure — Group C (Rev. 06/16) — Noritz America Corporation · 2016Manufacturer's manual · intended only for a qualified service professional; unauthorized use may void the warranty; residential flush-frequency table by hardness; >12 gpg (200 mg/L) not covered; 1 gal calcium/lime/rust product + 1 gal water, circulated 1 hour; read 2026-10-11
4. [4]How do I descale/flush the heat exchanger — Noritz America Corporation (knowledge base) · 2022Manufacturer's manual · manufacturer's help article: flushing is part of regular maintenance; pure vinegar or a calcium/lime/rust product; bucket, submersible pump, isolator valve kit; flush ports have 3/4-inch hose thread; separate procedures by model group; read 2026-10-11
5. [5]Error Code 94 — Noritz America Corporation (knowledge base) · 2022Manufacturer's manual · manufacturer's help article: exhaust temperature too high on condensing units; scale acts as an insulator and reduces heat transfer; may also cause EC20 or EC14; read 2026-10-11
6. [6]Tankless Flush Kit — Flushing Instructions for Paloma, Rheem, Richmond and Ruud Tankless Water Heaters (AP14429-2) — Rheem · 2012Manufacturer's manual · virgin food-grade white vinegar, no other solution; assumes service valves are installed, otherwise consult a plumbing professional; gas off for the whole flush; 2 gal, 45–60 minutes; copy hosted by a retailer; read 2026-10-11
7. [7]Hardness of Water — U.S. Geological Survey, Water Science SchoolGovernment or standards body · hardness is dissolved calcium and magnesium; heated hard water forms calcium carbonate scale; hardness bands in mg/L as calcium carbonate; read 2026-10-11