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Noritz · Tankless water heater troubleshooting

Water not hot enough or temperature fluctuates - Noritz tankless

What is going on

Customer says water is not hot, swings in temperature, or one fixture is cooler, with no error code. Use the maintenance monitors to split a heater problem from a plumbing problem.

Likely causes

  • Flow below minimum (0.4 GPM to start, 0.26 GPM to keep running on EZ/NRCR)
  • Demand above heater capacity for the inlet temperature
  • Clogged inlet filter or scale
  • Bypass servo stuck
  • Low gas supply
  • Plumbing/fixture issue after the heater

Check in this order

  1. Read MM14 flow rate during the complaint; most units need about 0.5 GPM to fire (EZ/NRCR: 0.4 GPM activation, 0.26 GPM minimum operating).
  2. Read MM30 inlet temperature and work out the demand: Btu = GPM x temperature rise x 500 (e.g. 0.6 x 28 x 500).
  3. Read MM31 outlet temperature if it is at set temp, the problem is in the plumbing or fixture, not the heater.
  4. Read MM32 heat exchanger temperature MM32 high (140) with MM31 low (100) = bypass servo stuck/failed; both low = burner not getting enough gas.
  5. NRC/NRCP read MM49 fan speed with the cover ON; above 100 means intake/exhaust restriction.
  6. Clean the inlet water filter and descale if the water is hard.
  7. Set temperature 115-120 F helps keep enough flow through the unit on small draws.
  8. Troubleshoot under the same conditions as the customer (cover on, same fixture).

Applies to: All Noritz residential tankless (EZ, NRCR, NCC199CDV, NRC/NRCP)

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Source: Noritz EZ/NRCR/CDV Level 2 Maintenance & Troubleshooting (Premix R2 V2), Maintenance monitors, p.18-21; Noritz Residential Level 2 Maintenance & Troubleshooting (NRC/NRCP and older, R2 V1), Maintenance monitors, p.12-15; Noritz EZ/NRCR Owner's Guide (Rev. 10-25), Troubleshooting, p.29. Written in our own words; check the manual for your exact model.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.