By Chad Edmondson
When it comes to domestic hot water recirculation design, OSHA safety provisions and ASHRAE 90.1 energy efficiency requirements have put plumbing engineers between the proverbial rock and hard place.
In an effort to minimize the energy penalty associated with mandatory domestic hot water recirculation, ASHRAE 90.1 has created operational parameters for these systems. The problem is that these parameters are in direct conflict with OSHA’s requirements for Legionella prevention.
What ASHRAE Says
As we noted in our last blog, domestic hot water recirculation is a requirement of ASHRAE 90.1 – 2010 and is on its way to becoming part of commercial building codes throughout the U.S. While recirculation is great for convenience and water conservation, it does involve additional energy usage, which is why the ASHRAE 90.1 also includes the following mandatory provisions:
Recirculating hot water systems must be equipped with automatic time switches orother controls that can be set to switch off the usage of the temperature maintenance system during extended periods when hot water is not required.
Temperature controlling means shall be provided to limit the maximum temperature of water delivered from lavatory faucets in public facility restrooms to 110°.
When used to maintain storage tank water temperature, recirculating pumps shall be equipped with controls limiting operation to a period fromt he start of the heating cycle to a maximum of five minutes after the end of the heating cycle.
What OSHA Says
While these are all prudent measures when it comes to energy conservation, there is a problem. In the interest of Legionella prevention, OSHA has different recommendations:
To minimize the growth of Legionella in a system, domestic hot water should be stored at a minimum of 140°
Domestic hot-water recirculation pumps should run continuously. They should be excluded from energy conservation measures.
OSHA’s position is not without basis. Legionella can survive in 122°F water and even higher in stagnant conditions where scale and sediment provide a safe haven for the bacteria. The most clear cut way to avoid this situation is to maintain the system at a higher temperature and recirculate the water continuously. A high/low flow mixing valve should be used in storage systems so the tank can be keep at 140°F to prohibit Legionella growth while providing 110°F water to the fixtures.
What’s an Engineer To Do?
All this puts plumbing engineers in the unenviable position of choosing between OSHA and ASHRAE on this particular point. Since Legionnaires' disease continues to be among the most deadly of preventable diseases, most engineers are likely to err on the side of OSHA. Disease prevention trumps energy conservation. However, there is reason to hope that a less prohibitive solution for engineers is on the horizon.
We touched on the development of ASHRAE Standard 188, a standard of practice for facility managers/owners to prevent Legionellosis, in a previous blog. If adopted, Standard 188 will shift some of the burden of prevention from engineers to owners and facility managers by requiring them to systematically identify and address their own risks. The standard has been in the works for many years, but there is reason to believe that it may go into effect as early as 2015. Presumably, system designers will be able to refer to this standard of practice to determine if their building water system design and engineering practices should be reviewed or revised.
More on ASHRAE Standard 188
ASHRAE Standard 188 specifies certain “practices” that are the responsibility of the owner to initiate and follow through with to reduce their own risks for Legionella. It is an elaborate protocol that requires building owners to establish a team with assigned responsibilities and accountabilities with regard to Legionella control. Once identified, the team must:
Describe how water is processed and used within the building, using process flow diagrams.
Perform systematic analysis to identify potential hazards throughout the entire water process.
Decide which if any hazards are significant, and if they are, determine what hazard control should be implemented.
Establish critical control limits.
Establish a set method for monitoring any potential hazards, along with a frequency protocol, and even corrective actions if critical control limits are violated.
Decide how it will confirm that the plan is being properly implemented.
This is indeed an extraordinary new requirement for building owners and facility managers – one that will no doubt involve a challenging learning curve. Nevertheless, it gives engineers the opportunity to decide on their own baseline preventive measures for Legionella (i.e. recirculation, temperature maintenance, etc.), while leaving it up to building owners to identify and address any further action that should be taken.