A properly sized unit should be able to maintain reasonable humidity levels (though maybe not at 78 degrees without a dehumidification mode). The dual-stage unit will help control humidity. The only way of extra dehumidification with the XL16i that I know of is with the TCONT803/Honeywell TH8321 thermostat, which will overcool to control humidity. Ask your contractor why he proposed the VisionPRO IAQ when the BK terminal is already in use by the XL16i, thus not allowing the dehumidification feature of the VisionPRO IAQ (which is great as it actually slows down the blower). If you want an alternative that can make use of the VisionPRO IAQ, I'd go with the XL15i. It's a single-stage unit, but sized properly it should control humidity fine with the added advantage of the VisionPRO IAQ. Here are possible setups (ask your contractor what size he would recommend--I would NOT go with a 3 ton for your size home). Now, there are different air handlers that can go with these units, but the 4TEE3F39 air handler is tall due to the Vortica blower. The 4TWX5024A1000A (2-ton XL15i) with the 4TEE3F31B1000A yields 13.00 EER, 15.25 SEER and 8.60 HSPF. The 4TWX5024A1000A (2-ton XL15i) with the 4TEE3F39A1000A yields 13.00 EER, 16.00 SEER, and 9.00 HSPF. The 4TWX5018A1000A (1.5-ton XL15i) with the 4TEE3F31B1000A (only rated match with the 1.5-ton outdoor unit) yields 13.00 EER, 15.00 HSPF, and 8.50 HSPF. Feel free to print out my responses if it will be easier for you to go over these possible issues with your contractor. Good luck.

2 separate units will do the job, but cost of maintenance and break downs of 2 units as opposed to one with zone controls will in the long run be more costly. You have 2 units to maintain, you have 2 units to repair when they break. Tag team event. One system breaks down one month. The other system breaks down the next month. Doesn't always happen this way... but it could. zoning has to be done right. So what does that cost? I don't know I am not there. 'Design, Design, Design' other wise 'Trouble, Trouble, Trouble'.

Ok I guess I misunderstood what you were saying. During heat season I would say not so much, run each level at the temperature you want. Heat will rise so if you're comfortable on the upper level with just the one system running, then no harm or foul during 'heat' season. The reason is: Heat rises. If you run the upstairs unit you may have to set it higher to get it to turn on anyway. Obviously this depends on the load of the structure... because as you know cool air falls. So if it is really cold out, you run the upstairs unit mostly to reduce the load on the down stairs unit... cool air falls... if the upstairs unit is off there is more cold air falling on the downstairs system... which contributes to the load on the system. I would say under most scenarios this event would happen at night... so if you're asleep it likely wouldn't become noticeable until you woke up. During cooling season this changes... because heat continues to rise... and there are only so many clothes that you can reasonably 'take off'... and the heat load happens when you are 'typically' awake. So under cooling season you run the down stairs unit to reduce the load on the upstairs system. I usually see a few calls every so often on this where the home owner tells me: The upstairs system just runs and runs and it doesn't get cool. I get there to find out the down stairs system is off. Some events this can be reversed it really depends on the home not to mention the load being put on it. IE: record heat wave and you have 20 guests come over, thinking that magic box on the side of your house is going to take everything you can give it. Now if it's heating season and you have 20 guests come over, you may be able to shut both systems off. Yeah, you and the likes of you are little heat pumps of your own design.

When we first moved here 25 years ago, I figured out that, because of irrigation, our summer water bill would be high, but that would stop in Oct, when our winter heating (gas - forced air furnace) bill would kick in. It has been that way ever since - they sort of balance each other out. We turn off the heat (literally - the furnace has a switch installed on it) in April, and usually turn on the irrigation then. Temps are almost identical to what you said, Nik. Of course, living in a benign climate the weather is never life threatening, as it is some other places. We have no air conditioning, because we live in a summer dry climate in a Victorian house which, by closing the windows and blinds in the daytime and opening the windows and using fans at night, I can keep at a reasonable temperature (low 70sF) even if it is over 90 degrees F outside. Every night it gets down into the 50sF outside, no matter how hot is was in the daytime, which allows the house to cool off. Can't complain. Jackie