ASHRAE Journal - May 2022 - 39
TECHNICAL FEATURE
higher than 1,000 ppm, in which case an additional
increment of 1,500 ppm would substantially overshoot
the 2,500 ppm target value. Furthermore, if the target
value were reduced from 2,500 ppm to the lower
range of Thom's work (and the European standard), the
acceptable incremental level would be cut in half.
Water Vapor
Moisture is not itself a pollutant in
the traditional sense. Its impacts can
be on building serviceability, thermal
comfort and facilitation of mold
growth-which can lead to health,
irritation and odor problems. ANSI/
ASHRAE Standard 55, Thermal
Environmental Conditions for Human
Occupancy,19 which was last updated
in 2020, sets an upper limit of 62.2°F
(16.8°C) for dew-point temperature
based on comfort conditions, but
dew points far lower than that can
cause condensation or microbiological
growth that would not be acceptable,
particularly in cold climates
where space heaters might reasonably
be used.
Relative humidity (RH) of a given
mass of air is dependent on its
temperature, so RH near a cold
surface will be higher than in the
bulk air, but the dew point is the
same. In fact, heating to a higher
setpoint while generating moisture
can lower relative humidity and yet
raise dew point. Since, however,
relative humidity is the more commonly
understood quantity, we will
use it with a typical winter indoor
(dry-bulb) temperature around 70°F
(21°C) and, thus, relative humidity
and dew point are directly linked.
ASHRAE Special Publication
" Damp Buildings, Human Health
and HVAC Design " 20 provides us
with some guidance on setting the
acceptable limit. This publication
recommends keeping the indoor
M AY 2 0 2 2 ashrae.o rg ASHRAE JOURNAL
39
humidity below 45°F (7°C) dew point in moderately cold
and mixed climates and lower in the cold and very cold
climates. A 45°F (7°C) dew point corresponds to a 40% to
45% relative humidity at room temperatures. During the
winter, homes in these climates likely have indoor relative
humidity in the 20% to 25% range. This suggests that
for the purposes of estimating unvented heater impacts,
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ASHRAE Journal - May 2022
Table of Contents for the Digital Edition of ASHRAE Journal - May 2022
Contents
ASHRAE Journal - May 2022 - Intro
ASHRAE Journal - May 2022 - Cover1
ASHRAE Journal - May 2022 - Cover2
ASHRAE Journal - May 2022 - 1
ASHRAE Journal - May 2022 - Contents
ASHRAE Journal - May 2022 - 3
ASHRAE Journal - May 2022 - 4
ASHRAE Journal - May 2022 - 5
ASHRAE Journal - May 2022 - 6
ASHRAE Journal - May 2022 - 7
ASHRAE Journal - May 2022 - 8
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ASHRAE Journal - May 2022 - Cover3
ASHRAE Journal - May 2022 - Cover4
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