Variable-speed compressor v. two-stage compressor
Building a house in Austin, Texas, and would like (1) a five-star green rating from the city and (2) a central AC with a variable speed compressor. The only problem: the system we’re looking at only comes in 1 ton increments. We need 3.5 tons to meet the city’s ratings requirement of square feet per ton (and a 3 ton system won’t be enough cooling for the house). Therefore, we (may) have to go with a two-stage 3.5 ton system.
My question: Does going from a 4-ton variable speed compressor to a 3.5 ton two-stage compressor make sense energy efficient-wise?
My understanding about the benefit of a variable speed system is that is runs at just the right speed (and uses just the amount of energy) to cool the house. In that case, does it really matter if the system is oversized?
Thanks in advance for the responses. And thanks for this site in general: It’s been an inspiration for the design of our house.
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Replies
Robert,
If your building envelope is tight (1-2ACH50), the ACCA Manual J is done right and the Duct design is correct, you should be around 1000-1200 sqft per 1 ton of AC in Austin. IF all those issues are taking care of, I would choose the 4 ton or 2-2 ton units, depending on layout and/or cost. A 4 ton unit on low speed may run like a 2-2.5 ton unit must of the time, saving you energy. Also, 4 & 5 ton units run less efficient than 2 or 3 ton units.
If you have any doubts or questions, you should consult with an independent consultant like Doug Garrett in Jarrell at [email protected].