We're in the process of remodeling a stand-up shower in our master bedroom (I'll call this one #1 in this discussion). The shower backs up to a shower/tub in another bathroom (I'll call it #2). The plumbers removed the old shower plumbing (#1) and replaced it with a new rough-in assembly. This assembly has a couple of jets and a hand-held unit in addition to a regular shower head. To locate the new assembly correctly, they had to re-route some of the vent line in wall.

Since then, we have been forced to use the old tub/shower (#2). I've discovered a strange phenomenon and need some help. When I open the hot water valve in the tub, the water never gets hot. It gets very slightly lukewarm; not warm enough to bathe in. If I open the cold water valve in the sink (in bathroom #2), the water flowing into the tub warms up enough to tolerate... it still never gets totally hot. Pressure seems OK. Read on:

Starting with everything off again, if I open the hot water valve in the sink (bathroom #2), cold water comes out. Ice cold. If I then open the cold water valve in the tub (#2), the water flowing into the sink gets very hot, like it should. If I close the cold water valve in the tub, the water flowing into the sink gets ice cold again. If I then open the hot water valve in the tub, cold water comes out of both it and the hot water valve into the sink.

However, this bathroom has not been used for bathing for many, many months. A month or so ago, our drain line from the house to the sewer clogged up, with sewage backing up into the commode and bathtub. Plumbers had to come out and clean the drain lines and install a cleanout port in the front yard. Not sure if this is related to this problem or not.

More info, useful or not: our hot water is on the opposite end of the house from all of this. It is approximately 15 years old (gas), and makes a lot of bubbling & groaning noises every time we use hot water.

The remodeling is still in progress. The new shower is not operational yet, the new sink is not plumbed up yet, and the new commode is not set yet.

Thanks for any help that anybody can provide!

I'll start with the water heater because I think it may be the source of your problem.
"our hot water is on the opposite end of the house from all of this. It is approximately 15 years old (gas), and makes a lot of bubbling & groaning noises every time we use hot water."
I'll bet the farm you don't flush your water out on a regular maintenance schedule. For 15 years it's been building up mineral deposits and now you're paying the price. If you do not flush,(not drain) your heater on a regular basis it will build up mineral on the bottom of the tank. When the flame comes on it turns the water trapped under the mineral to steam and the steam escaping is what you hear. Let me show you how to flush your heater. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) and some white grains,(lime or calcium carbonate). The whole thing shouldn't take more then a few minutes. Don't forget to flush it out every month. Your heater will thank you for it. You might want to consider cleaning the screens on your areators if you have a high mineral buildup. If thev problem still persists after you have flushed, you will have to drain the heater, remove the boiler drain and go up in the bottom of the tank with a wood or plastic rod, (not metal) and try to break up the mineral and flush it out.

Now on to the "mystery tub valve. One handle or two knobs? Unless your plumbers let you with a cross connection,( and that's a very real possibility they way you describe it) as they reconfigured the drainage and water they could have desturbed mineral built up in the pipe walls that has been sucked up into the #2 valve. To check I would shut the water off to #2 valve and open it up. Look for crud that may be blocking the inlet ports or seats. While you have the valve open check the plumbers work by first turing on the cold water. Did it come out cold? Now the hot water,(give it time to get from the heater to the valve). If you had hot or warm water coming out of the cold water supply or cold or warm water coming out of the hot supply you have been blessed by the plumbers with a cross connection.
Regards, Tom