Pressure drifting over 30psi

Discussion in 'Tankless Water Heater Forum' started by JackTownsend, Jan 1, 2019.

  1. JackTownsend

    JackTownsend New Member

    Jan 1, 2019
    Hi There and Happy New Year!

    We've got an oil fired Slant Fin Intrepid installed 2016. It's providing forced hot water baseboard heat for our 1400sf house and hot water with a tank-less coil setup.

    A month ago, the pressure relief valve started to leak. I figured it was a bad valve and finally got to replace it last weekend with a brand new 30psi watts valve. In the week since, the new valve has continued to leak about a gallon a day. This prompted me to look at the pressure more closely and I'm noticing that it's at about 20 most of the time, but then it will occasionally drifts up over 30,...and whooshe,....the valve does it's job and a quart of fresh hot water blasts out of the pressure relief valve into the bucket.

    Last night my wife complained that she couldn't get enough hot water for her shower.

    Any advice you can offer is appreciated.

    I'm in Massachusetts by the way.

  2. Dana

    Dana In the trades

    Jan 14, 2009
    Unless the radiator at the highest elevation is 40 feet above where you're measuring the pressure you don't need 20psi. Most 1-2 story houses should be set to 13 psi, some might need 15 psi. Measure that vertical distance on your system, multiply by 0.433 psi, then add 3psi for where you SHOULD be running the system.

    The expansion tank needs to be pre-charge with air to the target system pressure -0 psi/+3psi. This can only be done properly when there is no pressure on the water side of the tank.

    Tankless coils lime up over time and in hard water zones that can happen in less than once per year. They can be delimed/descaled to improve both flow and heat transfer efficiency. Tankless coils can also develop pinhole leaks, causing the system pressure to slowly rise over time as water flows from the potable side to the system side through the leak. That would be rare but not impossible on a coil this young.

    After resetting the system to a more appropriate pressure and properly pre-charging the expansion tank, turn OFF the isolating valve that's usually between the auto fill/PRV that adds water to the system. Auto-fill valves can seep if worn or if grit gets into the valve seats. With the isolating valve turned off after the system is up to pressure that seepage stops (unless the isolating valve also leaks.) Then monitor the system pressure daily to see if it's rising over time. If it's slowly falling there may be a leak out of the system to chase down, if it's rising over time the pinhole leak in the tankless coil becomes the prime suspect. It's probably still under warranty, but don't swap it out "just in case"- be pretty sure that it's the source of the problem first.
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