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Radiant Floor Heating

Potable water heating topics

Efficiency of the appliance
Efficiency of the system
Bacteria in drinking water
Scald protection

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

For a good overview of energy efficiency losses see: Bailes, A., The 3 Types of Energy Efficiency Losses in Water Heating, Energy Vanguard, Nov 04, 2011

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?


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:

BSR/ASHRAE Standard 188, Prevention of Legionellosis Associated with Building Water Systems
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)

Public Review Draft Standards / Online Comment Database

Suggested Reading:
2012 ASSE White Paper
scalding and bacteria risks

The American Society of Sanitary Engineering (ASSE) “Understanding Potential Water Heater Scald Hazards" describes scalding and bacteria risks associated with domestic water heaters.

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Our Current Position Statements on:

1. Using Hot Water Heaters as Boilers
2. Open Direct Systems used for space heating

Water heater efficiency - A hybrid approach
Copyright (c) 2012, Robert Bean, R.E.T., P.L.(Eng.), and content contributors

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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


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 water heater

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,

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 140F (60C) and the circulating hot water temperature be 124F (51C). Will this eliminate Legionella from distal outlets (faucets and showers)? The aforementioned study showed that peripheral sites remained heavily colonized despite elevated recirculation temperatures (>140F [>60C]). Legionella colonization was ultimately reduced in a Swedish hospital after it raised the temperatures even higher, to 149F (65C) at the tank and 133F – 142F (56C – 61C) 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 60C (140F); hot water temperature at coldest point in hot water heater, storage tank or distribution system at or above 51C (124F); cold water temperature in any part of system at or below 25C (77F).  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?".

ASHRAE Resources on water borne bacteria in domestic water systems:
Stout, J.E., Preventing Legionellosis, ASHRAE Journal, Oct. 2007
BSR/ASHRAE Standard 188, Prevention of Legionellosis Associated with Building Water Systems (note: public review draft only - link may become inactive once standard becomes published)
Freije, M.R., Legionella: Don't Assume, ASHRAE Journal, vol. 54, no. 10, October 2012

Bacteria risks are real - but so are scalding risks:
Scalding death investigation prompts changes in Alberta
Scalding shows AHS hasn't fixed problem
Elder Advocates Of Alberta Society

Healthy Heating's position statement on:
Using Hot Water Heaters as Space Heating Boilers
Water Heaters for Space Heating in an Open Direct System

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):

  1. ANSI/ASHRAE 137-2009 137-2009 -- Methods of Testing for Efficiency of Space-Conditioning/Water-Heating Appliances that Include a Desuperheater Water Heater 

  2. ANSI/ASHRAE Standard 118.2-2006 -- Method of Testing for Rating Residential Water Heaters

  3. Heat Pump Water Heaters: A Comment on Factors Affecting Operating COP, ASHRAE Transactions AT-2822, 1984, vol. 90, pt. 1A, Atlanta, GA

  4. ASHRAE/BSR 118.1-1988R 2000 Method of Testing for Rating Commercial Gas, Electric, and Oil Water Heaters

  5. Emerging Technologies: Gas Tankless Water Heaters, ASHRAE Journal, vol. 51, no. 12, December 2009

  6. Emerging Technologies: Condensing Natural Gas Water Heaters, ASHRAE Journal, vol. 46, no. 2, February 2004

  7. Emerging Technologies: Heat Pump Water Heaters, ASHRAE Journal, Vol. 47, No. 3, 2005

  8. Emerging Technologies: Condensing Technology For Home Water Heating, ASHRAE Journal, vol. 53, no. 1, January 2011

  9. Actual Savings and Performance of Natural Gas Instantaneous Water Heaters, ASHRAE Transactions, Volume 117, Part 1, Las Vegas, NV, 2011

  10. Input-Output Approach to Predicting the Energy Efficiency of Residential Water Heaters Testing of Gas Tankless and Electric Storage Water Heaters, ASHRAE Transactions, Volume 117, Part 2, Montreal, QC, 2011 

  11. Field Evaluation of Two Demand Electric Water Heaters, ASHRAE Transactions, vol. 112, pt. 1, 2006

  12. Cost of Increased Energy Efficiency for Residential Water Heaters , ASHRAE Transactions, vol. 106, pt. 2, 2000

  13. Energy Efficiency Design Options for Residential Water Heaters: Economic Impacts on Consumers, ASHRAE Transactions, Volume 117, Part 1, Las Vegas, NV, 2011 

  14. Field and Laboratory Testing of Gas Tankless Water Heater Performance, ASHRAE Transactions, vol. 114, pt. 2, Salt Lake City 2008  

  15. Comparing Water Heater vs. Hot Water Distribution System Energy Losses, ASHRAE Trans. 2005, vol. 111, pt. 2

  16. Application of a Linear Input/Output Model to Tankless Water Heaters, ASHRAE Transactions, Volume 117, Part 1, Las Vegas, NV, 2011

  17. Energy and Environmental Analysis of Residential Hot Water Systems: A Study for Ontario, Canada, ASHRAE Transactions, Volume 117, Part 2, Montreal, QC, 2011 

  18. A New Study of Hot-Water Use in Canada, ASHRAE Transactions, Volume 117, Part 1, 2011

  19. Hot-Water Distribution System Piping Time, Water, and Energy Waste—Phase III: Test Results, ASHRAE Transactions, Volume 117, Part 1, 2011

  20. Pilot Phase of a Field Study to Determine Waste of Water and Energy in Residential Hot-Water Distribution Systems, ASHRAE Transactions, Volume 117, Part 1, 2011

  21. Energy Efficiency Design Options for Residential Water Heaters: Economic Impacts on Consumers, ASHRAE Transactions, Volume 117, Part 1, 2011

  22. Emerging Hot Water Technologies and Practices for Energy Efficiency as of 2011, Report A112, American Council for an Energy Efficient Economy, Oct. 2011

  23. 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

  24. ENERGY STAR Hot Water Systems for High Performance Homes, Webinar slides, Sept. 30, 2011

  25. Designing a Highly Efficient Combination Space/Water Heating System, CANMET Energy,  ASHRAE Seminar 4, 2011

  26. Designing a Standard Set of Specifications for Combination System, Center for Energy and Environment, ASHRAE Seminar 4, 2011

  27. Combo Space/Water Heating Systems - "Duo Diligence", Building science Corporation, RR-0213, Sept. 2002

  28. Preliminary Modeling, Testing and Analysis of a Gas Tankless Water Heater, Joint NREL/TESS/DEG/BSC, Research Report - 1002, May 2008

  29. Testing Combination Systems to CSAP.9, "The Not-So Surprising Results", Natural Resources Canada, Seminar slides, ASHRAE Conference, June 2011

  30. The Performance of Residential Combination Hydronic Systems, Brookhaven National Laboratory,  ASHRAE Conference, June 2011

  31. The Performance of Integrated Hydronic Heating Systems, Brookhaven National Laboratory, (also in proceedings) 5th Aachener lwrme-Kolloquium, Aachen Germany, Sept. 13-14, 2006

  32. Payback analysis of design options for residential water heaters, Conference Paper, Proceedings - ACEEE 2000 Summer Study on Energy Efficiency in Buildings, March 2000

  33. Tankless water heaters: They're efficient but not necessarily economical, Consumer Reports, Oct. 2008

  34. Point-of-use Solutions for the Remote Plumbing clusters, for Environmental Building Solutions, Nov. 2007 (source:

  35. Heat Pump Water Heaters: A Better Way to Heat Water with Electricity? Energy Vanguard, Oct 25, 2010

  36. 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    

  37. Lower Water Heating Temperature for Energy Savings, U.S. Department of Energy, Energy Efficiency and Renewable Energy

  38. Hoover, B., Tank vs. Tankless Water Heaters: How's a Person to Decide?, presented at Better Buildings: Better Business, ECW, Schaumburg, Il, 2012

  39. Hoover, B., Hot Water 101 for Homebuilders and Architects, presented at Better Buildings: Better Business, ECW, Schaumburg, Il, 2012

  40. Hoover, B., What’s the Big Deal About Tankless Water Heaters?, April 2002

  41. Schoenbauer, B, Kingston, T., Residential Space and Water Heating: The Combined Approach, presented at Better Buildings: Better Business, ECW, Schaumburg, Il, 2012

Related pages:
Programmable Thermostats Part I
Programmable Thermostats - Part II (boiler efficiency)
Effectiveness coefficient, (Φ ) for temperatures in various countries 


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