I like the thinking or advice on this , is it a good ideal to install a 12 year old air conditioning unit that beennin a storage shed for 12 years .. it was removed from another house.
I need a new furnace and AC my wife wants to use this old unit instead of buying a new AC with the furnace..
I am thinking it be a very bad decision but wanted to ask the experts.. George
Replacement time is an opportunity moment to get the sizing right. Almost all hot air furnaces installed out there are 3x+ oversized for their actual design loads, and most AC is more than 2x oversized for their loads (often more). Yours is probably no exception. While oversizing the AC & furnace isn't a big upcharge in equipment costs (or even efficiency, assuming the ducts are reasonable for the cfm), it delivers a BIG hit in comfort. It's counterintuitive, but the
smallest furnace that still covers the design load is the most comfortable.
If you have a heating history on the place, use the fuel use against heating degree-day data to
MEASURE the 99% design heat load using this methodology. Odds are the BTU-out on the existing furnace is several times that big. ASHRAE recommends an oversize factor of about 1.4x the 99% load, which is more than adequate to cover Polar Vortex disturbance cold snaps with temps well below the 99% outside design temp. That delivers a ~71% duty cycle at the 99% outside design temp, and as it gets colder it's running even a bigger fraction of the time, with shorter off-cycles to let the chill sink in. But with multi-stage or modulating equipment it's often better (again, counterintuitive) to hold the line at 1.2x.
With oversized equipment the heat/cool is delivered in short bursts followed by an extended chill (or humid dwell, in cooling terms). With right sized modulating equipment it's running all the time at lower cfm, delivering a constant warm summer breeze (or cool sea breeze) rather than the hot-flash or arctic chill followed by an extended off cycle.
Before making any decisions on equipment it's worth taking a peak at Nate Adams' freebie down load chapters (of his book) & videos on his "Home Whisperer" page. H
ome Comfort 101,
HVAC 101 &
HVAC 102 are all pretty good non-technical primers on the topic of how equipment size makes or breaks making the house comfortable. (Nate's EnergySmart Ohio business is based in Cleveland- you're outside his normal service area, but he's spot-on with most of his approach. He also has a
video channel where many related topics are covered in greater depth.) Most people have never lived in a house with the mechanicals right-sized for the load, and haven't a clue about how down sizing is usually better for comfort. Bigger is the opposite of better, but when in doubt, that's where contractors & homeowners alike both go!
A right sized multi-staged heat pump is probably going to provide more comfort than a right sized 1-2 stage gas-burner + split AC, but may be more expensive to install. But even a 1-stage gas-burner that's right sized along with right sized 1-stage AC is in most cases still way more comfortable than what most people have been living with.
