New AC needed for addition?

[adriana]

How do I know if a new AC unit is needed for an addition we are planning or if we can someone tap into the existing unit?

some say look at sq footage of the new room, look up details on the unit itself and compare.

when is it not worth tapping into the unit and just getting a new one?

I don't want an AC unit on the wall...I think they look tacky - what are my options?

[roger g]

Contact a heating and ventillation company. A lot depend on what a/c you already have. How big your addition is. What side of the house it is (sunny?) Lots of windows? Insulation? Basement? 2 storey etc etc etc etc.
I would also look at ductless airconditioner/heat pump. They are amazing. A little bit pricey but may still be cheaper than adding to what you have even if you can do it which I really doubt. The ductwork might be extended but the a/c unit would probably be undersized.

Your questions
If your original a/c was sized properly then in proabably won't be enough. Don't forget the extra ductwork. Maybe you furnace isn't even big enough.

Square footage is only one small part of deciding and there isn't much on the unit that will help you the home owner.

A heating and a/c company would tell you the different costs.

A ductless fits on the wall but you have to see it and hear it to believe it.

Roger

[Rich]

As far as cost of having someone put in a new a/c unit - a rule of thumb is $1000 / ton of capacity. For a 2000sf home you're minimum requirements you would want would probably be a 4 ton unit. The biggest reason a/c units fail to provide the comfort level is not enough return air to the unit. In most new homes you'll see registers all over but very little return air. I purchased my own equipment (4 ton) and installed it myself for around $1000 - but my labor is free :)
And like roger said - if you are going to have someone size it they will need to know about exposures, volume of the home, etc..etc... to accurately calculate the size .. they should then upsize by at least a half ton.

[roger g]

You hit the nail right on the head Rich when you said not enough return vents. A friend of mine put one in every room except the bathroom and kitchen. Think about it, what use is a return air in the main hallway if you keep all the bedroom doors closed at night. A quick way to check to see if you have enough return air (just a rule of thumb) is count up all the square inch of hot registers and then count up all the square inches of return registers. I always like a little more return.
Anyways it is only a real rough way of doing at least one check.


Roger

[Dano]

As others have stated..depends on size of addition, exterior exposures, # of windows,etc. Is your existing unit a Heat Pump or just A/C? My experience is that installers typically oversize the units and I know that ALL designers oversize by a "Fudge" factor of 20-25%. This eliminates call backs to them on undersizing, etc. If your addition is "open" (more than a door size) to the existing house, you will get some natural migration from the tempered space and depending on your size of the addition an additional register under each exterior opening(window) @ 75 cfm should be adequate. BUT...alot of variables are involved in order to make a qualified decision.