When we had our lanai enclosed the county required heat/Ac survey to determine if our existing unit would be big enough to add approx 400SF with a NW summer exposure.This survey required by the county takes into consideration the size of your existing unit, sq footage to be added, type of windows to include such factors as heat gain and low E features, and exposure to the summer sun. Munns came out did the survey and it was approved by the county. Munns did the work and it was added directly to our Exchager in the garage. If you simply add an extennsion off you closest duct you will be robbing heat/ac from that room and not get the best results.
We are very pleased and have no problems with extending the existing system.
When your exisiting system cannot handle the additional square footage, some people have used a Mitisubiti system called Mr Slim. I hear they also work pretty good but you need a place to locate another outside compresor..
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