What is a safe / acceptable temperature for having hot water stored in a tank? I read that it should be at least 120 F to inhibit bacterial growth. How hot should it be?

Doesn't matter. After all what temp does the city store city water at in their tanks? The better question is how long is the water going to be stored - how frequently is it used?

Really temperature doesn't matter. Something’s grow better at some temps and others at others. Heck there is even bacteria growing in 300F+ sulfur vents at the bottom of the ocean.

If you want to store it long term I would suggest adding a bit of chlorine and get the water up to 5 ppm of free CL or larger amount of hard liquor (no kidding). On yachts we used to winterize some owners would leave a case (24 1 liter bottles) of gin to dump in the tanks after we rinsed and dumped three times. They swore they never tasted the gin in the spring and we never had issues with their tanks :)

I agree - I maintain domestic HW at about 110 for economy and safety. I've never had a problem with smell or bacteria.

That said, there is a strong recommendation and / or code requirement to avoid mixing hydronic water with domestic HW - risk of legionalla bacteria if system stagnates for months during non-heating season.

Legionella is a hot button issue for a lot of heating pros. I believe 140F or so is the minimum "safe" temperature for storing hot water. It's one of those things where 99.9% of people who keep their water heater at 115F (like engineer) will probably never have an issue, but it can be pretty damn nasty for that 0.1% that do have it. I'm just pulling numbers out of thin air, but I think it comes down to how much risk you're willing to accept. There was an outbreak at a nursing home near me earlier this fall that killed 2 or 3 people and made a lot more sick. I probably have a 100X greater chance of getting killed in a car crash than from Legionnaire's disease but I decided that for $10 per year in extra standby losses I'll keep my tank at 140 (with an anti-scald mixing valve to deliver the water at 115-120). This has the added benefit of allowing me to use a smaller tank.

heatinghelp.com has some good spirited debates on the subject if you look in the archives.

 

What is an anti-scald mixing valve?

Is it something that's added one-per HWH?  If so, for a 50 gal HWH, is it more economical to set the HWH's tstat to 140°+ (and rely on the mixing valve for safety)?

Or is it something required at every hot water faucet?

Many thanks for the help.

Best regards,

Bill

I think it depends on how critical water temp control is. I just have the one on the water heater, I think it was a Watts brand, $80 or so. I'll check. I think for hospitals and the like it is sometimes preferred to have them at point-of-use or maybe both for redundancy. Watts has some good literature on them on their website.

In terms of economics, are you talking about running a smaller tank at a higher temp vs a larger tank at a lower temp? The reduced surface area does reduce the penalty of the higher temp somewhat but not completely. It depends on the shape of the tank and insulation level. Of course for a GSHP there is a big efficiency penalty for the higher temps.

I'm simply thinking of installing an anti-scald valve at my existing 50 gal HWH, and then turning up the HWH's tstat to 140°, simply to save money (NG consumption).  Hence the questions.

Yes, I agree that running my existing 50 gal gas-fired HWH that's direct connected to my GSHP's DSH at 140° eliminates benefit of the DSH.  But in my case this is not a significant loss because my DSH doesn't run enough to provide more than about 5% of my total HW needs.

Thanks!

Best regards,

Bill

Bill,

I'm confused as to how turning up HWH tstat to 140 will "save money (NG consumption)" I must be forgetting an aspect of your system.

Running HWHs at higher temps reduces their lives - more scaling and corrosion.

Thanks for all the replies. I just had a dx-geo system installed, with a domestic hot water option, and the water is staged in an 80 gal tank. It gets used on a daily basis, but its coming out of the tap at around 107 at the moment. I'd like to get it a little hotter, for comfort. (For me, its just hot enough to take a shower with only the hot water on, full blast (no cold water on at all), and by the time I'm done, the temp usually drops in the tank, so its not comfortable for the next person's (wife's) shower.)

But I was worried, also about legionnaires. (I have my first 14 mo old too, so I'm a little paranoid.)

There is no mixing valve installed, so, I wanted to get a feel for how hot should I try and get it...economically, safely, etc.

Thanks!!!

Ok I was planning to leave this out because I personally believe 0 exposure to bacteria and and other microbials is bad , but if the concern is real , or the exposure high ( and the money is there ) the conversation starts and ends with a company RGF , there bread and butter is water treatment , and air treatment . There product line covers a wide range . I personally can testify for the ice maker system , the RGF phi air treatment ( if sized right ) , and there reverse osmosis ( about 4 gal a day ) they do make in larger whole house treatment systems which is supposed to handle as much water as you can throw at it .

Again everything they make is expensive , but it works

Is that device proven safe (RGF phi air treatment), with long term studies, over long term exposure? With the whole o-zone thing...I'm not convinced.

In my opinion they are more then safe , Basically whenever you expose hydrogen and oxygen to UV light or electricity you produce hydroxy radicals , 03 , hydro peroxides  all in the low ppm range. that being said a house should be pushing over 8 tons before I will put 2 in , ( my personal limit , manufacturer will say 2.5 per unit ) the O3 produced by them is very close to the O3 produced by an electrostatic filter , or standard UV . As far as do they work yes . Ive installed them on systems with black mold in the air handler , with in 6 months completely eliminated , customers with respiratory issues that claim real relief , One of our cust who had one installed on every unit ( 13 total ) had a independent air quality test done trying to show value for the 7 large ticket and had no complaints . again I do NOT recommend everybody in the world has them installed , but for germaphobes , people with mold problems , people prone to infections , hospitals , airplanes , subways , airports ( because they kill infectious diseases ) they have there place

Posted By engineer on 12/06/2008 7:46 AM
... how turning up HWH tstat to 140 will "save money (NG consumption)"

... Running HWHs at higher temps reduces their lives - more scaling and corrosion.

My thinking was that if a gas-fired HWH is run at a higher temp, less HW is needed to mix with cold water for the same durations and temps of showers / baths.  Which would result in the HWH's gas burner running less to recover after water usage.

Certainly the HWH's burner has to run more frequently to offset standby heat loss.  But my further thinking is that the savings from reduced burner running for water usage recovery more than offsets the increased standby usage.

Essentially, for a gas-fired HWH that's considerably used, I wonder where the best economical tstat setting is.  I'm thinking it's not a lower tstat setting.  And I'll guess that it depends on usage pattern.

(While I don't plan to spend time doing this, I think it would be possible for me to figure out the HWH's tstat setting for optimum savings.  I do have a measurement of the amount of time gas is flowing to the HWH - i.e., for this month so far, the burner has been on an average of 1.9 hrs/day.)

Best regards,

Bill

"My thinking was that if a gas-fired HWH is run at a higher temp, less HW is needed to mix with cold water for the same durations and temps of showers / baths. Which would result in the HWH's gas burner running less to recover after water usage."

Short answer - no. I'm struggling a bit to think of ways to explain this...

It is a matter of Btus, not simply flow. Suppose you need a 3 GPM shower at 110 deg. You could flow 3 GPM directly from the gas heater set at 110. Suppose cold water comes in at 60 F. 3 GPM * 50 deg rise * 8 lb / gal * 60 min / hr = 72000 Btuh from heater.

OK lets set the heater at 135 (that temp gives me a round number later in this calc - stay with me)

Now we can get a 110 shower with 1 GPM CW at 60 and just 2 GPM HW at 135. Whether you mix it at the water heater outlet or at the shower mixing valve makes relatively little difference in efficiency (slight effect from cooler hot water line from heater to shower)

Btuh math: 2 GPM * 75 deg rise * 8 lb / gal * 60 min / hr = 72000 Btuh from heater

Its the same heat from gas water heater. for each gallon withdrawn it'll run 50% longer to make the required 75 vs 50 degree rise.

Something I neglected to mention earlier - at a higher final water temperature gas heater's transfer efficiency drops - as the water temperature in the heater increases, its flue gas temperature will rise as well. Higher flue gas temperature means greater btu loss up the stack.

I would expect that raising a conventionally vented gas fired water heater's setpoint by 30 degrees might reduce its overall efficiency by 5% or more - a combination of increased standby losses and reduced heat transfer during the firing cycle.

Hope this helps.

Wow, thank you, engineer!  Very helpful!

So, in summary, for a conventionally vented gas-fired HWH, your math shows that there's no theoretical performance change running the HWH at a high versus low tstat temp.  Good - this is very helpful to know.

And when taking into account best efficiency, it's achieved at the lower tstat settings due to actual inefficiencies of a HWH.

Looks like it's a length of HW availability versus cost subject.  The higher the tstat, the longer HW is available, but at a higher operating cost.

(I understand that tank durability and safety (scalding) are influencing aspects to the subject.)

Many thanks!

Best regards,

Bill