New Takagi Not Heating Properly

[LCF]

Hoping for some insight that might help me figure this out. I installed a model TK-540P-PIH tankless today in my home. This is the third Takagi I have ever installed and never had issues like this before.

I plumbed the cold line into the recirc inlet and capped the cold inlet as per the instructions. I am not using a recirc line in this house.

The heater won't get the water hit enough. Have it set to 120 and it is only heating around 100 or so but is modulating up and down constantly.

I checked everything in the manual and there are no clogs in the filter or crossed lines and the gas lines are sufficient.

I am thinking that it might be a setting inside on the dip switches. Is it possible that because I am not using the recirculation that I was supposed to also change some dip switches? I went through the manual 20 times and couldn't find anything.

I also tried pressing the "max" button inside on the control board and sure enough the heater started working perfectly for a time, but then reverts back to not working correctly.

I appreciate any advice I can get. Thanks!

 

[FDNY/RETIRED]

https://www.takagi.com/media/54108/68A161.pdf

Look at pg 12 if you don't have this not sure it will help. But tis is all I see on Dip switch settings. I am not a plumber so I am at a loss for real advice. The other thing is this. But most of the stuff for water fluctuations you checked. BUT did you check what they say in this
https://www.takagi.com/professionals/troubleshooting-and-repair.aspx

If there is inadequate hot water or variations in temperature, check for plumbing “cross flow,” which means there is a connection somewhere between hot and cold. This may be as inconspicuous as a worn seal in a single-handle faucet. To check for this, shut off the cold supply to the heater and open either the pressure relief valve or any hot-water faucet. If there is a steady flow of water from the hot output after the pipe has drained, there is a cross flow somewhere in the plumbing that must be fixed.

 

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[LCF]

Thank you for the links! Unfortunately none of these issues are the cause. I suspected cross flow however once the water is drained from the hot lines while the cold supply is off, no more water comes out.

I also thought it might be altitude because my new home is in the Vermont mountains and I am at 1200 elevation but my dip switches are already set as shown in the diagram for my model.