I recently bought my first home, and the first thing we noticed is that the dynamic hot water pressure is pretty low compared to the cold water pressure. I purchased a water pressure gauge and found that the static hot water pressure is identical to the cold water static water pressure (about 95 PSI). When I turn on a cold tap, the dynamic cold water pressure reads at about 85 PSI, which is totally cool. When I do the same for the hot water, it drops waaay waaay lower.

When I attached a hose to my water heater to try to flush it, the water pressure was amazing, and the water was perfectly clear. So it doesnt appear to have a sediment problem.

Basically this results in a low hot water flow rate for the entire house. The water heater (probably near the end of life) is a 40 gallon gas heater. Any idea what might be causing this? Anyway I can try to fix this with out having to buy a new water heater or call a plumber?

1 Answer 1


There is a restriction, somewhere. You could use your gauge to verify that the pressure (while you are flowing from faucets) at the water heater is good (you've only done so qualitatively so far, and you have means to test it quantitatively, so....) But basically somewhere in the line is something that is restricting water flow - perhaps a clogged or under-sized tempering valve (if there is a tempering valve) or the one I keep mentioning because it was a bear to track down and semi-unbeliveable when I did, an elbow almost completely plugged with solder. Could also be pipes plugged with mineral deposits.

I happened to have camera and the fitting in question from my junk (perhaps "plumbing trophies") box and recall that I want to show it. Trigger warning "terrible plumbing."

Copper pipe elbow plugged with solder - image by me

The little gap at the bottom of this picture was the only passage for water in this fitting.

If you open the hot faucets and pressure at the water heater stays good, move the pressure test up the line - somewhere it will get worse, and between there and the last place it was good is the problem.

Incidentally, 95 PSI is on the high side for many fixtures - you might want to add (or adjust if there is one) a pressure reducing valve and get to down to 75-80 PSI MAX.

  • 1
    I recently replaced the old iron pipe risers off the top of my water heater. These pipes were more than half way clogged up with internal deposits and corrosion. Replaced with new copper and I improved my hot water flow rate some.
    – Michael Karas
    Jan 24, 2015 at 3:12
  • So to start off with, I would attach the pressure gauge to the water heater's drain, open up a hot tap somewhere and verify that the pressure didn't drop like crazy. If that's okay, then how do I go about moving the pressure test up the line? Sounds like the right road to go down, I might just need a few more details on how to go about it. I may have the wrong kind of pressure gauge, I have a Rain Bird P2A, which just goes on the end of a hose spigot. Maybe I need an inline one? Michael, I may have the same issue, I have old corrugated flexible hoses from my WH to the intake and outlet
    – Matt
    Jan 24, 2015 at 7:11
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    One of the things that makes this difficult is that there usually are very few places it's practical to test. Testing at the WH will tell you if it's the cold feed to the WH or not, and you have the drain there. Laundry connection is one place you'll find a hose thread. You can also get a hose thread adapter to attach to a faucet, but if all the faucets are noticeably poor, all that would do is confirm that they are all "past the blockage." Then you start having to cut (or unscrew, if yours happen to be threaded) pipes...
    – Ecnerwal
    Jan 24, 2015 at 16:13
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    Well, there's a good chance that it's the line coming out of the water heater (or the cold water intake) because I have a sink 3 feet from the water heater and a washer 5 feet from the heater and both of them have poor flow rate too
    – Matt
    Jan 24, 2015 at 18:41
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    The plumber replaced both supply lines, the hot water line was totally clogged and the cold water line was rapidly deteriorating. He replaced it with flexible copper lines and our hot water pressure is on par with the cold water. He also installed a pressure regulator and set it to 65psi which works great for us. All in $470 (ouch) but I'm glad it wasnt something worse, and now I know a bit more about plumbing :)
    – Matt
    Feb 11, 2015 at 16:35

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