Domestic Hot water Tank - Bacteria issue

[BPLBPL]

Hi all,
I'm designing a solar hot water system similar to the below layout.

http://www.solarforce.com/files/45-0147_MSTR_large.jpg

The storage tank is large - 1000gal and is supposed to store domestic hot water at 120-140F. Should I be concerned about any bacterial growth in the tank, though the stored hot water is constantly being drawn for domestic needs and new water being introduced daily? Is there any CODE guideline on this? Any information on bacterial growth in storage tanks would be very helpful. Thank you

bplwithu
NY, USA

 

[unclesyd]

Here is sort paper on the problem of MIC in water systems. It will be extremely hard to prevent their proliferation if the system is ever inoculated. As it states in this document have a simple a system as possible and start clean. Make sure your storage tank can be completely drained.

http://midlandcorrosion.co.uk/case-studies/108/Microbially-Influenced-Corrosion.pdf

Are you using chlorinated water?

 

[PEDARRIN2]

There is no code requirement for hot water storage. There are some code requirements for the temperature at the faucet.

But my advice would be to store your water at 140. Anything much less than that and you have the potential to have Legionella in your system.

Use master mixing valve to reduce temperature to 120 in the hot water loop or circulate 140 water and use point of use thermostatic mixing valves at faucets and showers.

 

[davefitz]

as pedarrin2 indicated, the main problem is Legionella. Same issue applies to domestic hot water tanks used for radiant floor heating of buildings- some plumbing codes will not permit such mixed used due to the legionella risk.

Try a google search using search terms {legionella, plumbing, "hot water"}.

 

[BigInch]

Something about storing at 140 and using at 120 that I don't fancy too much. I've been storing at 120 and using at 120 for many years with no ill effects. If you're not using for other purposes, radient heating etc., and if its more convenient to store at 120, you would not require those few extra square feet of collector and tank insulation to store at the lower temperature range. But in a lot of places with good solar levels, 140 is not going to be the problem. Keeping things cooler than 180 may be more trouble. What storage temperature do you have in mind right now?

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[BPLBPL]

NY city water is chlorinated and the temperature in the tank will reach above 150F atleast once a day. The annual storage tank operating range is 120F to 175F. I guess this should minimize the chances of Legionella growth.

Thank you PEDARRIN2, UNCLESYD and DAVEFITZ for the replies. Very helpful.

Wikipedia has more info. on different temperature ranges that affect the survival of the bacteria.

http://en.wikipedia.org/wiki/Legionella

 

[BPLBPL]

Thank you BigInch.
Since this is a solar system, the temperature in the tank varies depending on the weather - lowest would be around 120 during winter months. Glad to hear that 120F is a safe number. Most part of the year, my tank temp. would be above 140F.

 

[BigInch]

Not totally "safe"; maybe I'm just lucky. The ideal temperature range for Legionella growth is 68°F to 122°F and it does take a hot water temperature of 158°F to kill it, but as I said, you're probably going to be getting up well over that anyway. At least enough times to keep the possibilities down. Therefore I'd let it get up to 160 or a bit higher even, but not try to maintain it that high as a storage temperature at all times. Just put enough insulation on the tank to hold a minimum of 120-130 until it starts heating up again. That would let you unload that excess heat overnight.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[CMA010]

Different industry, but related: In the pharmaceutical industry they use a minimum value of 60 oC / 140 oF for their hot water storage and distribution systems (operated at 176 oF). For cold water storage they use periodical (depending on type of water) sanitization at 176 oF.

 

[PEDARRIN2]

It sounds like you will not have a bug problem, but you could have a scald problem if your hot water is for domestic use.

Therefore, be VERY careful about since you are in the 140+ range. You will be required to have thermostatic mixing valves (whether a central or local will depend on your use and AHJ requirements).

 

[BigInch]

Second that one. Over 125 is downright dangerous in the shower. 117 is pain threshold (on non-critical parts)

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[BPLBPL]

Thanks, I have got MIX valves that should take care of any scalding issues....that is not a concern.
I just don't want any bacterial infection on the potable HW tank and cause diarrhea to all the people in the 5-storey building

 

[hacksaw]

for interested parties

Temperature affects the survival of Legionellae as follows:
70 to 80°C (158 to 176°F)-Disinfection range
At 66°C (151°F)-Legionellae die within 2 min
At 60°C (140°F)-Legionellae die within 32 min
At 55°C (131°F)-Legionellae die within 5-6 hr

50 to 55°C (122 to 131°F)-They can survive but do not multiply

20 to 50°C (68 to 122°F)-Legionellae growth range

35 to 46°C (95 to 115°F)-Ideal growth range
Below 20°C (68°F)- Legionellae can survive but are dormant

 

[PEDARRIN2]

Just for curiosity, what type of mixing valves are you using.

Hopefully you are using a thermostatic valve and not a pressure balancing valve. With temperatures as high as you will have, the thermostatic valves are a must especially for domestic use.

 

[BPLBPL]

Good point, they are Thermostatic mixing valves.

http://www.plumbingsupply.com/thermostatic-mixing-valves.html

Just curious, anybody worked with large size HW storage tanks for domestic or radiant-heat usages? I assume radiant systems run often into bacterial problems because of their operating range ~ 110F, especially when the system is stagnant during summer.

Thank you everyone for your inputs.

 

[moltenmetal]

Radiant heating systems, at least the properly designed ones, are closed-loop on the water. They can go anaerobic and grow bacteria all they like, but they'll soon be starved of both food and oxygen- and nobody will be exposed.

"Open" systems such as yours, heating water that will be used for bathing and washing cooking utensils etc.- such systems have much more risk of causing harm to people.

I don't know if there are any NSF or other standards on this or not. Typical domestic hot water systems don't have the residence time that yours will due to the volume you need to store. You are right to be concerned, and others here are right to point out that higher temperatures minimize the bacteriological concern while raising others (ie. scalding etc.)