Rennai Tankless supplies HW to selective faucets only

[pondball]

Our son just moved into a 5 year old house.
Rennai Tankless water heater was installed at build - two story plus basement.
Hot water reaches the two 2nd story shower outlets within 10 seconds.
Hot water doesn’t reach any of the regular faucets, on any floor, including the laundry tub in the basement near the Rennai, or the kitchen sink directly above.
He has been on the phone with Rennai with little success. Brought in local plumber who knew little of tankless and had to phone Rennai - again with little success.
It’s his first house and after 3 weeks of no hot water from any sink faucets he is frustrated.
Shower lines are probably 3/4” and I’m assuming the rest are 1/2” - would these be coming from separate feeds from the Rennai.
Could the three rods inside the Unit somehow not all be functioning, with the dysfunctional ones feeding the 1/2” pipes to the faucets?
The flow is the same, and very good for both HW and CW faucets in all sinks…. But no hot water in these faucets is the issue.
Any tips as to why the showers receive near immediate hot water and the other faucets receive none?

 

[Geofd]

Not that it matters but was there an inspection before the sale,are there any error codes does he have paper work for the heater with trouble shooting instructions

 

[Geofd]

Shut off the hot pipe leaving the heater check each fixture, there should be no water flow from the hot side if there is there is a cross connection somewere

 

[pondball]

He definitely had an inspection, with a very lengthy report. I don’t recall anything in there about an issue with the water heater. We’re a couple of hours away, but we did meet him for a second open house where I remember testing a few taps for both pressure and hot water. I might not have let it run to scalding hot, but I kinda recall it being warm fairly quickly. He has the manual for the unit, and I’ve been through parts of the online version. One section says something about an app but wasn’t sure it applied to his unit. Is this where it would show the error codes - on the app? The only digital thing I saw in the unit itself was 120°

 

[pondball]

Shut off the hot pipe leaving the heater check each fixture, there should be no water flow from the hot side if there is there is a cross connection somewere

I’ll have him check this…