Two wall-mounted tankless water heaters with expansion tanks on an exterior wall
5 components covered

Field manual · Water Heater

Gas tankless water heater: field service manual (water side and diagnostics)

Covers: Residential non-condensing and condensing gas tankless units with digital controllers

Cross-checkedBrands: Rinnai, Navien, Noritz, Rheem, A.O. Smith, Bradford White5 components

Tankless units heat water only while it flows through a heat exchanger. A flow sensor starts the sequence; a fan, igniter and modulating gas valve set the firing rate. Water-side service (filters, descaling, flow problems) is field work; gas, combustion and venting are blocked and referred.

Safety first

  • Gas, combustion and venting work is blocked: refer to a licensed gas technician
  • Power off before opening the cabinet
  • Repeated vent/combustion codes: shut down and check CO alarms

How it works

When a tap opens and flow passes the activation threshold, the flow sensor signals the board, the fan pre-purges, the igniter fires and the flame rod proves flame.

The board modulates gas to hit the setpoint based on inlet/outlet thermistors.

Condensing units have a secondary heat exchanger and produce acidic condensate that must drain through a neutralizer.

Freeze protection uses small heaters and, on some models, burner firing; it needs both power and gas to work.

Components & tests

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Official manufacturer documentation

Always confirm values against the tech sheet for the exact model.

OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work. © 2026 OnCall Tech. Original content; not a reproduction of any manufacturer document.Generic part photos (not of this brand): Two wall-mounted tankless water heaters with expansion tanks on an exterior wall by Downtowngal, CC BY-SA 4.0

Diagnostic guides for this appliance

Guide · Water Heater

Heat-pump (hybrid) water heater: slow recovery, noise, errors

Hybrids need enough room air and a clean air filter, a working condensate drain and the right mode. Many complaints are mode settings or a dirty filter. Refrigerant faults are sealed-system work.

5 steps ~60 minmoderate
Guide · Water Heater

Tankless water heater descaling (flush)

Scale insulates the heat exchanger, causing overheat codes and fluctuating temperature. Flush with a pump and descaling solution through the isolation valves.

5 steps ~75 mineasy
Guide · Water Heater

Hot water recirculation pump problems

Check timer/settings, check valves, air in the loop and the pump itself. On tankless units with built-in pumps, a recirculation code may point to air or a leak in the loop.

5 steps ~60 minmoderate

Other water heaters manuals

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Rheem Performance / Performance Platinum electric tank (XE-series): quick sheet

Rheem Performance Platinum electric tanks sold at big-box stores with model numbers starting XE (e.g. XE50T12ST45U1 with 4500/4500 W elements, XE50T12EC55U1 with 5500/5500 W elements and LCD touch control, XE50T12CS45U0 / CS55U1 'Gladiator' trim with LeakGuard and built-in EcoNet). The '12' in the model is the 12-year warranty tier. Exact production years not confirmed; date the unit from its rating plate.

7 components4 common failuresRheem
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Bradford White RE (electric) and RG (gas) residential tanks: quick sheet

Bradford White residential upright and lowboy tanks: RE-prefix electric (e.g. RE250T6 = 50 gal tall) and RG-prefix atmospheric gas (Defender Safety System). Sold through plumbing wholesalers, not big-box stores. Newer ERE models with ICON E electronic control are a different platform. Date from the serial number letters (see below).

7 components3 common failuresBradford White
Manual · Water Heater

Rinnai RU / RUR (SENSEI) condensing tankless: quick sheet

Rinnai condensing residential tankless: RU-series (RU199iN/eN, RU180, RU160, RU130; indoor 'i' and outdoor 'e'), RUR models with built-in recirculation. Older non-condensing RL / V-series and RE models differ. Rinnai serial numbers encode year then month in the first four digits.

7 components4 common failuresRinnai
Gas tankless water heater: field service manual (water side and diagnostics) · OnCall Tech