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.
I’ll 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 didn’t make
any changes that were contrary to my understanding of State
Law regarding the licensing of learned
professions.
I’m 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 that’s the way I remember
it.
Nick Greco sat on the committee as the
representative from the New York
City Department of Buildings (DOB), and he’s 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 City’s 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.
I’m not at all clear that one can’t 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
Code’s 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 I’ve 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 I’d 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 men’s
or ladies’ 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 it’s hot and humid outside, it won’t
do good things to the size of the AC equipment you’ll
need.
This is a strange turn of affairs indeed. I’ve 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 DOB’s 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 IMC’s
increased ventilation requirements, rather than the NYC
Ventilation Index.
Is this an oops? I don’t know, but it could be.
I’ve 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:
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 “retain heat,”
commenting that the best they can do is “. . . fill in the
voids in system output when the boiler is off . . .“ and that
“. . . cast iron will cool to room temperature in rather short
order . . .”
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 I’d 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 couldn’t find out how
much a section of low profile cast iron baseboard
weighed.
I finally had to estimate the weight based on
cast iron’s density of .28 pounds per cubic inch, and the
estimated volume of a 9-7/8” high by 2-1/2” 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, it’s
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 it’s
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 wouldn’t 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, you’d have to know
about various types of heat transfer. Not in this issue.
