Chiller queston

Rule of thumb

load calcs are meaningless. It needs to be done the proper way taking into consideration the local climate, all construction/insulation methods of the house, glass area and shading, doors. 500/sq ft seem high for NY though lots of single pane glass can cause extra load.



Probably would be very expensive to rip out the system and go DX. Chances are the chiller has 2 or more stages so can reduce capacity as needed. So probably its install in 2008 was the wisest move.

Chillin'

As John said, the only way to know for sure is to perform a heat gain calculation, not just the building enclosure but the internal gains which contribute to the load. Within that there is a lot of diversity, meaning the probability of operation at the same peak time. Ventilation is also a large driver and with tighter homes, this is also engineered-in.



You will find that short of solar gains through glass, the heat transmission through walls and roofs is not a large load driver. The time delay from sunlit surface to space is often hours after the immediate solar gains through the glass areas are long gone. In other words, when the sun goes down, so does 65% of the space gains. So what big deal is a slightly warmer wall at that point?



A shorthand way to get a handle on this:

1. Does the house cool on a design day or fall short and where? (Fairly easy test, just ask.)



2. If you have a season for long term analysis, a check meter monitoring hours of operation can tell you a good deal.



3. If you have schedules and specifications on the air handling units, get the sum of peaks. Normally (and never as an absolute), the chiller capacity might be 75-80% of that number, knowing that the sun can only shine on two sides of the house at a time. The fluid temperature required (colder for more dehumidification), may skew the tonnage higher than it would be for more sensible cooling. In other words, for higher outside air loads, you may need 26 tons but in a dryer climate with less outside air, 22 tons might do, just an example!



Your rule of thumb, as limited as it is, is within range of what I would expect to see.



About ten years ago, heck that was 13 years now that I think of it, I designed the HVAC for a 20,000 SF house about 20 miles outside Boston. ("Exactly like my house, only different! tm  )

Anyway, this house had a 46 ton custom chiller with remote 4-circuit air cooled vane axial fan condensers, each moving 10,000 cfm quietly, built into the side of a hill. Lots of south and west facing glass and about eight air handlers plus an indoor and outdoor pool. (We used the outdoor pool as a condensing source too.)  Anyway, my point being, you are in the ballpark in capacity but YMMV!



Brad