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Old 01-17-2011, 06:10 AM   #1
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Pressure drop across water heater, low hot water flow


I installed a new electric water heater and there is a considerable pressure drop. I'm not sure if it was nearly this bad before, but even with the old water heater, when washer would cycle hot water flow on and off while someone's in the shower, it definitely caused unpleasant cold surges.

Washer faucets are right by the water heater and measuring there, Incoming cold water is ~40psi. It's hardly affected with water use downstream.

Hot water as measured at the washer faucet drops to ~20 psi when shower is turned and pressure read out is very jumpy. Pressure as measured at water heater drain valve is slightly higher.

Pressure at both points read about 40 psi, as expected, with no flow.

Before, I had flexible copper line with same I.D. as normal 3/4" copper pipes. Now, I'm using Sharkbite braided line with I.D. about the same as 1/2" copper line. I don't believe this matters much though. If I bypass the water heater by removing connections at the water heater and connecting the two hose ends, the pressure drop becomes negligible meaning that pressure drop is caused by the water heater itself.

Does this sound pretty normal?

Last edited by HVAC_NW; 01-17-2011 at 10:41 PM.
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Old 01-17-2011, 07:15 AM   #2
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Pressure drop across water heater, low hot water flow


I would go back to 5/8" or 3/4" inside diameter water lines anyway before doing more pondering and conjecturing.

A 1/4" ID line has about ten percent (theoretically one ninth) the cross sectional area of a 3/4" ID line.

I think it would take a Ph.D. in hydraulics to give a full explanation for your last paragraph.
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Old 01-17-2011, 10:43 PM   #3
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Pressure drop across water heater, low hot water flow


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Originally Posted by AllanJ View Post
I would go back to 5/8" or 3/4" inside diameter water lines anyway before doing more pondering and conjecturing.

A 1/4" ID line has about ten percent (theoretically one ninth) the cross sectional area of a 3/4" ID line.

I think it would take a Ph.D. in hydraulics to give a full explanation for your last paragraph.
I meant, the ID appears the same as 1/2, not 1/4. That was a typo. It's a SharkBite brand 3/4" braided lines intended for water heater hook up.
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