Hot water heater setpoint?

[jvithayapun]

Im a little confused on what the setpoint for say a standard electric hot water heater for an office building should be which serves sinks and such.

I have read in ashrae that 'service water temperature in the 140F range is recommended to limit the potential for Legionella growth'.

Ive also read from an ashrae 90.1 manual that '...can colonize in hot water systems maintained below 115F'.

So my question is...what is everyone using for temps? 140F or 120F?

thanks!

 

[CountOlaf]

I thought Legionella was more of an issue at the "tepid" temperatures normally seen in say cooling tower systems where it was first discovered (e.g., 95-105 deg F.)?

Anyway, regarding your question, anything over about 116 deg F is scalding to the touch, so 120 deg F at the heater is usually fine, unless you have other commercial type requirements/things connected to it such as mop sinks, commercial kitchen sinks, certain dishwashing stuff, etc. that needs a higher temp, that's when 140 deg F would be the way to go. Thermostatic mixing valves can always be employed to ensure you don't get over the 116 deg F mark at showers, lavatories, etc.

 

[jvithayapun]

yeah, i was thinking about just using 140F at the heater based on ashrae, and then running to mixing valve to bring the temps down.

but then it got me thinking why most factory set points of water heaters are 120F if in fact at this temp legionnella is still prone to growing?

I have read from

http://www.legionella.org/engineers.htm#a7

that the water should be set to 140F.

 

[PEDARRIN2]

Setting a water heater at 120 could lead to Legionella concerns.

Water in a tank heater should be at least 130 to minimize these concerns. 140 would likely eliminate them.

But if you store the water at anything greater than 120, you MUST have some type of tempering valve - either at the heater or at the fixture for scalding concerns.

You can either have a master mixing valve per ASSE 1017 or use a point of use per ASSE 1070. I would recommend the 1070 since they can be installed at the fixture with little issue.

 

[jvithayapun]

thanks for the replies. any comments as to why many heaters come preset at 120F though?

 

[CountOlaf]

I agree it's not a bad idea to set at 140F and "master mix" it down--one larger device is probably cheaper than a whole bunch of smaller devices. Plus, you can conceivably get away with a smaller tank size at 140 than you could if you design for 120.

Still, I think it's all overkill/hype (and certainly there's an energy cost associated with maintaining a higher setpoint)--probably 90% of residences have a 120 deg F setpoint and we're all not coming down with Legionaires Desease. If we want to go to that extreme, maybe these domestic water heaters should come with setpoints/capablilties up into the 160's (I forget the precise #, 161 or 167), that way we could be sure to kill 99.97% of bacteria as well!