How to Bleed Air from Navien Combi 240?

[Hope]

I have a Navien NCB-240E combi boiler for DHW and heating (baseboard+radiators) with 4 zones. I have had issues in the past with pressure dropping and boiler stopped working and giving error code E351. Majority of the time It’s fixed by draining the radiators on the second floor (sometimes I also have to up the pressure by using the manual feeder – marked by a red arrow in the first picture).


For the past week and a half, boiler has been giving problems and shutting off with error E-351. When this happens, I bleed air out of the radiators on the second floor, press reset button on the boiler and it turns on again and starts to work fine until another Error E-351 (sometimes on the same day or a day later) shuts the boiler off.

I suspected there is air in my system and I am not able to bleed it effectively due to lack of knowledge. Did some research and hooked up a hose with the drain, shut the boiler, and shut off each zone valve and drain water by hooking up a hose to the valve marked with white circle in second picture. This didn’t work and boiler displayed ‘AIR’ sign. So I bled air from two radiators from second floor. One of the radiator didn’t discharge water after the air was bled. (

Would like to point out that, one of the radiators on the second floor, bleeds air for the first few seconds but water never comes out of it. This is the radiator which has been turning hot the last and taking the most time to heat up. ) SO I manually fed the boiler more pressure and then bled the same radiator until it had water discharging. This was done with boiler on and all zones asking for heat.

I can tell I still have air in my system and boiler will shut off in a day or so and give E 351 error.

Can someone tell me whats going on? How to effectively bleed the air out of it?
Thanks in advance.

 

[Dana]

You need to pressurize the system using a pressure gauge to eyeball it while filling it, and the expansion tank needs to be pre-charged with air to the system's target operating temperature. The "I" parameter on the user display is the space heating system pressure measured by the NCB, and usually "good enough" to work with.

The minimum pressure to fill to would be 3 psi + 0.433 per foot of elevation between the location where the pressure is being measured to the highest radiator on the system. If the pressure isn't at least 0.433 x elevation it won't be able bleed air at the top, even if you managed to fill the system by other means. Independent of the elevation, pressurized the system to at least 13-15 psi at the boiler to prevent flash-boil on the heat exchangers, which cuts into efficiency (and can be really noisy.)

The maximum pressure on the boiler is 30psi, but don't set the system pressure anywhere near that high or you may damage system components or spring some leaks. The pressure relief valves on residential systems are normally set to 30psi.

Draining water from the system will only lower the overall pressure.

 

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[Hope]

Thanks for your feedback.
Boiler has been working fine. This morning the pressure was 18.1 PSI while running. Just now, the boiler stopped working and had the E351 error. I pressed the reset button and it restarted and is working fine now. Highest elevated radiator/baseboard is on the second floor which would be around 21' from the boiler in basement. The pressure calculates to 12.52 6PSI (3+(0.433 * 22)).

I am not technical when it comes to this stuff so I dont know how to pre charge the expansion tank?

Not sure why its happening. Could it be that there is other than air getting into the system? I have a feeling that my boiler looses pressure over time....few days ago it was running around 20-21 psi and this morning it was t 18 psi.
Any ideas/thoughts???

p.s. Just saw this video where the pressure sensor was changed to take care of E 351 error.

 

[Dana]

This video talks about pre-charging the expansion tank for a water heater, but the princple is the same. With the tank disconnected from the plumbing, pump the air to the target system pressure plus 1-3 psi. eg:

If the system's water pressure is set to 13 psi (at idle, not when hot at the end of a burn) set the air pressure in the tank to 14-16psi, not more, and not lower than 13 psi.

Then re-install it.

 

[Hope]

I may be calling a plumber to do that. Just for my own understanding, what do you think is causing the boiler to stop all of a sudden with E351 error and how pre-charging the expansion tank will fix it? Sorry for the silly question but you are dealing with a layman here.
Today I noticed that when I turn on the valve from the manual feeder to increase the pressure, it didnt work and the pressure didnt go up? could that possibly signal?
THanks.

 

[Dana]

Any time the pressure is low enough that the internal aut0-fill valve opens to add water for 5 minutes it will spit out that error and halt. If an improperly charged expansion tank causes the pressure relief valve to open up when the system is hot, removing water from the system, the pressure will drop when the system, perhaps low enough to cause the boiler to self-protect by adding water. Per the manual:


5.2.13 351Error

If the water pressure sensor senses low water level as the heating pipe pressure is low, the system stops operation of the boiler, and refills water automatically.

If water is supplied by auto feeder valve open over 5 minutes, the error(E351) is detected and displayed on the front panel. If this occurs, the boiler initiates shutdown.

So yes, the expansion tank or system being improperly pressurized could create that error condition.

 

[Hope]

Update:
So I had a Navien licensed plumber come take a look at the unit. He said the valve (display with a red arrow in the first picture in an above post) is shut…it is supposed to be in ‘on’ position. It is important to note that the valve has been in shut position for months and whenever the pressure dropped in the unit and it stopped working, I would turn it to on position and up the pressure. Secondly, he changed the water pressure sensor in the unit. Third, he increased the heating water temp from 150 to 180. (This may be why when it gets too cold, temperature in the house doesn’t seem to climb up to the desired thermostat temp.) He also drained the boiler of air.

Since then, the unit has been working fine with no issues. He did let me know that there is no manifold in the plumbing and I do need one because the unit cannot be serviced without it. Now the boiler has been running at around 27 psi. Is there any other benefit in having the manifold installed besides servicing the unit?

 

[Dana]

Running the heating system side of the combi boiler at 27 psi is a bit too high.

If you were/are running the boiler at a fixed temperature of 150F/180F you aren't getting any of the condensing efficiency this boiler is capable of. Using the outdoor sensor and adjusting the compensation curve you can get it to automatically run at 180F when it needs to, and 130F (for good condensing efficiency) when it's not as cold out. Before diving into that, run a bit of napkin-math analysis first. In most heating systems you can save more than 10% on heating fuel use and be more comfortable all the time if you have enough zone radiation to run it cooler without short cycling the boiler.

The manifold he was talking about is probably Navien's pre-engineered hydraulic separator. The system can run just fine with some other approach to hydraulic separation, and though I'm unable to visually untangle the plumbing in the pictures provided you almost certainly have some type of hydraulic separation in there, given that there are four zone pumps. The heat exchanger inside the Navien can't pass the high flow of all four running at once, and individual zones would be starved for flow if all are running at the same time.