Originally Posted by J_P_A
Met with our HVAC contractor today for about 2-1/2 hours. Good and bad news I suppose. The good news is, I'm still very impressed with him. He seems very knowledgable, even after meeting in person. And he brought up some very good points.
The big thing, is he's really pushing a two speed compressor to go with the variable speed air handling unit. The reasoning is the spray foam insulation works too well in some cases. For example, when it's 85 degrees outside, the AC can't run long enough to dehumidify the air because the compressor is making 5 tons of cold air all the time. The air handler can only run at a reduced speed for so long before it has to do something with the extra capacity. Obviously the problem is the $3,000 - $5,000 price tag depending on the unit.
We also worked out what looks like the only place to get a line to the basement. The plan is one 10" supply, and NO RETURN. Our stairway is open down to the basement (you can see from the foyer into the basement), and he felt like that would be sufficient. This is also partly motivated by the lack of areas to get a line to the basement. We used just about the only place for the one, and it comes with some compromises as it is.
And finally, because we only have one line going to the basement, we're not going to be able to zone the theater separately. We discussed options, and we can either use a remotely operated thermostat or possibly a thermostat with two temp sending units. The problem is that the air will be on in the entire basement regardless of which room needs the air.
Any thoughts on any of this? I'm having a hard time digesting everything we talked about at the moment.
Wow - major bummer. Here are my initial impressions based on this feedback:
First, he is absolutely correct with the two-stage unit with the variable speed on the air handler. Any HVAC system must have sufficient run-time to dehumidify the air, not just reach a certain temperature. The only alternative to this scenario is to downsize the tonnage of the unit to a more appropriately sized unit (because of the more efficient insulation) so you are essentially left with a smaller unit running longer because it needs to run to reach the temperature desired. The dual stage and variable air handling allow the system "intelligence" to maximize comfort IF
they run the outdoor and indoor temperature and humidity sensors to maximize comfort and efficiency.
Regarding only supplying your basement....just to clarify - can he zone the whole basement separate from the first floor? It sounds like he can from your description. And what is your basement square footage? A single 10" line looks really insufficient for the square footage. By way of comparison, my basement is approx. 1605 square feet and requires a 14" round RIGID to handle the supply lead-off which is stepped down to 12" and then 10" to maintain pressure over the supply length of 30 feet. If you are starting with 10", I can't imagine how choked off the supply would be. Second, an HVAC unit can only supply as much air as it can return. So if the basement is zoned separately and is being supplied, is ALL of the make-up air coming from the first floor returns? And if the first floor and the basement are running at the same time, it appears there are no additional returns to supply the required make-up air.
There will be some interplay of air "environments" of the basement and first floor, but creating air movement through a balance of supply and return is really the hallmark of a good system. I don't think the air will really seek out the stairs as much as he supposes. I would have a lot of concern regarding the basement comfort levels. Despite having a relatively open-concept design, there are still walls which impede a good mixing of air which will give you hot and cold spots throughout the space.
And I am sorry to say this, but you will not be able to overcome the laws of physics for your home theater, especially if you are provided only supply. Your theater room, if built properly, is going to be sealed up as tight as a drum - essentially a dry aquarium. As you pump AC into the room, there will be a slight raise in pressure (called positive pressure), but there will be an easily-reached limit (under a minute) before the pressure in the room raises to the point that supply in your theater room becomes severely diminished. You will still be able to feel air coming through your supply, but the CFM will be greatly diminished. One resolution is to keep the theater door open to allow for maximum air movement, but I can imagine this is not what you had in mind for the theater and keeping the sound in the theater. And don't even get me started on the accumulation of heat because of the extreme insulation....
If I were you I would find a way to get an appropriately-sized two-stage system for your main living area and just bite the bullet for a new separate and dedicated two-zone basement system with appropriate supply and return for each zone. One zone for the main basement area and one for the theater.
IMHO I don't think it is wise to rig the one system into trying to do what it really isn't designed to do.