Illinois DSH pressurizing hot water pipes

Discussion in 'Maintenance and Troubleshooting' started by illini150, Nov 15, 2020.

  1. illini150

    illini150 New Member

    Hey guys... not a Geothermal expert (inherited a Geocomfort system when I bought my house) but trying to learn and need a little help with my DSH. My system has an unpowered buffer tank that ultimately feeds a NG tankless water heater. Both the tank and the tankless were about 15 years old and the tankless was starting to act up so I had my plumber replace them both. The new tank is a standard AO Smith 40 gallon electric (not wired) and the new tankless is a Rinnai RUR199in - everything piped exactly as before.

    Had no issues to start but we were in between seasons so the system was not running much. Now that we're running heat a lot more often, I am noticing pressure in the hot water pipes when I first turn the shower on in the morning. The pressure relief on the tankless is also opening and leaking on the floor. This was not happening with the old setup.

    I've bled the system and not seeing any signs of air. Also talked to my local service/installer and he didn't have any ideas nor did he seem very concerned.

    I've read a few times about a part that needs to be removed on the cold water inlet to the buffer tank but I have not been able to track down any specifics - could that be the issue? What needs to be removed? There is a check valve on the cold water side before the DSH/buffer tank tee, just as before.

  2. SShaw

    SShaw Member

    Hot water expands significantly. Do you have an expansion tank plumbed into your buffer tank? It's code in most places to have a small expansion tank on a WH.
  3. geoxne

    geoxne Active Member Forum Leader

    SShaw is correct. Old school plumbing counted on the thermal expansion for a water heater to be released back to the cold water supply (either city water or well water expansion tank). Nowadays codes do not allow backflow so the chances of having a check valve in the supply system is great. This requires a potable water expansion tank sized properly for your system. It should be installed on the cold water supply before the hot water tank and after any check valve or shut off valve.
    You mentioned you had a check valve before perhaps it was leaking and allowing pressure relief.
    This is probably unrelated to your thermal expansion. Most modern water heaters have "heat trap nipples" at the cold inlet and hot outlet. These are provided as an energy saver to prevent thermal migration. There is usually a plastic insert with a flap (similar to a check valve but they are not positive stop) that must be removed from the cold water inlet in a typical system with a DSH. This allows the DSH pump to draw water from near the bottom of the tank through the cold water dip tube.
  4. illini150

    illini150 New Member

    I do not have an expansion tank. There wasn't one before the replacement either which is confusing because we had no issues prior to the replacement. The check valve I mentioned on the cold supply was not replaced when the water heaters were replaced - same as before so I don't think it was previously leaking and relieving the pressure.

    I am pretty convinced that I need to install an expansion tank on the system. Normal pressure is right around 65psi but after the heat runs overnight with no hot water usage, the system is at close to 140psi!

    I plan on going with a Therm-X-Trol ST-12 but need to make sure I put it in the right spot. I've seen some diagrams on here that place it on the hot water side and other information states that it should be on the cold, after the check valve. Cold side makes most sense to me as that is where it normally is on traditional hot water heaters. Should it be before or after the tee going to the inlet of my DSH? Also plan on replacing my check valve, just in case.


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