I'm in the midst of two bathroom renovations, including one powder room and one full bath. The powder room is not complete yet and the faucet is currently capped. The full bath is complete, and we've had a new Grohe thermostatic shower installed. Ever since the shower was installed, and the powder room was roughed and capped, we've had two problems: (1) we get scalding hot water coming out of all the cold taps in our house for the first 15 seconds of turning them on, and (2) the new shower never gets hot enough, even after turning it up to max 110 degrees.

Is it possible the new shower valve is recirculating hot water back into the cold pipes? Or is it some other issue, perhaps something going on with the capped faucet in the powder room? How can we get this fixed, AND get some hotter water to come out of the shower? As background, we have an oil-fired Weil McLain tankless water heater.

I'd appreciate any thoughts here! The plumber is on his way to check it out next week, but it'd be nice to get a second opinion.