junda wrote:there is a knob at bottom of machine for tank or tap. and a little switch at bottom to bypass empty tank

So was really a "wrong relay" as you mentioned, or else?

Yes. seller just told me VBM will send the correct relay part.

I actually don't get why VBM added the relay in the first place, it's supposed to be always on anyway, then what's the point for controllable switch?

If this is to switch on the power, you don't want to do that with a switch.

You have a switch that turns on a high current capable relay that relays the power to the consumer (heating element or whatever).

Every automobile has an ocean of relays for just this issue.

A switch that could handle the high current would be bulkier and more expensive.

Relays are cheap due to the high volume made.

HTH

Where are you seeing 5V? With a meter? The control board says 5W 12V

I am very surprised VBM are sending electronic parts without the seller having a look first. A water fill or not filling is quite a simple thing to diagnose.

yes, with multimeter on output pins. the control box has AC wired and converts to 12v for its own use, and outputs 5v to pressure sensor and the relay for AC to 24v for pump.

Pump isn't on because no 24v because no AC because relay isn't on. I told the seller the relay model and it requires 18v, asked seller to check w/ VBM how can the control box output 18v to relay, then got response VBM will send correct relay.