Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Boiler Pressure

mmvpp Member Posts: 8
Hi, i have a 2 story house with a boiler, water baseboard heat and indirect water heater. The boiler's MAWP is 50psi and it has a 50 psi blowout valve.
People say to run the boiler at 12psi cold and less than 20 when hot. So would it cause any issues if the system is run at a higher psi? Are there any advantages of running at a higher psi e.g. 30psi? Thx.


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,506
    Why the 50 psi relief valve? Are you sure it’s not 30 psi relief valve?
    Should be a 30 psi, and you probably only need 12 psi cold.
    No way should you run that system anywhere near 30 psi.
    Fill valve, expansion tank (if bladder type), and system should all be the same presssure.
  • mmvpp
    mmvpp Member Posts: 8
    Looks like a 50 psi. 
  • mmvpp
    mmvpp Member Posts: 8
    Fill valve is 12 psi. Just out of my curiosity, so any psi between 12 and 50 is ok? Would higher psi do any harms? 
  • mattmia2
    mattmia2 Member Posts: 9,469
    the only reason you would want higher pressure would be to prevent cavitation and boiling if you were running higher than normal domestic heating temps or if you had a particularly tall system and needed the pressure to force the water to the top of the system on fill.
  • Steamhead
    Steamhead Member Posts: 16,736
    Check the rating label on the boiler. It will show the boiler's maximum allowable working pressure. For the record, I have seen hot-water boilers rated 50 lb. MAWP.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • BDR529
    BDR529 Member Posts: 280
    Those pressures will requier a ASME rated expansion tank as well.
  • Zman
    Zman Member Posts: 7,551
    Running the higher pressure has no real advantages and quite a few disadvantages. The risk of leaks is higher, you also need to be sure the expansion tank is rated for the higher pressure and sized correctly for the higher pressure.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • retiredguy
    retiredguy Member Posts: 888
    edited August 2020
    The minimum cold pressure setting is the pressure that you need + a safety pressure of 2-3 psi to expell water from the highest radiator or heat emitter in your home. 1 psi of pressure will hold a column of water up 2.31 feet. So for example, if you measure the distance from the top of the highest rad to the gauge and that distance is 23.1 feet then the cold fill is 10psi + 2-3 psi. A cold fill of 12 psi works for most 1or 2 story homes. Note that the boiler pressure setting has no affect on boiler performance.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,606
    @retiredguy nailed it. If your boiler is rated for 50 psi then is able to be installed in the basement of a 5 story building. If the top radiator was 50 ft above boiler pressure gauge you would need about 24 psi of "Static Pressure" in order to force the air out of the top radiators. 24 psi cold water is too close to the standard 30 psi rated for most residential boilers. The relief valve may discharge regularly in that case.

    Since you have a 2 story home you don't need the extra pressure. If in the future, you need to replace the relief valve, a 30 PSI would probably be just fine.

    Water pressure has no effect on performance in your heating system as long as it is at least high enough to get the air out of the top radiators (like 12 PSI Cold fill pressure)
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • The 50 psi relief valve could have been installed by someone who was having trouble figuring out why the 30 psi relief valve was dumping. 
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • mmvpp
    mmvpp Member Posts: 8
    Thx. Guys. I learned a lot. Appreciated.