Plumbing – Kitchen faucet only gets lukewarm, never gets hot

When this happened at my house it was the antiscalding valve at the water heater. These mix cold and hot water together to reduce the temperature of the hot water to safe levels. Some places install them at each faucet, others install them at the water heater itself.

Turning on one faucet would just give cold water no longer how long it ran. But opening others, or sometimes turning the tap on and off rapidly a few times would bring the hot water.

The anti scalding valve was getting stuck, and the pressure changes of opening more faucets, or maybe water hammer effect from opening and closing would make it work for awhile.

If you installed a new one at your new water heater it might be defective. If you kept an existing one it may be worn out or have minerals trapped in it - sometimes things like that never cause a problem until you turn the water off and it moves around.

Also check under the sink to see if it has an anti scalding valve. It might need replacing too.

Hot water line is closer to the (substitute "source of cold" for "outside wall" in my comment) than the cold water line, and freezes first.

Mysterious water stoppages in cold weather are nearly always from pipes freezing.

The fact that it's the hot water line does not mean that it's full of hot water - after it's sat for a few hours, it's cold, and if it's sat for a few hours by something really, really cold, it will freeze.

Some plumbers can be quite oblivious about this - I've seen pipes clamped to the inside of the sheathing and all the insulation being put on over them - I've also seen those supply pipes, which I was assured would never freeze when I pointed it out, relocated to coming up through the floor inside the wall after a cold spell (not my house, someone else was "in charge" and it evidently took the pipes actually freezing before they would do the job correctly. It probably cost 3 times as much to do over as it would have to do right in the first place.)