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Old 09-12-2007, 11:04 AM   #1
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Water Temperature

When I got my tankless water heater installed, the company put in a controller that limited the water temperature to 120 degrees (for liability reasons).

I can pay $119 and get a controller that will let me set a higher temperature.

120 is fine, except that I'm wondering whether 130 degrees would be better for washing dishes in the dishwasher, germ-wise. I can use electric heat in the dishwasher, but that's inefficient, and makes the cycle much longer.

Thoughts?
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Old 09-12-2007, 11:11 AM   #2
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I turned my water heater way down, and my dishes are getting clean just fine. No botulism deaths so far, I wouldn't worry about it if I were you. When you hand wash pots and pans, how hot is the water?
I think the really hot water matters more in public eateries, where you need to sanitize for the germs of the masses, not like at home. We hand washed everything on the boat in cold water when we lived aboard, and never got sick, except from drinking too much beer.
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Old 09-12-2007, 11:15 AM   #3
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At 10 cents/kwh, that $119 will but 1190 KWH. The dishwasher heater is probably less than 1500 watts to run on a standard outlet, I doubt it would take 40 minutes to raise that water 10 F, but even if it did, that would be over 1,000 washing cycles to 'break-even'. Not something I would be too inclined to do. Buy another CFL

Can't you just set the knob to eleven?

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Old 09-12-2007, 11:17 AM   #4
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I have read (can't remember where) that 130 is ideal for killing germs in the dishwasher and hot laundry.

When my tankless was installed he said he wasn't allowed to set it above 120, but I can change it myself at any time.
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Old 09-12-2007, 11:18 AM   #5
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one other thought. When we got our new dishwasher, I was surprised how little water it uses. They recc that if you are a ways from the heater, that you run the hot water at the nearby sink until it flows hot, prior to starting the dishwasher.

That first gallon of cool/warm water running into the machine really lowers the average water temp if you are only using, say 3 gallons total (no, I'm not going to go drag out my manual and look up the water usage .

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Old 09-12-2007, 11:43 AM   #6
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Our dishwasher has a cycle you can choose that heats up the water. Never used it.
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Old 09-12-2007, 12:04 PM   #7
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TbAl/Martha. You guys must have some relatively older dishwashers. Newer ones have no issue at all getting the temperature up to 130-140 degrees (and do so as standard operating procedure under a normal wash), especially if its 120 degrees to begin with. In fact, as a hobby and interest, I often read at energystar.gov's website and they specifically say there that its better to set your water heater on a low setting (like 120) and let your dishwater do the extra heating, than it is to set the water heater higher just for the purposes of your dishwasher.

My understanding is the same as pbat's though; its recommended for 130 or better for dishes to be perfectly safe. My dishwasher's "pots and pans" setting gets my dishwasher up to 140.

> One tip though - run the hot water on the sink closest to the dishwasher till its hot before you actually run the dishwasher. That way the dishwasher doesn't have to heat up lukewarm water which would take even longer!
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Old 09-12-2007, 12:12 PM   #8
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The newer dishwashers with the Energy Star rating use between 7 and 15 or so gallons of water per cycle. Mine has a Sani-rinse option that raises the wash temp and gives the dishes a final 155 degree rinse. Frankly, I see no difference using this option or the regular wash at 120 degrees.

And yes, it is important to run the closest hot water faucet for a bit before turning on the dishwasher to maximize the water temp going into the machine.

(BTW, for those who think it's more efficient to wash dishes by hand, that's generally not the case. According to a National Geographic "Green Study": You'll use up to 35 percent less water by doing a full load of dishes, which haven't been pre-rinsed, in your dishwasher instead of by hand. You can also save about 15 percent on total dishwasher energy use if you select the air-dry setting or open the dishwasher's door instead of using its drying cycle. )
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Old 09-12-2007, 02:49 PM   #9
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Originally Posted by TromboneAl View Post
When I got my tankless water heater installed, the company put in a controller that limited the water temperature to 120 degrees (for liability reasons).
We use a thermal-control mixing valve on our water heater to limit its output to 135 degrees at the kitchen sink (the most distant outlet). Most humans find 125 degrees "hot" and heating above that starts to cause an uncontrollable reflex jerk. Of course older people might not sense the heat in time to avoid second-degree burns.

Having said that, I know your tendency for inquisitive research. You'll have to let us know your findings if you experiment with a thermometer, a sink of hot water, and various family members...

Quote:
Originally Posted by TromboneAl View Post
I can pay $119 and get a controller that will let me set a higher temperature.
We paid $90 + tax for a Watts 3/4" mixing valve in Jan 2006, so I'm afraid that price seems reasonable.

Quote:
Originally Posted by TromboneAl View Post
120 is fine, except that I'm wondering whether 130 degrees would be better for washing dishes in the dishwasher, germ-wise. I can use electric heat in the dishwasher, but that's inefficient, and makes the cycle much longer.
Our dishwasher manual recommends 120-130 degrees water supply. Regardless of what we think, the dishwasher heats the water to 130 if we don't. Unless you can find a thermostat you probably can't mess with that setting.

The "good news" is that most cohabiting family members share the same bacteria and don't sicken each other if the dishes aren't completely sanitary. It's only relatives & friends that risk illness when they're invited to dine at your house, but that risk factor is minor compared to the other pathogens (cold & flu virii) that everyone swaps during the party. This is why I prefer to sanitize my body as much as possible with alcohol, although some of my relatives inspire the same response.

The good news is that our dishwasher displays any "heating delay" and it's generally under a minute. It must only have to heat up a gallon or two. I don't know if we can see the difference in our power consumption or our pocketbook... you're probably deep into diminishing returns.

How close is your dishwasher's tankless heater? If it's just supplying the kitchen then it might be worth the $120 to give yourself 130-degree water. Heck, it might be worth the $120 just to gain an engineer's peace of mind that everything's been optimized. However, as has been pointed out, you'll wait a long time for the payback.
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Old 09-12-2007, 02:59 PM   #10
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In the winter especially, I sometimes have to set my water heater (tank type) above 120 degrees. Not becaues 120 degrees doesnt feel more than hot enough to me, but because I have to use more of the "hot" with less mixing of cold water to get a comfortable bath, shower, or the occasional jacuzzi and, thus, I (we) can sometimes end up using all of the hot water up with the temp set too low.
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Old 09-12-2007, 04:18 PM   #11
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Quote:
would be over 1,000 washing cycles to 'break-even'.
I considered that, and agree with it. But it takes our dishwasher a long time (25 minutes) to get the water from 120 degrees to 130, and it's noisily churning away that entire time.

Yes, we always run the faucet first, since it takes about two minutes for the hot water to reach the kitchen.

Thanks for the input. I guess I'll stay with my 120 degree controller, or perhaps keep an eye on Ebay for the hotter controller.
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Old 09-12-2007, 05:43 PM   #12
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Originally Posted by TromboneAl View Post
I can use electric heat in the dishwasher, but that's inefficient, and makes the cycle much longer.
I am a bit surprised it takes 25 minutes and it is running all that time (circulating the water past the internal heater maybe?) to go from 120 to 130F.

Just for what some of us call 'fun', I did the calc on the cost of electric heating. I wouldn't worry about it:

Assume 3 gallons being heated from 120F to 130 F (not all the water used for all cycles is heated, I think). A BTU = one pound of water changed one degree F. A Gallon = 8.333#. So that is 3G*8.333 = 25#. 130-120 = 10F delta. 25# * 10F = 250 BTU (see, isn't this fun?).

1W provides 3.4BTU/Hr, so 250 BTU = 74 W/Hours. That is .074 KWhrs. At $0.10/KWH, that is just $.0074 per cycle. OK, throw in some heat lost to poor insulation, heat soak from the dishwasher, etc (electric heat itself is close to 100% efficient), plus the motor I guess. Even at 10x, you are talking 7 cents per cycle, minus what gas would cost.

Whenever I do calculations like this, I wonder - how does my electric bill get so high? Refridge, freezer, dryer, well and a thousand small things.

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Old 09-12-2007, 06:54 PM   #13
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Originally Posted by TromboneAl View Post
I considered that, and agree with it. But it takes our dishwasher a long time (25 minutes) to get the water from 120 degrees to 130, and it's noisily churning away that entire time.
Thanks for the input. I guess I'll stay with my 120 degree controller, or perhaps keep an eye on Ebay for the hotter controller.
How old is that dishwasher?!? That's gotta be a problem. We could probably achieve the same results with a pot of water and a cigarette lighter...

I've seen ads for Energy Star dishwashers, and perhaps the money on a thermal mixing valve would be better spent on a new dishwasher.
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Old 09-12-2007, 07:00 PM   #14
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The dishwasher is from 1996. If you guys would stop helping me fix it each time it breaks, we could get a new one.
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Old 09-12-2007, 07:31 PM   #15
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The dishwasher is from 1996. If you guys would stop helping me fix it each time it breaks, we could get a new one.
Next time it breaks, don't ask.
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Old 09-12-2007, 09:00 PM   #16
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Just for what some of us call 'fun', I did the calc on the cost of electric heating. I wouldn't worry about it:

-ERD50
See, it's stuff like this that makes me glad I found this forum. That's my kind of "fun", too.
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Old 09-12-2007, 10:53 PM   #17
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First thing I do when I buy a new water heater is turn the temperature DOWN, you can save a good chunk of change each month by doing this, and then wrapping your pipes that come out of the water heater with something to hold the heat in. I set mine no higher then 115.

I need to figure out what temp we like our baths at and just set it at that, so I do not have to fool with the cold water.

Most dishwashers turn the heat up to 130 when you run them anyway. Or just disable the temp control on the washer. Spraying something hot on something with germs doesn't clean it anyway.
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Old 09-13-2007, 08:35 AM   #18
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If i had a big enough water heater to provide enough hot water to me and my family, I'd just set it to a comfortable temperature too. But as i mentioned earlier, apparently my tank is too small because if "we" run pure hot for all of us back to back, it occasionally runs out. Setting it hotter and mixing in cold seems to alleviate the problem.

.... which makes me wonder, what's most cost effective; Buying a huge tank and setting it to 115 or a smaller one and jacking it up to 130+?
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Old 09-13-2007, 01:14 PM   #19
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The dishwasher is from 1996. If you guys would stop helping me fix it each time it breaks, we could get a new one.
[DrewCarey] "Oh, why didn't you say so?" [/DrewCarey] Here, cut this out and tape to its door:
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"Al, it looks like REWahoo!'s the timer switch is bad. The only way you're going to fix this problem is to buy a new EnergyStar longboard dishwasher. Maybe the local electrical utility has a rebate program, too!"
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If i had a big enough water heater to provide enough hot water to me and my family, I'd just set it to a comfortable temperature too. But as i mentioned earlier, apparently my tank is too small because if "we" run pure hot for all of us back to back, it occasionally runs out. Setting it hotter and mixing in cold seems to alleviate the problem.
A couple comments here:
1. Have you looked into a water-heater blanket and an outlet heat trap?
2. If the water heater is over 10 years old then you can buy a high-efficiency model in a bigger capacity with a blanket and an outlet heat trap... you'll probably pay lower utility bills, you won't run out of hot water, and your water heater might qualify for a rebate from your local utility.
3. Two people showering together use less water than each showering individually. But if you're married then it's probably best to confine this practice to your spouse.
4. Different shower times for everyone?

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.... which makes me wonder, what's most cost effective; Buying a huge tank and setting it to 115 or a smaller one and jacking it up to 130+?
In this case it's approximated as the number of joules of energy that you can cram into the tank. The significant factor here, the difference in absolute Fahrenheit temperature, is only 15 degrees out of (459+115=) 574 or about 2.6%.

The "effective curve" probably looks like a "U": high at one end for a modern tankless water heater, and high at the other end for a larger heavily-insulated high-efficiency conventional water heater. You lose some garage space with the big tank but it probably has at least 20% more volume and will outlast even your teenage water buffalo.
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Old 09-13-2007, 03:50 PM   #20
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In this case it's approximated as the number of joules of energy that you can cram into the tank. The significant factor here, the difference in absolute Fahrenheit temperature, is only 15 degrees out of (459+115=) 574 or about 2.6%.
I'm not sure that's the applicable math (emphasis on 'not sure').

Rather than measuring from absolute temps, I think there are 4 factors:

A) Temp of incoming water.
B) Temp of ambient air.
C) Desired Temp.
D) Insulation.

If incoming is 60F you need to raise the water from 60F to 115F or 130F. The energy required is directly proportional to delta T. BTU = 1# water raised 1 Degree F. It will take 1.27x the energy to raise to 130 than to 115. This is completely offset by the fact that you then use less volume of hot water (if adjusting for a shower for example) at the higher temp.

B&C&D all impact how much energy it takes to hold the temp at that value when no water is drawn. It takes no energy to maintain 70F if ambient is 70F. However 130F is 60F over ambient, 115F is 45F over ambient. 60/45 = 1.5x. Heat transfer will vary with the square of the difference ( I gotta think this through with electrical equivalents), so 130 takes 2.25x the energy to maintain compared to 115F.

However, if you have really good insulation that may be 2.25 times a small number.

IOW, I don't know :confused:.

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