Noritz EZ Series & NCC199CDV Code 20
Heat exchanger high limit switch triggered. The high limit switch on the primary heat exchanger has tripped. Noritz's checks are whether the switch triggered, whether it is properly connected, and then one line that is worth the whole entry: check if there is scale build-up in the heat exchanger, because this error code may be caused by it. Noritz does not say that about any other code in this table.
Also displayed as: F20
Call a proThe verdict
Call a licensed plumber
Noritz names scale as a cause here, which makes this the code where a flush is the likely answer rather than a part. Cycling power clears it; if it comes back, the switch is doing its job and the heat exchanger needs attention, not the switch.
Typical cost: $100–$900
US estimate for parts and labor. Prices vary by region and by who does the work — see how we estimate costs.
What the manual says
The meaning above is transcribed from the manufacturer's diagnostic documentation for the EZ Series & NCC199CDV — not averaged across the brand.
Source: Noritz Service Manual — NRC663 FSV, EZ98DV, EZ111DV, NCC199CDV (Rev. Oct 2017) (p. 32), read Sep 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
Before anything else: if you smell gas, stop here, leave, and call your gas utility.
- Note when the unit was last flushed. If the answer is never, you probably have your cause.
- Check the inlet water filter is not restricting flow — low flow through a firing heat exchanger produces the same result.
- Cycle the power, which is Noritz's documented reset, and see whether it returns.
- If it returns, book a professional flush and inspection. Never bypass or jumper the switch.
Frequently asked questions
The unit already has a scale reminder code. Why does 20 mention scale too?
Because they catch it at different stages. The C1 service reminder is the unit counting hours and telling you maintenance is due. Code 20 is the high limit switch tripping because the heat exchanger is no longer moving heat into the water fast enough — which is what neglected scale eventually causes. If you are seeing 20, you are past the reminder.
Code 20 on other Noritz series
The same code number can mean something different on another series — make sure you're reading the page for your exact unit:
Don't confuse this with
One other Noritz EZ Series & NCC199CDV code points at a switch too. They are different components in different places, and ordering a part against the wrong one is the most common way this fault gets expensive: