Navien NCB-240 Combi Boiler - Not prioritizing DHW

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cm3939

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Hi All -

Having an issue with my NCB 240 combi boiler that I'm hoping someone can help with.

The boiler is not recognizing a call for hot water from our kitchen sink however this only happens when we have the radiant heat turned on and both sides of the boiler active. The strange thing is if I go to the basement and turn on the hot water in the laundry room sink (which is only a few feet from the boiler) the hot water will work there but then will also work in the kitchen sink afterwards. Also the kitchen issue seems to happen most frequently first thing in the morning after we wake up and haven't been running water in the house for several hours.

I'm somewhat handy and would like to address this myself before paying for someone to come out if possible. I'm guessing it's either a pressure issue at the kitchen sink (although the water pressure seems fine) or there is a part in the boiler which is supposed to recognize/prioritize DHW that is going bad.

Any help here would be very much appreciated.

Thanks!

Charlie
 

RS

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It sounds like your kitchen faucet isn't using enough water for the heater to turn on. Maybe just cleaning out the aerator will fix it. Every time I hear one of these stories about a tankless water heater I'm glad we don't have one!
 

Twowaxhack

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Turn the cold water off to the tankless water heater and then bleed the pressure off the water heater at a hot water faucet.

The hot side flow should stop completely.

If it doesn’t, you have a crossover condition

This is an easy test to do and would eliminate the possibility of this problem. Takes literally 5 min, no tools required.
 

cm3939

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Turn the cold water off to the tankless water heater and then bleed the pressure off the water heater at a hot water faucet.

The hot side flow should stop completely.

If it doesn’t, you have a crossover condition

This is an easy test to do and would eliminate the possibility of this problem. Takes literally 5 min, no tools required.
Thank you both for the response.

I did shut off the cold water to the DHW side of the boiler, turned on a hot water faucet and after a couple minutes the water had drained out and stopped so if I'm understanding correctly it's not the crossover issue.

Is this an issue of the kitchen sink not pulling enough water or having enough pressure to activate the water heater? I did remove the sink nozzle which has a built in sprayer to clean and soak in vinegar to see if that helps.

Thanks again for the help.
 

Twowaxhack

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Thanks for the feedback.

Cleaning out the aerator may have been the issue as it seems to be working for now. If it starts acting up again I'll plan on switching out the sensor.
Minimum hot flow for that unit is .5 gpm.
 

cm3939

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I checked out the flow while the faucet was running and it was showing .8 gpm so it seems to be pulling enough water. The issue only seems to come up when the radiant floor heat side of the boiler is running as well. From what I understand the boiler is supposed to prioritize the DWH side so I'm wondering if there is something malfunctioning that is stopping it from doing so. Could this be an issue with the computer?

Thanks again.
 
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