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Thread: Pressure Issues

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    Talking Pressure Issues

    I have two boilers that are fed from the same supply. The feed has an automatic fill and a back flow preventer. Now here’s the question; the pressure balance isn’t the same on both boiler. The boilers are approx 380,000 btu and a 60,000 btu. The smaller boiler is always at least 10 psi higher than the larger one.


    I didn’t plumb the boilers but my first thought is that providing each boiler with its own separate fed would eliminate the pressure difference. By this I mean from the cold water supply two branches each with a automatic feed valve and back flow preventer to each boiler.


    Am I seeing this right or am I missing something else?
    TIA

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    if they're both calling for feedwater at the same time there will be a pressure drop. but this is only feed water NOT the actual system pressure.

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    HayZee,

    Thanks for the reply. This is when there is no call for water. The small boilers pressure is always 10 psi higher.

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    please check out maybe two to three threads before yours. Lazypup seemed to explain it better than I can. Has to do with the boiling point of a fluid in a closed system.

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    Boilers used for home heating fall into one of two categories. 1. Low pressure saturated live steam and 2. Low-pressure Hydronic (circulating hot water) boilers.

    Per code;
    Saturated live steam boilers are limited to a maximum working pressure of 15psi
    Hydronic boilers are limited to a maximum working pressure of 30psi..

    While the maximum operating pressure of the boilers will be either 15psi or 30psi it must be remembered that the house potable water supply pressure will be in a range of 35 to 80psi therefore if the house supply water were to feed directly into the boiler it would put the boiler in a very dangerous overpressure condition. One the other hand, the feedwater system does need to be capable of supplying water to the boiler under the full range of operating pressures.

    In order to achieve this goal the pressure-reducing valve on the house cold water supply line is set to reduce the house water pressure down to the maximum working pressure of the boiler. After the water passes through the Pressure Reducing Valve (PRV) it is then fed into the boiler by means of an internal “Float control” that regulates the water level in the boiler in basically the same manner as the float control in your toilet tank.

    In nature most elements or compounds expand when heated and contract when cooled, but water has a rather unique property. Water reaches its maximum density (62.4lbs/cu.ft) at 39degF. It must also be noted that in all three forms, Solid (ice), liquid (water) or gaseous (steam) water is not compressible.

    When water freezes and expands in a pipe or vessel it will literally rupture the pipe or vessel, which explains why we find frozen water pipes that are ruptured or why we must put anti-freeze in our automobile engines.

    When water is heated above 39degF it will expand until it reaches a maximum expansion of approximately 10% by volume, then it will flash to steam. When water flashes to steam it expands by 1728 times by volume, thus one cubic inch of water will produce one cubic foot of steam.

    In a steam boiler the water level at cold startup is at about ¾ the volume of the boiler vessel. The upper ¼ is then called the steam header. There is a pressure switch in the steam header. As the water is heated the water expands upward into the steam header portion until the water reaches maximum expansion. Once it reaches maximum expansion the water will start flashing to steam, which then fills the remaining portion of the steam header. There is a pressure switch in the steam header that senses the pressure and turns the burner off when the steam reaches a preset pressure limit.

    In a Hydronic boiler the water vessel is completely full of water and the working pressure is actually controlled by a thermostat that senses the internal water temperature. We must then install an external “Expansion Tank” which has an air chamber inside. As the water expands it enters the expansion tank and the trapped air inside compresses to absorb the expansion.

    To answer the original question. Why is the pressure different on your two boilers?

    Answer: The actual pressure in your boilers is a direct result of the temperature of the water in the boilers and has absolutely nothing to do with the PRV. The difference in pressures between the two boilers is directly proportional to the individual boilers rate of cooling between fire cycles.

    CAUTION..Due to the sensitive nature of boiler controls and the potential hazards of improperly setting them, no one other than a trained professional should ever touch the boiler controls.





    .

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    Maybe a simple rephrasing of the question is in order. Should each boiler have it's own water supply with AFV and BFPV?

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    Providing that both boilers are of the same type and operating at the same maximum working pressure there is no problem with supplying them from a common manifold and feedwater pressure control.

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    LP,

    Here's the issue; The larger boiler operates at 22psi when firing due to it having 5 zones and one zone is located 26' above the boiler.

    This causes the smaller boiler to operate at above 30psi when fired. How can I maintain a 12psi cold state on the smaller boiler?

    TIA

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    Quote-“The larger boiler operates at 22psi when firing due to it having 5 zones and one zone is located 26' above the boiler.”

    First of all, let me make it perfectly clear that pressure is NOT the propelling force in a circulating hot water heating system, therefore it does not matter whether your system has 5 zones or 500 zones, the pressure would remain the same.

    That may seem difficult to understand but let us begin by examining how pressure propels water through a pipe. First of all, examine the face of your pressure gage closely and you will see that it is calibrated in PSI/G (pounds per square inch /gage). Keep in mind that the Earths atmosphere has a standard pressure of 14.7PSI at sea level. A gage that is calibrated in PSI/G is calibrated to read zero PSI at standard atmosphere and it indicates PSI above standard atmosphere. If a gage on you water line indicates 35psi/g it is telling you that the water in the pipe is at a pressure of 35psi greater than standard atmospheric pressure. If you then open a faucet on that line, the faucet is open to atmospheric pressure and you have a pressure differential of 35psi, which pushes the water through the pipe as nature attempts to reach equilibrium.

    Such is not the case in a circulating hot water heating system. A circulating hot water heating system is a totally closed loop, therefore no matter what the working pressure is, that pressure will remain constant at all points in the system, thus we have no differential to push the water.

    This then leaves us with two questions:
    1. What causes the pressure in a closed loop system?
    2. What propels the water through the system?

    If you will examine the attached illustrations you will see that there is an expansion tank above the boiler. When the system is initially filled the isolating valve to the expansion tank is closed and the air bleed valve on the top of the vertical riser is opened. The system is then totally filled with water. Once the system is filled, the air bleeder is closed and the expansion tank-isolating valve is opened. (CAUTION: The Expansion tank isolating valve absolutely must remain open when the system is in use).

    The moment that you open the expansion tank-isolating valve you will note a slight increase in the internal pressure. This is because water physically weighs 0.434lbs per vertical foot inch column, thus when the expansion tank-isolating valve is initially opened some of the water in the system will enter the expansion tank and slightly compress the air in the tank until the air pressure is in equilibrium with the physical weight of the water. As the water drops back down to the expansion tank this will actually leave a slight vacuum in the top horizontal line.

    You stated that your larger boiler has a 26’ vertical riser. If we then multiply 0.434psi per vertical foot x 26’ we get 11.28psi. (I suspect this is where you got the 12psi figure that you are looking for on the smaller boiler).

    As was mentioned previously, water reaches maximum density at 39degF and when heated above that temperature it will expand at a uniform rate until it reaches approximately a 10% expansion at the boiling temperature.

    Now here is where it gets a bit complicated. Everyone knows that water boils at 212degF but in fact, that is an incomplete statement. Water boils at 212degF at standard atmospheric pressure however if the pressure is increased or decreased there is a proportionate increase of decrease in the boiling temperature. By example, in a perfect vacuum (-14.7psi/g) the boiling temperature of water is 40degF. At 30psi, which is the maximum allowable working pressure for a Hydronic boiler the boiling temperature is 267degF. From this we can conclude that if a 60gal boiler were heated to 267degF the water would expand by 10% or approximately 6gal. If we fire the boiler to the standard 180degF the expansion is then 180/267 = 0.67% of the maximum or 0.67% of the 6gal, thus at 180degF the actual expansion of the water would be approximately 4gal. As the 4gal enters the expansion tank the air in the tank is compressed upwards thus the pressure will increase proportionately. In this case, the actual pressure would be the result of the total volume of the expansion tank and the amount of compression required to absorb the volume of the expanded water. From this you can easily see that if the expansion tank were critically sized to the volume of expansion the working pressure would be high, whereas if the expansion tank were larger the pressure would remain lower.

    Now, if we note that the pressure is mysteriously going higher than normal this is a good indicator that the boiler is slightly overfilled. In this case the first step is to turn the burner off and allow the boiler an hour or so to cool, then close the expansion tank isolation valve and open the expansion tank drain valve. This will drain the excess water off the system. Once the tank is completely drained you close the drain valve and open the isolation valve, then return the burner control to normal operation.

    Now let us consider some of the factors that might cause an over-pressure condition:
    1.There could be a pinhole leak in the upper part of the expansion tank that is allowing some of the air to escape.
    2.There could be a problem with the feedwater control that is allowing excess water to enter the boiler.
    3.If you have a diaphragm type expansion tank it may have a defective diaphragm.

    What is the propelling force for water circulation?
    Basically there are two types of circulation in a hot water circulation system, gravity or a circulation pump. A circulation pump system is basically self-explanatory, there is a pump on the circulation line that kicks on when the burner fires or is thermostatically controlled by the temperature of the circulation pipe.

    Although much less efficient than a pump system, never the less, there are many Hydronic heating systems that use a gravity flow system. As was stated before, water expands when heated, therefore it would stand that the specific weight for a given volume of hot water would be less than the weight of the same volume of cold water.

    Again look at the attached illustration. From the illustration you can see that some of the heat energy in the water is released as the water passes through the finned radiator, thus the water on the return drop on the cooler side is slightly heavier than the physical weight of the hot water in the riser on the right side. Even though the actual weight differential is quite small, nonetheless, it is sufficient to cause a circulatory motion in the system although it is only marginally efficient.
    Last edited by LazyPup; 03-14-2008 at 09:56 PM.

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    LP,


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