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.

Hi Rrekih:

Pressure, volume and temperature are all related to each other (I'll spare you the equations). Here, in a boiler system, if you have 20 PSI and then you heat the water to 190F the pressure could increase to say 25 PSI. With a pressure relief valve (all boilers have them) set to relieve pressure at 30 PSI you can see where a pressure exceeding 15-20 PSI would be a problem for the boiler system.

Further, boiler systems, radiators, etc. are simply not made to operate at pressures that exceed the 30PSI rating. Inevitably, if overpressurized these systems will fail at the boiler or the radiator sections.

Boilers are just big boxes of water and fire... low pressure is just another way to protect the homes from flooding or the boilers from failing.

Let us know if you want to discuss more...

MARK

PS: Don't forget to install a backflow preventer (see image) with that pressure regulator... keeps boiler water out of your drinking water... ;) They also sell a combination pressure regulator/backflow preventer (see other image).

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The check valve is installed in the heating box.
I am not using a boiler so that takes that out of the equation.
But the part about radiators etc. is a good point.
Heating the water to 190deg. Seems excessive, I will only being going to 140deg. Give or take.