Two water heaters

Until two weeks ago, my parents house used one water heater ( noted "1" on the picture) to provide two bathrooms and the kitchen with hot water. The bathroom on the first floor consists of a bathtub which is used occasionally as a shower. The bathroom on the second floor has a shower and is used often.

The pressure in the shower dropped when someone opened a tap to take hot water, so they decided to add an additional water heater ( "2" on the picture) to get rid of the problem. The two hot water network are now disconnected from each other, and this solved the problem of hot water pressure dropping.

There is now another problem. The pressure in the shower (2nd floor, coming from water heater 2) is very good for 65 seconds, then drops drastically. The water seems to just fall (vertically) from the shower head.

Can this be fixed ?

I'm not a plumber, and I'm not going to do it myself, but I may be tempted to (have someone) switch the output of the two water heaters; the water heater #1 is known to work perfectly on the second floor (pressure wise..), and the first floor shower doesn't really need a high pressure hot water (if that pressure problem can't be fixed easily).


About the picture: These are copper pipes painted in white. The red color has been added afterwards using Photoshop to help you visualize the two hot water networks.

I did a few tests this morning:

Time to fill a 17L sink: (17L is an approximation.)

  • with hot water only: 35s
  • with cold water only: 55s

Time until hot water pressure drops: 65s

Time to fill a 17L sink with hot water when there pressure is low: 75s

  • Does the pressure immediately re-bound if you turn it off and then back on or does it take a couple minutes? It sounds like the water in the heater is providing some gravity pressure but isn't filling fast enough to maintain it.
    – Comintern
    Commented Apr 11, 2014 at 0:03
  • It takes a couple of minutes to regain pressure.
    – alecail
    Commented Apr 11, 2014 at 7:59
  • First troubleshooting step then would be to see if using a lower-flow shower head increases the amount of time it takes for you pressure drop.
    – Comintern
    Commented Apr 11, 2014 at 13:05
  • A good test is to open a cold water faucet as close to the main supply as possible and measure how fast it supplies water. If it is good there work you way outward and see if you can find where it changes. Generally only one expansion tank is necessary, that you have three (small) ones is odd. Is the water supplied by a well or a community water system? Is there a pressure regulator that may have failed or needs adjusting?
    – mfarver
    Commented Apr 8, 2017 at 1:37

2 Answers 2


It appears to me you are building pressure using the bladder tanks. Once the pressure reserved in those tanks is depleted your pressure drops because the flow is restricted (possibly ahead of those tanks). I would agree with Justin that you have a restriction that is reducing the flow and after the bladder tanks are depleted you are only getting the flow through the main line.

The fact that it takes longer to fill the sink with pure cold water tells me your problem is a restriction in the line feeding the water heaters and the bladder tanks are compensating for this somewhat when you fill the sink with pure hot water.

Look for a restriction in your main cold water feed line, and then at other points in the system. Check the air pressure in your bladder tanks with the line drained. Lastly, the piping looks too small to me. I would upsize that piping for better flow.

  • Good advice, it sounds like a low flow system that can't keep up with the additional head pressure.
    – Ed Beal
    Commented Dec 14, 2018 at 17:19

Sounds like a partial clog/blockage of a pipe. It is most likely in one of the fitting that was added. Is shutoff valves all open completely? Is that pex piping? Is it kinked somewhere? Does the water pressure drop when using cold water? If you paid someone to add the tank tell them to fix it. They screwed up somewhere.

  • 4
    Plus adding a 2nd water heater makes no sense. You add a second hot water heater if you run out of hot water or if a house is large and run is too long to bathroom. If you have a drop in water pressure it means the city's water pressure is not strong enough or the pipes are undersized/corroded/clogged.
    – Justin K
    Commented Apr 11, 2014 at 2:34
  • So.. I know it's a XY problem. My parents trusted the person who installed the second water heater. Also, as I said in the question, the water flowed normally with only water heater #1. See my edit for pex piping.
    – alecail
    Commented Apr 11, 2014 at 8:22
  • 1
    So, remove the 2nd water heater and start over. Sorry they trusted.
    – Bryce
    Commented Apr 11, 2014 at 8:27

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