Navien CH-240 air starvation

[propofolboy]

Hi everyone, first post here. Maybe someone can help me.

I have a Navien CH-240 unit manufactured in 2010.

It has functioned well. The siphon started leaking so I got a plumber in to replace the siphon and well he was in we changed the DHW flow sensor pre-emptively. Easy replacements.

When we replaced the cover and tightened it up I started getting error 12 (flame loss) intermittently. We discovered that this only occurred with the case screwed tight. As long as the front case was left on loose it would obviously suck some air in around the case and it would never get an error 12. The problem was air starvation.

We examined the air intake vent. Its about a 30 foot run of 3" pipe, with one 90 degree elbow and a 45 angle, so well within the 100 feet allowed. With a scope it was completely clear of obstruction. The air intake filter was clear with no debris. There was no flue gas recycling (they are well separated). With the case on but not tight I can feel a good draw of air into the unit.

In my googling I have noticed many people have been troubled by intermittent error 12's, and some describe functioning well with the case off but not on. I also read someone describe that the Navien can 'learn' its operating conditions, including air intake so this can be calibrated..

We are at a loss right now for what to do. Everything seems fine, and it functions flawlessly with the case on but not tight. As soon as we tighten it it can't pull enough air via the intake. Anyone have any suggestions??
I don't want to start just replacing parts (PCB, fan motor, etc).

Thanks.

 

[Jadnashua]

If the incoming air ducting is clear (including the grill on the outside), the filter is not clogged, at least to me, the only other things left are either the sensor that detects it's not getting enough air is faulty, or it really isn't getting enough air, and then, if the later is true, it is likely that the blower fan is not running as fast as it should be. That could be because the tachometer output is not correct and it thinks the fan is running properly, or the fan itself (the tac logic may be inside the fan assembly) is not running fast enough. It should not matter if the case is on or off, since, at least in theory, the intake to exhaust is supposed to be sealed. It sounds like the inlet is leaking, and letting some air come in from the room verses sucking it all from the intake port through the pipe to the outside.

Just thought of something else...the burner may be clogged somewhat. A good test may be to check the CO, which looks at how efficient the burn is...an air or gas problem would cause that to be out of range.

With it running, and a smoking match, see where the air is being drawn when the cover is off. The pressure (vacuum) shouldn't be really high, and the thing might work correctly without leaks if you can figure out why it's not getting the proper air.

 

[Sponsor]

 

[propofolboy]

Thank you so much for input.
So just to clarify, the unit functions with the case closed for a while, and it does pull air from the outside intake (if i put a match it pulls the smoke, if i put a piece of paper on it it sucks it to the intake). However it eventually (and I suspect when it needs max BTU's) gets starved and fails.
With the case even slightly cracked open it never fails or goes into an error mode. It just takes the case being slightly loose. Its obvious it just needs a little bit of extra air occasionally to keep it from failing. Also if your not familiar with this unit the air intake is just a hole in the case, and the air enters the fan from the bottom of the fan. The fan pulls air from inside the case, and the negative pressure inside the case pulls air through the air intake from outside.
My guess is the air intake is just at its limit of length despite being within spec (< 100 ft of 3"). I haven't tried this but if I cut the air intake 5 feet from where it exits the house so it is 5 feet shorter then the problem would be solved (but it wouldn't then be pulling air from outside the house but from my crawlspace!).
The fan seems to be working correctly. And I don't want to just start replacing parts.
The other thing is I have seen other people have error code 12 in this unit and they never figure out a solution, or they figure they ultimately have to keep the case not sealed tightly. I'm suspicous that I'm not the only one with this problem and maybe the fan is undersized for any significant air intake run...

 

[Dana]

Every 90 degree ell of 3" PVC is worth 11', every 45 is worth 5', but it sounds like you still have plenty of margin on total length. If you disconnect the intake air pipe at the unit so that it's effectively zero feet, does it still spit an error with the case cinched tight?

 

[propofolboy]

Unhooked the air intake and it is running fine with the case tight - except for the big hole where the air intake goes!

 

[Dana]

So it seems pretty clear that the impedance of the intake venting is 100% of the problem, despite being within spec. There may be some value drift / aging issues in some components in the controls that detect the error, but I doubt even Navien certified techs could debug it to that level. It might a board swap to make it go away, but for now it seems as long as a TINY amount of additional combustion air is allowed it will work.

Consider drilling a 1/4" hole in the PVC near the connection to the unit to see if that fixes the symptom. It can be easily taped-over or plugged later if need be.

Are the air intake filters on these designed for periodic replacement?

 

[propofolboy]

The air intake has a removable filter that is to be cleaned periodically. Basically its a metal screen.

I was hoping someone was going to say there is a calibration mode or something for this unit depending on the air intake requirements but it doesn't sound like thats the case.

I'll consider the hole in the PVC, that would probably solve the problem of being able to close tightly the case but it doesn't really change that the makeup air will be coming from inside my house. I guess this isn't the end of the world provided its not causing negative pressure inside the house.

Thanks so much for replies.

 

[Terry O Pfeffer]

The air intake has a removable filter that is to be cleaned periodically. Basically its a metal screen.

I was hoping someone was going to say there is a calibration mode or something for this unit depending on the air intake requirements but it doesn't sound like thats the case.

I'll consider the hole in the PVC, that would probably solve the problem of being able to close tightly the case but it doesn't really change that the makeup air will be coming from inside my house. I guess this isn't the end of the world provided its not causing negative pressure inside the house.

Thanks so much for replies.

 

[Terry O Pfeffer]

I have a ch-240 had problems a long time, as other people would not run with front cover on. there is a flapper valve ( back flow preventer ) on the intake mine was stuck in about halfway position, cleaned that and all I could get to. ( it now works fine)