Comments on: An Air Conditioner Sizing Benchmark for High Performance Homes

By: Jerry

Jerry — Tue, 04 Oct 2011 06:52:19 +0000

I understand the whole high
I understand the whole high sf/ton what hasn't been touched on here is how to circulate the air in the home. If a 3000 sf home has 3 tons of ac 3*400cfm/ton=1200cfm. This won't circulate the air sufficiently without some zoning or dampers someplace. How do you get the proper circulation?

By: Timm Hamm

Timm Hamm — Tue, 04 Oct 2011 00:58:26 +0000

You mentioned "high
You mentioned "high performance homes". I agree about your sizing "rule of thumb". But what about non-high performance homes"? Homes that I go into to perform an energy audit that show air leakage 3 times the acceptable rate and duct leakage 4 times the acceptable rate?

By: Brad Kunkel

Brad Kunkel — Wed, 28 Sep 2011 04:04:43 +0000

Folks, read on. Meet the new
Folks, read on. Meet the new boss…same as the old boss!

http://www.linkedin.com/news?viewArticle=&articleID;=782781595&gid;=59247&type;=member&item;=71423674&articleURL;=http%3A%2F%2Fwww%2Egreenspirationhome%2Ecom%2F%3Fp%3D1069&urlhash;=dDNX&goback;=%2Egde_59247_member_71423674

By: Chandler Pernell

Chandler Pernell — Sun, 25 Sep 2011 08:22:52 +0000

Really like your flip on
Really like your flip on "bigger is better". We have been combatting this logic for 35 years and never considered flipping the numbers in order to perceive a properly sized air conditioner "bigger". EXCELLENT.

By: bob

bob — Sun, 25 Sep 2011 05:38:26 +0000

10' and 12' ceilings don't
10' and 12' ceilings don't have to add nuch to the cooling/heating load if done right. Use some of the space to house the ductwork inside conditioned space above hallways. There is only the additional 2'-4' of wall space to consider, it doesn't add much to the cooling load as the increased cubic ft would imply.

By: David Butler

David Butler — Sat, 24 Sep 2011 00:25:55 +0000

@Jim, I think you missed the
@Jim, I think you missed the point. System would still have to be sized based on Manual J (do I hear an echo in here?)

Enforcing a minimum SF-to-tonnage ratio would lead to more efficient homes as well as causing HVAC contractors to sharpen their pencils when doing MJ, exactly the opposite of what's happening now.

By: Jim

Jim — Fri, 23 Sep 2011 17:25:58 +0000

Your moving in the right
Your moving in the right direction. As we tighten the building envelope idealy using spray foam insulation to prevent air infiltration the old rules of thumb will cause serious short cycling problems. However, ANY rule of thumb can get you in trouble if you don't consider you are cooling CUBIC FEET NOT SQUARE FEET. An 8' cieling home will require far less tonnage than a 10-12' cieling home if both homes are 1,8000sq/ft. Pretty basic stuff.

By: bob

bob — Fri, 23 Sep 2011 06:19:04 +0000

excellent article !!
excellent article !!

By: David Butler

David Butler — Thu, 22 Sep 2011 07:38:22 +0000

Late to the party today, but
Late to the party today, but I do have a few comments on the comments...

@Allison, 2-stage AC doesn't really overcome the oversizing problem. It may mitigate the moisture impact a little bit, but *only* if the high stage is sized to the load, which is rarely the case in my experience. I don't know how many times I've heard of contractors selling 2-stage by touting the high stage as "reserve power" for those extra hot days or big parties. I guess it's ok to do this, but don't then expect the system to achieve its rated SEER. Dealers never reveal that little detail. And BTW, almost the same benefit can be accomplished with an $8 relay to reduce fan speed on dh call, assuming the extra latent capacity is needed in the first place.

@Chris, the SEER rating procedure does make an assumption about sizing. I don't recall the details, but it does not assume perfect sizing.

@Kent, the latent capacity which comes along with AC isn't free, but it is by far the least expensive way to remove moisture.

@PJ, as Allison said, the existing home issue is a bit OT, but you brought up a good point... HVAC dealers who specialize in retrofit should all own an use a blower door and duct blaster. Those that don't are just guessin'

That being said, as I've oft noted in other forums, a homeowner can get an accurate estimate of the home's true load by cycle timing, for those so inclined. Service contractors should be all over this by installing data loggers in their client's homes. That way, when it's time to replace the system, there's no question about what size is needed. If an envelope retrofit is planned, it's useful (although not always possible) to wait a year before changing the HVAC, so that cycle times can be monitored post-retrofit. (Sorry, didn't mean to get sidetracked.)

As for why worry about sizing so much when even a perfectly sized system is oversized 97% of the time... Well, full variable systems (VRF) get around that problem, BUT they are very expensive. As it turns out, a single-stage system sized properly has a much lower life-cycle cost and does a good job maintaining moisture levels (and comfort) in most of the country, if house is built correctly and system is properly designed. Even a VRF system won't overcome poor envelope and duct design.

BTW, a system that's 25% or 30% oversized will be somewhat less efficient, and will remove less moisture, but in many cases, will work just fine. The problem we're all twisted up about is the egregious oversizing we're seeing in high performance homes, where it's not unusual to see seeing systems that are 2x or even 2.5x the true load. Here's the reason... it takes the evaporator coil 8 to 15 minutes to start dripping water. If the system never stays on that long, it will not remove ANY moisture. In humid climates (most of the US), now you got problems. Not to mention the additional energy lost during startup, especially if ducts are in the attic.

What I like about the AC sizing benchmark is that it would finally force HVAC dealers and builders to collaborate on plan review, rather than "here are the plans, tell me how much". And at last, it would create value for proper system design.

By: nichole l reber

nichole l reber — Thu, 22 Sep 2011 03:03:08 +0000

2000 square feet per ton?
2000 square feet per ton? Crikey! Especially in GA, where people think they'll die if they experience a single drop of sweat.
Your idea to simplify programs like Energy Star's are also really good, especially in this time of overlapping green building programs. Too much information just confuses everyone, especially home buyers.
Cheers!