Comments on: I Use an Air Conditioner Sizing Rule of Thumb!

By: Allison Bailes

Allison Bailes — Mon, 15 Aug 2011 14:48:03 +0000

Paul S.:
Paul S.: Absolutely! If builders would sit down with the architect/designer, plumber, HVAC contractor, framers, and electricians before work began, the end product would be much better.

Jim: Thanks!

Mac S.: Yep, most architects have no idea how to make a complete HVAC system fit into a house. They don't give the air handlers enough space. They don't give the ducts enough room to turn properly. They put the systems in places where they shouldn't go. That's where we can help. We design homes and HVAC systems and know how to put it all together.

Steve E.: I'm with you 100%, even though I am one of the 'lettered people.' If you're new to our blog, take a look at some of the other articles we've written, and you'll see that we also moan and wail over those same issues you raised.

By: Steve Ebels

Steve Ebels — Mon, 15 Aug 2011 02:17:35 +0000

Ah yes............Builders,
Ah yes............Builders, GC's and architects Oh MY!

I am but a lowly HVAC contractor with no fancy letters behind my name. I find however that as far as understanding the building as a dynamic system which must function in widely varying conditions, it seems as though the many of the "lettered" people don't grasp that concept.
I cannot begin to enumerate the number of times I have been on a jobsite and made a suggestion to the builder or architect about the location of a duct or the equipment itself. It seems that function of the HVAC system and performance of the home take a back seat to aesthetic value. Little thought is given to it beyond the "look" of things.
Then they wonder and bitch about duct noise, moisture problems, condensation in walls, (one builder suggested using an exterior wall cavity as a return air duct)astronomical operation costs. ....I am about to throw in the towel unless I can work directly with a homeowner who will listen. Otherwise please spare me the grief.

Oh and BTW, HVAC ducts and equipment belong in the conditioned space. Not in an uninsulated attic or a damp, moldy and slimy crawlspace. Duct sealing and insulating can only do so much before the laws of physics take over and make a mess of your carefully thought out plan.

By: Mac Sheldon

Mac Sheldon — Tue, 09 Aug 2011 16:56:32 +0000

There’s even a greater
There’s even a greater disconnect between the architect/designer and the HVAC contractor. The architect will usually draw a rectangle and a circle side-by-side and label them “Furn” and “WH” and that’s as far as he’s thought about arguably the most important systems in the house. They exacerbate the problem by spec’ing joist hangers on beams so ducts can’t pass above them, and are often loathe to allow soffits in their 10′ ceilings. Sad but true.

Work with your architects and help them to understand that even the most beautiful home will be disdained by the homeowner if it is not comfortable. The Taj Mahal is one of the most beautiful buildings on the planet yet it has no windows, doors, HVAC or insulation. Would you want to live there? ….OK it’s a mausoleum so of course you wouldn’t but you get the point….. Comfort and Building Performance must be designed in and built in, they can’t be effectively added on.

By: Jim

Jim — Mon, 08 Aug 2011 22:19:44 +0000

good blog
good blog

By: Pauls S.

Pauls S. — Sat, 06 Aug 2011 21:52:27 +0000

It's not a matter of distrust
It's not a matter of distrust towards HVAC contractor, but simply a given when in new construction there is a disconnect between what the GC intended and what HVAC implemented. For a good end-product, the two better sit down together when producing a manual J report, as well as at least manual D. Prior to breaking ground on the project.

By: Allison Bailes

Allison Bailes — Thu, 28 Jul 2011 01:24:16 +0000

Mac S.:
Mac S.: Great points about HVAC and insulation.

Mark J.: That's something I'm in the process of learning more about myself, so expect to see something on that topic here in the blog.

David B.: Thanks for that reference. I hadn't seen that one.

By: David Butler

David Butler — Wed, 27 Jul 2011 06:40:28 +0000

Mark, here's a
Mark, here's a report that discusses this effect. It has to do with convective currents that set up in loose-fill fiberglass ceiling insulation as attic temperatures drop below freezing. In homes that have been air sealed, this should not be an issue in wall enclosures since they're encapsulated on all six sides.

The main take-away for me was that these convective losses don't kick it until attic-house delta-t exceeds about 38F (corresponds to attic temp of 32F in this study).

Although the negative impact of convection in loose-fill fiberglass can be significant at extreme (cold) temperatures, the seasonal impact in moderate climate zones is probably negligible.

Interestingly, the measured R-value for loose fill fiberglass in this study, especially @ R38, actually *increases* as temperatures drop, until convection currents set up. The chart at PDF page 31 is worth taking a look at.

By: M. Johnson

M. Johnson — Wed, 27 Jul 2011 06:06:51 +0000
This business about
This business about insulation losing its value due to being cold... I do not think that is generally known. Please consider telling us more about this.

By: Mac

Mac — Tue, 26 Jul 2011 18:00:04 +0000
I'm a spray foam guy so you
I'm a spray foam guy so you might ask why I'm commenting on the HVAC stuff. I spent about 20 years in the HVAC industry before the spray foam business and know the pitfalls pretty well. Sadly, I spent much of my HVAC career doing Manual J calcs by hand and over a 15-year period in residential work did about 4000 either by hand or with the software that emerged in the 90's. I agree with your rule of thumb and like you, found it useful as a "range-finder" before narrowing in on the details.

Two things came to mind while perusing your article; air distribution and insulation "sizing." As we properly (down) size our HAC systems we need to remember the "V" part of the acronym and we need to be sure we get adequate distribution and rotation of air into the farthest reaches of the structure. This is where good duct sizing is essential but is rarely employed. We also need to use the right materials, and unless you have two linebackers pulling flex duct straight, that product is not appropriate except as a sound absorption measure for no more than 6 ft at the end of the duct run. Very well sealed and insulated ducts located inside the conditioned enclosure are essential.....yes, we still need to be meticulous about air sealing ducts even inside the pressure envelope.

The second item elicited by your article is properly "sized" insulation. When we do our equipment selection, we base the performance on the ASHRAE 2-1/2% Design Temp for cooling and the 1% Winter Design temps yet we continue to use insulation that is "sized" for 75 degree mean temperature. Why are we not asking about the performance of our insulation at the same design temperatures we require of our HVAC equipment? For instance, we now know that some fibrous insulation loses nearly half of its insulating value at 0 degrees F, yet we continue to spec and use it! Anyway, I ask the question rhetorically because right now we have rules and codes to follow that don't require (or even allow) us to think, but I'm ever hopeful that this will change sometime soon as we introduce code change proposals to the International Code Council. For now, I challenge all the thoughtful people reading your Blog - don't forget the "V" and think about the efficacy of your insulation package if you have extreme temperatures.

By: Allison Bailes

Allison Bailes — Mon, 25 Jul 2011 18:32:05 +0000
David B.:
David B.: Unfortunately, getting builders to put right-sized systems on good ducts has been a lot harder than getting them to insulate and air-seal better.