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Why not run city pressure in a hot water boiler

My techs ask me why not run city pressure in a hot water boiler. I respond by saying because you don't have to. You can lift water three stories in a residential setting with 15 PSI. Why is it restricted to 30lbs. max? My question is this. Is there any other reason to keep the pressure as low as possible?

Comments

  • Zman
    Zman Member Posts: 7,435
    The rating of the boiler is the obvious reason. Residential models are only designed for 30#.
    Lower pressures are safer. Water boils at 212 f under atmospheric pressure. That boiling temp goes up rapidly as pressure increases.
    Your car radiator probably has a 12# cap. If you have ever taken the cap off an overheating car, you know how much energy is stored in the superheated water under pressure. Now picture a 60# boiler with faulty high limit controls.
    Ever watch myth busters?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    icesailorBobbyBoy
  • icesailor
    icesailor Member Posts: 7,265
    Zman said:

    The rating of the boiler is the obvious reason. Residential models are only designed for 30#.
    Lower pressures are safer. Water boils at 212 f under atmospheric pressure. That boiling temp goes up rapidly as pressure increases.
    Your car radiator probably has a 12# cap. If you have ever taken the cap off an overheating car, you know how much energy is stored in the superheated water under pressure. Now picture a 60# boiler with faulty high limit controls.
    Ever watch myth busters?

    Tell your helper that water boils at 244 degrees when under 12# PSI. If the pressure drops to 10#, the water suddenly wants to occupy 1200X that as steam.

    A very bad thing.

    (Square root of the pressure in PSIG X 14 + 198 equals the boiling point of water)

  • hot_rod
    hot_rod Member Posts: 18,268
    and the size of the expansion tanks grows with higher operating pressure.

    Many of the combined DHW, hydro air systems run on incoming water pressure, the appliance is rated for the pressure, and it has a T&P type safety valve.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • j a_2
    j a_2 Member Posts: 1,801
    Those techs should already know that answer….If they want to get ahead in this business tell them to READ...
  • j a_2
    j a_2 Member Posts: 1,801
    There are no stupid questins from home owners…But there are stupid questions , from ones who call themselves TECHS.. They need to be downgraded to helper...
  • Tinman
    Tinman Member Posts: 2,797
    edited February 2015
    One of my daughters is a school teacher and I taught for a short time too and if I learned anything from that experience is that different people learn in a different ways at different rates. The fact that they WANT to know is the important thing in my humble opinion.
    Steve Minnich
    Rich_49
  • Rich_49
    Rich_49 Member Posts: 2,708
    Ja,

    As Stephen has eluded to above , I agree . there are different levels of experience and stating that someone should not be called a tech and downgraded to helper is STUPID . Hell , I have been performing this work for over 30 years and I still don't know it all . But guess how I moved from tech to the tech I am now , I'd guess by asking stupid questions , if there is such a thing . people looking at me as if there were monkeys on my face was a quite prevalent thing for a number of years some time ago . I have some advice for you also , Keep reading and listening .
    Folks questioning why the standards are what they are and state what they state is what will ultimately save this industry . Please don't discourage anyone , we need all the techs we can train .
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    732-751-1560
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
    Tinman
  • rick in Alaska
    rick in Alaska Member Posts: 1,396
    Not sure about all cast iron boilers, but I know Weil Mclain boilers are only rated at a maximum of 50 psi. The relief valve is usually set at 30 psi. So, you do not want to run past those limits.
    Rick
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I used to have friends who operated steam railroad locomotives. These were small and ran at only about 150 PSI. That is about 360F.

    (At 40 psi, that might be a typical domestic water pressure, the boiling point is around 267F. My guess is that a pressure cooker runs at about 15 psi Gauge pressure, so the BP would be around 250F.)

    At the bottom of the boiler is a "blowdown" valve so you can drain off the solid crap that accumulates down there. You open the valve when the engine is at normal pressure.

    I saw them do this one day, and it was quite a surprise. Imagine a big tank, about 4 feet in diameter and 20 feet long, mostly filled with boiling water and 150 psi of steam above it. I expected a jet of crap and boiling hot water to shoot out the pipe. But no water came out at all, just steam that almost instantly turned to water vapor. (Steam is invisible; water vapor is that white stuff). The reason why no water comes out is because at atmospheric pressure, the boiling point is 212F around here (near sea level), but the water is about 360F, so it flashes instantly to steam. And the pressure in the boiler does not go down until the water in there cools down. That is why exploding a 150 psi steam boiler is a much greater disaster than a similar size tank with 150 psi air in it. With air, the pressure drops almost instantly. And the air does not burn the meat off the bones of people.
    Rich_49