Navien PSI drop NCB 240e

[TheNoobieHomeOwner]

Hey guys,

I have a combi NCB 240e navien system about 5 years old. In Nov 2025, the system has 13-14 psi consistently when not in use, but once the weather got cold, it dropped to 11 psi in Dec 2026 and 9 - 10 psi in January 2026. i have a couple of noobie questions, hoping someone can help and ease my mind.

1. Is this drop in psi normal in the cold, like how car tires drop in PSI?
2. The manual recommends 15 psi (when idle), so if I am at 9.1 should I be concerned?
3. should i open the water valve and add water into the system to increase its PSI?
4. kinda related to the above:
a) if I go down this route, how much water / PSI should i add (I want to be safe and is it safe to begin with?)
b) a long time ago, I drained the system to put in a new sensor, but as I add water back into the system to increase its PSI, after a certain point the system doesn’t in-take water anymore, is that normal or is the system reacting like this?

Any thoughts or comments are welcomed guys!

TheNoobieHomeOwner

 

[davnit32]

1. Yes, some PSI drop in cold weather is normal due to thermal contraction, but going from 13–14 down to 9–10 is a bit more than expected.
2. At 9 PSI, you’re slightly below the recommended range. It’s not an emergency, but it’s better to bring it back to around 12–15 PSI when cold.
3. Yes, you can safely add water using the fill valve to raise the pressure.
   4a) Add water slowly and bring it up to about 12–15 PSI with the system cold. Don’t overfill.
   4b) Yes, it’s normal for the system to stop taking water once the internal pressure matches the supply or the fill valve closes.

If the pressure keeps dropping, you may have a small leak or an expansion tank issue worth checking.