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re: insulating hot water pipes
7 oct 2005
wrote:
>"if you want to improve the efficiency of the water heater, then you
>should insulate these pipes. more than that much would not be useful,
>as the heat from the water heater is lost in the first few feet of
>copper pipe. that will also reduce by a couple of seconds how long
>you wait for hot water when you turn on the tap. "
>
>so, the water is back to ambient temp after running through the first
>few feet of pipe?
in a short while, with no flow... 20' of 3/4" pipe with c = 3.8 lb of water
and 4 ft^2 of surface might have a thermal conductance g = 1.5x4 = 6 btu/h-f,
so the time constant rc = c/g = 0.63 hours, ie the water would cool from 120
to about 70+(120-70)e^-1/0.63 = 80 f in 1 hour in a 70 f room.
>if that were the case, you'd never get hot water.
true, if there were no flow :-)
>and it isn't going to make hot water show up any faster, except in the
>case where it's reused within a reasonably short period, before it
>cools off again. but, for example, first thing in the morning, it's
>still going to take just about the same amount of time to get hot water
>with or without insulation.
the important energy savings and faster hot water might come because
the water heater keeps the first few feet of pipe warm by warm water
convection, even with no hot water usage. you might feel the pipe in
the morning, before any usage, and keep insulating it until the end
of the pipe emerging from the insulation no longer feels warm.
nick
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