You say heater is new,was everything ok before heater was replaced? Was anything else repaired/replaced same time heater was replaced? You say it started 3 days ago,what if anything was fixed or replaced just before this started?

The heater was replaced a year ago, so in the grand scheme of time, it’s new. Nothing has been touched since then. No changes. No issues. And the showers are still hot.

Let's look for "cross-over" between hot and cold water. Turn off either supply valve to washing machine then test sinks. If that doesn't cure the problem do the same to SINGLE HANDLE faucets. Some but not all single handle showers have screws on valve that are exposed when cover plate is removed. In the absence of means for turning shower supply off to single handle shower, try moving handle to various positions in hopes of hitting a sweet spot that stops cross-over. It's necessary to install repair kit in shower valve if it is crossing-over. Replacing fill valve on washing machine is the solution if it's found to be crossing over.

Just curious: What happens if you turn off the cold-water line valve underneath a sink and then turn on the faucet?

WDCCRUISE: two things happened… 1) water pressure barely there, and 2) took an incredibly long time to get hot.

What does that tell us??!!

I see that you did test wd asked for 3 days ago, what is status of test I asked for 7 days ago?

Hi, Just my 2 cents worth. If your sink faucet is connected to the pipe with a flex line shut the hot water angel shut off and remove the flex line from the faucet and put the end in a bucket and turn the hot water on and see which way your problem is, if comes out fast the problem the sink faucet if its slow the problem is back the other way.

Good Luck Woodbutcher