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My house is on a shared lot, and the water supply comes from a pipe supplied near the main house. We're not quite sure how everything is connected there, but from measuring pressure drops, we do know that there's (approximately) 100ft of 1/2" copper pipe leading to the supply at my house and that typically I see a 9psi drop (from 65psi) when running a showerhead (2.5gpm).

We also tested how my pressure varies when the main house is running water. I get extreme pressure drops when they activate a tub faucet. It goes from 65 to 49psi. I also typically get drops of around 5-10 psi when they turn on one or two showerheads (2.5 or 5gpm).

We're going to be redoing the plumbing in my place with descending lines from the attic since we have no real crawlspace access. I was wondering if there is a way to use a small holding tank (in the attic, around 4m above the supply entry) to buffer the water pressure so I don't experience quite as severe pressure losses when the main house runs water.

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Pressure drops are normal. That's just how it works. Even with much larger pipe, there'll be pressure changes as the pump kicks in and out.

I would start with a pressure regulator. Set the regulator for something like 35 PSI which the system can always provide. It would work even better if both houses had a pressure regulator. It goes without saying that this will reduce best-condition flow somewhat, in favor of more uniform flow all the time.

After you have a regulator (which implies a check valve), then a "pressure tank" will start to make sense. This buffers further. Although I have a concern with them in light of what we're learning about hot water heaters and legionella. If it's not good for water to be stagnant in a hot water heater below 140F, one of those pressure tanks has the same basic problem.

  • I understand pressure drops are normal. I am talking about reducing the drop from sudden, but short, loads, like a dishwasher or washing machine. I would rather buffer the cold input to the tankless than add a tank. – Hari Ganti Feb 4 '17 at 7:20
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    If water sitting in normal tanks at "room" temperature was a huge legionella concern, we'd all be dead. That is pretty much the norm for EVERY water distribution system from a single-house well-pump to a city reservoir and high-rise storage tanks. – Ecnerwal Feb 4 '17 at 16:30
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    I would just add a check valve and a pressure tank. This would provide several minutes of full pressure. Reducing the pressure to 35 psi would provide crappy pressure 100% of the time. +Ecnerwal 2 – Ed Beal Feb 4 '17 at 18:38
  • @Ecnerwal I'm talking about this. diy.stackexchange.com/a/108027/47125 Water in pipes moves through FIFO; and rarely dwells more than 24hrs. But a pressure tank is an absolutely huge dead leg. – Harper Feb 4 '17 at 21:09
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    Virtually all water distribution system have tanks, not just pipes. Some are more obvious and have the name of the town painted on them, some are less obvious and buried underground up a hill, or hiding on a rooftop or in a "utility floor" - until recent years every well pump had a pressure tank, and most still do. I think you are speculating well beyond established facts. – Ecnerwal Feb 4 '17 at 23:21
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The best method to buffer pressure swings on a shared pump is to first add a check valve and pressure tank after the check valve the larger the tank the longer the peak pressure will be maintained. To provide improved performance a booster pump would be added after the check valve this would assist in maintaining a higher pressure but with the long run of plumbing I would only invest in a small booster , less than 5 gallons per minute or it will cavitation when there is not enough supply.

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