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Thread: Typical Difference in Hot Water and Cold Water Pressure

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    DIY Junior Member psumichael's Avatar
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    Unhappy Typical Difference in Hot Water and Cold Water Pressure

    Hi all,

    I just installed a new Whirlpool electric water heater in my house. The hot water pressure is lower than the cold water pressure. To test/quantify the difference I filled a 2.5 quart pot with the kitchen faucet. Full cold it took 21 seconds. Full hot it took 26 seconds. So it took almost 25% longer with the hot water.

    Is this a normal effect of heat traps in a water heater - to drop hot water pressure that much below cold water pressure? I checked the inlet piping into the water heater. It's unobstructed.

    I'm ordering a water pressure gauge. If the home water pressure is below 50 or so psi I'll probably adjust the pressure reducing valve and consider it dealt with but if it's already at 60ish psi I will probably avoid raising it higher. Do those pressure thresholds sound about right?

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    In the Trades Gary Swart's Avatar
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    I believe you are confusing pressure with flow. The PSI (pressure) is governed by the supply line pressure in both the hot and cold. Realize that when you draw hot water, it is replaced in the tank by an equal amount of cold water. It is this incoming water that forces the hot water out of the tank. Flow is another matter. For instance, if you have galvanized pipes, the inside of the hot water pipe may be corroded to the size of a pencil. In that situation, the flow would be very low, but the pressure would be the same as if the pipe was unobstructed. Perhaps you have a shutoff valve in that is not fully open. Another possibility is if your heater was connected with flex tubing, perhaps the hot side was kinked. You should also check the aerators in the kitchen faucet. Debris could have been knocked loose during the installation. 50 PSI is ample pressure for a house.

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    Moderator & Master Plumber hj's Avatar
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    The hot water piping is typically "longer" than the cold which could give the results you see.
    Licensed residential and commercial plumber

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    DIY Junior Member psumichael's Avatar
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    Quote Originally Posted by Gary Swart View Post
    I believe you are confusing pressure with flow. The PSI (pressure) is governed by the supply line pressure in both the hot and cold. Realize that when you draw hot water, it is replaced in the tank by an equal amount of cold water. It is this incoming water that forces the hot water out of the tank. Flow is another matter. For instance, if you have galvanized pipes, the inside of the hot water pipe may be corroded to the size of a pencil. In that situation, the flow would be very low, but the pressure would be the same as if the pipe was unobstructed. Perhaps you have a shutoff valve in that is not fully open. Another possibility is if your heater was connected with flex tubing, perhaps the hot side was kinked. You should also check the aerators in the kitchen faucet. Debris could have been knocked loose during the installation. 50 PSI is ample pressure for a house.
    Thanks for the reply, Gary.

    I checked the cold water shutoff valve. It's a brand new gate valve I soldered in. Actually took that whole section of piping off yesterday to check that there wasn't an obstruction in it (like excess solder). It looked good. That's what has me asking about the heat traps in the water heater. The heat traps provide flow restriction much like a non-fully-opened shutoff valve, right? I've read comments from others that the heat traps used in some water heaters obstruct flow.

    The aerator would affect flow the same whether it's the hot water or cold water. I've tested other faucets and found the same situation; it's not specific to the kitchen faucet.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    There are at least a couple of different versions of heat traps. One uses a plastic ball, another uses a little flapper valve. There may be others. The more restrictions and pipe length can result is lower flow...rarely are the hot and cold runs identical.
    Jim DeBruycker
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    DIY Junior Member psumichael's Avatar
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    Quote Originally Posted by jadnashua View Post
    There are at least a couple of different versions of heat traps. One uses a plastic ball, another uses a little flapper valve. There may be others. The more restrictions and pipe length can result is lower flow...rarely are the hot and cold runs identical.
    Thanks! I wouldn't expect the hot and cold flow to be identical but I was expecting flow rate to be within say 10% of each other. What % difference would you consider "normal?"

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    Moderator & Master Plumber hj's Avatar
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    we don't even worry about what the "normal" pressure differential is, since there would normally not be any way to modify it anyway, and most people would either not notice it, or not obsess about it.
    Licensed residential and commercial plumber

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    DIY Junior Member psumichael's Avatar
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    Quote Originally Posted by hj View Post
    we don't even worry about what the "normal" pressure differential is, since there would normally not be any way to modify it anyway, and most people would either not notice it, or not obsess about it.
    The end result of all this, and why I care, is that the water pressure in the shower is reduced. Lots of folks care about water pressure in the shower.

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    Moderator & Master Plumber hj's Avatar
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    What kind of shower valve do you have. Most people just turn the hot on and adjust the cold to mix it.
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    Forum Admin, Expert Plumber Terry's Avatar
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    Like mentioned above by hj; a lot depends on how the pipes are plumbed in the home, and then the heater adds it's own restriction.
    You may also have undersized water lines and longer total length on the hot side compared to the cold.

    Sometimes the water heater is on the opposite side of the incoming cold, which can really add extra piping.

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    DIY Junior Member psumichael's Avatar
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    Quote Originally Posted by Terry View Post
    Like mentioned above by hj; a lot depends on how the pipes are plumbed in the home, and then the heater adds it's own restriction.
    You may also have undersized water lines and longer total length on the hot side compared to the cold.

    Sometimes the water heater is on the opposite side of the incoming cold, which can really add extra piping.
    Thanks for all the responses!

    I read you guys on the potential differences in piping layouts/sizes but there are two reasons I believe that's not the issue: 1) the water heater is right by the water inlet into the house and 2) I don't believe the issue with low hot water flow (and low shower water pressure) existed before I replaced the water heater.

    Now as far as 2) goes, I never had reason to measure the water pressure/flow with the previous water heater in place so I could be exaggerating the difference in my mind. That's what prompted me to ask for opinions on what's a "normal" or "acceptable" difference in flow rate for hot vs cold. Thanks again.

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