Navien combi does Startup/Post Purge cycle few times a day when there is no call for heat/hot water

Chris Alonzo

I am homeowner that recently had a Navien NCB 240/110H professionally installed. I work at home and the unit is in a closet next to my desk, so I hear everything it is doing. It's summer and the thermostats are switched off, and I can hear water running in case anyone using hot water. Several time each day I can hear it come on when it is not needed. When I check the Status on the panel, there is no flame, but it cycles between Startup and Post Purge for approximately 5 minutes.

I called Navien tech support and reported it and they said it is normal and it is doing a calibration. They couldn't provide me details of why it does it so often. Since then I have noticed when it is doing this, that there is a gas smell coming from the exhaust vent outside, which certainly does not seem normal to me.

Anyone heard of this?

HVACNUT

Has the installer been back? It would seem that they're the one's to call.
How often does it happen? Constant cycling through ignition without a demand doesn't seem right to me. They're not supposed to fire unless water flow is proven, so something is going on. 
You're sure there's ignition and it's not just a pump exercising prompt? Freeze protection if the supply pipe is 50°F or below?
And a gas smell at the exhaust isn't necessarily a bad thing. What were the combustion numbers after everything was dialed in?

hot_rod

Some of the combi boilers run short cycles to keep the DHW heat exchanger hot all the time. This reduces the hot water wait time. The burner would light off however if it is trying to maintain the HX

A few of the Lochinvar combis allow you to choose between cold start and always hot. Maybe look for that setting and toggle it off, if it is enabled.

Chris Alonzo

Thanks. I will check, but I don't remember seeing that setting. And as you said, the flame light should be on in that case.

I called Navien tech support and they suggested to call a licensed plumber and they would help them diagnose the issue.

Chris Alonzo

HVACNUT said:

Has the installer been back? It would seem that they're the one's to call.
How often does it happen? Constant cycling through ignition without a demand doesn't seem right to me. They're not supposed to fire unless water flow is proven, so something is going on. 
You're sure there's ignition and it's not just a pump exercising prompt? Freeze protection if the supply pipe is 50°F or below?
And a gas smell at the exhaust isn't necessarily a bad thing. What were the combustion numbers after everything was dialed in?

There is no ignition. I have a call into the installer.

Chris Alonzo

Finally figured it out! I don't have recirculation, so it never occurred to me to check the settings for Recirculation. But it finally dawned on me that I should check it, and sure enough, it was set to "Combi-Preheat" instead of "No recicurlation"!

Another tidbit: The Navien manual for the NCB has a 'default' value for every setting, except for this one. So, I don't know what that means. Is it a random value when it comes from the factory? Otherwise, why would it have come from the factory with it set to "Combi-Preheat". I would expect that most people are not using recirculation unless they have new construction.