Wiring conundrum for the water heater

[routlaw]

So I am finishing up the install of a new water heater, tankless version to replace the old Suburban. Retrofitting into the Oliver is NOT an easy task but more on that later. So on the old Suburban there are 3 12 volt DC wires connected to the 12 volt terminals, red for +, blue for -, and yellow/green for ground. However when I measure the voltage across the wires with HW switch turn on, the red to blue measures only 12.2 volts but when I measure the red to green/yellow @ ground I received the full SOC of my Epoch batter which is 13. 7 ± volts. 

The new Fogatti tankless only uses two DC wires, red and black, and no AC connection at all. I am inclined to think in my case to use the red and green/yellow wire for the full DC voltage available to me. 

Can anyone explain why the red to blue wires only return a reduced voltage of 12.2 volts DC? My gut instinct is the wiring was done incorrectly at the factory build swapping the negative terminal for the ground but this is just a stab in the dark not being totally versed in DC electronics. 

Thanks and appreciate any insight on this.

[Snackchaser]

You are measuring volts with WH switch on, which means your measuring the voltage drop of the internal WH electronics.  Disconnect the red, and volts should be 13.7.  The yellow/green wire is a non-current carrying ground due to the AC aspect of the WH, so it does not see the electronics voltage drop.   All seems normal.

Cheers, Geoff

[routlaw]

I probably did not explain my situation thoroughly @Snackchaser. The old Suburban is completely out of the Oliver, AC wires are taped off and disconnected. When measuring the remaining 12 volt wires they have not been connected to anything yet. If measuring with the HW switch off, crickets, I get nothing other than small microvolts. The switch has to be turned on to obtain anything close to 12 volts +. 

So if I connect the red + wire and the blue - wire to the new Fogatti it will only receive 12.2 volts of input. That amount of DC current is not enough to effectively run the Suburban furnace based upon past experiences and it might have a negative impact on the new WH with only 12.2 volts. Or am I missing something?

Thanks 

[Snackchaser]

That is different.  I'm on jury duty lunch break, this will be quick.  The WH switch I assume is a remote switch.  It likely is a specialty switch with some resistors or such causing a voltage drop, some LEDs will do that.  Unfortunately I'm not familiar with that suburban unit.  You will probably need to replace the switch for the new WH.  Check the old switch for internal resistance with an ohm meter.  Good luck and let us know what you find

 

[routlaw]

Understood good luck with JD. The HW switch I am referring to is the one remote switch that Oliver installs on the interior not the switch at the new WH. IOW's I'm still confused. Thanks for trying though. Thus far nothing is connected to the new WH other than the propane. I am trying to figure out which wires to connect to the new WH.

Many thanks.