Looking at the service manual for this system, there are so many potential problems (with the gas, plumbing, electrical connections, computer control board, mixing valves, etc) that it's practically impossible to diagnose from a distance. What I can give as an answer is the list of issues that the service manual says to consider when the water is completely cold, or not hot enough:
- Water shutoff valve not fully opened
- Hot water faucet is not fully opened
- Water pipes are frozen
- Electrical or water supply cut off
- Unit not switched on
- Temperature set point is too low
- Mixing valve malfunction
- Error code displayed on control unit
- Not enough water demand
- Water inlet filter clogged
- Fixture aerator is clogged
- Scale buildup in the heat exchanger
- Hot and cold water lines reversed
- Gas valve is not fully opened
- Gas supply pressure too low
- Bleed over in one of the hot water fixtures
As you can see, this is a fairly long list to work through (although some of the items can be ruled out quite quickly).
In addition I note that the system has to be calibrated for the right operating altitude, although as you are fairly low this is unlikely to be the issue.
In addition, as I noted in my earlier comment, I see that the system requires a minimum water pressure in addition to a minimum flow rate. When there are multiple units in parallel, each individual unit needs to see the minimum pressure (suggested to be 40 psi). The manual specifically warns against use in low pressure systems. I recommend you investigate, to ensure that each unit is seeing at least the minimum pressure and flow.
P.S. If and when you have a final diagnosis from your technician, can you add a self-answer to share the solution with us?
