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  1. #1
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    High Pressure in Domestic Water Heater During Recovery (100 liters)

    Hi All,

    Working on a job site (apartment 8-storey) wherein we have multiple domestic hot water heaters installed ranging from 50L to 100L storage capacities.

    In one particular apartment located on the 2nd floor we're facing an issue wherein the 100L domestic water heater is having pressure shoot up during it's recovery stage to approximately 8-bar which is causing the P&T valve to relieve itself. The P&T valve is operating as intended but for some reason the pressure spikes are rather odd and the relief valve is intermittently opening.

    We called the manufacturer onto the job site and they replaced the heater completely under warranty but the issue still hasn't been solved. We also tried changing the safety valve 2 - 3 times but that didn't solve anything either.

    The system parameters are as given below:

    1) Inlet pressure from mains to the water heater: 3-bar (the pressure is being maintained by a pressure reducing valve from the common water meter room on the floor).

    2) Inlet temperature from mains to the heater: 31 deg C (summer conditions).

    3) Outlet temperature when P&T valve opens up: 45 deg C (set point is 60 deg C, but we can't get it there because the P&T valve keeps opening).

    4) Outlet pressure when P&T valve opens up: 8.2 bar

    5) The piping used in the apartment is PEX and on the inlet and outlet lines of the heater we have water hammer arrestors and isolation valves. This particular heater is installed above the false ceiling in a washroom and it serves 3-fixtures - wash basin, standing shower and bathtub.

    6) The P&T valve is installed on the cold water inlet line of the heater and underneath is a drain pipe to collect the discharge.

    7) No abnormal whistling, gurgling sounds or noise observed during operation.

    8) There is no expansion vessel installed in the system.

    I've been studying the situation for a few days now but unable to come up with a better solution than installing an expansion vessel on the inlet. But just the idea of suggesting this seems rather odd because I've never heard of anyone installing an expansion vessel especially for 100L capacity, usually I see it more on the larger calorifier systems around 500L++.

    The problem to me sounded like thermal expansion and the flow being restricted from going the other direction which is causing the pressure to build up but if I were to use this logic the same problem should be appearing in all the units where the heater is installed.

    Not sure if anyone's come across something like this before but appreciate any and all advice if possible.

  2. #2
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    It really doesn't make sense..... every time I've had a tank overpressure it's been a thermostat issue.
    Could it possibly be that you have a bad stat on the new tank?
    Is there any way for you to get a temp reading from the safety valve body?
    My Motto lately is.... just because it's new doesn't mean it's good.

  3. #3
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    Why is the P&T valve installed on the cold water side? This being a small system it may need an expansion tank because the volume available for thermal expansion may not be enough even though you don't need much.
    No man can be both ignorant and free.
    Thomas Jefferson

  4. #4
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    Thread Starter
    Quote Originally Posted by Restaurant mech View Post
    It really doesn't make sense..... every time I've had a tank overpressure it's been a thermostat issue.
    Could it possibly be that you have a bad stat on the new tank?
    Is there any way for you to get a temp reading from the safety valve body?
    My Motto lately is.... just because it's new doesn't mean it's good.
    Thermostat issue in what sense? We're not achieving the set point temperature so I don't think the thermostat is faulty because the pressure is increasing but the temperature rise is barely budging.

    Would thermostat operation affect the pressure outside of the proportionate increase due to heating? I understand that temperature increase will lead to an increase in pressure but it seems the pressure in this case is sort of an exponential curve rather than linear if that makes sense.

    In the original tank + replaced tank we're getting a temperature rise of 10 - 15 deg C MAX but almost 5-bar pressure rise which is extremely odd.

    The temperature from the safety valve is approx 3 - 5 deg C less than the outlet/fixture temperature.

    I don't think the issue is the heater could it be our piping? Would the distance between this heater and the water meter room come into account?
    Last edited by jd1993; 06-30-2020 at 01:18 AM. Reason: quote

  5. #5
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    Quote Originally Posted by WAYNE3298 View Post
    Why is the P&T valve installed on the cold water side? This being a small system it may need an expansion tank because the volume available for thermal expansion may not be enough even though you don't need much.
    The arrangement of this water heater doesn't have the P&T valve connection on the body, there are only two connections inlet and outlet for this heater. So it's normal for us to keep the P&T valve on the inlet, it's also part of the manufacturer's O&M.

    Please see the attached photo of the heater connections for your reference. It's above the false ceiling so it's a bit difficult to get all the angles properly.

    Piping Arrangement Zoom Out.jpgInlet Outlet Connections.jpgIsolation Valves.jpg

  6. #6
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    If all the info you have given is accurate there is zero room for thermal expansion. A small expansion tank will solve the problem.
    One question though, assuming you are actually measuring the tank temperature where are you taking the reading? It seems to me with the P&T valve on the inlet pipe it is subject to being fooled. The heater on a small tank will come on the vast majority of the time while using hot water.
    No man can be both ignorant and free.
    Thomas Jefferson

  7. #7
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    Quote Originally Posted by WAYNE3298 View Post
    If all the info you have given is accurate there is zero room for thermal expansion. A small expansion tank will solve the problem.
    One question though, assuming you are actually measuring the tank temperature where are you taking the reading? It seems to me with the P&T valve on the inlet pipe it is subject to being fooled. The heater on a small tank will come on the vast majority of the time while using hot water.
    So we don't have any gauges on the line itself but in order to take the readings we take it from under the wash basin sink. We remove the flexible pipes under the sink and directly connect the gauges to the hot water and cold water lines (for checking pressure).

    Once the gauge hits around 8.0 - 8.5 bar on the gauge we can see the dripping start from the valve above the false ceiling.

    For taking the outlet temperature readings we just turn on the taps and use a digital thermometer to measure. For the inlet temperature in this particular we literally turned off the heating elements, ran the hot water from the tap until we were getting a constant value that wouldn't drop further which suggested this was the inlet temperature to the heater.

  8. #8
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    It's good that you are getting well thought out readings. The heater appears to be at the high point in the system. If it is even though you started out with an air pocket at the top of the tank water moving through the tank will slowly mix the air and water. Eventually the air in the tank will be gone. You don't need much room for thermal expansion but you do need some. I think the problem is you don't have any air which is why you need a small expansion tank. A small bladder tank will solve the problem.
    No man can be both ignorant and free.
    Thomas Jefferson

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