I just thought that if there is a long distance from your hot water
heater or boiler coil to the kitchen maybe a smaller diameter pipe would
get the hot water there faster. Maybe 1/4" or 3/16" tubing would be
better. I may try this.
Simple job, assist the assistant of the physicist.
I seriously doubt that you'd get hot water at the remote faucet any
faster, but you would waste less energy by having a smaller amount of
hot water left in the pipe when the faucet was closed.
I'd 'spect you'd find the flow rate unbearably slow. Look up and use
flow rate and pressure drop tables to see what the flow rate would be.
I replaced a 80 ft run in an apartment building and went fromn 1/2 to
3/4 thinking more is better because supply was poor, it now takes
longer to get the hot water to the end. pvc will not cool down as fast
as metal and the foam insulation will help even more preserving heat.
If your pipe is old the inner diameter is likely scaled up and the
diameter is smaller than original, my galvanised pipe you could not
see through it was so scaled up. What you have now could be equal to
3/16" so I would not just rip it all out, study it.
A smaller diameter pipe (within reason) will be cleared of cold water
quicker and thus you'll get hot water to the fixture faster.
3/4 vs 1/2 ....... the 1/2 line will get you hot water in ~1/2 the
time fora 3/4".
Overall performance is driven by available water pressure & flow at
As long as the run is of reasonable length such that pressure drop for
the smaller line is not excessive.....
the smaller line will give you hot water quicker at reasonable flow &
Wirsbo has pressure drop tables based on diameter, run length &
flow. IIRC ..... 3/8 is the smallest PEX line recommended to serve
You know this is very true. I have a newly constructed house. The
plumber did a nice job of using larger diameter pipe for the trunk
lines and then branching out to smaller sizes. Well, with the new
low flow faucets everywhere these days, it takes forever to get hot
water at the end of a long 3/4" pipe. Even at my kitchen sink,
which is directly over the water heater, it takes a goodly amount of
time to get the water hot. The faucet companies have done a good
job in duping the user to think there is a lot of water flowing by
using really nice aerators and sprayers. But try filling a jug or a
pot and you will know that the flow is really quite low.
And that's the key point in the whole discussion. If the end of the
pipe were open, you would get hot water faster with the larger pipe --
assuming in turn that the supply to the HWH is at least as large.
Think about it -- at a given pressure, the flow velocity in a 3/4"
pipe is faster than in a 1/2" pipe (for the same length). More waste,
However, as Art points out, the pipe is not the bottleneck -- it's the
faucet. You can't open any normal faucet to a 3/4" opening, or to a
1/2" opening, or even close. So the flow velocity in the pipe will be
determined by the pipe diameter and the faucet characteristics.
So as long as the pipe is large enough to supply the faucet without
restricting its flow, you will get hot water faster with a smaller
pipe. Once the pipe gets so small that it significantly restricts the
flow with the faucet full open, the smaller pipe will take longer to
deliver hot water.
Using the calculator at pipeflowcalculations.com, and assuming a 50'
run with the input at 80 psi, and smooth pipe, I get these figures:
1/2" pipe: 3.84 gpm, 6.3'/s
3/8" pipe: 1.22 gpm, 3.5'/s
1/4" pipe: 0.24 gpm, 1.6'/s
3/16: pipe: 0.08 gpm, 0.9'/s
My guess from this is that at common municipal water pressures, a sink
faucet will be the bottleneck using 3/8" pipe, but the pipe will be
the bottleneck using 1/4" pipe. For a bathtub faucet, you need 1/2"
pipe to prevent the pipe from being the bottleneck. This hardly comes
as a surprise, since we know what size pipes are used in residential
plumbing. Of course, many runs are shorter. Even if LSMFT accepts the
low volumetric flow rate, the low velocity in 1/4" and 3/16" pipe
means he'll be disappointed if he expects the smaller pipe to get the
hot water to him more quickly.
At my last house, a modular, they used 3/4" pipe as a manifold and when
on site, you cut and hook up the water where ever you like on the 3/4"
manifold. The boiler hot water coil and fittings are only 1/2" so it
went from 1/2" the opposite way, then to 3/4" the right direction, then
down to 1/2" again near the bathroom and kitchen.
I tore it out and ran two separate 1/2" lines right from the boiler, one
to the bath and one to the kitchen. It took about 1/4 the time to get
hot water to the bathroom, even less to the kitchen sink.
And to help with someone else's reply, hot water to the kitchen doesn't
mean it's being used to cook with, it can be used for washing dishes also.
Oh, you can COOK with water from the water heater, just don't CONSUME water
directly from the water heater. For example, don't use it to wash vegetables
that will be EATEN raw (i.e., turnips).
There are some reports that even cooking with water from the heater will
turn you cranky, but I don't hold much with that idea.
I had a long wait for hot water to the kitchen sink at least 30
seconds with sorry volume, I was thinking an under counter HW heater to
shorten the wait for hot water but one day I bit the bullet, crawled
under the house and ran as straight a shot as possible from the HWT to
the kitchen faucet. I used 1/2 inch CPVC with 45s instead of ells etc.
Now it takes a quart of water and the water is coming through hot,
maybe 5 seconds? less than 10 seconds for sure. It's probably a 30 foot
run. The old pipe was 1/2 inch crusted up galvanized with some black
pipe, and it kind of meandered around under the hose through tee's an
ells, this also eliminated chunks of rust from the old pipe that would
One of those projects that took me a couple of hours to do but 15
years to get around to doing it, when I was done and things were so much
better I wondered why the hell did I put up with it the way it was as
long as I did.
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