On Apr 4, 7:08�am, wrote:
... be certain to insulate all hot water lines

How much energy would we save by adding R2 insulation to 100' of 1/2" pipe,
with 6 10 gallon hourly 110 F water uses per day at 3 gpm (an SRCC standard
test condition)?

The pipe is only used for about 3 minutes per hour...

Hint: not much, since there isn't much water in 1' of pipe,

... 62.33Pi(0.5/24)^2 = 0.085 pounds.

so it cools fast between uses, even with lots of insulation.

L' of ri' radius pipe with an r' insulation radius and a k Btu/h-F-ft
insulation conductivity and an airfilm conductance h Btu/h-F-ft^2 would have
thermal resistance R = (1/(hr)+ln(r/ri)/k)/(2PiL), so 1' of bare 1/2" pipe
with ri = r = 0.3125/12 = 0.026' and an h = 1.5 slow-moving airfilm would
have R = 1/(1.5x0.026x2Pi) = 4.1 F-h/Btu, so RC = 4.1x0.085 = 0.35 hours,
ie 21 minutes. After 57 minutes in 70 F house air the 8.5 pounds of water
would cool from 110 to 70+(110-70)e^(-57/21) = 72.7 F, losing (110-72.7)8.5
= 317 Btu of heat.

With 1/2" of R4 per inch (R48 per foot, ie k = 0.021) foam insulation,
r = 0.6875/12 = 0.0573' and R = (1/(1.5x0.0573)+ln(0.0573/0.026)/0.021)/2Pi
= 7.8, so RC = 0.67 hours, ie 40 minutes, and the water cools from 110 to
70+(110-70)e^(-57/40) = 79.6 F, losing (110-79.6)8.5 = 258 Btu.

The net savings is 6(317-258) = 352 Btu per day or 129K Btu (38kWh) per year,
worth $3.80 at 10 cents/kWh.

OTOH, some pipe insulation near the heater where the pipes are warm most of
the time could pay for itself quickly.

Nick

remember in the winter the lost heat helps warm your home............

in summer it can be a loser if you air condition.

definetely insulate the lines but PEX is much less conductive heat
wise than copper, so its naturally a insulator.

life is full of standby losses..........

every electrical device in your home thats not 100% off when
off.......

cable boxes, computers, pilot lights, clocks, night lights, security
lights, TVs, the list is endless..........

one shouldnt get too concerned wany single one.

better to insulate and weatherstrip your home for better
savings............

you will likely save more