How large a Cental Air conditioning unit do I need?

[usawargamer]

My house is about 2,000 Sq. Ft, in New Jersey. We have Vinyl Siding. The contractor suggested a 2.5 ton unit (30,000 BTU) based on the Windows, size, etc.

I wonder if thats large enough?

We currently have a number of individual units, and their total BTU is around 45,000. Even using 20* 2,000 SQ/Ft = 40,000 BTU

The contractor is reputable, and seems knowledgeable. I don't think a larger unit costs much more and generally contractors will be happy to sell a more expensive unit, as they make a bit more.

I'm just concerned that 30,000 is not enough.

[barry]

Could be. Honestly, can't see from here. Heat gain rate depends on skin-area, not floor, and R-values; air-infiltration rate; internal sources; etc.

There are standards for calculating this. Which did he use, and did he show you the calculations?

Window units are quite leaky, so they are a poor basis for comparison.

In fact, smaller A/C is better for dehumidification, which is significant in much of NJ.

Second opinion would be good, but ask to see calculations this time. DAGS on this also- it's discussed ad nauseam.

J

[Doug Kanter]

I'm no expert at this, but two contractors have told me that if you go too large, the system will get the temp down nice and fast, but it won't run long enough to remove humidity. I worked in an office where some major renovations were being done down the hall. It required temporarily re-routing the AC ducts. One of the maintenance guys, who was VERY good with HVAC work, warned me ahead of time that it might be a bit clammy for a week or so. He was right. Long story.....but anyway:

Perhaps this is part of the reason for your contractor's recommendation.

[udarrell]

Trenton New Jersey has almost identical Summer Outdoor Design Conditions as Madison, Wisconsin (88-dry bulb, 74-wet bulb) I have a mere 6,000-btu/hr half-ton window unit that cools my entire first floor over 900-sq.ft. area, extremely well.

Do NOT oversize as the efficiency will go way down due to short cycling, each cycle it takes 7 to ten minutes before it begins to remove heat with any efficiency. You can throw the SEER rating out the window! Normally, -- a properly installed 2.5-ton system ought to handle the heaviest heat loads. We have 2-ton systems on 2500-sq.ft. well insulated and constructed homes around here. udarrell

[Bob]

You can't go by home size at all. You must do a heat gain calculation. Most of the heat gain calculations I've done on that size homes came up with 2 tons with the same climate.

[scott21230]

How many other contractors have you had over and what size do they recommend?

[Kathy]

We have Vinyl Siding.

BTU) based on the

and their total BTU is

knowledgeable. I don't think a

contractors will be happy to

more.

Call a contractor that will take the time to do a load-calc.

the e-mail might not work so call

[WM]

My brother-in-law like to keep his house cold as a meat-locker.

48,000 would barely meet his requirements. We are all different.

[Anonymous]

There are 2 books one to size duct work the other to caculate heat gain/loss that takes into account insulation, windows, occupants, direction of house, appliences etc. I think waat you want is called form D but might be mistaken. It should be available at any tech school. If you do not mind a bit of work you can figure out exactly what you need. Not hard only tedious. Some HVAV folk only do a ball park quicky estimate. My house is in atlanta ga. and has 2500 ft on first floor and did fine with 2.5 ton unit rather than 3-3.5 that was suggested. Temps here can get to 100 and stay there for days with high humidity. I used window units in 2 bedrooms at night to save energy but never needed them during the time central unit unit was running during the day and they were there to suppliment if needed.