How long do you let the water run? If the water has not been used all night and you are the first one using it, it could be cooling off in the riser and you need to wait for 16 floors of cold water to clear. During the day other condos are using water and keep the riser warm.

Agreed. Assuming you don't have any hot water recirculation system, then if the "hot" water is flowing cold then either -

1) You've got cold water flowing in from the boiler or
2) You've got hot water flowing in from the boiler but it needs to push 16 floors worth of cold water out of the way to get to your flat.

I would guess that the circulating pump for the building isn't circulating on that riser.

There is an easy test for the condo managers. Simply put a thermometer with a remote sensing unit on the hot water line. That will let you know if water is being circulated in the riser.

Thanks for all your replies. The building is divided into two and 15th floor is end of upper water line so we are near the end of that run and we are early risers. We have 3 year old high efficiency boilers which have never worked as well as the old ones but I'm fairly sure they are not specifically at fault. I think the circulating pump idea might be worth following since it does seem to be in that one riser. I feel like knocking on the doors above and below and just asking if they have problems. I know the building hasn't done so and I think it would help a lot in terms of resolving the matter.

Since my first post I've spoken with the plumbing company that did the install and with the manufacturer of the shower fixture. We are considering that the thermostatic shower might be "crossing over". Not sure what it means but the experts think it's viable. We've decided to wait until it happens again at which point I'll call, they'll send their best guy ASAP. If it turns out to not be a crossover problem then at least he can do some snooping while the problem is happening. As it usually goes away by evening it's been hard to follow up so hopefully a quick response will make it easier to trace the problem.

My bet is your thermostatic valve.

pjb, I have a nagging feeling the thermostatic shower valve is somehow involved but I don't understand how it would affect the separate tub area. The line comes in first at the tub then runs over to the shower. (FYI-It's a Natphil Zucchetti SP07. Not ridiculously expensive but not cheap either. The tub has a much less fancy Toto pressure balanced valve) I'm anxious for another one of our morning surprises so I can get on the horn and call that plumber and hopefully get this all resolved.