multi vs single heat pumps?

I am in the processing of adding the last section of my house which was designed to be built in 3 parts. The first part of the house has a GE (now with an American Standard outside unit) that has been quite satisfactory. I have a Rudd heat pump in the 2nd part (single story, 1246 sq ft) of which I'm not too fond. The third part of the house is 2 story (1414 down + 840 up floored + 574 up "open" sq ft). Before talking to the local hvac pros, I'm trying to get a little background and a feeling on whether it would be better to go with another heat pump for the 3rd section or to replace the 2nd part's Rudd with a larger Amer Std that can be zoned to handle house part 2 & 3. General opinions?
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design of the system in regard to loads and capacities. I personally prefer multiple systems for zoning, especially for two stories. Take the advice of your competent professional installer. Don Young
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Didn't intend to imply that changing brands would solve anything. Just trying to convey my feeling that the unit & hvac design for the 2nd part of the house were not 100% satisfactory and thus were potentially "fair game" for redesign if a single unit was a reasonable option. I had that 2nd unit installed by a man who'd been doing a good job with my a/c repairs for a long time and I trusted his opinion. However, he was retiring & selling his business, so he brought in the new owner to assist with the job. In hindsight, I don't think they did a very good job with the design. Later, I found that the new owner to be less than satisfactory with repairs. Does make question this time whether use my very reliable repair pro with the new work or to look at a firm that does new installations regularly... My inclination is to go with my "regular" since he'd be the one to make repairs later, but I'd like to go in with a little background education first.
Diane
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how old is the existing unit? you might be better off replacing the existing one if a new unit is much more efficent...
beyond that multiple zones are nice
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Ignore the age of the current system. What I'm trying to determine is whether the new 3rd section (read as "nothing there now") and the existing 2nd section should use each use a separate unit or to use a larger, single unit with zoning for both.
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Diane,
It kind of sounds like you're looking for an ironclad "yes/no" answer here when the reality is that like many things, there are tradeoffs involved and you need to make your own choice as to what's important to you.
A single unit with zoning has 3 primary advantages: lower cost, cheaper, and less expensive. Both to install and maintain.
Separate units have pretty much everything else going for them. Better performance/temperature control, redundancy (if one unit fails you can still get heat/cool from the other), less noise when only 1 unit is operating. Installation can be easier since the ductwork for each unit covers a smaller area. Also in a retrofit case like yours, it means there's no modification needed to the wiring/ductwork/etc. of the existing system.
Cost-of-operation-wise, I'm not sure which approach wins. On paper, larger units are sometimes more efficient than smaller ones, which would favor the zoned system. However I'm not so sure that's true in the "real world" when the zoning-related factors are added into the equation.
Eric Law

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Eric, I realize there are many trade-offs and that there is probably no clear-cut "right" answer that fits all cases. Would make the decision much easier if it were so. Just making a stab at identifying the pros/cons & seeing if anyone has had "real-life" experience with zoning, particularly with a 2 story. FWIW, I'm near Austin, TX, so cooling is primary.
Thanks for your reply. Diane
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age or more better efficency rate of existing unit, if new unit is higher efficency then it makes upgrading to a single unit better
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