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HVAC fella wrote:

Well, here in SW WI even at 104-F Heat Index we are cooling an 1800-sq.ft. home with some shade and proper insulation with properly installed 2-Ton systems. If you do what ought to be done, I am betting properly installed 3-Ton system will do the job more efficiently. With 1800sq.ft., that is only a hair over 600-sq.ft. per ton. A 4-Ton system is 450-sq.ft. per ton! What is it, all windows with some of them cracked open?
In Ohio some are cooling 2400-Sq.ft. homes with 2.5-Ton systems. Is the ductwork sized properly, insulated properly and sealed tight? Why do you need 4-Ton? What are the ambient Heat Index Temperature ranges? http://www.udarrell.com/proper_cfm_btuh_duct_sizing_air_conditioning_systems.html
DISCLAIMER: I assume NO responsibility for the USE of any information I post on any of my Web pages or on NGs. All HVAC/R work should always be done by a licensed Contractor & properly licensed Techs'! This information is only placed on these pages for your understanding & communication with contractors & techs. This information is also for the edification of Contractors and Techs. I am NOT liable for your screw-ups, you are liable for what you do! - udarrell
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'Well, here in SW WI even at 104-F Heat Index we are cooling an 1800-sq.ft. home with some shade and proper insulation with properly installed 2-Ton systems.'
REPLY: Yes, but what temp. are you maintaining inside ? 80 f ? The average comfort temp. is 74-76 f. according to ASHRAE.
'With 1800sq.ft., that is only a hair over 600-sq.ft. per ton. A 4-Ton system is 450-sq.ft. per ton!'
REPLY: That is not unusual for very hot climates like Florida , etc... With the summers getting hotter virtually everywhere in the nation, undersizing an a/c system is more ludicrous than ever. People arent going to be happy waiting 4-5 hours to get their house cool after coming home from work.
'Is the ductwork sized properly, insulated properly and sealed tight? Why do you need 4-Ton? What are the ambient Heat Index Temperature ranges?'
REPLY: All these things, and more, are definetly worth considering and investigating. Providing the OP's contractor did his homework and arrived at between 3.5 and 4.0 tons of cooling ...id opt for the 4 ton given the extreme climatic conditions that exist and will continue to increase.
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Wow...
Who on God's big greeny sizes cooling based on the Heat Index? We've got -60 & colder wind chill factors here, but damned if I'm gonna add another 40 degrees to my heat load calculations. Size for design conditions... the 2.5% Ashrea column is what I use for cooling... Heating 1% (but we Minnesotans like our heat in the winter) Most run loads & add 10-15% for heat Some loads software based on the latest ACCA standards had issues as they started adding a % for cooling & the systems were being oversized... (use the calculated cooling tonnage & not the "recommended" tonnage on Elite Vers. #8)
ACCA data can be a bit large, especially when having to guess at any amount of "worst case" construction materials, but it's the industry standard, & even those who write there own load software base it on Man J.
Sizing for shade trees & awnings can be risky too, construction materials are the only constant. I lost the 300 yr old oak in Sept 2005 to a tornado, lost 75-80% of my 10am to 7pm shade on the roof & back yard, my 2-1/2 ton system is running longer & I'm rebalancing rooms as the heat is much greater in a few areas. I'm upping it to a 3 ton 2 stage heat pump when the dead presidents allow. On the up side, I can now see my house on google earth, & we've planted a garden for the first time since we bought the place.
As for "skin effect" uh...???
Grab a Psychometric chart & measure the actual heat content of a "pound of dry air" at the different conditions. It's called enthalpy (h). Mpls design (89db/73wb) h= prox 36.3 room temp (72db/ 50% RH) prox 26.1 The wet bulb in Salt Lake & Vegas require adding moisture to reach 50 Relative Humidity. I've seen systems in the high desert sized for 300-400 sq ft per ton residentially because there's no humidity, Boy you can cool a house down quick that way... If you sized like that in our area, you'd have cool clammy & cantankerous customers, & probably very little word of mouth or repeat business. Most residential systems in my area (new or "updated" construction) tend to fall in the 750 -850 sf/ton. Undersizing for longer run times will wring out more moisture, & even if the system starts to spiral up when the temps exceed design for extended periods, it's dry air & isn't nearly as uncomfortable... Of course it's good to explain this to your customer & get their okay. Show them some Bin Data & the actual hours the temps exceed the design. This will really explain the Ashrea 2.5% & 1% columns...
Finally, what did you pay for? If you paid for a 2 stage, you ought to get it, though, if it's not in writing, that may be tough. 2 Stage will allow for oversizing with the nuts to cover temps exceeding design, & the lower capacity to cover the majority of the cooling hours. If your sizing a 2 stage heat pump, by all means oversize, you'll get more heat in the winter months, drive down the balance point of the system, & still cover all the afore mentioned summer cooling needs.
But then again I could be wrong...
goodluck geothermaljones st.paul,mn

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