Hot Water Plumbing Problems

[sarahharris]

Hey there.
So we have an old home (1902) that is 3 stories.
We currently have a tenant in our ground floor apartment. She just informed me that she is unable to get the hot water to come out of the bathroom sink tap.
She could let it run and run and it will not get hot at the sink tap.
Her kitchen sink tap works fine.
Our kitchen sink tap works fine.
Our 3rd floor bathroom sink gets hot pretty quickly but
our 2nd floor bathroom sink tap has the same problem that out tenant does...
Our 2nd floor bathroom sink hot water has NEVER gotten hot (unless I take a shower in there beforehand) and we've just learned to live with it, since our primary bathroom is on the 3rd floor...
Anyway, this is a new problem ( we think) for our ground floor bathroom sink as our previous two tenants never mentioned it.

We have a tank (blech) water heater 80 gal on the ground level of the house, so her bathroom is actually the closest to the source of hot water.

We can't figure this out.

Any ideas oh plumbing gifted folks out there? Thanks!

[Redwood]

I would suspect a hot/cold cross connection allowing cold water to flow toward the hot side.

Similar to a faucet where the hot and cold are both turned on but the outlet is turned off.

This is also possible if there are single handle Moen or, Mixet valves involved.