For a good overview and discussion on water
heaters see: Holladay, M.,
All About Water Heaters, Musings of an Energy Nerd, Green
Building Advisor, Feb. 2012
The heat exchanger for the featured hybrid is typically a
brazed plate type as shown below.
Click to enlarge
The brazed plate type heat exchangers have a long
history in providing compact reliable heat exchange at a
relatively low cost. Photo above is from an actual hybrid
application as described on this page.
A domestic dilemma
Click for larger image
Bacteria vs Hot Water Scald Burns, Time - Temperature Relations,
2nd & 3rd Degree Burns, Adults & Children
Acceptable risks? Two opposing questions:
Should you operate water heaters at temperatures low
enough to enable better efficiencies but support microbial growth?
or...
Should you operate water heaters at temperatures high
enough to control microbial growth but introduce scalding risks
and reduce efficiencies?
One answer can be found in the "precautionary principle":
The Canada Consumer Product Safety Act, April 8,
2008, states, “Whereas the Parliament of Canada recognizes
that a lack of full scientific certainty is not to be used as a
reason for postponing measures that prevent adverse effects on
human health if those effects could be serious or
irreversible..."
Scalds can be prevented with approved mixing
valves however bacteria in a potable water system is an unknown.
So in the absence of certainty, it is best (in the opinion of
those who study bacteria) to create conditions which do not
support the growth of bacteria, i.e. using high temperatures and
solve the scalding risk with approved safety devices.
For more details on prevention of Legionellosis associated with
building water systems obtain the document below:
Proposed new standard: BSR/ASHRAE Standard 188, Prevention of
Legionellosis Associated with Building Water Systems (Public
review draft only - not official and not for use until released
by ASHRAE)
The American Society of Sanitary Engineering (ASSE)
“Understanding Potential Water Heater Scald Hazards" describes
scalding and bacteria risks associated with domestic water
heaters.
There has been a lot of discussion on the web on
the efficiency of water heaters. Rather than add to that
discussion we've listed at the bottom of this article some very
good resources by reliable researchers. What we'll do here is
describe a hybrid system for those applications requiring a
boiler. In this approach a typical tank type water heater is
adapted or converted to an indirect fired water heater for use with a
heating boiler for winter time operation. During summer time,
boiler is turned off and water heater operates as a normal fuel
fired appliance.
Why do this? If you
follow the research (BNL report, T.A. Butcher), one of the
lower efficiency applications for heating domestic water in a
single family home with relatively low to moderate draw is the
operation of a boiler during the summer months to heat domestic
water in an indirect fired tank or tankless heater. The hybrid
below can avoid this by taking advantage of the boiler
combustion and its associated efficiency during the space heating periods
and the water heater combustion and its associated efficiency during the non space heating seasons.
Secondly, such a set-up can provide for
periodic disinfection of water borne bacteria in the
plumbing system and can be used for emergency space heating in
the event the boiler is shut down for maintenance or repairs.
Is it for everyone? No, like all solutions, there
is no one size fits all. This hybrid is one solution where there
is a need for a boiler due to choice in heating system and works
to enable the best efficiency for each device during the space
heating and non space heating seasons. It can be implemented in
new systems or can be adapted to exiting systems.
A professional engineer/technician and competent
HVAC/plumbing contractor can provide the control strategy using
off the shelf components.
Figure 1 Hybrid system for using a typical fuel
fired water heater as an indirect for winter operation, and
emergency stand by. Boiler can also be used to provide periodic
disinfection of potable water system (see scald
warnings).
Figure 2 Hybrid system (larger view): the system
can also be reversed during the winter for emergency use should
the boiler be shut down for repairs or maintenance.
Choice
of heaters: A note on domestic water temperatures for
controlling bacteria
Figure 3 Bacteria vs Hot Water Scald Burns, Time
- Temperature Relations, 2nd & 3rd Degree Burns, Adults &
Children (click adjacent thumb for larger view). Overlay are
typical temperatures for radiant heating systems.
Without a doubt one of the contentious issues in
domestic water heating addresses bacteria such as but not
limited to Legionella. As evidence of this issue make note
of the following text (Stout 2007);
"Many guidelines recommend that the hot water
temperature at the tank be 140°F (60°C) and the circulating hot
water temperature be 124°F (51°C). Will this
eliminate Legionella from distal outlets (faucets and showers)?
The aforementioned study showed that peripheral sites
remained heavily colonized despite elevated recirculation
temperatures (>140°F [>60°C]). Legionella colonization was
ultimately reduced in a Swedish hospital after it raised the
temperatures even higher, to 149°F (65°C) at the tank and 133°F
– 142°F (56°C – 61°C) at the outlets."
Two of the challenges with these high
temperatures (though effective as a disinfectant) is risk of
scalding and destruction of heater efficiency.
Recommendations in pending standards is to mitigate
bacteria by maintaining hot
water heater outlet temperature at or above 60°C (140°F); hot water
temperature at coldest point in hot water heater, storage
tank or distribution system at or above 51°C (124°F); cold water
temperature in any part of system at or below 25°C (77°F).
This operating range requires scald protection devices (see
mixing valve in Fig. 2)and could reduce the choices in water
heaters.
Unless high operating temperatures are mandated
by local authorities having jurisdiction, consumers will have to
decide what are acceptable risks to them and their families when it comes to water borne
bacteria (see side bar). It really is the million dollar
question, "does energy trump health?".
Suggested reading (for authors name and
affiliation and access to reports, Google each citation as
desired), some must be purchased through the ASHRAE bookstore,
others are readily available and free (research
reports on Legionella):
ANSI/ASHRAE 137-2009 137-2009 -- Methods of Testing for
Efficiency of Space-Conditioning/Water-Heating Appliances that
Include a Desuperheater Water Heater
Performance of Integrated Hydronic Heating Systems, Project
Report for New York State Energy Research and Development
Authority and National Oilheat Research Alliance, Brookhaven
National Laboratory, December 2007
Shapiro, C., Puttagunta, S., Owens, D.,
Measure
Guideline: Heat Pump Water Heaters in New and Existing Homes,
Consortium for Advanced Residential Buildings (CARB) prepared
for Building America Building Technologies Program, Office of
Energy Efficiency and Renewable Energy, U.S. Department of
Energy, February 2012
Schoenbauer, B, Kingston, T.,
Residential Space and Water Heating: The Combined Approach,
presented at Better Buildings: Better Business, ECW, Schaumburg,
Il, 2012
