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    What is the safety zone in PSI for boiler pressure (and temp if important) for hot water Boiler heating system? (14 Posts)

  • Rocco Rocco @ 6:40 PM
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    What is the safety zone in PSI for boiler pressure (and temp if important) for hot water Boiler heating system?

    What is the "safe" operating pressure in PSI for a hot water boiler heating system, please specify high and low pressures. Also what is the "safe" range in term of Farenhite Degrees for boiler operation?
  • Ironman Ironman @ 7:02 PM
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    Safety Zone...

    The highest pressure the boiler should ever see is stamped on the boiler tag - Usually 30 or 50psi.

    The pressure the system is designed to operate at is usually 12 - 25 psi for a two story house and 18 - 25psi for a three story.

    Operating high temp should not be over 200deg, but 180 is normal max design temp. There are are alot of variables depending upon the type of system and boiler you have and many times the operating temp can be much lower.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • icesailor icesailor @ 10:35 PM
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    Boiler Water Pressure

    The water pressure in a hydronic boiler shouldn't be over 12# to 15#. It should have only enough pressure to raise the water a few feet over the top of the highest heat emitting unit on the system.12# is 28' above the gauge or fill valve. 15' is 34. At 12#, the boiling point at the fill valve/gauge is 246 degrees. At 28', it is 212 degrees. If the height of the water isn't high enough from lack of pressure, the suction pressure on the loop may cause the first water to boil that comes from the boiler on a warm start boiler. If you set the system pressure higher than needed, you decrease the amount of room for system expansion. If you have a one story ranch with a basement, 12# should be all you ever need. A two story Cape, shouldn't need more than 15#. A Cape is usually 34' from the ground to the peak of the roof. 
    This post was edited by an admin on December 26, 2010 10:37 PM.
  • Jean-David Beyer Jean-David Beyer @ 7:28 PM
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    You must read the manufacturer's specifications.

    Most residential hot water heating systems run at 5 psi more than the pressure required to raise the water from the pressure gauge to the highest point in the system. In my Cape Cod house with the boiler on the floor of my garage, the pressure gauge reads between 12 and 15 psi (dial too small to read more accurately). The maximum pressure I am allowed it to get to is 30 psi, where the pressure relief valve will open. I believe this is typical of residential hot water boilers.

    As far as temperature is concerned, my boiler will shut off if the temperature exceeds 205F (if I remember correctly) THey do not watn the watre in there to boil ever.

    Of more importance is the minimum return water temperature. With a conventional boiler, you do not want condensing, so the return temperature should niormally be over 140F to prevent the boiler from rusting out and the chimney from failure. With a condensing boiler, you want the return water as cold as possible, short of freezing, to get maximum condensation and the resulting higher efficiency.
  • pressure pressure @ 5:05 PM
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    Boiler pressure too high

    I have had a problem for a couple of days with my boiler.  It is leaking from the safety pop off.  I have replaced the expansion tank the safety pop off and the automatic fill valve.  The pressure runs close to 30 psi while heating.  The water is heated to 180 before the boiler shuts down until it needs to reheat the water.  After it shuts down it somehow continues heating to around 190.  If there isn't any call for heat from one of the thermostats while the water is at temperature the pressure builds until the pop off either leaks or blows like it is designed to do.  I had a contractor add a loop to my garage.  It is a unit that hangs from the ceiling with a fan behind it.  This was added in December and I haven't had a problem until now.  How do I get the pressure back in line where it is supposed to be?
  • Mark Eatherton Mark Eatherton @ 7:36 PM
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    Does your boiler also produce your DHW? If yes, you could have a leaking heat exchanger in your DHW tank.

    If no, and the system pump is pushing water towards the expansion tank connection, AND the make up is connected to the inlet side of the pump And it's a high head pump AND flow is restricted, you need to move the make up connection to the expansion tank.

    To the original poster, a safety factor of 90% of rated pressure would be considered the normal maximum. So if its a 30 # valve, then 27 would be the normally acceptable maximum.

    Why do you ask?

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • pressure pressure @ 9:44 PM
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    Boiler pressure too high

    This system has been in for 15 years.  The only thing different is the loop in the garage.  The water needs to pump higher than before because it is a ceiling unit.  Would this increase the pressure?  If the DHW had a leak would the hot water be hotter than the thermostat would call for or be rusty?What is the best way to check for a leak?  Would I shut off the valves to this zone and unhook the thermostat?  Is it possible to have too much water in the boiler?  Thus leaving too little room for expansion?  Or is the boiler always completely full because of the automatic fill valve and all the expansion done in the expansion tank?
  • Mark Eatherton Mark Eatherton @ 9:55 PM
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    The fill pressure is typically 1/2 PSI per vertical foot of system elevation above the boiler, plus 5 PSI.

    A qood overnight test would be to turn off the cold water to the DHW tank, turn the aquastat on the tank down, and watch the system. If the system rises overnight then you need to start looking at the system configuraton as I pointed out earlier. Don't forget to get up early enough to turn the DHW tank back on and up so you get hot water for showering.

    All expanding and contracting fluid goes to the expansion tank. It is possible that the diaphragm on the tank has failed, and you have no expansion tank, but that is rare. WHat kind of expansion tank do you have? Ceiling hung, or smaller one hanging off a pipe?

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • icesailor icesailor @ 11:30 AM
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    Replaced Expansion Tank:

    When the expansion tank was replaced, was it replaced with the same rated tank or was it (perhaps) replaced with a smaller, cheaper one?
    Asking some "Experts" might tell you need only need a smaller tank when in fact, you need a bigger one. Or at least one that is equal. Especially at on-line or big box stores.
    I've seen a lot of problem systems with Expansion Tanks too small. I've never found a problem system with a expansion tank that was too big. Where the larger tank was the cause of the problem.
  • Jean-David Beyer Jean-David Beyer @ 1:47 PM
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    I've never found a problem system with a expansion tank that was too big.

    Other than the waste of money, what is the problem with too big an expansion tank? Is it like having too much venting on a steam main?
  • icesailor icesailor @ 3:20 PM
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    That;s true:

    That's true.
    But it didn't make the Relief valve blow or any of the other symptoms.
    I would say that once cold start oil boilers came into vogue, and they still used #30 Extrol tanks on the cold start's where they always used #30's on warm starts, they should have been using #60's because of the higher water expansion rate. If that makes sense.
    I went to a Greet & Eat at the supply house that Amtrol put on. They gave us this little wheel calculator. According to the calculator, almost every Extrol installed was undersized. Especially on cold start boilers. That application is a cold start application with less water content than a normal boiler.
  • pressure pressure @ 7:06 PM
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    Boiler finally working

    I seem to have fixed the problem.  After replacing the expansion tank going from a 30 to a 60, the safety pop off and the automatic fill valve all to no avail, I had to try something else.  I would drain the boiler down so the pressure was around 20 and the next time it ran it would go over 30 again.  I took the automatic fill apart and adjusted it down.  I am not sure how much range there is in the adjustment, but I eventually had to go all the way down.  I drained the boiler down again and my pressure has stabilized at 25 lbs.  So in answering my previous question there is such a thing as having too much water in the boiler.  It seems that the city water pressure must have gone up above anything I had received in the past, therefore causing my leak.  I intend to buy a water pressure gauge to see what I have.  It seems I spent a lot of money for something that was out of my control.  If I have any more problems I will have to install a water regulator in front of my automatic fill valve.  I am generally lucky to have any water pressure to speak of as I am at the end of the line.
  • icesailor icesailor @ 7:21 PM
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    Fix It:

    There is almost nothing that I will not try to fix. I've tried to fix boiler fill valves. I've never been successful. Watts 1156F pressure reducing valves are cheap dates. If you're going to replace any gauges, start with the Tridicator gauge on the boiler. Or buy a pressure gauge and adapt it to a hose fitting and put it on the boiler somewhere. If you let water out of the boiler and it registers 8#, the fill valve should automatically fill it to 12# to 15# without any help from you. It is automatic. If it is a cast iron valve, replace it with a brass one. If you get scuzz between the seat and the washer, it will leak bye. There are some who say to never leave them on.
  • M Lane M Lane @ 11:41 PM
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    Simple answer

    To your question is the MAWP rating on the boiler. Maximum allowable working pressure.
    Ideally someone chooses a boiler with the right MAWP for its application.
    Basic design guidelines:
    * Like Mark said, you need 1/2 lb. per foot of elevation water pressure in order to lift the water vertically. 5 lbs. per floor. A 10 story building needs 50 psi minimum to assure the water reaches the top floor. We might add a few pounds to make sure there's enough to add some static to the radiators so that the bleeders work better.
    * Accordingly, the boiler's MAWP then must be greater than 50 psi. This is primarily reflected with the relief valve.
    * The system's expansion tank must have an equal air charge to the operating pressure. Check it with a tire gauge at the schraeder valve.
    * The make-up water assembly then needs a PRV that can be set at 50 psi.
    * For baseboard, we usually default operating temp. (setpoint) to 180*. It can be as low as 160 or as high as 200, but your are pushing limits and risking damage an/or injury. Below 140 the boiler will start to condense, with a non-condensing boiler this will ruin the heat exchanger. Here in Denver, above 212 you get steam. But it wont be steam until it is exposed to atmosphere. Which is what happens if the relief valve lifts or something blows apart.
    * One of the safeties with the boiler is the high limit switch. We normally set it 40 degrees above setpoint; or 220 with a 180 degree setpoint.

    Edit- I started typing before reading all the posts. Sorry if my post seems a bit rudimentary. you didn't mention the actual problem in the OP so I just went general principles.
    This post was edited by an admin on March 20, 2014 11:51 PM.
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