Hot water line pipe size trade-offs

[artnking]

I'm pluming in a new shower and sink that is 77' from the hot water heater. There are two 90-degree bends in the run. The local plumbing supply people tell me I need to use 3/4" line (i'm going with PEX). I'm afraid that 3/4" line will lead to a LONG wait for hot water. Could I get away with using 1/2"? I'm quite certain there will never be any additional faucets on this run.

Any other suggestions for getting hot water to the shower fast?

 

[frodo]

using pex?
if you are using 3/4 pex than the ID of your pex pipe is .681
it is just bigger than 1/2''

 

[bmDhillon]

If you really want instantly hot water then use recirculating pump

 

[Mr_David]

I was bored so I did the math.

1/2" pex ID = 0.485"

A= 3.141 x (0.485 x 0.485 ) = .738 sq.in
A x L= V
L = 77' x 12" = 924 "
924" x .738sq.in. = 681.91 cu.in.

1 cu.in. = 0.004329 gallons

681.91 cu.in. x 0.004329 gal = 2.95 gallons in a 1/2 pex line 77ft long.

3/4 pex ID = .681" . . . . . . . . . 5.82 gallons in a 3/4 pex line 77ft long

Now all you have to do is figure out the flow rate of the shower head or faucet.
let's say 2 gallons per minute and that can vary depending on the pressure.

Maybe 1.5 minutes to get hot water on a 1/2' line and almost 3 minutes on a 3/4' line.

You might be better off using a 1/2" line.

wow! That was a bit of a challenge for me. Did I do it right?
I used to love doing trig and calculus in high school.
That was a looong time ago.
Now I have to add $hit with my fingers sometimes.

 

[frodo]

David, did you factor in a sink and a shower?

 

[Mr_David]

No I just factored the amount of cold water sitting idol in the 77ft line and how much time it would take to displace that with hot water through a outlet at 2 cu.ft./ minute. I suppose if you open the sink faucet and shower at same time you'll get a faster time.

It was kind of an exercise to compare the difference in using 2 different pipe sizes . I suppose if I had more information like water pressure, how many fittings in line, actual fixture ratings, water temperature, elevation above sea level and the PH of the water I can get a more accurate estimate.

 

[frodo]

No I just factored the amount of cold water sitting idol in the 77ft line and how much time it would take to displace that with hot water through a outlet at 2 cu.ft./ minute. I suppose if you open the sink faucet and shower at same time you'll get a faster time.

It was kind of an exercise to compare the difference in using 2 different pipe sizes . I suppose if I had more information like water pressure, how many fittings in line, actual fixture ratings, water temperature, elevation above sea level and the PH of the water I can get a more accurate estimate.

you sure that would be enough information?
i think you would need to know what was on the radio at the time

 

[Mr_David]

you sure that would be enough information?
i think you would need to know what was on the radio at the time

Oh, Absolutely!