House is about 1+ yrs old built on concrete slab. For a guest bedroom on 1st floor, we had to have a separate gas hot water tank placed in an attic above a closet. It feeds 1 full bath (tub/shower + a sink). Sink gets plenty of hot water (quite hot), but tub only gets lukewarm water regardless how long you run the water. It just never gets very hot. These are the only items this hot water heater feeds.Pressure + flow seem fine. Any suggestions as to what may explain this + how to resolve. Thanks.

I'm not such an expert at this (to say the least), but my first guess would be (esp. if you have a single-handle shower) that the shower valve mechanism has some sort of leak - i.e. when you turn on the hot water only, some water from the cold water supply pipe is also coming out, even though the cold water should be off at the time. Maybe a fauly waser, or whatever? I don't know. Like I said, I'm no expert - wait for Tom or someone to help you...

To test my theory, try shutting off the cold water supply to the shower (or the whole bathroom), and see if the water comes out any hotter. If it does, post back and we'll take it from there.

Cheers,
Moishe

Evinx ; if the shower valve is I single handle style it should have a temp limit stop behind the handle that is adjustable if you turn the cold water off in the house nothing will probably work but with your house being new you must have pressure to both hot and cold for valve to operate the limit stop limits the amont of travel to the hot side of the faucet which in turn gives you more cold than hot if not adjusted properly this is a simple procedure in most cases what type of valve is it I hope this is useful

I did a bit of checking + we have the Kohler Thermo Mixing Valve T-9494)(http://www.us.kohler.com/onlinecatalog/detail.jsp?item=543202&prod_num=T9494-7&module=Bath+%26+Shower+Faucets) where you can adjust temperature on this valve.

The shower/tub faucetry is a 5 piece configuration. Tub spout, diveter-volume control, temp control (T-9494) shower diveter-volume control, and shower head on a slide bar. A little too complex for my thinking.
Anyway, get plenty of cold + some hot water but no way hot enough. Water to lav sink comes in plenty hot + both fed from same (+ only) hot water heater. Any further thoughts most appreciated.

Did you try my previous suggestion of turning off the valve supplying cold water to the shower/tub (or the whole bathroom, if necessary) but leaving the hot water supply on and then turning on the shower? If you do, and less water comes out, but it is hotter, that would confirm that it's just a matter of adjusting your mixing valve.

This test may not accomplish much, since we seem to have determined that it's the mixing valve anyway, but hey - it's a easy test that takes less than a minute to do, so it can't hurt to confirm it, can it?