Heating, Air Conditioning, Fridge, HVAC/Variable Speed AC Question


We are looking for a new AC system.  The AC company did a manual J calculation and told us our home needs a 3 Ĺ ton system.  They want to up size to a 4 ton system because it will be a variable speed system and does not come in Ĺ ton sizes.  Is it proper to up size a variable speed AC system?  Iíve read it should not be done with single stages and can cause problems.  
Thank you for your help.

Mold Growth within Fibrous Glass Duct
Mold Growth within Fib  

Never upsize the unit. To do so is to invite a situation wherein the unit never runs at high speed, cycling on low speed only, with resultant poor humidity control. You'd be much better off with a three ton two stage: Long hours at low speed, cycling up to high speed during very hot weather. Temperature may "float" a degree or two, but there will be continuous air movement and humidity control will be excellent.

Be sure they perform the required duct survey (ACCA Manuals J & RS), and report any sanitary (mold? dirt?), design or installation issues (undersized ducts, high resistance fittings, restricted flex, etc.) that need to be corrected. This is especially important if you have any fibrous glass duct (see image).


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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.

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