What is the recommended pressure in a hydronic radiator closed circuit? By "closed" I mean there is a valve to fill it and then it can be closed, after which the circuit is not connected to the water supply. The system consists of a 130K BTU gas boiler and 8 differently sized radiators throughout the house.

I currently have 20 PSI. Should I adjust the pressure reducing valve to lower it?

  • Is that Absolute or Gauge pressure?
    – Solar Mike
    Oct 22, 2021 at 6:26

1 Answer 1


There is usually a pressure regulator in the form of a giant cylinder with a diaphragm and a bike-style air valve that you can pump up. The actual calculation depends on how high the highest radiator in the house is relative to the boiler.

One example of the calculation is here: https://www.slantfin.com/building-height-and-hot-water-system-pressure/ - they are a boiler maker, and this has generally worked well for me.

There is also a bell-shaped automatic water feeder / pressure reducing valve. Most hydronic systems start at around 12PSI cold and go up to 30+ when hot; having the original pressure too high will cause the T&P valve in your system to blow off excess water, and when the system cools down, the water feeder will send more water in. So you'll know that the water pressure is too high if you start seeing water on your floor when the boiler gets hot.

I've never seen the need to touch it myself, though most people do keep them at around 15 PSI.

  • 2
    No, that's an expansion tank, not a pressure regulator, which looks like a bell and is right next to another bell, which is a relief valve
    – amphibient
    Oct 22, 2021 at 3:00
  • If this does answer the question (and I don't know enough to know if it does or not), it is essentially link only. Copying the info here and pointing to the source is expected by the SE rules. Links go dead, companies rearrange their web sites, they close or get bought out, then this info is lost.
    – FreeMan
    Oct 22, 2021 at 15:06
  • 1
    @amphibient I updated my answer. But basically, if you aren't blowing water out of the relief valve, I wouldn't touch it.
    – gbronner
    Oct 22, 2021 at 19:41

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