Question: isn't pressure supposed to remain when shutoff?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by eduncan911, Aug 12, 2019.

  1. eduncan911

    eduncan911 New Member

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    Northeast
    Isn't the water pressure supposed to remain the same if I shut off the water supply to the house?

    Also note that the hot water is not heated yet. It's just filtered well water and I havent turned on the heater yet, so temp changes shouldn't be much of an issue with pressure readings (I think - it does warm up to 75°).

    I've finished the plumbing for my hot water system in this foreclosure that had several busted baseboard hearing pipes. Most of the house plumbing seems fine on cold.

    Now it's time for the hot water.

    However, I've noticed that if I fill the hot water lines with water, bleed the air out of most facets, and let it sit at 56psi as measured at the heater inlet while turning off the hot water supply with a new ball valve, the water pressure drops from 56psi down to 40psi in about 12 hours.

    I haven't checked the cold line (just noticed this pressure drop this morning). Normally i'd do an air pressure test in the system first and measure the drop overnight. However, the system still had a lot of water in it from previous work. So I didn't think an air test would be accurate.

    So I'll check if the cold supply drops overnight tonight if I shut off the cold supply.

    Yes, perhaps there's a leak in the hot lines - this house has had lots of freeze damage. However, I'm not see any leaks yet - and I've even cut out the shower valves and capped them off, and have most access to most lines and they visually seem fine.
     
  2. Reach4

    Reach4 Well-Known Member

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    With limited elasticity in the plumbing, and no pressure tank, it doesn't take much to make the water pressure change a lot. It could be a little cooling happening.

    With 56 psi water, you would usually want the thermal expansion tank to have a precharge of maybe 58. But for this, maybe drop the precharge down to say 40. That tank will act as a small pressure tank. Restore the water pressure to 56, and watch for leaks.

    Move the air precharge in the thermal expansion tank back to 58 to 60 after testing.
     
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  4. plumber69

    plumber69 In the Trades

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    Seems like a lot of you guys have pressure tanks on your domestic systems. Why is this? there are none here unless they have a booster pump
     
  5. Reach4

    Reach4 Well-Known Member

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    Houses with private wells have pressure tanks.

    Then there could be some that mistakenly call a thermal expansion tank a pressure tank. They look pretty much the same.

    With water being mostly incompressible, you cannot really test for leaks by cutting off the supply and monitoring water pressure, unless you have something that serves as a pressure tank.
     
    Last edited: Aug 13, 2019
  6. James Henry

    James Henry In the Trades

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    You didn't bleed the air out of the shower valve lines. That might have an effect.
     
    Last edited: Aug 13, 2019
  7. hj

    hj Moderator & Master Plumber Staff Member

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    If you are testing for a pressure leak, you DO NOT want to have any "compression type" tank connected to the system, because that would just "mask" any small leaks and not show a "short term" pressure drop
     
  8. Reach4

    Reach4 Well-Known Member

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    Yet you are not going to say that turning off the water supply valve and monitoring the water pressure is a good leak test without a tank, are you?
     
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Any air in the system will try to expand/contract more than the liquid water will. 16# indicates a leak to me, regardless of what expansion/contraction might have added. If you have an autofill valve in a hydronic system, it could be a slow leak there, and the water is going to replace that lost in the heating system. It doesn't take much water loss to drop the pressure. FWIW, air tends to change pressure at about 1-pound per 10-degrees F in a closed system.
     
  10. eduncan911

    eduncan911 New Member

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    Thanks. That makes sense for an expansion tank; but, I do not have one.

    There is a well pressure tank, but it is upstream and not behind the shut-off valve.

    There isn't much cooling/heating. Well water was 50-ish degrees, and it warms up to about 70 degrees in the house (the hot water heater is not on yet).

    All pipes are copper, except for about an 8 foot section that is 1" pex.

    I did find a tiny drip-per-20 seconds on the laundry, but I think it's on the cold side not the hot.

    I was thinking that perhaps some of the mixer valves at the facets were leaking hot to cold. However, the cold side is still open and stays at 56 psi. So it would go the other way, cold to hot if that was the case.

    I'll play around a bit more as I work on other projects.
     
    Last edited: Aug 14, 2019
  11. Reach4

    Reach4 Well-Known Member

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    The pex adds some expandability with pressure, and that continues with time. See https://www.uponorpro.com/~/media/Extranet/Files/manuals/PLU_PressureTest_IS_0812.aspx?sc_lang=en for a graph in a non-leaking system. In the graph, at the vertical lines of pressure change, they are adding or removing air.

    I would not put significance in the drop from 56 to 40 in this test. Temperature of the water could have dropped. The valve to the WH could have had a minuscule leak. Did the drop continue continuously in the next 24 hours?
     
    Last edited: Aug 14, 2019
  12. eduncan911

    eduncan911 New Member

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    The reason why is to limit the on/off cycle of your well pump.

    Well pump == $1000s and $1000s to drill and replace. You want to limit it's cycling as much as possible.

    A pressure tank will have a high/low pressure switch attached, that will turn on the well pump at the low setting and off at the high.

    This system has a 60/40 switch, so it kicks on the pump when pressure drops to 40 and turns off at 60 psi.

    This allows about 40-50 gallons of water to flow before the well kicks back on to refill the pressure tank.

    Some people have several tanks connected, to try to get entire showers (150 gallons) without cycling the well pump. My next house will have multiple pressure tanks.
     
  13. eduncan911

    eduncan911 New Member

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    Yep, drip continues. But I think it's on the cold side, not the hot side that I'm testing. Unfortunately the valve is one of those combination valves with a single lever that turns on/off both cold and hot outlets for the washer.

    FYI, the WH I just installed is an ondemand electric. No expansion or leak or anything. It's just copper tubing cycling through a few tubes.

    Here's a pic of exactly my Rehm (input is on the bottom right, output is at bottom left).

    [​IMG]
     
    Last edited: Aug 14, 2019
  14. Reach4

    Reach4 Well-Known Member

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    Looks small. How much power?
     
  15. eduncan911

    eduncan911 New Member

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    It's actually the largest they make in this home segment, 36kWh. Enough to heat 5x simultaneous showers, dish washer, and clothes at the same time. And yep, it's a fraction of the size of say a gas-fed unit.

    Uses 4x 40 AMP 220V circuits. Yep, you read that right. 160 AMPs across 4x 40 AMP breakers, in a 200 amp panel. Had to re-wire half the panel to make room for it. That's 8x 8 gauge wires at the right there that I had to run to the panel.

    It's known as a "4-element" unit where each element is about 9 kWh. Due to the radiant flooring of this 3600 sq ft home, and the need for 3x showers, we were borderline on the 27 kWh unit (3x elements). Went ahead with the larger one so we'll never be starved.

    It's all variable using PWM with up to 255 steppings per element. IOW, it only uses just enough energt to get the temp up to the setting (130°) and no more.

    So far I've measured only 29 AMPs to go from 75° pressure tank temp of water to 130° at 5.5 gpm. I havent finished calibrating the power monitors though so it could be off a bit.

    A bit more about the house, in case you were wondering... Next spring (don't have the funds this fall), I'll be installing either a solar heating tank or solar panels. The tank would be free heat year around, where the panels would give us electric credit. I want to wait a while to see the bills first before I decide on ROI.

    The radiant system is an Open Primary Loop system. Meaning, fresh water always enters the radiant floors first when a hot tap is open. This gives me about 130 gallons of hot radiant water under our feet as kind of a water tank. The fresh water ensures no stagnent water year around growing bacteria. This pre-heated floor water enters then enters the water heater, where it just bumps the temp up to where it needs to be, before leaving to enter the house.

    IOW, the heater is at the end of the manifold and pre-heated water is only heated at the end of the loops, where little energy would be used to bump the temp up before entering the house. Radiant loops are only heated as rooms call for it, and each loop being about 40 or so gallons of all circuits, so not much to re-heat when called on and cycles.

    The "primary loop" portion means it cycles the hot water produces by the heater in a very small 4 foot loop of 1.25" copper pipe. This is so the heater is only working to heat a tiny fraction of the water in that tiny loop. Once heated water enters into the heater again, the heater has nothing to heat. Then the floor pumps slowly mix the old 90° floor water in with the 120° water to increase the heat. So the heater always sees pre-heated water, which further cuts down on heating costs.

    Pretty excited to see the electric bills this winter so I can decide exactly if panels are worth the investment, or to go with a solar heating system and oil tank to preheat the water. Well, not excited to pay the costs... But to understand the exact ROI for this house and location for other homes I plan on renovating. Would love to get a wood burner but local building codes won't allow it.
     
    Last edited: Aug 14, 2019
  16. wwhitney

    wwhitney Member

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    Location:
    Berkeley, CA
    That's 36 kW. A kW is a unit of power (rate of energy use/transfer), and a kWh is a unit of energy.

    1 BTU will heat 1 pound of water 1 °F. 1 gallon of water weighs 8.35 lbs, and 1 BTU = 0.293 W-hr. That means

    55 °F * 5.5 gpm * 8.35 lbs/gal * 60 mins/hr * 1 BTU/lb-°F * 0.293 W-hr/BTU = 44 kW.

    But 29 A * 240 V = 7 kW. So something is off .

    Cheers, Wayne
     
  17. eduncan911

    eduncan911 New Member

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    Northeast
    Holy crap, I knew I was in the right place! been wanting to create that formula, thank you!!

    Ah, I see it is the difference in temp for 55° is of heating. Humm, that exceeds the max rating of the unit though.

    You know, that could explain why I am only seeing 120° in the outlet gauge (the unit is set to 130 so I was assuming some heat loss).

    45 °F * 5.5 gpm * 8.35 lbs/gal * 60 mins/hr * 1 BTU/lb-°F * 0.293 W-hr/BTU = 36 kW

    Nevermind the amp meters (it was a raspberry pi project). This has me concerned over winter months when that room may drop in temps well below 75° that is now summer. Given the radiant floors should be mixing in 90-110° water for the tap demands, that can easily be delutted with a couple of back to back showers.

    Looking at my RPi setup, I think I found the issue. The current transformer leads I'm using are 40 AMP max; so, I only wrap it around one of the two poles on the breaker box for each of the 40 AMP dual-pole breakers. The idea was to multiple x2, in order to get the correct amp reading for fixed-loads that are only 3-wire 220VAC (this would not work on 4-wire 220VAC because the neutral would allow each circuit to be variable, so you need to monitor both circuits, like for wall ovens and 4-wire clothes dryers, etc).

    I do this for all breakers in the box, including all 4x 40 AMP ones for the heater.

    It looks like I was only averaging the values. :(. Which makes sense to read 29 A with a fudge factor to recalibrate to 40 A under full load.

    I just wasn't totalling all 4 breakers, but averaging in my software. Whoops. Now that I use Total (SUM), it reads a whopping 148 AMPs. Ouch! Still need to calibrate but that makes much more sense now.
     
    Last edited: Aug 16, 2019
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