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This is somewhat of a complex (and long question). Sorry.
I am a Homeowner. We live in central Virginia. We have a 2000sqft home that is well insulated. We have hydronic RFH. For cooling we have a slightly unusual system. We have a 2 ton chiller (Aquaproducts)which feeds cooled fluid to a 2 ton Spacepak high velocity system and small diameter ductwork. The chiller also cools water that is circulated in the radiant system for some degree of radiant cooling. The system is just barely adequate. I think that the fault is that the chiller is too small, as the fluid temperature into the Spacepak system can be as high as 55F. Because of this the chiller runs continuously during the summer. The house is just barely OK except for runs of several days over 95F.
Anyway, we want to build an addition. It will be 3 stories, with each floor being about 320 sqft (a little over 1000sq which includes a finished basement room). We have now gotten 2 proposals for cooling the addition and helping the rest of the house. I am also wondering about other options.
Proposal #1: Increase the size of the chiller (5 tons)Unico and add an additional 1.5 ton Unico fan coil/distributor. Cool the addition with high velocity ductwork. The estimate for this is expensive (>$14k)
Proposal #2. Increase the size of the chiller to 4 tons (Multiaqua). Use some of the chilled fluid to a) feed the current Spacepak, b) feed chilled water to 2 air handlers in the addition situated in such a way as to primarily cool and condition the Addition as well as help the rest of the house and c) supply cooled water for the radiant system. I have gotten a quote for this and it is expensive (>$10k).
Other ideas (mine):
1) 2-3 zone Mini-split system to cool and condition the Addition (and some of the house). This would hopefully take some of the load off of the current system.
2) Install a small conventional heatpump/AC with conventional ductwork in the addition which would hopefully also take some of the load off the house. Right now the plans do not take into consideration conventionally sized ductwork, but I imagine we can change that.
I do not know if anyone has done a Manual J calculation (but will be asking)
Thanks for letting me ramble.