Heating, Air Conditioning, Fridge, HVAC/loft hvac sizing


i am looking to purchase a particular 2 story loft which is 2150 sq ft. About half of thee unit is the great room with 22 ft high ceilings. the back of the great room is floor to ceiling glass getting morning sun.
When i visited the unit i saw space heaters in two room. It has a 3 ton, 36000 btu a/c heat pump
the compressor is on the roof.
I am inclined to think it is undersized but see all kinds of sizing guidelines.
This unit is in raleigh NC
Does the 3 ton unit seem to be an appropriate size in your opinion.
thanks in advance

I've never worried too much about morning sun: It occurs at the coolest time of day and then goes away. For example, east-facing glazing peaks at 0800, loses about 40% of its intensity by 1000 and ends up at about 8% of the 0800 value by noon. Then there's the fact that Manual J requires us to include window treatments (blinds, shades, curtains, etc.) in the calculations. You didn't mention the age of the glazing, but if it's new there should be no concern.

The space heaters probably indicate an air distribution problem (branch ducts too small, restrictive duct fittings, imbalance, etc.), as opposed to an insufficient capacity problem. That's usually the case.

Furthermore, the loft may require zoning: The room with all the glass may be thermally incompatible with the "two rooms".

As for your installed capacity, 717 SF/Ton doesn't concern me, assuming the loft is relatively new (20 years or less). Manual J - it's a book - tells us today's homes can be 1,200 SF/ton (but cautions SF/Ton is not the way to calculate cooling requirements).

You may hear from someone in the "500 SF/Ton crowd" - those people who've "always done it that way" and "never had a problem" - saying 3 tons is nowhere near enough for 2,150 SF, and you need another ton or so. Well, good luck if you do. You'll still need to fix whatever underlying distribution problems there may be.

You might want to commission a blower door test to identify any air leakage problems. Avoid "for free" tests by people who are selling home improvements.

AND do find someone to perform an accurate load calculation before going too much further.

Heating, Air Conditioning, Fridge, HVAC

All Answers

Answers by Expert:

Ask Experts


Fred Weldin


I can't help you if your whatsis won't work, people (Especially if it's a refrigerator); I have no experience with appliances, and I haven't been involved with H&AC service and repair since March 08, 1996 (Thank the Lord); I always send a "standard" reply to appliance questions and H&AC "Service and Repair" questions (about 20 a month), so if you want to know why your whatsis isn't working like it used to, ask someone else. A lot of my answers arenít read by the questioners; in that event, I always send reminders to read the answers for a month or more (the word "idiots" comes to mind - actually there's another more appropriate word, but discretion prevents its use here). If you have questions about how big a unit you need, if one room's warmer than the others, if you want an opinion oas to scope of work on bids received, etc. just ask. Do visit www.askweldin.com, there's a lot of good information on sizing, ductwork, efficiency, as well as some techniques accomplished DIY-ers can use to troubleshoot and improve their H&AC. Finally, I want to thank all of the kind, serious people whom I meet here: I enjoy working with you. My wife says I'm "snarky" from time to time; as for you others, please use your imagination as to the responses swirling on the snarky side of my brain when I read your questions.


53 years in the business. See www.askweldin.com


B of ME U of D 1965, numerous classes, seminars and a lot of "Hands On" learning since.

©2016 About.com. All rights reserved.