Will check valve work to fool Delta scald guard shower valve?

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We have a Rinnai tankless water heater, natural gas powered, new 2021. Set to 130 degrees F. Home has good water supply 60PSI very consistent. No issues with water heater. Four sink faucets produce hot water perfectly. 2 baths have Delta shower faucets, new 2021, with scald guard. I understand how the scald guard works; sensing and compensating flow of hot vs. cold.

Problem: colder months of the year the cold water supply reaches temperature so low that the Delta faucets‘ point of balance between the hot and cold result in a max temperature that is not hot enough. By October/November we have the shower faucet handle all the way over to hot and it’s still just shy of where we’d like the temperature. Summer months it’s fine. I learned a lot about how the Delta scald guard valve works by experimenting with the hot and cold feeds - we have a manifold system. This is not a recirculating loop problem, it is a problem with temperature fluctuation in the cold water, as it comes into the house, seasonally. The max available temperature coming from the shower faucets correlates to the general outdoor temperature. No other performance problems of any kind anywhere else in the system - temperature, pressure, or anything else noteworthy.

Reducing cold water flow to the shower by partially closing the valve at the manifold just reduces the shower flow.

Closing the cold water valve completely at the manifold closes the hot water valve in the Delta unit = zero water.

Question: if I join the hot and cold lines of the shower faucets with a tee, near the faucets, and install a check valve in the connecting tee that allows hot water into the cold water line, will that help solve the problem? What I’m able to deduce is that making the cold water a bit warmer will allow the Delta faucet to do its thing while producing a higher temperature at the final outcome. Wondering if I’m missing anything about how a check valve would perform in this application. Thoughts, criticisms or other ideas welcome.

I am not installing any booster heaters or raising the temperature setting on the Rinnai. I am not replacing the Delta units or cutting open walls to tinker with them.



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New York
Set the Scald guard for a higher temperature and forget the check valve and cross connection stuff

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