Comments on: U R A ΔT, and Other Building Science Blandishments

By: phil

phil — Wed, 16 Nov 2011 17:47:09 +0000

OK,I think I've got it.&
OK,I think I've got it.

To avoid the horrors of oversized equipment we use math and science and our manual J spreadsheet to select on/off equipment that is oversized 99% of the time.

To those folks that elect to choose a heating source that delivers just the amount of capacity they need we tell them their perceived benefits are due to a post hypnotic suggestion.

And then we call ourselves building scientists.

You gotta love it, What a business!

pj

By: David Butler

David Butler — Wed, 16 Nov 2011 10:04:04 +0000

I never said single speed is
I never said single speed is "as comfortable", but when compared to what most folks are used to, I doubt many people would be able to tell the difference.

You said...
>I do not know anyone with 2 stage AC units who would ever go back to a single stage unit.

I would argue that this is because their dealer did a great job brainwashing them (sorry, couldn't resist). Seriously, it's because their point of reference was likely so bad. After spending the money, and if their new system is designed and installed properly, then of course they're going to feel that way. Multi-stage and variable capacity equipment is good stuff. Really. It's just too expensive for what it provides. And has some long term service issues.

I have literally hundreds of clients throughout the country for whom I've designed and specified HVAC who are thrilled at the comfort they have (check the testimonials on my website about-us page). My clients too often come from bad homes with bad HVAC systems. I wonder if they could really appreciate the subtle difference. It's like going from 1950's TV via rabbit ears to HD-720P (single stage done right), then to fully modulating (1080P).

And BTW, I never specify ground source heat pumps. I've yet to find a situation where they're cost justified, even with 30% tax credit. In my own home, my 15 SEER Lennox air source HP only consumes around 900 kWh in winter, and 2200 kWh in summer. A ground source (assuming it would even work here) might save $150 (40%) off what I spent for hvac before I went net-zero energy.

I believe the most important prerequisite for comfort is a good envelope. Perhaps it is you that hasn't experienced how comfortable one can be with single stage HVAC.

By: phil

phil — Wed, 16 Nov 2011 08:46:26 +0000

Thank You for your response.
Thank You for your response.

You are the first person I have ever met, ever, claim single speed AC and Heat is as comfortable as modulating equipment.

Since loads are constantly changing why would a single speed AC/Heat make any sense? Would you buy a car with an on/off switch for a gas pedal? Full speed or stopped?

In many towns, Boston for instance, carrier will state that their 2 stage AC will remove 30x the moisture, run 84% of the time on first stage, and provide far superior comfort. IAQ devices only work while units are running. Your manual J sized single stage furnace will short cycle all the time except when it is -10 outside.

I do not know anyone with 2 stage AC units who would ever go back to a single stage unit.They would certainly not agree their money was "wasted"
Is someone buying a geo system wasting their money? What if I don't like unsightly outside units, or noise.What if I'm a Greenfreak? How can you make this call for someone else.

Fact:It is impossible to know just how comfortable you can be until you are comfortable. Most folks with single stage units think that is all comfort is.
Since your last 2 Homes had singe stage units this would seam to apply in your case.You may not have experienced comfort in your home yet, A tragedy.

I am sure I can find a contractor buddy that would be happy to get you a nice price on a 2stage HP, $50 says you change your mind.

Pj

By: David Butler

David Butler — Wed, 16 Nov 2011 07:16:51 +0000

Phil:
Phil: While I agree with most of your points (although I would hardly refer to Manual J as being a crude tool for modeling design loads), it is not necessary to have modulating equipment to achieve comfort. I've heard that argument many times before, mostly from the very folks who sell this (very expensive) equipment.

The reason most homes are uncomfortable is *not* because the equipment is oversized during part-load conditions, but rather because of poor design and installation practice (especially duct systems) and egregious oversizing, along with a litany common envelope flaws. The fact is, a poorly designed and installed modulating system in a home with a faulty envelope will not deliver comfort.

Let's get the basics right before we start talking about higher performance equipment, lest we waste the client's hard-earned money on the small potatoes.

BTW, my current home and previous home (both with single-stage heat pumps) are the most comfortable and economical I've ever lived in.

By: phil

phil — Wed, 16 Nov 2011 00:14:27 +0000

Mr. Bailes,
Mr. Bailes,

1. When would modulating equipment not be the "best" solution?

2. Why use a crude approach like manual J with all the flaws when better tools are available?

By: John Mattson

John Mattson — Tue, 15 Nov 2011 23:57:29 +0000
Good article overall. Small
Good article overall. Small point, all else being equal (which never is) a house with larger SURFACE AREA will loose/gain more. "Size" can mean volume as well as surface area so be careful using "size". Your equation comes out in heat per unit of surface area. Second, and a really small point, of interest only to physics types, it is Grad-T, the temperature gradient, not simple difference. They are subtly different.

By: Allison Bailes

Allison Bailes — Tue, 15 Nov 2011 04:05:29 +0000
George R.:
George R.: Good eye! In this case the walls were undersprayed intentionally because they're not your standard 2x4 walls. I'm pretty sure the one in that photo is a 2x8 wall, so it can still be Grade I as long as the rater doesn't overstate the R-value.

By: George Reynolds

George Reynolds — Tue, 15 Nov 2011 03:01:12 +0000
The picture of the spray foam
The picture of the spray foam insulation with the "nominal" thickness variation (which exceeds the Version 3 grade 1 spec, raises a more difficult question than balancing equations. An IR analysis will normally yield quantitative variation of 25%+ unless the cavity is full and flush. Is there any guidance from EPA or Resnet that excuses spray foam from having voids in cavities?

By: Allison Bailes

Allison Bailes — Mon, 14 Nov 2011 20:42:25 +0000
Phil: You
Phil: You make a lot of good points, of course. Yes, there's a lot of uncertainty in modeling existing homes. Yes, equipment that can change its capacity can perform better. It's also more expensive to install and repair. As with anything, there are pros and cons. Don't get me wrong. I like variable capacity, but it's not always the best solution.

John P.: No, no. That comes next week, when we solve the partial differential diffusion equation.

By: John Poole

John Poole — Mon, 14 Nov 2011 19:53:22 +0000
I don't see any partial
I don't see any partial differential equations here, but this is a good start. Deltas are good. Little deltas even better! :-D