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Old 03-06-2009, 12:07 PM   #1
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Change in pipe diameter at water heater


Hello: I just remodelled my house and had 1 inch pipe installed in the crawl space for good flow (to allow several showers, tubs, etc to be run at once without starving flow).

So my main water lines (cold and hot) are each one inch. My plumber has installed a 3/4 water heater, so my one inch pipe feed pipes neck down to 3/4 going in and out of the water heater, then back to 1 inch.

It is my opinion that the flow in my system is limited to the narrowest pipe in the network. I know my fixtures are 1/2, but in terms of the feed volume, am i correct that if you have a pipe that is 1 inch, then it necks down to 3/4, then back up to 1 again, the flow at a constant pressure is no greater than the narrowest section can accomodate (i.e., in and out of the water heater)?

This makes sense to me, because if you take an extreme example it becomes clear: If a fire hose from a hydrant necks down to a garden hose for a few inches, then back up to the full size fire hose, you will not have very good flow at the nozzle!

I have a one inch water main and one inch pipe from the street to the one inch house feeds.

Thanks, Dan

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Last edited by blacksheiladog; 03-06-2009 at 12:29 PM.
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Old 03-06-2009, 12:51 PM   #2
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Change in pipe diameter at water heater


The water heater necks down to 3/4" no matter what you do. Especially since you have a 1" main whereas a 3/4" is more commonplace, I don't think you'll notice any difference at all. We're talking volume here, not pressure. With 3/4" pipe, there's no way that any fixture in your home will be starved for water delivery.

I wouldn't give it a thought.

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Old 03-06-2009, 01:12 PM   #3
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Change in pipe diameter at water heater


thanks for the quick reply. We have 5 bathrooms, and the master shower has the multiple rain head, shower, and handheld. We often have 2 bath tubs/showers going at once plus toilet flushings etc.

The 75 Gal water heaters (it will fit in the closet) have 1 inch threads and several plumbers have said that having paid for 1 inch feed pipe throughout the house, using a 3/4 water heater just negates all that investment.

Is the premise correct that your pipe system flow is no better than the narrowest choke point? Thanks again

Last edited by blacksheiladog; 03-06-2009 at 01:21 PM.
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