SZ…KELY ENGINEERING
Tom Szťkely, P.E., LEED AP

EXPLANATIONS & EXAMPLES - Vol. 8, No. 1
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January 8, 2008

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A Funny Thing Happened on the Way to the LEED Exam, or, Construction Codes and the Law of Unintended Consequences

 

Regular readers may recall that I wrote a series of four newsletter issues (Vol. 7 No. 1 through Vol. 7 No.4 ; on the Newsletter Archives page of my website) early last year where I explained the LEED rating system , one of the motivations for which was self-preparation for the LEED-AP accreditation examination.

 

Iíll finally be sitting for that examination on January 11th, but back in 2001 when I sat on the Technical Subcommittee which hashed out the New York State version of the International Energy Conservation Code (IECC), it was as a representative of the New York State Society for Professional Engineers and my prime focus was to make sure we didnít make any changes that were contrary to my understanding of State Law regarding the licensing of learned professions.

 

Iím happy to say that my fellow committee members were such a well grounded and levelheaded bunch that I only felt the need to speak up on two or three occasions, and each time it was for some minor technical clarification.  I can just see those of you who know me shaking your heads in disbelief regarding my professed reticence, but thatís the way I remember it.

 

Nick Greco sat on the committee as the representative from the New York City Department of Buildings (DOB), and heís the reason for 101.3.3 in the Code where it makes the NYC Building and Electrical Codes applicable within the five boroughs of New York City whenever the State Energy Code makes reference to Building and Specialty (Plumbing, Mechanical, etc.) Codes.

 

The NYC Department of Buildings has formalized the steps necessary to show compliance with the NYS Energy Code only since September of 2007, but the Cityís Building Code has required compliance with the former via Reference Standard RS 13-1 of the latter, probably since 2002 or so. However, when the NYS version of the IECC was first issued, things started to get a bit sticky.

 

Iím not at all clear that one canít end up using more energy rather than less if one complies with the Energy Code via the prescriptive requirements therein.  I first had inklings of this state of affairs when I set up a block load HVAC spreadsheet based on the ventilation requirements of the International Mechanical Code (IMC), which is referenced in Chapter 8 of both the IECC and the NYS version: The Energy Conservation Construction Code of New York State . 

 

If you go with the prescriptive requirements of chapter 8 rather than the one-sentence reference to

ASHRAE/IESNA 90.1 which comprises chapter 7 of these codes, you could end up with ventilation or exhaust make-up air of such magnitude as to seriously affect the sizing of air conditioning

equipment when compared to the ventilation requirements imposed by the  NYC Building

Codeís Ventilation Index (a dimensionless number calculated on the basis of how many persons are in how big a space).

 

I began to see this because Iíve long had an engineering spreadsheet set up to do block loads and calculate NYC Ventilation Indices for me, and I refined it some time back to make it iterative.  That is I could try different percentages of outside air to meet the minimum ventilation CFM required for a given space, and not infrequently discovered Iíd require less air conditioning if I used a larger percentage of outside air (notwithstanding the fact I had to cool and dehumidify that air) because of the relative size of my internal cooling load.

 

The prescriptive per person or per square foot outside air requirements included in the energy codes via reference to the IMC are actually substantially greater than those in the Building Code of the City of New York, as was hammered home to me on a project I just completed, where I calculated up 25 tons of AC and 2015 CFM of outside air for an 8200 square foot commercial space in the Bronx, using the Ventilation Index in the NYC Building Code, and 36 tons of AC and 4648 CFM of outside air when using the IMC for the same space.

 

Rather than bore you any more than I already have with more minutiae of how I got there, take this away with you:

 

The NYC Building Code requires 50 CFM exhaust for a single water closet or urinal, or 40 CFM each in a room with a battery of fixtures.  A five-position menís or ladiesí room thus requires 200 CFM exhaust. 

 

Table 403.3 of the IMC mandates 75 CFM per fixture, period, or 375 CFM for a battery of five.

 

That exhaust air has ultimately to come from the great outdoors, and when itís hot and humid outside, it wonít do good things to the size of the AC equipment youíll need.

 

This is a strange turn of affairs indeed.  Iíve spent much of my professional life lamenting the needless restrictiveness of NYC Codes, only to discover that certain aspects of the codes of the International Code Council are worse.  This is the reason I said things began to get a bit sticky with the DOBís September implementation of compliance verification;  one of the methods of verification permitted is via the prescriptive requirements using software or worksheets, which I expect are built around the IMCís increased ventilation requirements, rather than the NYC Ventilation Index. 

 

Is this an oops? I donít know, but it could be. Iíve not had time to look at the worksheets, and will let you know what I think after I do.

 

OK, on to another subject, one which I have pontificated upon at some length in the past:

 

Cast Iron Radiators and Retention of Heat (Again)

 

Two years ago, in Vol. 5 No. 1, I reiterated my problems with steam heating systems, and I spoke of the popular belief that cast iron radiators ďretain heat,Ē commenting that the best they can do is ď. . . fill in the voids in system output when the boiler is off . . .ď and that ď. . . cast iron will cool to room temperature in rather short order . . .Ē

 

As is often the case when I shoot my mouth off (okay, as is occasionally the case), I began to feel uncomfortable enough with the generalizations Iíd made to want to look at some numbers.

 

I should have known better.

 

First, I had to find out how much heat had been pumped into such a unit to get it up to operating temperature, which was no easy task.  No matter how hard I searched, I couldnít find out how much a section of low profile cast iron baseboard weighed.

 

I finally had to estimate the weight based on cast ironís density of .28 pounds per cubic inch, and the estimated volume of a 9-7/8Ē high by 2-1/2Ē wide by one foot long section, allowing for the voids between the cast-in fins and the absent material resulting from the water or steam flow passages.

 

My flying leap of calculations came up with about 28.3 pounds for a one foot section.

 

Regular readers already know that a BTU is the amount of heat that will raise the temperature of a pound of water a degree Fahrenheit.  The specific heat of a substance is how much heat would be required to do the same thing to that substance, and in the case of cast iron, itís 0.11 BTU.

 

When a steam baseboard is operating at 215 degrees F and a room temperature of 70 degrees F, it has (215-70) x 28.3 x 0.11 BTU, or about 451 BTU stored in a one-foot section at the moment the boiler shuts off.  While the thing is operating with live steam flowing though it, it delivers 750 BTU per hour of heat to the space itís in.

 

If everything were linear, and actual heat transfer rates did not depend upon temperature differences (which change as an object cools off), one could infer that something that could deliver 750 BTU in an hour would dump 451 BTU in 451/750 x 60 minutes, or 36 minutes.

 

The reality is that because the temperature difference between a recently inactive radiator and its surroundings decreases as time goes on, the amount of heat transferred to the surroundings by that radiator also decreases, so that while the radiator may remain above room temperature for a somewhat longer time, it wouldnít be doing much in the way of keeping the space warm. 

 

To really nail this down, one would have to know about heat exchangers and LMTD (Log Mean Temperature Difference), and before you could do that, youíd have to know about various types of heat transfer.   Not in this issue.

 

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