Very little hot water at bathroom sink. Normal (good) flow at all outlets (tub, toilet, shower, sink). Tub will get to 100% hot. Shower only gets 70% hot. Sink gets very little hot water: about 10% hot. Under the sink, I turned off the cold water valve. I still get good normal flow but still only 10% hot. I assume cold water is getting mixed in somehow at the shower control (Delta with that big blue/white cartridge). Is that right? Water pipes run straight to the tub. On the way to the tub, there is a T to the sink... then further up a T to the shower.

Kudos and congrats for correctly analyzing your problem as a cross connection in a cartridge, To be absolutely sure which one turn off the cold water, one at a time. To the other fixtures and then check the sink each time for hot water. When you get hot water you have located the culprit. Pull the cartridge and clean the ports. If that doesn't work replace the cartridge after first flushing out the supplies and open valve body. Good luck, Tom

Thanks Tom! I can't perform your suggested test (by turning off cold water to other fixtures) as there are no cutoffs for the shower or tub. However (1) The shower is the only cartridge on this side of the house and (2) the wife reports that the single knob on the shower moved to the 1/2 way point and hung up twice... then she was able to turn it all the way. There was something there that caused it to stick that is not there now.

My question now is --> how can the cartridge in the shower provide a cross connection when the shower is turned off? I suppose I'll find out when I pull the cartride out.

ANY one handle valve in the house could cause a crossconnection but Id check , and change if necessary, the shower cartridge. Do you have shut offs on your other one handle valves? Back to you, Tom