Zone balance with one large zone and two small zones

[solslund]

I have a 2-story, roughly 2,000 square foot home in Northern California. 3 years ago my HVAC contractor installed a new Bryant Evolution Heating and AC system. The house was divided into 3 zones (bedrooms, living area and upstairs office). The contractor has been unable to properly balance the system and as a result, on a sunny, hot day (let's say 85 degrees), if we want the three zones at 75, by the time the office reaches 75, the bedrooms will drop below 70 and the living area will struggle to get below 80. If all we want is to serve one zone such as the office zone or the bedroom zone it will blow (and sound) like a hurricane (especially in the small office).

I have done Manual J calculations and found that the unshaded, 2nd story 144 square foot office has 5,000 BTU heat gain, the south/west facing 1,000 square foot living area has 19,300 BTU heat gain and the north/east facing 730 square foot bedroom area has 6,100 BTU heat gain.

I also did some rough Manual D work and found the supply and return duct work to be designed for 300 CFM in the office zone, 865 CFM in the living area zone and 450 CFM in the bedroom zone.

My AC unit is a 4-ton single stage Bryant Preferred Compact. My furnace/blower is a Bryant Evolution 90i (100,000 BTU) with a 1HP variable speed blower operating between low at 1,100 CFM and high at 1,600 CFM.

I had another HVAC firm come out and inspect the system and they suggested that I needed a bypass for situations when only one zone were being served and I also needed to up-size the office zone duct. Made sense to me, but then I read in the Bryant manual that it should never have a bypass because it uses a sophisticated algorithm to dump excess CFM into unoccupied zones or whichever zone is the closest to set point and that a bypass would compromise the systems intended function. So now I am confused and waiting for the HVAC contractor to come back with some additional information. One more piece of info; my Evolution Control system tells me that if all zones are on then office zone will get 21% of capacity, living area zone will get 42% and bedroom area will get 21% (16% is duct leakage).

Can you advise? Is there any way to balance this and making it comfortable, i.e. quiet and correct temperature in all zones? Should I just get a separate mini-split system (or something like that) for the upstairs office?

Thxs in advance!
Svein

[beenthere]

No bypass allowed on that zoning system.

Might want to set the controller up for comfort. that will lower air flow. Your seeing the effects of over sizing the equipment, and under sizing the duct work.

Larger duct work, and more supplies is what most of your zones would need to make that 4 ton close to working ok.

[motoguy128]

IF anything, when you zone a house, you lean towards a slightly undersized system if you're borderline between sizes. Especially a large well insulated home. They heat up and cool off slower, so tolerate undersizing a bit better.