Navien Combi Heats home but no DHW

[Brooklyn]

The Navien is a recent purchase and this issue started when it got really cold. The combi starts up to supply hot water to radiators but does not supply hot water. The water is running cold in all the shower and faucet. There is no error code. The flow should be high as I dont use faucets that have constrained flow but how do I know whether its flow issue. How do I fix navien to heat water as i think the system and water flow is fine?

 

[Fitter30]

Whats the model.number? When running the hot water does the display show flow/ gpm?

 

[Sponsor]

 

[Brooklyn]

Whats the model.number? When running the hot water does the display show flow/ gpm?

It's a NBC 150e and its connected to 2 showers and 4 sinks. I shut off hot water valves for 2 faucets ... I keep running dhw but hot water indicator on navien screen does not come on indicating its not heating water. The heating system works however. The screen shows psi but not gpm. Psi is around 21.6. I keep running flow at shower and sink full blast but sometimes it has triggered navien to heat but most times not. It was all fine before cold weather hit. I dont understand why mavien she not coming on. The gpm in the mode info settings shows 0 gpm.

 

[Dana]

It's rare for a unit so young, but a bad or stuck 3-way valve can leave you with no hot water, but it should spit out a code.

If it's not showing gpm it could be that a valve somehow got closed preventing flow, or the flow sensor is disconnected or toast. Look up the installation manual for your exact model online.

You may also get some insights and better idea of what's going on under the cover by running a web search on the terms [mikey pipes navien ]. This guy is a Long Island plumber who services and installs dozens of Navien NCB & NHB every year, and posts many videos of service calls on a popular video hosting site.

The -150 is a bit underpowered for a 2-bath house, as it it won't support 2 full flow showers simultaneously this time of year (but could be OK with 2 low-flow showers as long as no other hot water draws are made during showing). During a tub fill in one bathroom using a shower in the second bath could be next to impossible.

 

[Brooklyn]

My plumber said the 150 should be fine but I think you are right as the problem did happen in past when we run multiple water lines. However now it just does not generate hot water for DHW. I looked up Mikey Pipes and his calls do not relate to this issue.
Could heating system work if flow sense or toast or disconnected? I suspect its a flow issue but can't figure out where the issue arises. Where can I find the valve so can see if it closed?

I might just buy another unit to just run DHW and leave the 150 for heating as I suspect you are correct.
As well, I think there should be more warnings for people to know how these Navien's are finicky as I think when you live in wildly fluctuating climates with varying water lines and pipe sizes and city supply flow rates fluctuating due to weather, I think they just don't make sense like for where I live in old Brooklyn. I truly regret getting these systems. My big fear is that natural gas prices will rise substantially as LNG supply infrastructure increases and exports of LNG will raise natural gas prices in North America but maybe I should have looked for something as good as option for cost management but easier to maintain....

Thank you for any answers you can provide.

 

[Fitter30]

Flow sensor is mounted just above the dhw connection inside of cabinet try tapping on it with water running. E on display is flow rate. Also there is a filter and a orfice at the dhw connection.
Condensing water heaters need yearly service filter need to be cleaned and dhw side.

 

[Dana]

Could heating system work if flow sense or toast or disconnected?

Yes- the potable water flow sensor has no effect on the heating side.

I might just buy another unit to just run DHW and leave the 150 for heating as I suspect you are correct.

You probably don't have enough capacity at the meter to run a pair of tankless water heaters.

As well, I think there should be more warnings for people to know how these Navien's are finicky as I think when you live in wildly fluctuating climates with varying water lines and pipe sizes and city supply flow rates fluctuating due to weather, I think they just don't make sense like for where I live in old Brooklyn.

When properly installed Naviens aren't really very finicky at all. They are certainly better than many older tankless water heaters.
The biggest mis-match for most Brooklyn houses & apartments is on the heating side. Combi boilers usually a bit too big for the design heat load, and often too big for the available heat emitters, though the -150 still has a reasonably low minimum firing rate compared to most. When the boiler is oversized for the radiation, unless there is a decent amount of thermal mass in the system (such as big cast iron radiators rather than fin-tube baseboard or other convectors) combi boilers are prone to short-cycling due to insufficient radiation. The napkin math on how to analyze that issue can be found here.

 

[Fitter30]

It's a NBC 150e and its connected to 2 showers and 4 sinks. I shut off hot water valves for 2 faucets ... I keep running dhw but hot water indicator on navien screen does not come on indicating its not heating water. The heating system works however. The screen shows psi but not gpm. Psi is around 21.6. I keep running flow at shower and sink full blast but sometimes it has triggered navien to heat but most times not. It was all fine before cold weather hit. I dont understand why mavien she not coming on. The gpm in the mode info settings shows 0 gpm.

With no flow no burner look at picView attachment 70951

 

[Fitter30]

this is from the service manual

 

[Brooklyn]

Its not the valve and my plumbers says its the flow. He says it wont trigger the sensor but I just don't believe it. When I put on my faucet or shower, the flow of the water coming out is strong. Am I missing something?

 

[Fitter30]

Its not the valve and my plumbers says its the flow. He says it wont trigger the sensor but I just don't believe it. When I put on my faucet or shower, the flow of the water coming out is strong. Am I missing something?

Did you read my post above yours. Without flow no burner also read my other.post to show you where it's located

 

[Dana]

Its not the valve and my plumbers says its the flow. He says it wont trigger the sensor but I just don't believe it. When I put on my faucet or shower, the flow of the water coming out is strong. Am I missing something?

I haven't read the spec on the NCBs recently enough, but most better-class tankless water heaters these days 0.8-1 gpm is PLENTY of flow for starting an ignition cycle, and most won't flame out after lighting until the flow is 0.5-0.6 gpm. The mixer on a shower would be mixing in some cold so it's not a good tap to bucket-test the flow with a stopwatch, but the sink would be.

If the domestic hot water setpoint temp on the NCB is too high you can run into flame-out issues, and if the tankless is set high but has a tempering valve down stream of the water heater mixing cold water into the distribution pipes it may have problems lighting up. Programming the NCB to 110F-115F would fix some of those issues.

But if the displayed flow is 0 gpm it'll never fire up. A reading of 0gpm means there actually ISN'T any (or barely any) flow going through the water heater (closed valve? Scaled-up solid from hot water & crud blocking the screen filter?), the sensor isn't sending, the sensor is disconnected from the control board, or (rarely) part of the control board is fried.