Navien 240A

LagoVista

My second (slave unit ) has gone dead, no screen, no reset. The prime unit is fine, but is throwing an error code due to the loss of connectivity with the slave unit.

I was on vacation for two weeks and typically power down both units using the screen power off button. When returned from vacation, the slave unit came to life with the normal powere button, but a week later is now dead. The unit is 2018 install, and has had minimal use.

I called tech support, ZERO help.

I have a volt meter and some technical knowledge, but looking to avoid an Easter Egg Hunt.

pedmec

Did you make sure it wasn't unplugged? Someone might have needed power for a tool or something and just never plugged it back in

109A_5

If I had to guess from here I say the power supply (PWM / SMPS) that runs the logic (and other stuff ) has failed, the Orange arrow. I'd check the fuse first. Would not surprise me if there are bad capacitors. I would verify the 5, 12, 15, 24 VDC rails are absent. The problem is they may not be labeled. On the primary side of the SMPS (Switch Mode Power Supply) there are lethal voltages. Attempting and /or doing this type of work is not for everyone, you need to know how to work and use the meter safely.

109A_5

You did verify the actual (hardware) power switch and that fuse. Fuse locations seem kind of vague.

Alan (California Radiant) Forbes

"I would verify the 5, 12, 15, 24 VDC rails are absent."

What do you mean when you say this?

109A_5

The on board power supply, SMPS (Switch Mode Power Supply), on the control board converts 120 VAC to 5, 12, 15, 24 VDC rails. If the SMPS is dead all low voltage DC rails will be missing (or absent). If only the 5 VDC is missing (or absent) the logic will be non-functional and the unit will probably appear dead.

The on-line pictures of that control board, it looks like the control board is placed in a plastic holder and potted or submerged in a clear conformal coating. This makes diagnostic measurements on the control board more difficult. Also the low voltage rails are probably not labeled on the board, but shown on the basic drawing.

Since SMPS repair is not a skill set found with most HVAC techs and/or home owners, the actual repair of the board on site is not likely. So the most pragmatic option is to replace the control board, if defective.

Verifying that the 120 VAC is actually making it to the control board and the DC rails are missing (or absent) helps focus the accuracy of the diagnoses. Which may be replacing the control board.