Online educational resource on achieving indoor environmental quality with radiant based HVAC systems
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water stagnation, bacteria, cross contamination, operating temperatures, need for sterilization vs. scald risks

Bacteria and space heating systems using domestic water heaters.

"Infectious diseases caused by pathogenic bacteria, viruses, protozoa and helminths are the most common and widespread health risk associated with drinking-water."
source: World Health Association

"Like many microorganisms, legionella bacteria have the ability to anchor themselves to the interiors of pipes, faucets and shower heads. Once attached, the bacteria replicate in a sticky substance called a biofilm. As water flows past, it dislodges some of the biofilm, dispersing bacteria throughout the water system."
Source: Mayo Clinic

"I’m not a microbiologist nor a physician and I don’t understand what “slight risk” means except if someone in my family gets sick then the risk for them was I defer to the Precautionary Principle and my personal choice for my family is to operate our indirect water heater at 140°F (60°C) - and no further defence of that choice is necessary."
Robert Bean, R.E.T., P.L.(Eng.)

Government and Association Statements

Open direct combination heating systems

Alberta Government requirements for combination heating systems

Design requirements for hydronic systems

Alberta Government requirements for the design of hydronic heating systems

Alberta Government requirements for single and double wall heat exchangers.

CIPH Information Bulletin on the use of Open Direct Systems using Water Heaters

CIPH Information Bulletin on the use of Water Heaters

Requirements for Water Heaters Used for Combination Heating Systems

MCA Saskatchewan, Requirements for Water Heaters Used for Combination Heating Systems

B149.1 Gas Installation Codes of Practice - SASKPower Gas Inspection Division

Illinois Plumbing Code, Water Heaters Used for Space Heating.

Part 890 Illinois Plumbing Code, Section 890.1220 Hot Water Supply and Distribution

Contact us to add your AHJ statement here...

A caution to do-it-your-selfers


Our Current Position Statements on:

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

Definition of open direct system: one where the water used for space heating is also used for bathing, cooking, cleaning and for consumption.

For additional support visit our visitor services page.

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1. Using Hot Water Heaters as Space Heating Boilers

We are not aware of any jurisdiction in North America which permits domestic water heaters furnished with relief valves rated above 30 psi to operate as stand alone space heating devices. We will be happy to acknowledge any written proof offering a different position.

We are not aware of any water heater manufacturer which furnishes its product with relief valves rated above 30 psi to recommend its product be used as a stand alone space heating appliance. We will be happy to acknowledge any written proof offering a different position.

We are aware of the availability of water heaters manufactured and rated for dual purpose which may or may not be permitted in one's municipality.

It is important to mention:

1. The availability of dual purpose products does not necessarily mean it's acceptable by all jurisdictions across the country.

2.The availability of these products and their acceptance by some jurisdictions does not imply zero risk.

We are aware that any field alteration of a factory certified product to circumnavigate local codes or knowingly using a water heater for something other than what it was otherwise intended exposes one to liability issues which in the event of illness, injury or death could give cause for criminal charges.

Some parts of North America also hold past property owners (seller) liable to new owners (buyers) for components, systems, and assemblies which did not meet local codes and standards at time of the sale.

Sample statements from authorities having jurisdiction:

  1. United States

  2. Canada.

Our position at is to contact a lawyer specializing in negligence and the insurance company that underwrites ones home owner policy to see what they are prepared to cover - particularly if ones decision contravenes local codes, standards and manufacturers recommendations.

Read this article by Dave Yates (F.W. Behler)

2. Water Heaters for Space Heating in an Open Direct System

Areas of concern: water stagnation, bacteria, cross contamination, operating temperatures, need for sterilization vs. scald risks.

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..."

Suggested studies: The Precautionary Principle

If you are browsing the internet asking people for their opinions on open systems and specifically the potential for Legionella, here is our position:

Legionella and countless other bacteria can be a health risk in plumbing and HVAC systems - this, no one of any credibility debates.

With regards to getting advice on Legionella and designing plumbing and HVAC systems - we say it’s probably unwise to ask a lawyer, doctor or insurance agent on how to design, specify and install HVAC and plumbing systems; likewise it’s probably unwise to rely on a do-it-yourselfer, contractor, distributor or manufacturer for advice on waterborne pathogens when it affects ones health, insurance and legal liabilities.

A professional would be obligated by their Code of Ethics to state, " hey Jill and Jack homebuyer, I am not qualified in matters of medicine, microbiology, industrial hygiene, law and insurance as it relates to recommending water heaters for use in open system for space heating, as such I am ethically bound to recommend prudence."  This is an application of the The Precautionary Principle.

Our position at is:

...if a space heating load is low enough to be serviced by a single water heater furnished with an internal or external heat exchanger isolating the potable water from the space heating fluid, then we would find this as an acceptable first solution for combined indirect heating systems provided the equipment and system meet all local code requirements established by the jurisdiction having authority.

If a jurisdiction allows combined open direct systems then know water maintained continually above 60 deg C (140 Deg F) has a bactericidal effect, BUT also be aware that at these temperatures there is the potential for harm via scalding of unprotected skin. Additionally, know that these higher temperatures are often above what may be necessary for radiant floor heating.

Therefore taking the risk of an open systems requires the addition of scald protection valves for thermal safety of occupants and quite possibly the addition of a temperature control valve for the radiant floor systems.

Finally any dual purpose open system should also have a timed flush and purge process with fail safe controls on the entire system to ensure fluid volume in the space heating system is treated to a high temperature at least once in a 24 hour period.

Most radiant heating systems operate within 95 deg F to 125 deg F, which is also hospitable for the colonization of pathogens. This is why the prudent HVAC designer will offer other choices which may not be as low cost as an open system but they will be safer.

Guidelines for Drinking-water Quality - World Health Organization

Guidelines for Drinking-water Quality  World Health Organization

Legionella and open direct water heaters

 Canada Safety Council



Water heaters and radiant floor heating

Spokane Building Services


Open direct water heaters used for space heating

Nova Scotia Labour and Workforce Development


Making decisions on what is right - is safer, healthier and less risky - than doing what is cheap.

Suggested readings and links:

ASHRAE Position Document on Legionellosis, reaffirmed 2012

A Canadian Perspective on the Precautionary Approach/Principle

Precautionary Principle, World Commission on the Ethics of Scientific Knowledge and Technology (COMEST), March 2005, UNESCO

A Canadian Perspective on the Precautionary Approach/Principle Discussion Document

Residential water heater temperature: 49 or 60 degrees Celsius? Benoît Lévesque, MD FRCPC, Michel Lavoie, MD FRCPC, and Jean Joly, MD FRCPC, January/February 2004, CJIDMM, Volume 15 Issue 1: 11-12

Balancing the risks: Legionella pneumophila pneumonia and tap water scalds in the home,  Richard S. Stanwick, MD, FRCPC, CMAJ. 1986 December 1; 135(11): 1251–1252. PMCID: PMC1491392

Reducing the incidence of tap-water scalds: strategies for physicians Dirk W. Huyer, MD; Sonya H. Corkum, BSc (HEd), CPH, Can Med Assoc J, 1997;156:841-4

Risk factors for contamination of domestic hot water systems by legionellae. M Alary and J R Joly, Appl Environ Microbiol. 1991 August; 57(8): 2360–2367. PMCID: PMC183576

Legionella and domestic water heaters in the Quebec City area. J Joly, Can Med Assoc J. 1985 January 15; 132(2): 160. PMCID: PMC1346747

Occurrence of Legionella in hot water systems of single-family residences in suburbs of two German cities with special reference to solar and district heating, Int J Hyg Environ Health. 2008 Mar;211(1-2):179-85. Epub 2007 Apr 3.

Prevalence of Legionella species, serogroups, and monoclonal subgroups in hot water systems in south-eastern Germany, Zentralbl Hyg Umweltmed. 1993 Feb;193(5):450-60.

Biofilm formation and multiplication of Legionella in a model warm water system with pipes of copper, stainless steel and cross-linked polyethylene, Water Res. 2005 Aug;39(13):2789-98

Factors predisposing to Legionella pneumophila colonization in residential water systems, Arch Environ Health, 1988 Jan-Feb;43(1):59-62.

Legionellaceae in the potable water of Nova Scotia hospitals and Halifax residences, Epidemiol Infect. 1994 Feb;112(1):143-50

Risk factors for domestic acquisition of legionnaires disease. Ohio legionnaires Disease Group, Arch Intern Med. 1996 Aug 12-26;156(15):1685-92

Opportunistic pathogens enriched in showerhead biofilms, Feazel, L.M., et al, PNAS 2009 : 0908446106v1-pnas.0908446106.


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