ASHRAE Journal - October 2022 - 6

LETTERS
Impacts of Unvented
Space Heaters
The May 2022 ASHRAE Journal
article, " Impacts of Unvented Space
Heaters " by Max Sherman, Ph.D.,
Fellow/Life Member ASHRAE;
Philip Fairey, Life Member
ASHRAE; Roy Crawford, Ph.D.,
Life Member ASHRAE, has some
problems.
One involves water vapor and
relative humidity (RH). The
authors correctly identify that RH
is dependent on temperature but
fail to mention that the relationship
between the two is inversely
proportional. As temperature
increases, relative humidity
decreases, and the air will actually
become drier. This is especially
true during the winter when
unvented heaters are typically
used.
While using stoichiometric combustion
calculations may show an
increase in the absolute value of
water vapor, actual measurements
would likely show RH decreasing as
air temperature increases. In fact,
in winter adding a little humidity
may improve conditions, not
diminish them.
We have all experienced shocks
from static electricity in the winter.
That's because the air is dry, which
in turn can lead to health problems
including increased risk of infection
from germs and viruses. So,
adding some moisture to cold, dry
air is a benefit, not a detriment, an
important point overlooked by the
authors.
A probabilistic assessment
of unvented heaters on indoor
RH was conducted in 20021 and
6
examined 20,000 simulations
over five climate zones. The results
showed that even with additional
water sources and other variables
(considerations beyond what the
Journal article included), all were
below the 70% threshold for mold
growth. What's more, the article's
mass balance model assumes an
ACH of 0.35 and below. That is a
very tight building envelope, especially
when the 2021 International
Energy Conservation Code allows
for 5.0 ACH-14 times the value of
what the authors used. It would be
difficult to find " poorly insulated
homes " in such well-constructed
residences.
Additionally, a 2008 study2 measured
actual RH and dew-point
temperatures of 30 homes in
Illinois and showed particularly
good results with regard to water
vapor generation from unvented
heaters. Both dew point and RH
measurements were well below
the thresholds in the vast majority
of measurements taken in various
locations in the home. These
real-world results contradict the
authors' assessment.
Taking a one-dimensional
approach to analyze the contribution
of water vapor from stoichiometric
combustion is flawed. There
are too many variables that need
to be taken into consideration.
This article does not do justice to
its topic and instead uses selective
facts, excerpts and calculations to
condemn unvented space heaters.
References
1. Whitmyre, G.K., et al. 2002.
" Assessment of The Potential Impacts of
Vent-Free Gas Products on Indoor Relative
Humidity. "
2. Francisco, P.W., et al., 2008.
" Combustion Product Concentrations of
Unvented Gas Fireplaces. " HUD Grant
Number ILLHH0125-04.
Eric Adair, P.E., Arlington, Va.
The May ASHRAE Journal article
on unvented space heaters omits
one very important detail in the
combustion process: the production
of heat. We know from basic
chemistry that heat is required to
produce the reaction that converts
methane into carbon dioxide and
water vapor. The authors overlook
this important aspect of combustion
in their analysis.
Why is heat important?
Considering the operation of an
unvented heater, 100% of the heat
is delivered to the space where
the heater is located. That heat
affects temperature rise, and more
importantly, the duration of operation.
And that will have a dramatic
influence on the contaminant levels
delivered during typical heater
operation.
ASHRAE JOURNAL ashrae.o rg O CTO B E R 2022
https://www.ashrae.org/
ASHRAE Journal - October 2022

Table of Contents for the Digital Edition of ASHRAE Journal - October 2022
