Navien NCB 240e Combi Intermittent Short Cycling

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braindrain22

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Hi everyone,

Aside from the internal circulating pump failing in September, the unit has been running without issue since its install 5 years ago.

Everything was in order during my annual service 3 weeks ago by a Navien certified plumber.

Starting 2 days ago, I noticed the thermostat calling for heat for an hour--which was very odd. The boiler can typically get the temp up in about 5-10 minutes, never an hour (outside temp wasn't much lower than 30 which is pretty typical here in New England).

When I went down to unit I noticed the boiler firing for about 1 minute until hitting the temp setting (140) then ending the fire until the temp dropped back down to about 115, then firing again in the same pattern.

The temp of the supply was definitely 140, right until it hit the T in the Navien manifold, at which time the hot water blended with the circulating water and was cooled. Return temp was very cool, maybe 100 deg F.

In the morning the house temp was 5 degrees below thermostat setting (63 deg F, set at 68 deg F). So the unit was calling all night, and sending warm water to rads, just never hot.

Randomly later that morning the unit began firing as normal (nice long 10 minute fire with appropriately warmed return water temps).

The problem went away for about a day, then returned again. This time I shut the unit down completely. Following the full shutdown it worked fine for a day, then fell into the short cycling pattern intermittently.

I imagine a sensor is beginning to fail somewhere, just not sure where or which. Since it's the beginning of heating season here, lead time for a tech is a couple weeks. Perhaps this could be a setting update (during the last service the tech did mention turning the unit down to use about 80% capacity).

Anyone see anything like this with a Navien?

edit, the unit has no errors; domestic h/w works fine.
 
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Tuttles Revenge

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it should be showing an error code. Best way to resolve is to call Navien and have one of their techs help diagnose the issue.
 

braindrain22

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Follow up in case anyone else has this issue:

No error codes, no functional issues aside from the burn times being short. See below for cause.

Oddly enough there is a back flow prevention valve following the zone circulating pump (single zone). The valve is getting stuck intermittently. The symptom here is supply and return temps rising simultaneously within about a minute until the set space heating temp is reached (values A and B in INFO). Return should rise more slowly given full system traversal is required. Unit stops the burn due to the temps being tight and the return temp being so high. Basically the unit does not know the water never circulated.

Local service tech figured this one out. Solution is to remove the back flow prevention valve. Several plumbers at the company researched the necessity for such a valve with Navien systems and never heard of the requirement--they are a Navien recommended company.

Solution in this case is to remove the valve.
 
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Tuttles Revenge

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Thanks for the follow up. I just fired up 2 naviens on Wed in prep for families wanting to move in and use their almost remodeled homes for the holidays..
 

braindrain22

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Following up again on this.

Turns out the back-flow valve wasn't it.
The problem was the three way valve.

I held off calling the plumbers back to get more data. I was also fortunate to get a more experienced tech the second time around. After a few calls to Navien I finally get a pretty solid tech specialist to talk to me. Fortunately the problem was occurring while on the phone.

The number one piece of diagnostic info was random clicking sounds coming from the unit--both the experienced plumber and navien said this. This apparently is the valve sticking and beginning to fail.

The second piece of info has to do with the 'Info' values on the unit.
Value A and B stacked at same temp and rising together along with an increase in value C during a call for heat is indicative of the three way valve.
Basically the valve is sticking making the unit think the return temp is high (B) when really its not able to circulate out causing it to dump into the lower heat exchanger (value C is DHW temp).

So that's it. Three way valve.

Good luck everyone!
 
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