We just had a new furnace and AC unit installed. We never had a humidifier before and had them add one to our furnace. Now we have half the water pressure in our shower and bath as we did before. Is this normal or should they come back to adjust this?

There is only one possible way that I can think of that would tie the new furnace install to low water in shower. Since you didn't have a humidifier before, they may have shut the house main water off to install a shut off valve for the hum. Any time the main water is shut off there is a chance of debree in the pipes breaking loose and plugging various fixtures etc, when the water is turned back on. That's really the only thing I can think of. Other than that, there is absolutely no connection between the new furnace-a/c unit and your water system. Most likely just a coincidence. If you had the new furnace installed by a plumbing and heating company, you may have a shot at them coming back to check things out. If you had the unit installed by a strictly heating and cooling company, they most likely will advise you to call a plumber at your own expense. Good luck, and please let us know what is found.

Thank you SO much. I will call them today and see what they say. The other thing that is different is that my neighbors downstairs(we are in condos) asked if I had a heat pump installed. The reason they asked was because of the drastic increase in water in their water pan? Apparently our units' all send this from the condensation to this one pipe and they empty it? I am assuming this actually does make sense because of the humidifier? I never knew about that pipe though. Any insight?

You bet, that humidier will put out water any time is it run. Also, if you got a 90+ eff furnace, it will also create condensation and put out water any time the furnace is running. So there would be more water in the drian than before, completely normal.