HVAC first, then plumbing. But whomever is supervising the build should make sure each know what's going where. Aside from the location of the main panel, electricians have the greatest flexibility.

Plumbing first. The DWV system cannot necessarily be installed where ductwork is already in place.

If you think altering HVAC is hard, try to get water to flow uphill.

We had all three in there at the same time. Sometimes it is nice to have them there together to help coordinate their plan of action. I would say HVAC is probably the hardest just because their duct work is the largest. We used PEX piping in our house so I think it is a little more flexible. Electrical should be the easiest to adapt but HVAC really has to plan their system and returns or your system wont function well.

Listen carefully, students. Plumbing in a house is really two systems. One is DWV, Drain, Waste, Vent. It carries away both liquid and solid wastes, vents the noxious gases and accommodates the changing volume of air within the system as liquids/solids enter and exit that system. It is intended to have NO cross connection to the other plumbing system, the Supply system. DWV installers (and those who schedule them) must keep in mind that feces and urine (insert your own four letter words here) does not flow uphill! And the DWV goes in before the Supply piping. Air in ducts is forced uphill quite often, as is water in the supply system. Thus HVAC installers have some flexibility that DWV installers do not have. Having all the mechanical contractors on site at once can work, but the plumber MUST have first choice of available spaces.

Let the HVAC go before the electrician. My GC let the electrician come in before the HVAC guy. Big mistake, as some electricial had to be moved around and moved completely due to ceilings being dropped, etc. Electrician was not happy.

lryan - your GC is an i****. Electrons don't care about gravity and travel in very small spaces.

the plumber MUST have first choice of available spaces.

Colour my cheeks red! That's right. And that's who comes in first. Though a good plan accounts for those runs beforehand.

The best GC I ever worked with had worked for 7 years for a large plumbing, HVAC and electrical contractor--- they did all three. I was doing the electrical on that house. The framing was done so that no joists had to be cut by the plumbers or HVAC. This GC understood the various trades.

Agree that electrician is last no matter what. Good to have plumber and HVAC coordinate but not always necessary. I would say MUST is a strong word as any compentent GC and HVAC can plan for the probable DWV routes. Thicker floor trusses is an advantage.

Competence is more difficult to find than one might imagine. Read the horror stories on this forum.