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efects of presure too high

DrHeat Member Posts: 5
I need to explain to a customer why a hot water system should run at 12 to 15 psi. If you have a good concise answer that does not start with "the manufacturer says" It would be helpful.

Here are the particulars. Another contractor has been servicing this account for three years. They have replaced the circulatory and have now proposed to replace seven zone valves. I first looked at the boiler last night to give a second opinion.

There are three 165,000 btu cast iron boilers running on the same plumbing.  with nine zones seven are solenoid controls off a B&G100 circulator   the others have there one pump.

When I first enterd the room I saw water stains on the floor near the three relief valves. I then saw the water pressure on all three boilers was 40 lbs I drained from a boiler drain and all the tridecators read 12 lbs.

My question for the team here is will high pressure lead to early fails on circulators and solenoids?


  • icesailor
    icesailor Member Posts: 7,265
    High Pressure:

    Does a bear poop in the woods?

    What does the system have for expansion control? That would need more than a few #30 Extrols. How many floors does the boiler service?

    Are the boilers all cold start? Warm start?

    Maybe the "other" contractor thinks he found a cash cow. Maybe he didn't.

    We LOVE photos. Send a few. What kind of zone valves are there installed. Are they are ones known to be leakers?

    Maybe somebody else doesn't understand the concept of water expanding when heated and if the pressure in a system is higher than required, it may expand enough to raise the pressure above the pressure relief valve. Are the boilers rated for 40#? If they are rated at 30#, why do they have 40# relief valves on them?

    Oh well, just asking
  • Gordy
    Gordy Member Posts: 9,546
    High Pressure

     Is the pressure high because the other service tech set it that way? Or is it because the expansion tank is undersized?

     As far as high pressure causing circs, and zone valves to fail early. I would say no to the zone valves, and no to the circ. Unless the weeping off the relief valves is causing a lot of make up water to be taken in to the system frequently. Most zone valves, and circs are rated at higher than 40PSI.

     Higher PSI can keep air in solution in the system. But then that is a band aid for proper air removal to begin with.

  • DrHeat
    DrHeat Member Posts: 5

    I think the pressure is too high because someone added water through the bi pas.

    The reliefs are 30# but they seem to hold till the gauge is at #40

    The compression tank is a bladder tank it is quite large it is bigger than the #90 I have in my shop but I cannot find any size on the tank.

    I am not a wet head yet Getting there I have been a scorched air guy for thirty years. Only been in the Hydro side for three years. So, I am confused about the effects of pressure on a system. It seems to me that apart from the obvious danger that pressure posses and the leaks  it would make little real difference. The pressure is the same everywhere  both sides of the circ both sides of the solenoid etc. If someone can clarify this it would be very helpful.

    What  difference  would it make for example if I run five pounds instead of twelve? 
  • Gordy
    Gordy Member Posts: 9,546
    edited November 2011

    You need pressure to raise the water to the systems highest point from the boiler. So you take how many feet to the highest emitter from the systems fill point divided by 2.31 plus an extra 4 psi.  So if its a 2 story home baseboard emitters with the boiler, and its fill point in the basement. Take 16' / 2.31=7.5+4 to get it over the top= 11.5 psi. 

     12 psi is the ambient fill pressure NOT while the system is operational. The pressure on a bladder tank should be checked with the tank isolated from the system.  Usually this is sufficient for a 2 story home.

    Circulation is created by pressure differential of the circulator, not psi in the system. But most circs need at least 4psi to operate.  PSI in the system is there to raise the water to its highest point. So if you only ran 5 psi in a system with a high point of 16' you would only raise the water to the first floor level. To low of PSI will create cavitation at the circ. No flow through the emitters at the highest level. and disolved air will come out of solution.

    May I suggest visiting Dans library. Lots of good reading material on old, and new systems there.

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