No Hot Water

[randynavarro]

I don't think the thing about hot water pipes freezing faster is B.S., though.

I can't quite cite something, but I remember it from one of my building science classes in the past.

If I can find the gumption, I'll see if I can cite a source.

[Marc]

I don't think the thing about hot water pipes freezing faster is B.S., though.

I can't quite cite something, but I remember it from one of my building science classes in the past.

If I can find the gumption, I'll see if I can cite a source.

We're all familiar with convection heat losses in air. Can't that happen to liquids too?

Not a rhetorical question.

Marc

[kurt]

Is it ok to copy an paste stuff like this here?

http://www.scientificamerican.com/artic ... -hot-water

"To the first part of the question--'Does hot water freeze faster than cold water?'--the answer is 'Not usually, but possibly under certain conditions.' It takes 540 calories to vaporize one gram of water, whereas it takes 100 calories to bring one gram of liquid water from 0 degrees Celsius to 100 degrees C. When water is hotter than 80 degrees C, the rate of cooling by rapid vaporization is very high because each evaporating gram draws at least 540 calories from the water left behind. This is a very large amount of heat compared with the one calorie per Celsius degree that is drawn from each gram of water that cools by regular thermal conduction.

"It all depends on how fast the cooling occurs, and it turns out that hot water will not freeze before cold water but will freeze before lukewarm water. Water at 100 degrees C, for example, will freeze before water warmer than 60 degrees C but not before water cooler than 60 degrees C. This phenomenon is particularly evident when the surface area that cools by rapid evaporation is large compared with the amount of water involved, such as when you wash a car with hot water on a cold winter day. [For reference, look at Conceptual Physics, by Paul G. Hewitt (HarperCollins, 1993).]

I think it's OK. You gave it an attribution/reference, and you aren't using it to make money.

Maybe I got that wrong, but I don't think so.

[Erby]

Randy, which professor did you believe?

[SNations]

I'm new to this forum, and I've been perusing old posts. So nobody may see this, but here goes anyway.

In some cases, adding insulation to a cylindrical or spherical object can increase, rather than decrease, heat flow. (I don't believe this would ever happen to a water heater, but I suppose it's possible.)

There are two basic types of heat flow that we need to consider. Conductive and convective. (Radiation plays no part in this discussion.) Conductive heat transfer will always be reduced by adding insulation. But convective heat transfer can actually increase, because you're increasing the area available for heat transfer.

Convective heat transfer from a pipe can be greatly affected by the orientation of the pipe (vertical, horizontal). But as a simple approximation the critical radius of insulation (beyond which you're actually hurting rather than helping) is = k/h, where k is the thermal conductivity of the insulation and h is the convective heat transfer coefficient. Notice that as h goes to zero, the critical radius goes to infinity. Meaning that if conductive heat transfer dominates then you can just keep piling on insulation. This is the condition we encounter in a home inspection -- particularly if the outer shell of the water heater is at the same temperature as the home's environment (always the case, right?).

So the myth is true, although not something we'd ever encounter in a home inspection.

Here's a good link:

http://www.cdeep.iitb.ac.in/nptel/Mecha ... 2.6.4.html

Steve

[Erby]

Steve. Yes, people will see it. Most of us who've been around for awhile check out the "Whats New" link at the top instead of going through each forum.

A new post to even a years old thread shows up on the Whats New" list.

Point to the Whats New, right next to the Home on the black bar and at the left side of the blue bar right below that it shows Active Topics. Click that link and you'll see a link to every post that's had a new message on it since the last time you logged in. You can change the time period to just the last few minutes or even the last few days..

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