ASHRAE Journal - July 2014 - 10

Letters

would like to provide the following data in response to
your questions. In the interest of brevity, we will not
repeat the questions, but include ADEG recommendations in parentheses.
Space Conditions The temperature setpoint for radiant
and VAV system is the same, 75°F. The ceiling fans are
installed on both sides of the building. Both of these,
the temperature setpoint and ceiling fans, are standard
practice in Indian office buildings.
Ventilation Airflow The DOAS supply air dew point for the
radiant side of the building is 50°F. We achieve this using
46°F chilled water by designing the air handler with a
low face velocity and close approach. We do supply a
little more air than is needed by Standard 62.1 to control humidity. The sizing of the DOAS AHU on the radiant side is equivalent to 17 cfm/person. The VAV system
uses fresh air only to the extent specified by 62.1. The
VAV system fresh air design was slightly higher than 13
cfm/person.
VAV System Parameters The VAV system in the building was
designed to perform better than the AEDG. AEDG recommendations are shown in parentheses and the building VAV design parameters are shown after.
Maximum Fan Power (0.72 W/cfm) 0.39 to 0.42 W/cfm
External Static Pressure (2.0 in. w.c.) 1.44 to 1.72 in. w.c.
Energy Recovery (80% sensible, 70% latent, 75% total) total
effectiveness of 79.4%
Fan Pressure Optimization (Yes, required by 90.1 since 1999)
Yes.
Supply Air Temperature (50°F, reset to 58°F for economizer
effectiveness) 56.5°F is the design supply air temperature
Demand Controlled Ventilation (Zone DCV in response to occupancy) Yes, in response to CO2
System Ventilation Reset (Using Standard 62.1 multiple space
equation and diversity) We did not implement ventilation reset control (VRC) due to its limited applicability
and benefit in a hot climate where the building has a
predictably high-load occupancy.
DX System Replacement A low-cost DX system for dehumidification could have worked. We needed one that can modulate smoothly to meet a variety of loads. Instead, we
had one that had only four stages. Even if a more expensive modulating DX chiller were used, the cost of the
radiant system would have been equivalent to the VAV
system. Lower energy use at the same cost is still a favorable outcome. Once we switched to using the chilled
water for dehumidification, after taking into account
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the chiller energy, the energy savings of the radiant side
increased to 42%.
Economizing We beg to differ that the VAV system would be
more efficient than radiant in more moderate climates.
With a well-designed waterside economizer that we have
used on many award-winning projects in the U.S., radiant systems still significantly outperform well-designed
VAV systems with airside economizers.
Peter Rumsey, P.E., Fellow ASHRAE, Oakland, Calif.

n In May's "VAV vs. Radiant," the authors state, "This
comparison, based on actual measurements instead of
simulations, is more reliable and accurate than studies
that use energy modeling software." Their conclusion,
not validated by any evidence, is exactly the opposite of
my career experiences.
I used my first building simulation in 1973, and have
used a variety of methods and on-site comparisons. In
general, I've found typically ±50% variance between simulation and actual energy. Every time I've investigated in
more detail, I find the building use and the choices made
by occupants and maintenance personnel make such a
large difference that I always use a disclaimer that economic comparisons are good tools for choosing between
alternatives, but simply cannot anticipate the changes
made after construction for "guaranteed utility bills."
The comforting thing is that on those occasions when
we tracked most of the changes, the decision between
choices did not alter; only the magnitude of the root
numbers (predicted energy use for example) changed.
Percentages (between system choices), paybacks, ROI
were all remarkably stable. The simulations are very
dependent upon the user's experience with the software
for final result accuracy, but they are nicely neutral to
unpredicted occupant demands.
Unless one is willing to do very intrusive (and expensive)
hands-on field verification of use/abuse conditions, "real
utility bill" results are not good system comparison tools.
Pete Menconi, P.E., Life Member ASHRAE, Sun Lakes, Ariz.

The Author Responds
You do make an important point when you say "I've
found typically ±50% variance between simulation
and actual energy." In my experience I have found the
same thing. Where we differ is in our assessments of
the cause of the variance. You conclude that the reason has to do with operations and people not acting

ASHRAE Journal - July 2014

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