One zone or two?
With a conventional ducted HVAC system, how many zones should my house have?
We are building a pretty-good-house for our retirement in Virginia. The main level is 1500 square feet, with cathedral ceilings, divided into a master suite and a great room. Two people will live in this space. The lower level is also 1500 square feet, but only 1100 are finished and about half of it is underground. Zero people will live in this space, except at Christmas.
My HVAC contractor initially recommended a single zone, but is now proposing two zones. Having two zones adds $4k to the cost, but more importantly it means that the two air systems will not intermix. And that means headaches because I had been counting on appliances on the main level – a spot ERV (Panasonic Whispercomfort) and a direct vent fireplace – to help condition the whole house.
We have additional exhaust-only air changes (a Panasonic Whispergreen 110 CFM exhaust fan on each level), so a two zone system is “legal”. But I kind of liked the idea of rolling over the air in the whole house.
I think the building envelope will perform well. We used Zip-R6 on 2X6 walls and blown-in foam under the rafters, and we got a blower door test of 0.44 ACH@50 Pa (Woo-hoooo!) on our first try. So the heating loads will be small. However, we have a lot of glass on the west (a lake view) and will have occasional high cooling loads in late afternoon, on both levels.
Most people would run two zones, as I originally expected to do, but now I am tempted to run just one. Is that realistic or plain wack-o?
Any reactions?
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Replies
If you have two zones, you can run your bedrooms with different parameters from the rest of the house. I have my own house setup this way and it’s nice to be able to have different day and night temperature setpoints that somewhat follow the occupants throughout the day.
It’s possible, but less common, to setup multiple zones with powered dampers instead of two separate HVAC systems. That might be something to discuss with your contractor.
Bill
Without the room by room, zone by zone load numbers it's hard to make this call.
The heat loss/gain characteristics of fully above grade space are pretty different from rooms partially or nearly fully below grade, which tips it toward zoning by floor, but every case is different.
Having a separate ERV per floor isn't a bad idea, nor is a single ducted ERV (with recirculation capability if mixing is something you actively want.)
Since you will be mostly living on the main floor and there are no additional floors, I don't think the cost of two zones will be worth it.
I think the best would be to get a modulating or two stage furnace and two stage AC that is sized to handle the whole house. Run the lower level on a separate supply trunk with damper you can shut off when not in use. The damper can be manual or electronic wired to a thermostat for the lower level.
Roughly guessing based on the size difference, the 1st stage would be in the ballpark with heating/cooling the house white the lower level closed off. Also having the extra bit of AC will help in the summer with your solar loads if needed.
Probably the biggest challenge will be finding a small enough two stage based on the house description.
If you want to circulate the air, you can do the hybrid ducting with your ERV where you share the return duct with the HVAC and have separate supply ducting. This way even with the lower zone shut off, you still get air circulation through the entire house.
I think a single zone will be fine as long as you understand the basement is likely to be slightly cooler than the main floor.
Walt
https://www.energyvanguard.com/blog/duct-design-6-determining-zones
This is a great blog.