Comments on: We Are the 99% — Design Temperatures & Oversized HVAC Systems

By: Richard

Richard — Sat, 12 May 2012 16:15:16 +0000

To comply with the IRC 2009
To comply with the IRC 2009 code a Manual I,for load, a Manual S for the size of the equipment, and a Manual D for the sizing of the ductwork is required to be submitted when you build to the Uniform Construction Code in New Jersey. Sadly this is never done. If a system is not installed according to these design standards then this is a code violation. I expect this to be the new hot button for home inspectors. I just wish homeowners would get this information when they buy replacement equipment. I just got a permit application where the house was getting 10 K in insulation and a high efficiency furnace the same BTU rating as the old unit. I wonder how it will work out.

By: Elaine

Elaine — Sat, 12 May 2012 02:45:10 +0000

I agree with Geoff on all
I agree with Geoff on all counts.

By: Thomas Anreise

Thomas Anreise — Fri, 11 May 2012 23:22:10 +0000

Great one, Allison. I
Great one, Allison. I thoroughly enjoy "occupying" your blog when I have time. With regard to Brett M's question about verifying proper duct sizes, having the contractor's duct design (if there is one) would be the way to do this, assuming they have used reasonable inputs to create it. The SpecPro software has a page in the report for duct sizing, which tells the installing technician what size duct needs to be run to each location. Of course, duct size is only one component of proper design and installation (flex duct compression, distribution box design, and fitting selection all have enormous impacts on airflow as well.)

By: David Butler

David Butler — Fri, 11 May 2012 22:03:13 +0000

Allision, I was talking about
Allision, I was talking about oversizing the condenser (the real problem), not the blower. When the 13 SEER rule took effect in 2006, one of the things manufacturers did to improve efficiency was to reduce the horsepower of the blower motor. Of course, if the condenser is oversized, the matching furnace or air handler is will have more blower capacity, but remember, the larger condenser also requires more airflow to keep the evaporator happy. So if the ducts are restricted, then its a zero sum game.

By: geoff hartman

geoff hartman — Fri, 11 May 2012 16:13:00 +0000

CAUTION: Duct Calculators
CAUTION: Duct Calculators are not all equal. Some are set up for only one type (Smooth metal, etc.) of duct, and they are still useless until after you know what he design static is, based on fan curve, duct design, materials, etc.

These can be even scarier than improper load calcs. I see technicians and energy auditors pull these things out and pick a random static (0.1? 0.08?) for calc. then they look at the duct size, exclaim "this room needs xxx CFM, slap CFM & static into the calculator, and Vuala! A required duct calc.

Garbage in ='s Garbage Out

Complete the whole process. Use software! you can't do this in your head. Trust the results, but step back and do a "sanity check" to make sure the results are reasonable.

By: Allison Bailes

Allison Bailes — Fri, 11 May 2012 15:49:50 +0000

Geoff H.:
Geoff H.: You're absolutely right. Without going through the full HVAC design process, the results will be less than stellar.

John R.: You're doing a great service to your clients by gathering all those data. When they need to replace their system, they'll be armed with the data they need to get the right system.

Mike L.: Variable capacity systems are great, but they're generally more expensive and not what the industry is used to right now. I'm glad you like the blog!

Jamie C.: Well, if I point out everything in the article itself, I won't get as many comments like yours. ;~) You're absolutely right, of course, that MJ8 results in oversizing even when you do everything correctly.

David B.: You said, "In particular, ducts that are unbalanced, restricted, and more often than not, both. Oversizing does nothing to fix these problems." You know more about all this than I do, but I wouldn't say oversizing does nothing to solve these problems. A bigger blower can push more air through a restricted duct system. But yes, my remark probably overstated the benefits of oversizing.

Brett M.: You're right - Manual J gives you not only the Btu/hr you need for each room but also the air flow in cfm. As both Geoff and David said, though, there's no simple chart you can use to see if it's correct. You need to know the static pressure in the system, the blower's fan curve, the friction rate... There is something called a ductulator which some contractors use for sizing ducts that relates flow, friction rate, and duct size.

By: David Butler

David Butler — Fri, 11 May 2012 13:02:44 +0000

And even if you have the room
And even if you have the room-by-room CFM targets, there's no way to correlate to duct size. The ability of a duct system to deliver the correct airflow depends on the relative friction rates of each duct pathway, the blower curve, and the overall duct system static.

Also, there are many different duct design solutions that will produce a good result, and many more than won't.

If you really want to check your mechanical contractor's ducts, you can invest in a good balometer (or anemometer) and a manometer, or just pay a 3rd party T&B; technician. But as Geoff said, without the target CFM's, there's nothing to verify.

By: geoff Hartman

geoff Hartman — Fri, 11 May 2012 08:45:34 +0000

Brett - yes, the primary
Brett - yes, the primary reason for Man J, after sizing, is to get the cfm. You need to complete Man J, S, T and D correctly to get these numbers.

when your rater tests, verifies and completes QA checklists, they mus have correct cfm, or the process is not going to get the correct results.

By: geoff hartman

geoff hartman — Fri, 11 May 2012 08:41:57 +0000

Brett - No, it does not work
Brett - No, it does not work that way. sorry = you really do need to do the proper design and installation. Variables change with each installation.

why do we keep trying to use a "rule of thumb" to substitute for science and best practices.

The reference chart is the design specs provided by your design contractor. If you don't have this information, you are SOL and guessing at best.

Right is right! (John Tooley)

By: Brett Moyer

Brett Moyer — Fri, 11 May 2012 07:43:05 +0000

Great article Allison.&
Great article Allison.

Correct me if I'm wrong, but one of the benefits of doing Manual J calcs is to get a cfm per room, which helps to determine the amount of heat/coolth that goes to each room.

Now, for us builders (who are admittedly not HVAC pros), is there a simple chart that breaks down duct type/size/length per cfm?

It would be nice to be able to reference this chart on the job site to verify that the HVAC contractor is installing the appropriate ductwork.