We have 3.5 baths. Water heats up in every faucet/shower/tub except for tub/shower in guest bath (guest bath is on 2nd flr of home, as is main bath; water heater is in basement, 2 floors below). Water in tub/shower in question is Luke warm at best and frequently shifts from warm to cold. Faucets in same bath produce hot water with ease. What likely is problem? What is solution?

Check to see if the faucet for the shower has a temperature control or limit built into it. Many do. If it does rset it higher.

Hi Digitalmo

If this is a new issue then I would suspect that the hot water side of the shower valve cartridge is clogged/defective and needs to be cleared or replaced. If this is the "guest bath" as stated, then this valve is infrequently used and further compliments the idea that the cartridge is the culprit.

A name brand would be helpful here.

If this is an issue from the day you guys installed the shower valve then the issue could actually be the limiting device as presented by Handyman2007.

Back to you...

Mark