The humid weather is resulting in "sweaty" cold water pipes in our
I have read the suggestion to insulate the pipes just like with hot
water pipes. However, I am concerned that while this may reduce the
sweating, there still will be some condensation, but now the
condensation will be trapped against the pipe.
Should I be worrying about this? If so, what should I do to minimize
the potential for mildew and mold in this trapped moisture?
Second, I find that the condensation only occurs on the first few feet
of pipe in from the grounde. Is it possible that conduction causes the
water in the first few feet of pipe to stay cold all the time leading
to heavy condensation while beyond that the water for the most part
has warmed up (except when the water is actively running) so that
there is much less active condensation?
Not in any measurable way. I assume you are meaning the heat the cold
water picks up in the line while waiting for some draw. It is minimum.
Even a few degrees rise in a pipe run of 40 ft doesn't add up to a
Care to expound on why? The amount of water contained in a 3/4" pipe
say 20 ft long is not very much. About 5 cu in/ft (after heavy
rounding off) unless my long ago math is out of whack. Water in the
cold intake pipe also doesn't usually sit there long enough to pick up
much ambient heat.
That depends on how fast the heat flows... 20' of 3/4" pipe has about
4 ft^2 of surface. With still air and no condensation, it might have
a thermal conductance G = 1.5x4 = 6 Btu/h-F, so the time constant RC
= C/G = 1 hour, ie a 3.8 lb slug of water would warm from 50 to about
57 F (1/e th of the way to 70) in 1 hour in a 70 F room.
Recall Charlie Wing's TV show in which he built a tempering tank in
a basement, a 20' length of 6" PVC pipe tucked up under the rafters?
Well pressure tanks can warm water too.
And condensation might raise G to 1000, so flowing water can warm to
the dew point in real time as it moves along the pipe. As I recall,
the OP described "a few feet" of condensation near the pipe entrance.
I don't recall that the entire pipe had condensation when water was
flowing. If not, the pipe warmed flowing cold water to the dew point.
With no cold water pipe insulation, we have 4 potential savings in
water heating: the small warm slug of still water before flow begins,
the large flowing gain to the dew point, the small flowing gain from
the dew point to room air, and the effect of having warmer cold water
at a sink or a shower, which can allow using less hot water in a mix
to achieve a certain temperature. The latter may come from a lot more
cold water pipe in the house, unrelated to the pipe that goes into
the water heater. Harvard physicist William Shurcliff has written
about these savings.
Good discussion of the -theorectical- savings. Just how much savings
dollar wise do you think it comes down to in practical life? You won't
see it on your power bill. Most of your warming is coming from having
water stand still in the pipe for long periods. That doesn't happen in
an active house. The tempering tank could have some small advantage. I
wonder about the well tank (basically same as tempering tank). Think I
will test my incoming water temp as opposed to the tank out temp after
sitting all night.
The biggest might be from condensation on active pipes. If we spend
(say) 50K Btu/day warming 1000 pounds of 60 F well water to 110 in
a water heater, and insulating the cold water pipes lowers the 60
to 50, we have to spend 1000(110-50) = 60K Btu/day, ie 20% more, eg
10K/3412 = 2.93 kWh/day or 1070 kWh/year, eg $107 at 10 cents/kWh.
Nick, according to my calculations, there 5.3 cubic inches of water in
a foot of pipe, that is equal to .0229 gallons. At 8.33 pounds per
gallon, that is .191 pounds of water per foot of pipe. Assume 10 feet
of 3/4" pipe that sweats that is 1.91 pounds of water. To warm that
from 40 degrees incoming temperature to 70 degrees, at 1 BTU times 30
degrees times 1.91 pounds = 57.3 BTUs. At 8 cents per therm (100,000
BTUs) the cost savings gained by leaving the insulation off is.0045
cents every time you have to warm the water in the pipe. If the people
in the house use the water 40 times each day, the total savings per day
is .183 cents per day. Times 30 days per month is 5.49 cents (per
month). Nick, that is not enough to worry about. Even if you allow
for inefficiencies, it is still less than 10 cents per month. Nick,
that is not worth the time I just spent on it. Or do I need to write
that into a BASIC program to get the point across to you?
Thanks. My last physics was in HS 50 years ago. I was beginning to
think I'd have to do some real research to make the point. I was
running off of common sense and practical application. There are a lot
of things out there that look good in theory but don't work out in
practical application. Of course Nick can argue that any savings is
worth it but to me dripping pipes trumps that one.
Common sense and practical application don't seem to apply with Nick.
His math may be correct, but his assumptions often go awry. He has
some interesting points, but if he had customers that had to live with
his assumptions, he would soon go broke.
You might check your math for glaring errors and read more carefully.
I said this savings is small compared to warming an active pipe with
condensation, which might save 20% on a water heating bill.
And you might modify your arrogant attitude and be more polite.
Apologizing is optional.
You assumed oil heating the water, I assumed gas. Although my gas
rates may be off a bit, he did not say what he is heating the water
with. Also the calculations assume the water is completely warmed to
room temperature each time, which is not likely. There are a lot of
suppositions involved here, most of them are overblown. The daily
savings are likely exagerated to the extreme. Also, pipe insulation
slows down the rate of heat transfer and increases the area of surface
area exposed to ambient conditions. It does not stop heat transfer.
As well, as the temperature in the pipe approaches room temperature of
the basement, heat transfer slows down, so it will take too long for
all this heat transfer to tane place. Certainly each complete
temperature change is not likely to take place in in 36 minutes. So an
apology is not likely. Still not enough cost to worry about.
Even using Nicks figures there is a glaring assumption that way
inflates the figures. Although you did point it out, I will do so
again. Water in the intake pipe is -not- going to warm up to ambient
in less than several hours. In my house with just the two of us, water
in that pipe will only remain there until the pump kicks in again.
Given normal useage that will occur at least once per hour, oftener
when watering. In a house on city supply, there will be an exchange of
water in that pipe any time a faucet is opened or toilet flushed, etc.
That will occur several times/hour for zero savings.
I am on oil for heat, electric for water heating.
A factor of 10, but that was the UNportant savings...
No. Please reread them more carefully.
OK Nick, you got me on this one! I WAS off by a factor of 10 on my gas
costs. Please pass the salt with the crow!
CHEW, CHEW, CHEW, CHEW, ..... Boy, this stuff is tough!
CHEW, CHEW, CHEW, CHEW....................
Now you might ask yourself what's really going on here:
I think the OP is saying the incoming water is warming up to the dew point
of the basement air. If condensation never occurs beyond the first few feet
of pipe, even when water is flowing, the pipe is warming flowing water in
real time, which is a much more important saving than the warming of small
amounts of water between uses.
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