Domestic Hot Water and Legionnaires

[DBNH22]

I had a Nyeltherm Heat Pump water heater installed last week and I'm in the process of adjusting the aquastat that controls it to the ideal temperature. My plumber recommended that I set the aquastat so that water at the tap is 125F at it's hottest. He mentioned that there are two concerns when fine tuning in the temp.

1. It shouldn't be set too high or scalding can occur.

2. It shouldn't be set too low or the legionella bacteria may develop.

Whenever I've heard Legionnaires mentioned in the past it was always with regards to AC units.

I did a little reading on the subject today and found some interesting stuff. Many sources refer to the temperature of your hot water but do not say whether it's in the tank or at the tap. I'm assuming when they do not do this they actually mean at the tap. I'm also assuming that the water is bound to lose a few degrees as it traverses your plumbing between the tank and the fixture. I remember seeing tables for heat loss values per foot of copper pipe, uninsulated vs insulated and all that.

Most of the sources that focus on preventing Legionnaires state that the hot water should be set to 140F while most of the sources that focus on or discuss hot water safety recommend keeping the water at no higher than 120F I thought it was interesting how the two were at odds. I also saw one source that recommended keeping the water temp at roughly 120F for normal every day use but also raising the temp up to 140F and running the water at the fixtures for 1/2 hour. To me that seems like a an awfully energy wasteful way of preventing Legionnaires. It would almost make more sense to raise the temp in the tank and exercise caution at the fixture.

I know this isn't a boiler topic but there didn't seem to be a fittign category to post this under.

 

[Clarkbug]

You are right that its a conflict of guidance. In commercial applications, the tank is typically kept at 140, and then there is a recirc line for any runs that are over 100 feet. This keeps hot water at the tap, and also reheats any water that cooled down to help with legionella. Depending on the end use, there is typically a mixing valve right at the discharge of the heater to temper things down to 120 or lower, which is silly to me, but thats the way it goes usually.

If you are very concerned about legionella, you can set your heater to 140, and then install point of use tempering valves at each fixture, or adjust the limit stops for your shower (note, you can still get scalded if someone flushes a toilet if you arent using a tempering valve)

Unless you have a system that you know is filled with the bacteria, or someone who has a compromised immune system, you are probably OK with the lower temps. The 140 degree water run at the tap for half an hour would be to sanitize your piping system, and you are right, is incredibly wasteful. But easier than trying to chemically disinfect things.

 

[skfire]

my tank is set to 140, then I have a mixing vlv set to 120 and cycling back into it. I am off well water and take no chances. To boot when I built my house all my fixtures were set up with tempering vlvs, which my wife wanted pretty much maxed, cause she has no concept of hot....
Scott

 

[maple1]

I considered this too. Lots of conflicting info out there.

My routine has me normally keeping my DHW tank at around 120-125. Bit less at the bottom, bit less as storage gets almost depleted. I burn every 3-4 days to heat my storage up. When I'm doing that, I run the DHW tank up to 145-150. I figure the dose of high heat every 3-4 days should help to kill any bugs if they might be there - but who really knows. I also have one mixer, just outside my DHW tank - but it's a cheaper one that doesn't work the best & I likely shouldn't have used it for that. But I had it here so I put it in. It doesn't mix much at all - so the hot is getting into my pipes too. Although not quite as hot as what's in the tank - there is still a bit of mixing going on.

Everyone at our place learned a long time ago that wood makes water hotter - nobody complains about the water being too hot, and nobody has burned themselves.

 

[Fred61]

Much has to do with useage. A former co-worker of mine developed Legionaires several months after his wife passed away. They had kept their water at 120 over the years but due to the volume decrease with only one person in the house, the bacteria was given more time to develop in the system. I believe the fix was to raise the temperature to 140 or more and install a mixing valve.

 

[tom in maine]

This congers up the question about electric water heater controls topping off at 130F.

 

[skfire]

https://www.osha.gov/dts/osta/otm/legionnaires/hotwater.html

 

[tom in maine]

If you research plumbing with plastic, you find a lot on the bioslime that grows on the inside of plastic domestic water tubing.
This does not occur in copper. My suspicion is that copper and materials containing copper deter this growth.

 

[skfire]

Various plumbing materials have been evaluated for their ability to support the growth of Legionella (and other bacteria). Copper pipe is inhibitory, whereas plastic (PVC) pipe will support the attachment and subsequent growth of bacteria- including Legionella. If the hot water system temperature is maintained at or above 130oF, growth will be inhibited regardless of pipe materials.

from:
http://legionella.org/faqs/engineers/

excellent..thank you Tom

Scott

 

[velvetfoot]

Hah, I knew PEX was too good to be true.

 

[WiscWoody]

In my chemical treatment classes I've taken for cooling towers they said that after years of working on towers those who were in the class probably have some form of legionella but it will never rear it's ugly head. I can't remember how many strains of legionella there were but it was many, somewhere in the tune of 30+ IIRC.

 

[jebatty]

You know, I am sure, that this topic has been hashed and rehashed to death (no pun intended), including on hearth.com. The web has whatever info you want to follow or believe.

 

[heaterman]

I have a good friend in the industry who experienced Legionella firsthand. Let's just say his water heater runs 140+ and to heck with the increased energy costs.
It darn near took him and it is nothing you would ever want to experience according to him.
My own tank sits at 145* and I mix it at point of use where needed.

PS. potable water piping in my humble adobe is all copper.

 

[Ashful]

https://www.osha.gov/dts/osta/otm/legionnaires/hotwater.html

Some interesting info in there, particularly for those of us with some very long runs of plumbing to appliances that see very infrequent use. Some of our sinks on the longest runs might only get used once / twice per year, which seems scary, after reading that site.

 

[WiscWoody]

I have my hot water maxed at 150 at the tap which is right above my water heater. I like my hot water hot and there's no one to scald here except myself. The dishwasher likes really hot water too.

 

[bro-tek]

all the info on the post are good, I have some experience on this field. The tank, low flow , low temp is the main problem. a good system would be to bump the temp up to 140 twice a day. it would be agood seller item. everything point to a shower mist inhale and not drinking the water. leave your shower on a few minutes will also help, but not 100% safe if tank is low temp

 

[Ashful]

We have a separate hot water heater for our third floor bathroom, in which some takes showers maybe only a few times each year. I'm now wondering what I should do about this. Just keep it set over 140F? It's in an I heated attic space, so it must be kept on all winter, lest I chance freezing pipes.

 

[BoilerMan]

If you research plumbing with plastic, you find a lot on the bioslime that grows on the inside of plastic domestic water tubing.
This does not occur in copper. My suspicion is that copper and materials containing copper deter this growth.

This is why I do NOT like PEX for and potable water. Also the MTBE can leach into the water with little useage. Copper is king!


No numbers or good ol' scientific experimentation, but my Nyletherm would have a really hard time getting a tank up to 140F, as it is it runs for around 8 hours a day@ 125F for 3 showers a load of dishes and some light cleaning. Still costs less than $15/month to operate.

TS

 

[tom in maine]

I think the advice of heating water to 140F is extreme. Legionnaires is nothing to trifle with, but I suspect leaving systems stagnant at lower temps is likely a much
more significant issue than most others. If a storage tank is to be left unused for a while, it makes sense to flush thoroughly into a sink. Using copper in the system just helps keep things clear.

 

[hobbyheater]

Interesting thread!
In our system DHW is preheated while it passes through a heat exchanger 150 to 195 F then is stepped down to 160 F before it enters an electric hot water tank. Through periods where not a lot of hot water is used, this tank can drop to about 140 F. Is this system still at risk for Legionnaires?

 

[tom in maine]

Interesting thread!
In our system DHW is preheated while it passes through a heat exchanger 150 to 195 F then is stepped down to 160 F before it enters an electric hot water tank. Through periods where not a lot of hot water is used, this tank can drop to about 140 F. Is this system still at risk for Legionnaires?

I doubt it.

 

[goosegunner]

Interesting about the copper. I have considered switching to pex because our water is slighly aggressive. I have been under the impression that copper is not good for you in drinking water.

So what is better to deal with?

How do you deal with aggressive water and copper?

gg

 

[Ashful]

I have been under the impression that copper is not good for you in drinking water.

As far as I know, copper is the ONLY material to have ever been used for drinking pipes for more than 50 years, without someone having found some major mechanical or health concern with the material.

 

[velvetfoot]

There was that lead solder thing.

 

[tom in maine]

I have no major concerns using PEX especially given aggressive water attacking copper.
If the system was left off for a while, I would just thoroughly flush the hot side.