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Thread: pressure and hydronic radiant heat

  1. #1
    DIY Member rrekih's Avatar
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    Default pressure and hydronic radiant heat

    From what I understand the pressure for hydronic radiant systems is about 12psi.
    What is the reason for this being so low?
    The pex pipe is rated much higher even at high temp.
    Does it have anything to do with the supply to the boiler?

    I have a tank less hot water heater system that is going to go into a heating box that supply's domestic hot water and the heating system, it is closed loop system with heat exchangers.
    I plan to install a pressure regulator on the supply to the heating manifold after the heating box so the domestic hot water will still run at about 60psi while I can drop the pressure to the heating system.

  2. #2
    In the trades Dana's Avatar
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    Default

    Quote Originally Posted by rrekih View Post
    From what I understand the pressure for hydronic radiant systems is about 12psi.
    What is the reason for this being so low?

    The pex pipe is rated much higher even at high temp.
    Does it have anything to do with the supply to the boiler?

    I have a tank less hot water heater system that is going to go into a heating box that supply's domestic hot water and the heating system, it is closed loop system with heat exchangers.
    I plan to install a pressure regulator on the supply to the heating manifold after the heating box so the domestic hot water will still run at about 60psi while I can drop the pressure to the heating system.
    It only needs to be high enough pressure to keep oxygen from entering into the system to create corrosion- any higher is overkill. But there are many waterproof materials that are gas-permeable, and constantly replenishing O2 into the heating system leads to early pump death and general gunk & rust. While 12psi gives you a bit of margin, in taller buildings you may need to bump it up to avoid low pressure higher floors.

    The higher the pressure, the more water you lose from any leaks (but we know heating systems NEVER develop leaks, right? ) Run it at 12psi and be happy.

    If the tankless was on the low pressure side of the heat exchanger, if the boiler loop pump pumps AWAY from the boiler you might have to raise the pressure to keep bubbles from forming on the heat exchanger (a combination of a pressure low point and high heat). While the sizzle & hiss from low pressure isn't dangerous, it does reduce efficiency.

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