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  1. #1
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    P74 Sequence of Operation to Prove flow in HHW Sys.

    Can anyone briefly explain how a P74FA-5 proves flow to a heating coil for an air handler. Hot water is is modulated from a 3-way mixing valve, to the circ. pump and then to the coil. Pressure inputs are on the suction and discharge side of the pump. CO is set at 20 psi. The circ. pump is running and there is proof of run status. Alarm on EMS states no flow.
    Need a simple explanation of how the switch should work and be set. I believe the fixed diff. is 1.5.
    Thanks in advance.
    "Nearly all men can stand adversity, but if you want to test a man's character, give him power."
    -Abraham Lincoln

  2. #2
    Join Date
    Dec 2006
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    183
    http://cgproducts.johnsoncontrols.co...px?main.aspx&2

    http://cgproducts.johnsoncontrols.co...&hc=23&req=p74


    Pressure setting depends on how much pressure in system.
    We usually set them at the lowest pressure but enough to close
    contacts while pump is running.

  3. #3
    Join Date
    Dec 2006
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    183
    Sorry, I just seen where you stated The circ. pump is
    running and there is proof of run status but you have a
    no flow alarm. Do you have a point that states the
    pump status is ON, well what about the pump command
    Do they match?
    Other than that you;re no flow alarm may not be mapped correctly.
    Did it work before?

  4. #4
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    Quote Originally Posted by d_d_c_jci View Post
    Sorry, I just seen where you stated The circ. pump is
    running and there is proof of run status but you have a
    no flow alarm. Do you have a point that states the
    pump status is ON, well what about the pump command
    Do they match?
    Other than that you;re no flow alarm may not be mapped correctly.
    Did it work before?
    Useful links. Thank you.
    I'm certain that there is a point stating pump status as well as pump command. I'll check into to that to be sure. If there is, they should match.
    I'm new to the job site, so I am not certain if it did work before.
    I'll have to check the HHW loop pressure. With a switch setting of 20 psi, my loop pressure should be at or above 20 pounds, right? If the loop pressure is lower than 20, should I adjust the pressure setting down accordingly? I would think the loop pressure would be 20 or above. This is a three story building with six air handler units.
    I'll check to see how the no flow is set up too. I'm assuming that is what you are referring to as mapping.
    Appreciate the help. Again, thank you.
    "Nearly all men can stand adversity, but if you want to test a man's character, give him power."
    -Abraham Lincoln

  5. #5
    Join Date
    Sep 2007
    Location
    Michigan
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    Quote Originally Posted by whec720 View Post
    Useful links. Thank you.
    I'm certain that there is a point stating pump status as well as pump command. I'll check into to that to be sure. If there is, they should match.
    I'm new to the job site, so I am not certain if it did work before.
    I'll have to check the HHW loop pressure. With a switch setting of 20 psi, my loop pressure should be at or above 20 pounds, right? If the loop pressure is lower than 20, should I adjust the pressure setting down accordingly? I would think the loop pressure would be 20 or above. This is a three story building with six air handler units.
    I'll check to see how the no flow is set up too. I'm assuming that is what you are referring to as mapping.
    Appreciate the help. Again, thank you.
    If I'm following you, aside from the switch problem, your not sure the loop DP is correct? Open all your valves wide open, boost pumps running, and take your diffrental between the supply and the return where the control sensor is. check that against your DP reading on the sensor just to compare, should be close. Then set your control point to whatever your DP sensor is reading. If the system was balanced nothing would have changed, and you'll end up with the same number the balancer did, In theory. I know some building guys play with the control point to try to save on running the pumps. Have a 12 story building with 6 very large AHUs running a DP of 14, the size of the building really has nothing to do with the DP.
    "Yeah I can figure out whats wrong with it, but you were here first and there isn't room for two, plus it's cold up here, I'll be in the van"

  6. #6
    Join Date
    Apr 2007
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    little rock ak
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    so your saying that the size of the building doesnt matter
    then the more pressure drop the lower the dp setoint doesnt make any sense

  7. #7
    Join Date
    Sep 2007
    Location
    Michigan
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    You can't equate the size of the building to the DP. I'm not going to go back and forth, someone else can chime in here.
    "Yeah I can figure out whats wrong with it, but you were here first and there isn't room for two, plus it's cold up here, I'll be in the van"

  8. #8
    Join Date
    Sep 2007
    Posts
    71
    If you have proof of run status then the switch should be adjusted properly. Sounds like somebody mapped either the wrong status or command point to the no flow alarm. Make sure the status drops out when the pump isn't running.

    The DP switch is sensing not the system pressure, but the differential between the suction and discharge side of the booster pump. If that differential is less than 8psi then it's lower than the lowest setting on that particular switch. If the switch isn't working you may need to go with a P74EA-8C which is 2-26psi adjustable range. The P74FA-5C has an 8-60psi range. The 1.5 differential is how much the pressure needs to drop once the switch makes to change the switch state.

  9. #9
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    Western NY
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    Quote Originally Posted by foyler View Post
    If you have proof of run status then the switch should be adjusted properly. Sounds like somebody mapped either the wrong status or command point to the no flow alarm. Make sure the status drops out when the pump isn't running.

    The DP switch is sensing not the system pressure, but the differential between the suction and discharge side of the booster pump. If that differential is less than 8psi then it's lower than the lowest setting on that particular switch. If the switch isn't working you may need to go with a P74EA-8C which is 2-26psi adjustable range. The P74FA-5C has an 8-60psi range. The 1.5 differential is how much the pressure needs to drop once the switch makes to change the switch state.
    Good points. I'll verify the status dropping out when the pump is shut off, like you said. I'll also force the circ. pump on with the HV fully open to the coil and take a DP reading at the pump, as keepitcool stated.
    I am now starting to see how to correct the problem. Thanks to all, for your input. I'll post the results.
    "Nearly all men can stand adversity, but if you want to test a man's character, give him power."
    -Abraham Lincoln

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