I have a few (OK, a lot of) questions for the experts here about the system I'm about to have installed. Basic info:

40 year-old house in Northern Virginia, ~2600 square feet in two floors, plus an unfinished (and unconditioned) basement.

Heat pump: Trane 4TWX6048B1000B
Coil: 4TXCD050BC3HCA
Natural gas backup / air handler: TUH2D120A9V5VA
Humidifier: THUMD500APA00A
Stat: Vision Pro IAQ

I definitely like and trust the contractor, but for this kind of $$ it doesn't hurt to get additional opinions. Just to get past a few issues that usually get brought up in this forum:

Yes, he's going to do a Manual J -- these sizes are his initial guess, but they may change once he's done the math.
Yes, I really do want the 95% efficient furnace, even though I expect to be using the heat pump most of the time. I'm willing to spend the extra money up-front.
Yes, I've considered other brands such as Carrier (the Infinity control is sweet!). I picked Trane as a result of picking the contractor I trust the most.
Yes, I'm aware that larger Trane heat pumps are a bit low on the BTU range for the quoted tonnage.

Now my questions:

- Any obvious issues with this proposal?

- My current furnace vents up the same chimney flue as my natural-gas fired water heater. Since I'll be going to a condensing furnace, the contractor says that they'll need to install a flue liner, since the water heater will be the only thing venting up the flue. I must confess I don't really understand the reasoning -- any opinions?

- My current whole-house humidifier plugs into an electrical outlet on the outside of the furnace. This outlet is, I believe, switched to go on/off with the fan (or possibly fan + heat? I need to doublecheck). A humidistat in the return provides control. A pretty standard setup, I suspect. But is there a better way to do it when using the IAQ? I assume what many people do is ditch (or just disconnect) the humidistat in the return, and control the humidifier using the HUM1/2 contacts. Since the IAQ can be configured to operate the humidifier only when the fan (or fan+heat) is on, should the humidifier be plugged into an unswitched outlet? Or is it safer to stick with a switched outlet? Also, is it really OK to allow a humidifier to operate when fan is on but heat is not? Are there issues with condensation?

- On the flip side, what about dehumidification in the summer? I've seen threads here that suggest removing the factory jumper between R and BK on the air handler, then running wire from R/BK to DHM1/2 on the IAQ, configured in NC mode. Does that sound right? Should Comfort-R mode be left enabled? Anyone have any experience with this setup?

- How does heat pump defrost work? I have to assume it's under the control of the heat pump, not the thermostat. Do they run a line from the H.P. to one of the W terminals of the furnace, to allow the H.P. to force aux heat on? Does the H.P. handle the required sequencing and lockout timing of switching the reversing valve, cycling the compressor, etc?

- I'm under the impression that the following connections (using the IAQ's labels) go to all three components (stat, furnace, and heat pump): R, C, O/B, Y, Y2, possibly AUX (see above). Is that right? Any others? I'm sure my contractor will get it right, but I really want to understand how it all works together.

- Do contractors typically hook up the L (equipment monitor) line from the heat pump to the IAQ? Should I ask him to? Should I care?

- OK I'm done