Using PEX in Solar Water
In the article "Solar Hot Water 101" (10/05), the author
states, "We don't use PEX because in California it's illegal to
use it for potable water — plus the high temperatures
found in the closed loop of a glycol system could easily be too
hot for it."
This statement concerns me. If the product's illegal for
potable water, that makes it unusable, in my opinion.
Shawn McFarland, AIA
Author Gary Gerber responds: It is my understanding that
PEX has not yet been made legal in California for potable hot-
or cold-water use. It is legal in many other states, and in my
opinion it should be legal everywhere.
My main caution for using PEX in a solar system is that the
material softens at higher temperatures, so care should be
taken not to exceed the manufacturer's temperature and pressure
limitations. In some solar hot-water systems, temperatures can
exceed 300°F at the collector during stagnation.
I wouldn't worry about using PEX for connecting to the tanks
of thermosiphon systems, but in a system where the PEX could be
directly connected to an empty glazed collector, that
connection could experience extremely high temperatures and
pressures should the collector be left empty in the full sun.
One solution might be to use copper pipe for some distance from
the collector — say, 10 feet — and then
transition to PEX.
Solar Hot-Water Storage
I appreciated Gary Gerber's article on solar hot-water
systems. I'd like to offer another viewpoint on one area,
however. He gives an optimum amount of storage in gallons per
square foot of collector, and says that this optimum varies
with location (climate). For example, he states that the
optimum for my part of the country, the Northeast, is 0.75
gallon of storage per square foot of collector. My experience
is that on a clear day a square foot of collector can put 800
or more Btu into the storage tank. This amount of energy would
raise 0.75 gallon of water 128°F! Since the collector
runs more efficiently when the water is at a low temperature,
it is actually best to have more storage. This keeps the
temperature of the fluid returning to the collector lower, and
increases the energy harvested.
I size solar domestic water systems in the range of 1.5 to 2
gallons of storage per square foot of collector. This sizing is
based on clear-day solar gain. In the Sun Belt, there's more
solar gain to be had, but the difference is mostly due to the
proportion of clear days, not the amount of clear day
insolation. So don't undersize your storage tank, even if you
live in the Northeast or Northwest.
Marc Rosenbaum, PE
How Does Hot-Water Recirculation Work?
I was interested in the article on the retrofit pump for
hot-water recirculation (Kitchen & Bath, 8/05), but have
a question: How can a pump circulate water through a
pressurized system? Does the system account for the street-line
pressure pushing against the pump?
The editors respond: Since both sides of the supply system
— hot and cold — are under street pressure,
the difference between them is effectively zero. Therefore it
doesn't take a large pump to move water from the hot side
through the cold line back to the water heater. The hot water
coming out of the tank is simply replaced by the cold water
coming back, so there is no head of pressure to
Leak Testing Questioned
In "Rainproofing Stucco Trim" (10/05), the author describes
using the ASTM E1105 standard for testing window trim and
stucco cracks against water penetration. This is a stand-alone
test and has not been adopted by either the stucco or
window-installation industries. Using it to evaluate a trade
that hasn't adopted the standard is irresponsible. The wood
trim in this application was a poor design, whereas the stucco
is performing within industry standards but was tested with an
invalid testing procedure.
San DiegoAuthor David Dobson responds: ASTM E1105 was created for
the American Architectural Manufacturers Association as a
standard to test windows for certification and to set
performance standards. The test uses a spray rack to simulate
wind-driven rain equal to 8 inches per hour at a wind speed of
33 mph for 15 minutes.
Not all experts agree to the use of the calibrated spray rack
in the field to test windows. However, when window and wall
leaks are present during construction defect litigation,
experts for both the plaintiffs and the defendants often use
spray racks calibrated per ASTM E1105 as a diagnostic test to
locate leaks — a fact that shows it is an accepted
standard for determining the source of leaks.
Occasionally experts and attorneys cry foul when the spray
rack is used to test windows that have been in place for
several years, claiming that the spray racks simulate a rate of
rainfall that has never been recorded. While this is in fact
true — to my knowledge, there has never been 8 inches
of rain per hour recorded anywhere — all structures
should be designed with a safety factor to withstand forces and
conditions they are never expected to meet, to ensure adequate
performance for the life span of the structure or