We have been back and forth and up and down about the HVAC design for this house. Original plans just called for a single unit in the basement in the fancy new furnace nook we built. Contractor wanted us to install two units: on upstairs and one down. This is more efficient because less ducting (especially no vertical ducting all the way from the basement to second floor). This is more expensive, but would be great to have a really good working system.

UNFORTUNATELY… the contractor did not measure new attic space– there is no room for the unit in the attic so he wants to take over my linen closet. This is bad for 5 reasons: 1) I wanted a linen closet! 2) this costs more and our budget is already busted after overages on windows 3) the HVAC unit has to have ventilation so would have to have a weird vented door which would look ugly next to the other normal doors 4) the HVAC closet would be sharing the wall with my bedroom meaning I would have to hear the furnace rumble on and off all night and 5) I am irrationally uncomfortable with have a gas appliance right next to my bedrooms.

So back to the one unit plan.

Furnace Options: Now, there is the option to design a system with zones (i.e. electronic dampers in the duct system that allow for separate thermostats for upstairs and downstairs). I scoured the internet for info and advice on this and my conclusions is that a variable speed furnace (no bypass needed) with 2 zones is the best option. HVAC installer doesn’t seem to think bypass is a big deal, but all comments on the web consider it a recipe for disaster–I later found out that per the Herrs Inspection Guidelines bypass are NOT ALLOWED because they are incredibly inefficient.

AC unit: the actual AC unit has to sit outside and because our side yard is so narrow we need a special low decibel unit per Palo Alto city code.

Return Air: You need to have a approximately 20 x 20 vent in the house that pulls the air back into the system to avoid negative pressure build up. Since we moved the furnace location and had to install new foundation footings it is harder for the HVAC installer to use the old location (in the living room). Again, I scoured the internet for advice and got a lot of variable information. We had some discussion about putting it up high on the wall near the ceiling but, no matter how functional it might be, it seems terribly ugly. We compromised on a return in the floor of the family room.

I didn’t realize this was so complicated, and that HVAC design was so important for function–I would have liked to get a couple alternative estimates/advice from reputable HVAC companies. Really crossing my fingers that this will work come the first 95 degree day next summer!

UPDATE: We were almost ready to install the single unit when I started second guessing the HVAC contractor’s suggestions. Luckily I was able to talk to a couple people over the phone and one guy from another comapny even came over to give me a quick assessment. He determined that we really don’t have room for the large ducting required to install one unit with the proper air volume to actually cool the upstairs. The new HVAC contractor suggested a heat pump upstairs (for heating and cooling) which solves my closet problem. The heat pump is requires less room and less venting so we can place it in the attic. Most expensive solution but should be more straightforward to install and will be much more effective and efficient. We will just use a smaller traditional gas furnace in the furnace nook for the ground floor heating needs.