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Hot water line sizing help

Posted by nolaslumlord (My Page) on
Mon, Mar 3, 14 at 10:46

Background: House is 2000 sqft, built in 1975, in the south, all copper.
The copper hot water line in the slab must have burst at some point because the previous owner has installed 1/2 flex copper in the attic. ALL 1/2 from the outlet of the water heater. The feed to the heater is still the original 3/4 slab copper. So far everything has worked in regards to flow and pressure.

My issue: Im looking to move the water heater to a location that wont have a 3/4 cold supply. Basically, I want to move to my garage that has access to the master bath wet wall. Can i just use the 1/2 branch for the master bath to supply the water heater? Also, this will move the heater farthest away from the kitchen sink. Is this an issue as well? I figure, since the whole hot water line has been replaced with 1/2, a 1/2 feed shouldnt be an issue. Just thought id try here first.

Just created this account for this question but i usually search this site for all my projects. Yall have been helpful everytime. Thanks.


Follow-Up Postings:

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RE: Hot water line sizing help

Depends on distances, BUT not likely to be a good idea unless you have a postage-stamp of a house. You will add substantial pressure drop to your feed pipe in addition to increasing velocity in the line, which will cause excessive erosion at fittings and elbows, particularly any time you use more than one hot water fixture at a time.


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RE: Hot water line sizing help

Alice, would it make a difference that the output of the water heater is already all 1/2?


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RE: Hot water line sizing help

I think you will find that the delay in the kitchen wil drive you crazy. In the master bath you use hot water a couple times a day. In the kitchen you use hot water frequently. As well, in the master you can turn on the shower and move the water more quickly. The kitchen faucet will likely have a lower flow rate and a longer delay.


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RE: Hot water line sizing help

Alice, would it make a difference that the output of the water heater is already all 1/2?

No. Undersizing the feed on your hot water will have a greater affect on pressure drop and velocity than undersizing the outlet because the feed side has to supply more water than each individual outlet.

1/2" copper pipe should carry a maximum of 2.55 gpm hot water or 4 gpm cold water in order to avoid corrosion/erosion from excessive water velocity through the pipe (5 fps hot, 8 fps cold). A bathroom faucet will use about 2 gpm, a kitchen faucet about 3 gpm, a shower about 4 gpm. Not all of that will be hot water, but just at your kitchen sink, if you are waiting for hot water you'll likely have it turned all the way to the hot side and exceed the pipe rating. If anyone runs another sink, a dishwasher, washing machine, etc at the same time, your problem becomes worse, both in the water heater feed line and the trunk existing the heater. You may not see a problem for a while - you will likely get the amount of water you need from the plumbing, BUT your plumbing will fail prematurely, whereas correctly sized and installed copper pipe (assuming your water chemistry isn't overly aggressive) should last nearly forever.

The previous owner made a mistake if s/he plumbed a copper trunk and branch system with all 1/2" pipe. Don't compound the mistake by making the feed line too small as well.

This post was edited by aliceinwonderland_id on Tue, Mar 4, 14 at 11:13


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RE: Hot water line sizing help

Jack/Alice, thank you for your replies. Based off this info i have decided to install the heater to its original location and spring for a new one in the attic when I get a few more dollars. That way it has an equal run to all drops and is out of the way completely. Thanks for your help.


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